Exam Preparatory Manual for Undergraduates: Fixed Partial Dentures Lovely M
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1

Essays

  1. Discuss the importance of diagnosis and treatment planning in fixed partial denture.
  2. List the ideal requirements of an abutment tooth and describe the steps taken to maintain biological integrity while preparing teeth for fixed partial denture.
  3. Describe various methods used for gingival retraction and add a note on the advantages of fluid control and tissue displacement.
  4. Describe the methods used to classify fixed bridges. Add a note on types of retainers used in fixed partial dentures.
  5. Define and classify retainers. List the advantages and disadvantages of partial veneer crown over other types of retainers. What are the factors that come into play in the selection of retainers?
  6. Define retention and resistance form in fixed partial denture. What are the factors affecting retention and resistance in posterior tooth preparation?
  7. Classify finish lines. Enumerate the functions, indications, advantages and disadvantages of finish lines. Add a note on selection of type of finish line to be used.
  8. Illustrate with diagrams, the tooth preparation on right upper central incisor for receiving porcelain jacket crown. Add a note on the advantages, indications and contraindications of all-porcelain jacket crown.
  9. Define a crown and mention the advantages, disadvantages and indications of a full metal crown. Describe in detail, with diagrams, the step-by-step procedure in the preparation of a full metal crown on a mandibular first molar.
  10. Describe in detail, with diagrams, the step-by-step preparation of a maxillary canine to receive a three quarter crown. Explain and justify different impression techniques for fixed partial denture.
  11. Name the component parts of a bridge. Define and classify pontics. Add a note on selection of pontic design in anterior and posterior teeth. What are the requirements of a pontic?
  12. What are the biological, mechanical and esthetic considerations in designing a pontic? Add a note on fabrication of pontics.
  13. Describe different types of provisional restorations. Describe various techniques used for making provisional restoration for anterior and posterior tooth preparation.
  14. Describe ideal requirements of luting agents and various luting agents used in crown and bridge cementing. Explain the procedure and care to be taken to lute porcelain jacket crowns.
  15. Classify adhesive bridges/resin bonded bridges. Cite the advantages, disadvantages, indications, contraindications and preparation steps. Add a note on types of bonding to metal and tooth.
  16. What are the requirements of dies? Describe materials used in preparation of dies and few techniques used for preparing dies?
  17. Classify ceramics. Enumerate its advantages and disadvantages. Explain the mechanism of bonding and describe the laboratory steps involved in fabrication of different ceramics.
  18. What are the types of veneering materials? Describe their advantages, disadvantages and indications. Add a note on the recent advances in veneering materials used in fixed partial denture.
  19. Discuss in detail failures in fixed dental prosthesis.
IMPLANT DENTISTRY
  1. Classify and discuss on various impression procedures involved in implant-supported prosthesis.
  2. Discuss on implants for fixed prosthesis work.
  3. Classify types of implants and discuss the role of surface topography of implants in osseointegration.
  4. Explain procedural options for single tooth implant prosthesis
  5. Discuss the immediate loading and progressive loading of endosseous implants.
  6. Occlusal consideration for implant-supported prosthesis.
  7. Recent advances in surgical techniques.
  8. Discuss implant failures and precautions that need to be taken.
3
ESSAYS
1. Discuss the importance of diagnosis and treatment planning in fixed partial denture.
Diagnosis is the examination of the physical state, evaluation of the mental or psychological makeup and understanding the needs of each patient to ensure a predictable result.
Treatment planning means developing a course of action that encompasses the ramifications and sequelae of treatment to serve the patient's needs.
 
 
Diagnosis
Diagnosis in fixed partial denture includes:
  1. Chief complaints.
  2. History taking of the patient.
  3. Examination.
    • General examination
    • Temporomandibular joint examination
    • Extraoral examination
    • Intraoral examination
    • Occlusal evaluation
    • Abutment tooth evaluation.
  4. Making of diagnostic casts.
  5. Full-mouth radiographs.
Chief complaints
Are mainly of four categories:
  1. Comfort (pain, sensitivity, swelling).
  2. Function (difficulty in mastication or speech).
  3. Social (bad taste or odor).
  4. Appearance (fractured or discolored tooth).
Comfort
Pain
Location, character, severity and frequency of the pain should be noted as well as the first time it occurred, the factors increasing pain (e.g. hot or cold things) and any changes in its character.
Swelling
The location, size, consistency and color change during inflammation and the duration, and frequency of the swelling needs to be noted.
Function
Difficulties in chewing can be due to a fractured cusp or generalized malocclusion. Speech difficulty may be due to local cause or systemic problems.4
Social
A bad taste or smell may be due to poor oral hygiene or periodontal disease.
Appearance
  • Missing or crowded teeth
  • Fractured tooth or restoration
  • Malpositioned or discolored teeth
  • Congenital anomalies of dentition.
History
  1. Personal details (name, age, sex, address).
  2. Medical history.
  3. Drug history.
  4. Dental history.
    • Periodontal history
    • Restorative history
    • Endodontic history
    • Orthodontic history
    • Removable prosthodontic history
    • Oral surgical history
    • Radiographic history
    • TMJ dysfunction history.
Medical history
  • Any cardiac ailments, requiring antibiotic premedication before treatment, CNS disorders or other systemic diseases affecting treatment method. Hypertensive patients and coronary disease patients should not be given epinephrine
  • Any previous radiation therapy, blood disorders, terminal illness affecting treatment plan
  • Systemic conditions with oral manifestations
  • Infective diseases as aids, hepatitis and syphilis needs to be evaluated.
Drug history
Previous medication history, drug allergies and if patient is taking any medicines routinely should be noted.
Dental history
Periodontal history
Oral hygiene status, any previous oral hygiene prophylaxis or any previous periodontal surgery is noted.
Restorative history
All restorations of amalgam and tooth-colored restorations along with time of these restorations are noted.
Endodontic history
If the endodontically restored tooth is a prospective abutment tooth then a radiographic evaluation of the periapical health is noted.5
Orthodontic history
If radiographic evaluation shows root resorption, it can be due to previous orthodontic treatment. Occlusal adjustment with minor tooth movement can promote long-term positional stability of the teeth and reduce, or eliminate, parafunctional activity.
Removable prosthodontic history
Previous removable prostheses must be carefully evaluated and the duration of wear needs to be noted.
Oral surgical history
Missing teeth and period of edentulousness should be noted.
Radiographic history
Previous radiographs and current diagnostic radiographic series aids to assess the progress of the disease. It also aids in locating impacted tooth, root tips and cyst and tumours.
TMJ dysfunction history
  1. Pain or clicking in the temporomandibular joints.
  2. Tenderness to palpation.
  3. Difficulty in opening the mouth.
  4. Deviation while opening.
  5. The above symptoms with any treatment done earlier for the dysfunction as occlusal appliances, medications, or exercises should be noted.
Examination
General examination
General appearance, gait, weight skin color (anemia or jaundice). Vital signs, such as respiration, pulse, temperature, and blood pressure, are measured and recorded.
Temporomandibular joint examination
  • Bilateral palpation anterior to the auricular tragi while the patient opens and closes the mouth, can locate disorder in the posterior attachment of the disk
  • Tenderness clicking, or pain is noted
  • Jaw opening of less than 40 mm indicates restriction
  • Deviation from midline is also recorded
  • Maximum lateral movement can then be measured (normal is about 12 mm)
  • Masseter and temporal muscles are palpated for signs of tenderness and classified as mild, moderate, or severe.
Extraoral examination
Includes noting of:
  • Facial asymmetry
  • Cervical lymph node palpation
  • TMJs and the muscles of mastication
  • Lips: Smile line, negative space between the maxillary and mandibular teeth when the patient laughs, missing teeth, diastemae and fractured or poorly restored teeth are noted.
6
Intraoral examination
Soft tissues, teeth and supporting structures as the tongue, floor of the mouth, vestibule, cheeks and hard and soft palates are examined and findings noted.
Periodontal examination
  • Oral hygiene status assessment
  • Examination of gingiva, periodontium and the response to the host tissues (BPE)
    • Healthy gingiva is pink, stippled and bound to the underlying connective tissue
    • The texture, size, contour, consistency, position and color are noted. Any exudate or pus is examined for
    • The width of the keratinized attached gingiva around each tooth is assessed.
Examination of teeth
  • Absence of teeth, dental caries, any restorations, wear faceting, fractures, abrasions, malformations and erosions are noted
  • Pocket depths (usually six per tooth) are recorded on a periodontal chart.
Occlusal examination
  • General alignment
  • Lateral and protrusive contacts
  • Centric relation
  • Jaw maneuverability.
General alignment
The teeth can be evaluated for crowding, rotation, over eruption, spacing, malocclusion and vertical and horizontal overlap.
Lateral and protrusive contacts
The presence or absence of tooth contact in eccentric movements is verified with a thin Mylar strip. Tooth movement (fremitus) should be identified by palpation.
Centric relation
The relationship of teeth in both centric and intercuspal position is assessed. If a slide from CR to IP is present, its horizontal and vertical components can be estimated and a note made of any lateral deviation.
Jaw manoeuvrability
The ability and ease with which the patient moves the jaw and the guiding movements should be assessed.
Check for habitual occlusion.
Abutment tooth evaluation
  • Abutment teeth need to be strong enough to withstand the forces directed to the missing teeth in addition to those usually applied to the abutments
  • Abutment teeth should not exhibit mobility
  • An asymptomatic endodontically treated tooth can be considered for an abutment provided it can withstand the forces transmitted to it7
  • The supporting tissues surrounding the abutment teeth should be healthy and free from inflammation.
Evaluation of abutment teeth includes:
  • Crown–root ratio
  • Tooth configuration
  • Periodontal surface area
  • Vitality testing.
Crown–root ratio
An abutment teeth should have a combined pericemental area equal to or greater in peri-cemental area than the tooth or teeth to be replaced (Antes law).
Favorable crown-root ratio is 1:1.
Tooth configuration
  • Root shape: Short conical roots give less support. Divergent multiple roots give good support.
Periodontal surface area
Root surface area: Larger teeth will have greater surface area and will handle stress better.
Vitality testing
Prior to any restorative treatment, pulpal health must be assessed by measuring the response to percussion as well as thermal and electrical stimulation.
Diagnostic casts
  • Articulated diagnostic casts aid in planning treatment procedures, provide information about static and dynamic relationships of the teeth and help to view several aspects of the occlusion not detectable within the confines of the mouth.
Advantages of diagnostic cast
  • Changing the arch relationship before orthognathic procedures
  • Changing the tooth position prior to orthodontic procedures
  • Modifying the occlusal scheme before attempting any selective occlusal adjustment
  • Trial tooth preparation and waxing can be done before fixed restorative procedures
  • Selection of an optimum path of withdrawal of a fixed partial denture can be assessed.
Radiographic examination
  1. Periapical radiographs.
  2. Bitewing radiographs.
  3. Panoramic films.
  4. Transcranial exposure, serial tomography, arthrography, CT scanning, or magnetic resonance imaging in case of TMJ disorders.
Periapical radiographs
(14 periapical radiographs help in complete examination)
Uses
  • To assess the extent of bone support, quality of supporting bone8
  • Detailed root morphology of each abutment tooth
  • Width of periodontal ligament space
  • Bone resorption (vertical, horizontal)
  • Inclination of teeth
  • Continuity of lamina dura
  • Pulpal morphology and previous endodontic treatment
  • Any periapical pathology can be evaluated
  • Evaluation of crown–root ratio
  • Evaluation of the shape, length and direction of root
  • Helps in checking for periodontal situation of the abutment tooth, as widening or thickening of lamina dura, occlusal prematurities or trauma can cause thickening.
Bitewings
Use
Evaluation of caries on proximal surfaces and secondary caries on previous restorations.!
Panoramic films
Uses
  • Evaluation of bone resorption, pattern of bone resorption and quality of bone support
  • To check for presence of retained root tips, impacted tooth
  • To determine the thickness of soft tissue on the ridge in area of pontic placement
  • They do not provide a detailed view for assessing bone support, root morphology, or caries.
Treatment Planning
Treatment planning helps to design and select the material of choice for a particular situation.
The design and material choice depends on:
  • Amount of tooth structure present
  • Esthetics
  • Plaque control.
Choice of restoration
In following situations, fixed partial denture is contraindicated and the restoration of choice is removable partial denture.
  • Large edentulous space
  • Edentulous space with no distal abutment
  • Bilaterally edentulous with no distal abutment
  • Grossly tipped teeth
  • Periodontally weak teeth
  • Teeth with short clinical crowns
  • Severe bone resorption
  • Young age
  • Large tongue
  • Extensive caries with poor oral hygiene.9
2. List the ideal requirements of an abutment tooth and describe the steps taken to maintain biological integrity while preparing teeth for fixed partial denture.
Alternate form of the question
What are the biomechanical principles used in tooth preparation for a fixed partial denture?
Requirements of an Abutment Tooth
  • Abutment teeth need to be strong enough to withstand the forces directed to the missing teeth in addition to those usually applied to the abutments
  • Abutment teeth should not exhibit mobility
  • An endodontically treated tooth can be considered if it is asymptomatic and if it can withstand the forces transmitted to it. The supporting tissues surrounding the abutment teeth should be healthy and free from inflammation.
 
Factors to be considered in abutment selection
  • Crown–root ratio
  • Tooth configuration
  • Periodontal surface area
  • The length of the pontic span
  • Forces
  • Oral hygiene measures.
Crown–root ratio
  • An abutment teeth should have a combined pericemental area equal to or greater in pericemental area than the tooth or teeth to be replaced (Antes’ law)
  • The optimum crown–root ratio is 2:3. A root ratio of 1:1 at least needs to be present for a prospective abutment
  • If coronal structure is less, core build up or crown lengthening needs to be done to fulfil the crown–root ratio.
Tooth configuration
Root shape and angulation
A molar with divergent roots, single-rooted tooth with an elliptic cross section (broader labiolingually than mesiodistally), a long root or multiple roots provide better support than a root surface area with circular cross section.
Periodontal surface area
Root surface area
Larger teeth will have greater surface area and will handle stress better.
Bone support
Teeth with vertical and horizontal resorption give less support.10
The length of the pontic span
  • Two abutment teeth can support two pontics
  • Failures due to abnormal stress have been attributed to increased length of the span.
Forces
Varying the occlusal scheme by altering the occlusal table or plane can decrease the load on abutment teeth.
Oral hygiene
Teeth with slight mobility can be used as abutments if stabilised and if oral hygiene is maintained.
Principles of Tooth Preparation
  1. Biologic considerations.
  2. Mechanical considerations.
  3. Esthetic considerations.
Biologic considerations
(Are factors that affect the health of the oral tissues)
  1. Prevention of damage during tooth preparation to:
    • Adjacent teeth
    • Soft tissues
    • Pulp of the tooth being prepared.
  2. Conservation of tooth structure.
  3. Margin placement.
  4. Finish lines.
  5. Occlusal consideration.
1. Prevention of damage during tooth preparation to:
Adjacent teeth
If the proximal contact area is damaged during preparation it needs to be reshaped and polished otherwise it is susceptible to dental caries.
To prevent damage:
  • A metal matrix band needs to be used around the adjacent tooth
  • A thin tapered diamond is used to break the inter-proximal contact.
Soft tissues
Damage to the tongue and cheeks can be prevented by careful retraction with an aspirator tip, mouth mirror or flanged saliva ejector.
Pulp
Extreme temperatures, chemical irritation can cause pulpal damage.
Prevention
  • Assess morphology of the dental pulp chamber before preparation with the help of a radiograph
  • Use new and perfect abrasives while reducing the tooth. This reduces the heat that is generated11
  • Apply gentle pressure while preparation of tooth
  • Use copious amount of water spray directed at the area of contact between tooth and bur. This removes clogging and prevents desiccation of the dentin
  • All retention grooves and polishing need to be done with a slow-speed handpiece with adequate amount of water spray
  • Avoid use of chemical agents for cleaning.
Conservation of tooth structure:
  • Use of partial veneer crowns instead of full veneer crowns
  • Minimum taper between axial walls
  • Occlusal surface reduction following anatomic planes
  • Tilted tooth to be repositioned, so that less tooth structure is removed during preparation
  • A conservative margin finish
  • Supra gingival or crest of the gingival margin finish line.
Failures due to improper preparation of tooth:
  • Insufficient axial reduction with an over contoured restoration can cause periodontal disease or dental caries
  • Inadequate occlusal reduction can cause occlusal dysfunction and poor margin placement
  • Excessive axial contours can cause gingival inflammation.
Margin placement
Requirements
  • Ease of preparation without overextension
  • Easy to identify in the impression and on the die
  • Easy to finish on wax pattern
  • Sufficient bulk of material
  • Preserve tooth structure.
Types
  1. Supragingival.
  2. Subgingival.
  3. At the crest of the gingiva.
Supragingival margins
  • They can be easily finished
  • They are more easily kept clean
  • Impressions are more easily made with less potential for soft tissue damage
  • Restorations can be easily evaluated at recall appointments.
Subgingival margins
  • Subgingival margins are often on dentin or cementum
  • They are done when cervical erosion or restorations extend subgingivally and when a crown-lengthening procedure cannot be carried out12
  • A well-designed preparation has a margin that is smooth and will provide the patient with a longer-lasting restoration.
Finish lines
Types
  1. Featheredge or shoulderless crown preparations: Conservative but not to be used.
  2. Chisel edge: Only on tilted tooth.
  3. Chamfer: All metal restorations.
  4. Shoulder: All ceramic restorations and not conservative.
  5. Sloped shoulder: Reduces possibility of leaving unsupported enamel and leaves sufficient bulk to allow thinning of the metal framework to a knife-edge for good esthetics.
  6. Shoulder with bevel: In subgingivally extended finish lines due to cervical erosion. A bevelled shoulder margin is used for the facial surface of a metal–ceramic restoration where a metal collar is to be used.
Advantages of a bevel
  • Easy burnishing of the cast metal margin
  • Decreases marginal discrepancy
  • Protects unprepared tooth structure from chipping.
Occlusal considerations
If occlusion is disrupted by supra-erupted or tilted teeth either uprighting of tooth or a modified restoration should be considered after endodontic treatment.
Adequate occlusal clearance is required for an optimal occlusion.
Mechanical Considerations
  1. Providing retention form.
  2. Providing resistance form.
  3. Preventing deformation of the restoration.
Providing Retention form:
This is the quality of a preparation that prevents the restoration from becoming dislodged by such forces parallel to the path of withdrawal.
Factors affecting retention:
  • Magnitude of the dislodging forces
  • Geometry of the tooth preparation
  • Roughness of the fitting surface of the restoration
  • Materials being cemented
  • Type of luting agent
  • Film thickness of the luting agent.
Magnitude of the dislodging forces
Great removal forces occur with sticky food. The magnitude of the dislodging forces depends on the stickiness of the food and the surface area and texture of the restoration being pulled.13
 
Geometry of the tooth preparation
Fixed prostheses depend on the geometric form rather than on adhesion for retention as majority of the luting agents are non-adhesive like zinc phosphate.
Factors
  • Taper
  • Surface area
  • Stress concentration
  • Type of preparation.
Taper
  • Maximum retention is obtained if a tooth preparation has parallel walls
  • Smaller degrees of taper have more retention
  • As the taper increases, however, so does the free movement of the restoration and retention will be reduced
  • The recommended convergence between opposing walls is 6 degrees.
Surface area
Provided the restoration has a limited path of withdrawal, its retention is dependent on the length of this path in sliding contact. Therefore, crowns with long axial walls are more retentive than those with short axial walls.
Stress concentration
Round margins may reduce stress concentrations and hence, increase the retention of the restoration.
Type of preparation
Retention is increased by adding grooves and boxes to a preparation with a limited path of withdrawal.
Roughness of the surfaces
Retention is increased if the restoration is roughened or grooved by air-abrading the fitting surface with alumina.
Materials being cemented
  • More reactive the alloy, more adhesion with certain luting agents
  • Base-metal alloys are better retained than less reactive high-gold content metals
  • Cement adheres better to amalgam than to composite resin or cast gold
  • Crowns adhere better with composite resin than with amalgam cores.
Type of luting agent
Adhesive resin cements are the most retentive, with less bond during long-term.
Providing Resistance form
Lateral forces tend to displace the restoration by causing rotation around the gingival margin which is prevented by any areas of the tooth preparation that are placed in compression called resistance area.14
Resistance depends on:
  • Magnitude and direction of the dislodging forces
  • Geometry of the tooth preparation: Increased taper and rounding of axial angles, short tooth preparations with large diameters reduce resistance
  • Physical properties of the luting agent: Zinc phosphate cements have a higher modulus of elasticity than polycarboxylate cements, hence polycarboxylate cement depends more on the taper of the preparation than zinc phosphate cement.
Preventing deformation of the restoration
Restoration must have sufficient strength to prevent it from being permanently deformed during function.
Factors
  • Alloy selection
  • Adequate tooth reduction
  • Margin design.
Alloys
Type III or Type IV gold alloys. High-noble metal content metal–ceramic alloys have a hardness equivalent to Type IV golds and nickel–chromium alloys are more hard and can be used in long-span fixed partial dentures (FPD).
Adequate tooth reduction
A minimum alloy thickness of about 1.5 mm over centric cusps (buccal in the mandible and lingual in the maxillae) and 1 mm in non-functional cusps. Occlusal reduction should follow the morphology of tooth.
Margin design
Depending on the type of restoration the ideal margin design needs to be followed.
Esthetic considerations
  1. Complete examination and assessment of teeth during smiling, talking, and laughing.
  2. Find the patient's esthetic requirements.
  3. Type of restoration selected.
Partial-coverage restorations
Proximal margin
Place proximal margin just buccal to the maximal contact area where metal will be hidden by the distal line angle of the neighbouring tooth.
Facial margin
The facial margin of a maxillary partial-coverage restoration should be extended just beyond the occlusofacial line angle. A short bevel is needed to prevent enamel chipping. A chamfer can be placed where appearance is less important (e. g., on molars) because this will provide greater bulk of metal for strength.
Metal–ceramic restorations
Facial tooth reduction
A minimum reduction of 1.5 mm is required for sufficient bulk of porcelain and metal for strength.15
Labial margin placement
Supragingival margin placements are easier to prepare properly and easier to keep clean.
After observing the patient's smile, margins should be placed.
Margins should not be placed subgingivally encroaching on the attachment. If it extends within 1.5 mm of the alveolar crest bone resorption tends to occur.
3. Describe various methods used for gingival retraction and add a note on the advantages of fluid control and tissue displacement.
 
Definition
Gingival retraction is a process of exposing margins when making impressions of prepared teeth.
 
Other names
Tissue dilation, tissue retraction and tissue displacement.
 
Advantages of gingival retraction
  • Duplicating subgingival margins in impressions
  • Copying the unprepared tooth surface to aid in evaluating the marginal finish line accurately
  • Helps in accuracy of wax pattern fabrication and location of finish line
  • Final restoration tends to have better marginal adaptation
  • To alter the contour of the gingival tissue around the teeth or edentulous ridge.
 
Classification of Gingival Displacement
Generally classified as non surgical and surgical
Nonsurgical
  • Mechanical: Retracted by mechanical methods
  • Chemical: Dilation with help of certain dilatants
  • Mechanical–chemical: Retracted with a cord impregnated with a chemical for hemostasis.
Surgical
  • Rotary
  • Conventional (gingivoplasty, gingivectomy and periodontal flap procedures)
  • Electrosurgery
  • Lasers (CO2, argon, diode and Nd:YAG).
 
Mechanical methods
  1. Retraction by modified temporay crown or impression caps.
  2. Modified impression techniques
    • Oversized copper bands with elastomeric impression materials
    • Temporary acrylic coping
    • Modified custom tray technique
    • Matrix impression system.16
  3. Mechanical retraction
    • Gingival protectors
    • Matrices and wedges/rubber dam
    • Strips (hydroxylate polyvinyl acetate)/paste
    • Syringed PVS/aluminium chloride.
  4. Mechanochemical methods
    • Retracting with hemostatic agents
    • Retracting with paste and hemostatic agent.
Mechanical methods
Impression techniques
Oversized copper bands
  • Are placed on the prepared tooth and either impression compound or elastomeric impression material supported by acrylic with adhesive is used to make an impression of the prepared tooth which retracts the gingiva under pressure
  • Time consuming and can cause trauma to tissue.
Temporary acrylic coping
Acrylic coping is relieved by 1 mm and an adhesive is used followed by an elastomeric impression material.
Matrix Impression system (MIS)1
  • Three impression procedures using three viscosities of impression materials
  • Technique—A matrix fitting the tooth preparation is made using a semi-rigid elastomeric material before gingival retraction
  • After gingival retraction a definitive impression is made with the matrix using a high- viscosity elastomeric impression material
  • Pick up with the matrix impression seated a stock tray filled with a medium-viscosity elastomeric impression material is used to make the entire arch impression
  • Advantage—The design of matrix gently forces the high-viscosity impression material into the sulcus, which does not allow it to collapse as the medium-viscosity material in the stock tray is seated for the pick-up impression
  • Disadvantage—Increased chair side time.
Modified custom tray Technique2
  • Custom tray is modified by intra oral relining with autopolymerising acrylic resin
  • The relined custom tray is trimmed by 2 mm to obtain clearance for elastomeric impression material
  • In case of subgingival finish lines that area is only trimmed by 0.5 mm so that the elastomeric impression material is directed into sulcus
  • Advantages—Time saving, accurate and retraction cord can be avoided.17
Chemical methods
  1. Anti-sialogogues.
  2. Local anesthetics.
  3. Anticholinergics: Methantheline bromide (50 mg), propantheline bromide (15 mg), atropin, etc.
  4. Antihypertensives as clonidine hydrochloride (0.2 mg).
Chemical methods are done when mechanical methods cannot be achieved. Each of the drugs has side effects, which need to be critically evaluated.
Mechanical–Chemical Methods
This technique consists of using impregnated cords with chemicals.
Chemicals that are used are:
  • Racemic epinephrine: Contraindicated in cardiac, hypertensive and diabetic patients
  • Aluminium potassium sulphate (least inflammation and little sulcus collapse after cord removal)
  • Aluminium chloride and ferric sulphate (inhibits set of polyvinyl siloxane and polyether impressions)
  • Phenylephrine hydrochloride
  • Others as inert matrix polyvinyl-siloxane (generates hydrogen that causes expansion of material against sulcus walls)
  • Injection of 15% aluminum chloride in Kaolin matrix (easy, no pain, no trauma but inhibits polyether and polyvinyl siloxane).
 
Armamentarium
  1. Saliva ejector.
  2. Mouth mirror.
  3. Explorer.
  4. Scissors.
  5. Cord packing instrument.
  6. Retraction cord.
  7. Hemodent liquid.
  8. Cotton pliers.
  9. Cotton rolls.
  10. Dappen dish.
  11. Cotton pellets.
  12. 2 × 2 gauze sponges.
 
Method of Gingival Retraction
  1. The prepared tooth area is dried and isolated with cotton rolls.
  2. A retraction cord of two-inch length is drawn out from the dispenser bottle held with sterile pliers and cut with scissors.
  3. The retraction cord is dipped in 25% aluminium chloride solution or 8% epinephrine.
  4. The excess amount of aluminium chloride is squeezed out with a gauze piece.18
    zoom view
    Fig. 1: The cord is first secured in the mesial interproximal area and the distal interproximal area with a cord-packing instrument
    zoom view
    Fig. 2: Cut the excess amount of cord in the mesial interproximal area
    zoom view
    Fig. 3: Complete the placement of cord on the buccal side from the distal end to the mesial side until it overlaps the mesial portion
  5. The cord is made into a “u”, and looped around the prepared tooth. Only the end of the cord is to be touched (Fig. 1).
  6. The cord is first secured in the mesial interproximal area and the distal interproximal area with a cord-packing instrument.
  7. After the cord is secured in the distal interproximal area, the cord is inserted from the mesiolingual to distolingual corner. While tucking in the cord the tip of the packing instrument should be angled towards the area where the cord has been placed.
  8. Cut the excess amount of cord in the mesial interproximal area (Fig. 2) and complete the placement of cord on the buccal side from the distal end to the mesial side until it overlaps the mesial. A minimum bulk of 0.2 mm sulcular width is essential to make an undistorted impression (Fig. 3).
  9. After 5–10 minutes, the cord is gently removed with the sulcus around the prepared tooth exposed and hemostasis maintained.
  10. This is followed by the impression of the arch.
 
Double cord technique
Advantage—Declines the tendency of gingival cuff to recoil and partially displace the impression material as it sets.19
This is a technique in which a thin cord is placed initially and over which a large cord is placed. Thin cord remains during impression making. Hansen et al. observed that 98% of prosthodontists use cords of which 48% use a dual cord technique.3
Surgical Tissue Dilation
Electrosurgery/Surgical diathermy
(Introduced by D'Arsenal).
An electrosurgery unit is a high-frequency oscillator or radio transmitter which uses either a vacuum tube or a transmitter for delivering a high-frequency electric current of at least 1 Mega Hertz.
An electrosurgical unit consist of oscillator, active electrode and a ground electrode for the safety of the patient.
Types of electrode
  • Coagulating electrode: Controls hemorrhage
  • Small wire-loop electrode: Used for sulcular enlargement
  • Round electrode: Removes gingival tissue
  • Large loop electrode: Is used to remove large amount of tissue
  • Straight electrode: By angling the working electrode at approximately 15–20 degrees and carrying the tip through the tissue, a small wedge of tissue can be removed. In anterior quadrants, the angle of the working electrode is positioned parallel to the long axis of the tooth.
Posner Electrode
With the AP 1 ½ electrode, the insulated portion of the electrode is directed around the tooth, removing the gingival sulcular epithelium. If less trough depth is desired, part of the tip is removed to create the desired depth at 0.5 mm, 0.75 mm, or 1.0 mm.
Types of current
  • Monoterminal: Used for fulguration, removal of papillomas and fistulous tract
  • Biterminal: For coagulation, removal of granulation tissue
  • Unrectified damped current: Not used in dental treatments
  • Fully-rectified full-wave modulated current: Good for gingival enlargement
  • Fully-rectified filtered current: Is the best current source.
Method
  1. Profound local anesthesia is given and a pleasant smelling aromatic oil is applied on the vermilion border of upper lip.
  2. Plastic suction tips and plastic-mounted mouth mirrors are used. Odor is controlled by an outside ventilated oral evacuator system.
  3. An adequate power is set on the unit. Electrode is passed quickly over the tissue to be removed. Adequate time interval between each stroke needs to be followed.
  4. Fragments of tissue are removed with an alcohol-soaked sponge.
Indications
  • Minor tissue removal before impression procedures20
  • Removal of granulation or inflamed tissue around a given tooth
  • For enlargement of gingival sulcus in some cases
  • Crown lengthening.
Contraindications
  • In cardiac pacemaker patients
  • Not to use on thin attached gingiva.
Rules
  1. Profound anesthesia.
  2. No metal instruments should be used.
  3. Proper grounding should be done.
  4. A fully-rectified filtered current should be used.
  5. Electrode should not make contact with any metal restorations in the patients mouth.
  6. A light stroke with a 5 second time interval between applications of the electrode.
  7. If the tip drags, the instrument is at too low a setting and the current should be increased.
  8. If sparking is visible the current level is set high and need to be decreased.
  9. A cutting stroke should not be repeated within 5 seconds.
  10. The electrode must remain clean of tissue fragments.
  11. The sulcus should be swabbed with hydrogen peroxide before the displacement cord is placed.
  12. Maintaining the biologic width after tissue healing.
  13. After the impression is made tincture of myrrh and benzoin is placed till healing completes in 5–10 days.
Rotary curettage/Gingettage
(Introduced by Amsterdam in 1954)
  • This is a troughing technique to remove limited amount of epithelial tissue in the sulcus while chamfer finish line is prepared.
  • This technique needs to be done on healthy, non-inflamed gingiva to avoid tissue shrinkage after healing of the diseased tissue.
Lasers4
Diode lasers, such as neodymium: yttrium-aluminum-garnet (Nd-YAG), Erbium: yttrium-aluminum-garnet (Er:YAG) and CO2 laser are commonly used for gingival retraction around natural teeth.
Advantage
Less bleeding (CO2 laser), gingival recession, painless and sterilizes sulcus.
Disadvantages
  • Er:YAG laser is not as good at hemostasis as CO2 laser
  • CO2 laser provides no tactile feedback hence, can damage junctional epithelium.
Advantages of Fluid Control and Tissue Management
  • Patient comfort21
  • Safety for patient
  • Good visibility while doing clinical procedures
  • Clear the area of saliva and water, for better visibility.
Fluid control attained by:
  • Rubber dam
  • Cotton rolls
  • High-volume vacuum
  • Saliva ejector
  • Svedopter
  • Vac-ejector
  • Moisture absorbing cords.
 
Rubber dam
Punch holes are made in the area of preparation site of the rubber dam and clamped in position.
Uses
  • While removing old restorations
  • While preparing onlay or inlay
  • For endodontic procedures as a safety measure
  • While using pin-retained restoration.
Disadvantages
  • Difficult to use while preparing crowns and FPD
  • Reacts with polyvinyl siloxane impression materials.
 
Cotton rolls
Absorbent cotton rolls are placed in the area where saliva pools (in maxillary arch, a single cotton roll is used in buccal vestibule and in mandibular arch in lingual sulcus).
Placement of cotton roll
One or two cotton rolls are placed vertically against the horizontally placed cotton rolls or one single horse-shaped roll can be used.
Limitation
The entire saliva and water soaked roll needs to be removed each time.
 
High-volume vacuum
Can be used as a retractor as well as for clearing saliva and water during preparation.
 
Saliva ejector
  • Useful for maxillary arch along with cotton rolls
  • Placed in the corner of mouth opposite the quadrant being operated with the patient's head towards that side.22
 
Svedopter/Speejector
Svedopter consist of a metal saliva ejector with a tongue deflector. Effectively used in mandibular arch. Effective fluid control along with cotton rolls. The patient is seated in an upright position.
Positioning
Placed in the incisor region with the tubing under the patient's arm.
Disadvantages
  • Access to lingual aspect is limited
  • Metal surface can cause tissue irritation
  • Can cause gagging in some patients
  • Cannot use in mandibular tori patients.
Vac-ejector
Tongue control and high-volume evacuation along with a bite block. It aids in removing large volumes of fluid. Tongue deflectors and bite blocks are available in several sizes. The tongue deflectors are made of plastic.
Moisture absorbing cords
Consist of pressed paper wafers covered on one side with a reflective foil. The wafer side is placed facing the tissues. Used along with cotton rolls to control saliva and retract cheek laterally.
 
References
  1. Livaditis GJ. The matrix impression system for fixed prosthodontics. J Prosthet Dent. 1998;79:208-16.
  2. Ortensi L, Stocchi ML. Modified custom tray. J Prosthet Dent. 2000;84(2):237-40.
  3. Hansen PA, Tira DE, Barlow J. Current methods of finish-line exposure by practicing prosthodontists. J Prosthodont. 1999;8:163-70.
  4. Martin E, Parker S. The use of lasers in fixed prosthodontics. Dent Clin North Am. 2004;48:971-98.
4. Describe the methods used to classify fixed bridges. Add a note on types of retainers used in fixed partial dentures.
Classifications of Fixed Bridges
  • Classification depending on edentulous spaces/site
  • Classification based on type of connector
  • Classification based on type of abutments
  • Classification based on retention
  • Classification based on materials used
  • Classification based on temporary and permanent nature of bridges
  • Classification based on span length.
Classification depending on edentulous spaces
Class 1: Posterior edentulous space.
Class 2: Anterior edentulous space.23
Class 3: Anterior and posterior edentulous spaces.
Each of these classifications has divisions and subdivisions.
Divisions
To get idea on the type of support.
Division I
Abutments on one side of the edentulous area are capable of rendering support (Cantilever FPD).
Division II
Abutments on both sides of the edentulous space are capable of giving support. (Conventional FPD).
Division III
A single tooth in the center with edentulous space on either side (Pier Abutments).
Sub-divisions
Denote the position of a prospective abutment tooth.
Sub-division I: Ideal abutments.
Sub-division II: Tilted abutments.
Subdivision III: Periodontally-weak abutments.
Subdivision IV: Extensively damaged coronal structure with good bone support.
Sub-division V: Implant abutment.
Groupings
Each of the subdivisions are further grouped into A and B.
Group A: A single abutment on one side of the edentulous space each is sufficient.
Group B: More than one abutment support is required from each side of the edentulous space.
Another classification based on site
Anterior Bridge—Unilateral (not crossing midline) or bilateral (involves both sides).
Posterior Bridge—Only in posterior region.
Complex Bridge—One of terminals beyond canine.
Classification based on type of connector
  1. Rigid connector.
  2. Semi-rigid connector.
  3. Removable fixed partial denture.
Classification based on type of abutments
  1. Ideal abutment.
  2. Cantilever abutment.
  3. Pier abutment.
  4. Abutment receiving pin support from coronal structure.
  5. Telescopic crown.
  6. Endodontically-treated abutment with cast post or post and core support.24
  7. Resin-bonded bridges.
Classification based on retention
Simple and compound.
Simple
Fixed-Fixed—Fixed at both ends.
Fixed-Movable—Nonrigid connector at one terminal (limited movement).
Cantilever—Free end bridge.
Spring cantilever—Pontic at the end of resilient curved arm with support from a distant abutment.
Resin bonded—Presently a single wing is commonly used.
Compound bridge/Hybrid
When more than one type of simple bridges are employed.
Classification based on materials used
Metallic, nonmetallic and combined
Metallic
  • Precious (gold alloy)
  • Semi precious
  • Nonprecious (base-metal alloy).
Nonmetallic
Resin/Ceramic.
Combined
  • Metal with labial/buccal ceramic veneer
  • Metal with fullveneer
  • Metal-reinforced bridges.
Classification based on temporary and permanent nature of bridges
Provisional
Temporary—After preparation till, a definitive prosthesis is given.
Immediate—Made before extraction and placed immediately after extraction.
Interim prosthesis, splints.
Definitive/Permanent
Fixed partial dentures (single, complex and pier-abutment prosthesis).
Implant abutments.
Classification based on span length
Short span—One missing tooth
Medium span—Two missing tooth
Long span—More than two missing tooth.
Types of Retainers
Complete crowns
  1. All-metal crowns.
  2. All-ceramic crowns.25
  3. Metal–ceramic crown.
  4. Acrylic-fused-to-metal crowns.
  5. Metal-free (fiber-reinforced composites) crowns.
Partial crowns
Posterior tooth
  1. Mesial one-half crown.
  2. Three-quarter crown.
  3. Modified three-quarter crown.
  4. Seven-eight crown.
Anterior tooth
  1. Three-quarter crown.
  2. Variations of three-quarter crown (Selberg crown).
Complete crowns
All-metal fixed partial dentures
  • They are used for replacing maxillary and mandibular posterior teeth
  • They also provide proper guide planes for removable partial denture
  • Aids in occlusal plane modifications.
Advantages
  • They have the maximum strength and durability
  • Less tooth structure is reduced as compared to all porcelain (chamfer margin).
Disadvantage
They are not esthetic.
Indications
  • For posterior tooth with short clinical crowns
  • For grossly caries or fractured tooth.
Preparation steps
  1. Occlusal reduction: Using round-end tapered diamond.
  2. Functional cusp bevel: Round-end tapered diamond.
  3. Buccal, lingual, axial reduction: Chamfer diamond.
  4. Proximal reduction: Short thin and chamfer diamond.
  5. Seating groove: No. 170 bur.
Metals that can be used
Gold
Base-metal alloys.
All ceramic fixed partial dentures
  • All ceramic partial dentures are brittle and less fracture resistant compared to all metal partial dentures
  • Castable and alumina-reinforced porcelains have more strength compared to conventional porcelains.26
Advantages
  • Can duplicate tooth color exactly
  • Good translucency
  • Different shades of luting agent give the retainer a natural appearance
  • Good as anterior retainers.
Disadvantages
  • Reduced strength
  • Brittle
  • More expensive
  • Cannot be used on extensively-damaged teeth
  • More amount of tooth structure need to be removed compared to other restorations
  • Large connectors cause impingement of inter-dental papilla
  • Wear of opposing natural teeth
  • Two opposing retainers in porcelain can cause a clicking sound
  • Not advisable in posterior tooth if heavy loads have to be applied.
Preparation steps
  • After placing depth orientation grooves approximately 1.0 mm on labial surface and 2.0 mm deep on incisal half is reduced
  • Incisal reduction: 1.5–2.0 mm with flat-end tapered diamond
  • Labial and lingual axial reduction: With flat-end tapered diamond
  • Lingual reduction: Small wheel diamond
  • Axial reduction: Flat-end tapered diamond (aids in retention, resistance and structural durability)
  • Shoulder margin: Helps in resistance and marginal integrity.
Types of ceramics
  1. Conventional platinum matrix ceramics.
  2. Alumina-reinforced ceramics.
  3. Slip cast alumina (Inceram).
  4. Castable ceramics (Dicor).
  5. Mica glass ceramic (Machinable ceramic: CAD CAM).
  6. Heat-pressed ceramics: Leucite crystals dispersed ceramics (Optec HSP, IPS empress).
  7. Resin-bonded ceramics.
Metal–Ceramic fixed partial dentures
This retainer consists of a core of metal with a ceramic external surface.
Two types
  1. Metal is fused with porcelain on all the surfaces.
  2. Only the lingual and occlusal surface is of metal, whereas the labial and gingival surface is formed by porcelain.
Advantages
  • Esthetic and also incorporates the strength of metal
  • Characterization can be done by internal and external stains.27
Disadvantages:
  • Not conservative preparation
  • For better esthetics, the facial margin needs to be extended subgingivally. This can cause gingival destruction
  • Failure at the metal–ceramic junction can occur.
Preparation steps:
  • For type one with a complete metal and porcelain, preparation is same as for all porcelain
  • For type two in which the lingual surface is of metal, a chamfer margin is sufficient, with a shoulder margin on labial aspect to accommodate for the bulk of porcelain.
Acrylic fused to metal
  • Poor wear resistance
  • Easy to fabricate and adjust
  • Esthetically pleasing
  • Less expensive.
Fiber-reinforced composite resin bridges
Materials used
Polymer or resin matrices reinforced with glass, polyethylene or carbon fibers. The reinforcing fibers may be unidirectional (long, continuous and parallel), braided or woven.
Classification
  • Pre-impregnated with resin (e.g. Fiberkor)
  • Impregnation required with fiber (e.g. Ribbond).
Contraindications
  • Cannot be used in long-span bridges
  • Patients with parafunctional habits
  • When opposing tooth is unglazed porcelain.
Advantages
  • Optimal esthetics
  • Metal free
  • Decreased wear of opposing teeth.
Tooth Preparation
  • Lingual reduction: Flat end-tapered diamond
  • Groove preparation on the palatal surface of the edentulous side
  • A shoulder or chamfer margin can be prepared.
Partial crowns
Posterior tooth
1. Mesial halves
For tilted molar abutments to obtain a parallel path of insertion and to alter the occlusal plane.28
2. Three quarter crowns
  • Only the lingual aspect is prepared extending till mesiolingual and distolingual transitional line angles
  • Proximal grooves are prepared perpendicular to the prepared surface and the buccal wall is flared with tapered carbide bur
  • In case of additional retention offset is prepared extending from proximal grooves along the buccal cusp.
3. Modified three quarter
Done on premolars with the preparation extending buccally.
4. Seven-eights
Includes lingual, occlusal and distal half of buccal surface.
Anterior tooth
Three quarter
Refer to posterior tooth three quarter crown preparation.
Pin ledges
Indications
  • When large amount of tooth structure is lost with intact buccal walls
  • For restoration and alteration of occlusal surface.
Contraindications
  • In short clinical crowns
  • In long-span FPD
  • In endodontically treated tooth
  • In patients with active caries
  • In malaligned abutment tooth.
Advantages
  • Conservation of tooth structure
  • Supragingival preparation
  • Better visibility of seating and cementation.
Disadvantages
Less retentive
Not esthetic if metal is displayed.
5. Define and classify retainers. List the advantages and disadvantages of partial veneer crown over other types of retainers. What are the factors that come into play in the selection of retainers?
 
Retainer
Definition
Any type of device used for stabilization or retention of prosthesis—GPT.29
Function
Is to support and connect the body of the bridge with the abutment. It also restores the form, function and esthetics of the abutment.
Classification of Retainers for Fixed Partial Denture
Class I—Extracoronal restorations
A. Full-coverage crowns
  1. All-metal crowns.
  2. All-ceramic crown.
  3. Metal–ceramic crown.
  4. Acrylic-fused-to-metal crown.
  5. Metal-free composites.
B. Partial-coverage crowns
  1. Posterior tooth
    • Mesial one-half crown
    • Three-quarter crown
    • Reverse three-quarter crown
    • Seven-eight crown.
  2. Anterior tooth
    • Three-quarter crown
    • Variations of three-quarter crown (Selberg crown).
Class II—Intracoronal restorations
  1. Inlays.
  2. Onlays.
  3. Pin ledge.
  4. Combinations.
Class III—Radicular retainers
  1. Cast core.
  2. Blue island posts.
  3. Para post techniques.
  4. Kurer technique.
Class I—Extracoronal restorations
  • The preparation of the tooth and its cast retainer lies externally to the body of the coronal portion of the prepared tooth and restores a tissue-compatible contour for the crown
  • The retention and resistance to displacement is developed between the inner walls of the casting and the external walls of the prepared tooth (complete crowns).
Class II—Intracoronal restorations
  • The prepared cavity and its cast retainer lie within the body of the coronal portion of the tooth30
  • Retention and resistance are developed between the casting and the internal walls of the prepared cavity.
Class III—Radicular retainers
  • The dowel type of retention is confined to the root portion
  • Retention and resistance to displacement is developed by the extension of an attached metal dowel into the radicular portion of the tooth.
 
Partial Veneer Crowns
  • A partial veneer crown is a restoration covering two or more surfaces of a tooth
  • The surfaces usually covered are the lingual, proximal, occlusal, or incisal
  • The retention and resistance to displacement depend on the internal surfaces and the auxiliary retentive means, such as grooves, boxing, and pins.
Types of partial veneer
Three-quarter crowns
Covers three fourths of the gingival circumference of the tooth, leaving the facial surface intact.
Advantages
  • Esthetically pleasing
  • Can serve as a single unit or as an FPD retainer
  • Can be used on anterior and posterior teeth as retainers.
Reverse three-quarter crown
  • Is when the preparation is done on the buccal aspect leaving the lingual surface of a mandibular posterior tooth intact
  • Commonly done on mandibular molars with severe lingual inclination.
Seven-eighths partial veneer crown:
Covers seven eighths of the gingival circumference of the tooth leaving the mesial aspect of the buccal surface intact.
Indications
  • For maxillary molars and premolars with extensive carious involvement or restoration on the distal surface with intact mesial surface
  • Used on a mandibular premolar with the preparation extending from the distal finish line to the mid-facial surface.
Mesial half crown
Is a three-quarter crown rotated 90 degrees, preserving the distal surface of the tooth while covering the remaining surfaces.
Indications
  • In tilted mandibular molar abutment
  • Used as a single retainer in drifted or tipped mandibular molar to alter the occlusal plane.31
Contraindication
If distal surface of the tooth is damaged by caries or large restoration.
Advantages of partial veneer crowns over complete crowns
  • Conservative tooth reduction
  • Good esthetics compared to complete veneer cast crown
  • Better biocompatibility with supportive tissues as there is less gingival involvement than with complete coverage
  • Due to supragingival finish line margin, accessibility for finishing and cleaning is good compared to complete crowns
  • Easy verification of complete seating of the casting
  • Seating is complete compared to complete crowns as the luting agent has space to flow out (less hydraulic pressure)
  • Electric pulp testing can be conveniently accomplished on the intact enamel surface
  • Reduced pulpal and periodontal trauma during tooth preparation
  • Access to supragingival margins is easy and hence, better for oral hygiene as compared to full crowns
  • Due to direct visibility, cement removal is easy.
Disadvantages
  • Less retentive than complete veneer crown
  • If good preparation is not carried out, there will be display of metal with the partial veneer crown
  • The partial veneer crown preparation can be done only on intact teeth with average crown lengths
  • Less resistance form as compared to complete crowns
  • The placement of grooves, boxes, and pinholes requires good skill otherwise difficulty occurs in seating of casting
  • Not recommended in caries prone patients with poor oral hygiene
  • Unseating is common if excessive loads are applied.
Indications
  • Intact or minimally restored teeth with intact buccal wall
  • To re-establish anterior guidance
  • Used as retainers for a fixed partial denture when alteration of the occlusal plane is needed
  • To splint teeth
  • Can be prepared on teeth with average crown length
  • Prepared when teeth have normal anatomic crown form
  • Anterior teeth need to have adequate labiolingual thickness.
Contraindications
  • High caries rate (More for partial veneers where both the un-veneered surface and the margin to finish line interface are susceptible to decay
  • Teeth with extensive core restorations32
  • Deep cervical abrasion
  • Short teeth
  • Teeth that are severely constricted at the cervical require more axial reduction to provide adequate groove length
  • In long-span FPD
  • Should not be given on endodontically treated posterior teeth if the buccal cusps are weakened by the access cavity, or on teeth with an extensively damaged crown
  • Should not be placed on teeth that are proximally bulbous
  • Not recommended in thin teeth of restricted faciolingual dimension
  • Not recommended in poorly-aligned teeth
In all of the above situations complete veneer crowns need to be given. When there is difficulty in establishing adequate retention and resistance form, complete veneer restorations are more suitable.
 
Selection of Types of Retainers
  • The selection of the types of retainers depends on the oral-hygiene habits or the DMF rate of the patient, or both
  • If caries index is low any type of retainer is safe provided other factors as periodontal status crown length are all present
  • The length of the bridge span and the type of bridge determines the type of retainer selected
  • Retainer selection for terminal abutments is a crucial decision.
Retainer selection is affected by the following factors
  • Age
  • DMF rate
  • Edentulous span
  • Periodontal support
  • Arch position of the teeth
  • Skeletal relationships
  • Interocclusal and intraocclusal conditions, such as crown length
  • Existing and projected oral hygiene of the patient
  • Vitality of the potential abutment.
 
Ideal Retainers
  • An ideal retainer needs to support and connect the body of the bridge with the abutment
  • It should restore the form, function and esthetics of the abutment
  • Should be biocompatible with tooth and its adjacent tissues
  • Retainer should be constructed without injury to pulp and supporting structures
  • Should protect and maintain the pulp against thermal and galvanic shock
  • Should provide safety for the tooth during the lifetime of the restoration
  • Should provide self-cleaning property
  • Should be resistant to corrosion and tarnish
  • Should be a conservative preparation with uniformity of reduction of the abutment tooth33
  • Load should be dispersed to the more receptive areas of the abutment
  • There should be large surface contact between the abutment and a retainer
  • Selection of an adequate luting agent is important.
6. Define retention and resistance form in fixed partial denture. What are the factors affecting retention and resistance in posterior tooth preparation?
Definitions
Retention
Retention prevents removal of the restoration along the path of insertion or along the path of the tooth preparation.
It is the quality of a preparation that prevents the restoration from becoming dislodged by such forces parallel to the path of withdrawal.
 
Resistance
Prevents dislodgement of the restoration, by forces directed in an apical or oblique direction and prevents any movement of restoration under occlusal forces.
Factors Affecting Retention
  • Magnitude of the dislodging forces
  • Geometry of the tooth preparation
  • Roughness of the fitting surface of the restoration
  • Type of restorative materials being cemented
  • Type of luting agent
  • Film thickness of the luting agent.
Magnitude of the dislodging forces
Factors influencing dislodging forces:
  • Stickiness of the food
  • Surface area of the restoration
  • Texture of the restoration being pulled.
Of all the type of forces sticky food exhibits large dislodging forces.
Geometry of the tooth preparation
Factors influencing geometry of tooth preparation:
  • Taper
  • Surface area
  • Stress concentration
  • Type of preparation.
 
Taper
  • Maximum retention is obtained if a tooth preparation has parallel walls
  • Smaller degrees of taper have more retention
  • Two opposing surfaces each with a 3° taper provide a 6° taper for the preparation
  • Ideal convergence between opposing walls is 6°. This amount of convergence is required to restrain the cemented restoration.34
Factors decreasing retention in regard to taper:
  • Less amount of taper will form an undercut causing divergence between opposing axial walls, in an occlusocervical direction
  • An increased amount of taper increases free movement of the restoration and retention will be reduced (Experimented by Jorgensen in 1955).
Preparation rule:
A rotary instrument of the desired taper held at a constant angulation produces the amount of taper required.
The rotary instrument of ideal taper when moved through a cylindrical path during tooth preparation will produce the desired axial wall taper on the completed preparation.
 
Surface area
  • The greater the length of the clinical crown, the more is the retention
  • In short clinical crowns, the surface area needs to be increased with the help of grooves and box preparation
  • The greater the width of the clinical crown, the better the retention
  • A restoration with limited path of withdrawal is more retentive depending on the length of the surface area in sliding contact.
 
Stress concentration
  • Round margins reduce stress concentrations, which in turn increases the retention of the restoration
  • Stresses are generally concentrated around the junction of the axial and occlusal surfaces
  • Changes in the geometry of the preparation (e.g. rounding the internal line angles) reduce stress concentrations and hence, increase the retention of the restoration.
 
Type of preparation
  • Additional grooves and boxes to a preparation increase the retention as the surface area is increased
  • Retention is double for complete crowns than for a partial crown
  • Luting agent is only effective if the restoration has a single path of withdrawal
  • The occluso-axial line angle of the tooth preparation should be a replica of the gingival margin geometry
  • Fixed prostheses depend on the geometric form than on adhesion of the luting agents for retention.
 
Roughness of the surfaces
  • Retention is increased if the restoration is roughened or grooved by air abrading the fitting surface with alumina
  • A smooth internal surface of a restoration can cause retentive failure at the cement-restoration interface.
 
Materials being cemented
  • The more reactive base-metal alloys have more adhesion with certain luting agents than less reactive high-gold content metals
  • Cement adheres better to amalgam than to composite resin or cast gold.35
 
Type of luting agent
Adhesive resin cements are the most retentive.
Factors Affecting Resistance Form
Lateral forces tending to displace the restoration by causing rotation around the gingival margin is prevented by areas of the tooth preparation that are placed in compression called resistance area.
Resistance depends on:
  • Magnitude and direction of the dislodging forces
  • Geometry of the tooth preparation
  • Physical properties of the luting agent.
 
Magnitude and direction of the dislodging forces
  • Patients with abnormal biting forces should be given a complete metal crown. A proper design and preparation can help to resist these displacing forces
  • In patients with parafunctional habits and pipe smoking to prevent large oblique forces from being applied to a restoration, additional retentive grooves and the restoration should be luted with adhesive cements.
 
Geometry of the tooth preparation
  • Increased preparation taper and rounding of axial angles tend to reduce resistance
  • Short tooth preparations with large diameters have less resistance form
  • A partial-coverage restoration has less resistance when compared to complete crown.
Resistance can be increased by:
  • Preparation of boxes or grooves with walls that are perpendicular to the direction of the applied force
  • U-shaped grooves and flared boxes provide more resistance than V-shaped ones
  • Pinholes increase resistance as they prevent rotational movement and subject additional areas of the luting agent to compression. The buccal and lingual walls must meet the axial walls at 90° to resist rotational forces.
 
Physical properties of the luting agent.
Factors affecting resistance to deformation of luting agent are as follows:
  • Compressive strength
  • Modulus of elasticity.
 
Compressive strength
  • Zinc phosphate cement should have compressive strength above 70 MPa at 24 hours
  • Silico-phosphate cements have a higher compressive strength
  • Glass ionomer, resin, polycarboxylate and reinforced zinc oxide-eugenol have lower values
  • Increase in temperature during manipulation, decreases the strength of the luting agent.
 
Modulus of elasticity
Zinc phosphate cements have a higher modulus of elasticity than polycarboxylate cements, hence, polycarboxylate cement depends more on the taper of the preparation than zinc phosphate cement.36
Conclusion1
  • A prepared teeth should have the following charecteristics as 10–20 degrees of total occlusal convergence, an occlusocervical dimension of 4 mm and occlusocervical to faciolingual ratio greater than 0.4
  • Facio- and lingoproximal line angles should be preserved if not auxillary features as box or grooves need to be added
  • All line angles should be rounded and supragingival finish lines are more acceptable in case of esthetic requirements finish lines at gingival level or sometimes subgingival if restoration are present.
 
Reference
  1. Goodacre CJ, et al. Tooth preparations for complete crowns: An art form based on scientific principles. J Prosthet Dent. 2001;85:363-76.
7. Classify finish lines. Enumerate the functions, indications, advantages and disadvantages of finish lines. Add a note on selection of type of finish line to be used.
Definition
Finish line/margin is the part of tooth preparation/restoration that is in close proximity to the periodontium and that forms the most important and weakest links in the success of full-coverage restorations.
It is the outer edge of a crown, inlay, onlay or other restoration.
Other names
Finish line, finish curve.
It is a line of demarcation, peripheral extension of tooth preparation and planned junction of restoration.
Classification Of Gingival Finish Lines
Basic types
  1. Knife-edge/Chisel-edge/Shoulderless cervical finish line.
  2. Chamfer.
  3. Shoulder.
 
Variations
Variation in knife-edge/Chisel-edge/Shoulderless cervical finish line
Feather edge.
Variations in chamfer
  • Hybrid
  • Ski-sloped
  • Round shoulder
  • Heavy chamfer
  • McEwen's chamferette.
Variations in shoulder
  • Full shoulder with bevel37
  • Full shoulder with bevel and butt joint
  • Radial shoulder.
 
Functions of Finish Lines
  • Helps in evaluating the amount of tooth structure removed during tooth preparation
  • To evaluate the accuracy of impression of the tooth preparation
  • To evaluate quality of die
  • To evaluate the marginal adaptation of wax pattern
  • To check the seating of the final restoration
  • To evaluate the marginal adaptation of the final restoration.
 
Characteristics of an ideal finish line has
  • Good marginal adaptation
  • Biocompatibility with supporting structures
  • Adequate contour
  • Sufficient strength.
Feather-edge/Chisel-edge Finish Line (Fig. 4)
 
Definition
Knife-edge preparation is a tapered preparation that has maximum tooth reduction at occlusal, incisal surfaces and tapers to zero cutting at the gingival termination.
 
Indications
  • In young patients
  • Pin ledge three fourth crowns
  • In tipped teeth
  • In long clinical crowns
  • In inaccessible areas in the oral cavity
  • When finish lines are on cementum
  • As proximal finish line for inaccessible areas.
zoom view
Fig. 4: Feather-edge finish line
38
 
Advantages
  • Easy to prepare
  • Conservative tooth preparation.
 
Disadvantages
  • Location of margin difficult to detect
  • Over contouring of restoration is possible
  • Over or under extended crown due to difficulty in locating the finish line
  • Decreased marginal adaptation.
  • Decreased retention
  • Wedging effect due to internal stresses
  • Distortion of the margin is possible during lab procedures as it is thin
  • Knife-edge finish line is not a recommended finish line.
 
Chamfer Finish Line
Definition
This is a marginal finish line either curved or formed by a plane at an obtuse angle to the external surface of a prepared tooth (Boucher).
It is a concave extracoronal finish line that possesses greater angulation than knife-edge with less width than shoulder finish line (Fig. 5).
 
Instrument used
Round-end tapered diamond.
 
Indications
  • Ideal gingival finish line for cast-metal restorations
  • Lingual finish line for metal–ceramic crowns
  • For metal collars.
 
Advantages
  • Provides a slip joint
  • Provides gingival area with acceptable stress distribution, adequate seal and minimum uniform tooth reduction (El-Ebrashi et al)
    zoom view
    Fig. 5: Chamfer finish line
    39
  • Aids in accurate die trimming
  • Can be terminated as subgingival preparation
  • Adequate bulk and tooth contour
  • Easy to prepare.
 
Disadvantages
  • Marginal distortion during firing of porcelain
  • Reverse lip can form if the entire diameter of instrument is used
  • Less room cervically than shoulder preparation, therefore cannot be prepared with low-speed cutting instrument.
 
Variations
 
Hybrid
When the chamfer bur is inserted to one third of depth of stone a hybrid preparation results.
It is a preparation between chamfer and knife-edge.
 
Ski-sloped
When the chamfer bur is inserted to half the depth of stone a Ski-sloped preparation results.
 
Rounded shoulder
When the entire depth of chamfer bur is used in preparation a rounded shoulder results.
 
Heavy chamfer
Used in restorations where more axial reduction is required due to caries or previous restoration. As heavy chamfer forms a butt joint, bevel is added to make it a slip joint.
 
McEwen's chamferette
Finish line between chamfer and feather edge.
Shoulder Finish Line
 
Definition
When the external line angle of the preparation is perpendicular to the long axis of the tooth, a shoulder finish line results (Fig. 6).
 
Instrument used
Flat-end tapered diamond and flame diamond for gingival bevel.
 
Indications
  • Facial margins of metal–ceramic preparation
  • Preparation margin for all ceramic restorations
  • Margin for castable and injectable ceramics
  • 135° shoulder can be used for long clinical crowns.
 
Advantages
  • Least stress in cervical area compared to other margins
  • Better resistance to occlusal forces
  • Bulk of porcelain can be accommodated
  • Esthetic40
zoom view
Fig. 6: Shoulder finish line
  • Easy to identify margin and fabricate wax pattern
  • Increased retention
  • Good marginal adaptation
  • Less marginal distortion
  • For proper contouring of restoration with a straight emergence profile
  • Easy to check seating of restoration.
 
Disadvantages
  • Least conservative preparation of all the other preparations
  • If a metal collar is not used labial porcelain butt fit is more difficult to achieve
  • Difficult to prepare
  • Can cause adverse pulpal involvement
  • Difficult preparation.
 
Variations
 
Full shoulder with bevel
In erosion of tooth structure, gingival finish line for proximal boxes and for porcelain fused to metal restorations (Fig. 7).
 
Full shoulder with bevel and facial butt joint
Bevel on proximal and lingual surfaces and butt joint on facial surfaces to avoid metal display on labial side.
 
Radial shoulder
Shoulder with internal line angle rounded.
 
Complete shoulders
Finish lines for all porcelain crowns and injectable ceramic restorations.
 
Bevelled shoulders
Used for veneers and selected posterior teeth.41
zoom view
Fig. 7: Bevelled shoulder
Functions of the bevel:
  • Seals restoration against cement leakage and subsequent bacterial invasion
  • Easy finishing and burnishing on die or tooth
  • Provide circumferential rigidity
  • Iinitiate reproduction of the contour and provide control of the emergence profile during framework try-in.
 
Chamfer margin
For posterior crowns and the lingual surface of anterior metal ceramic crowns.
 
Knife-edge finish lines
Used for younger patients, pin ledge three-quarter crowns, inaccessible areas of the oral cavity, and finish lines on the cementum.
 
Principles in Determining Type of Finish Lines
  1. Type of restorative materials.
  2. Location of finish lines.
  3. Configuration.
  4. Marginal Fit.
 
Type of restorative materials
 
Gold alloys
  • Good burnishability.
  • Chamfer margin.
 
Base-metal alloys
  • Lack of burnishability
  • Chamfer margin.
 
Porcelain
Shoulder margin.42
 
Metal–ceramic materials
Buccally/labially-shoulder margin and lingually chamfer margin.
 
Location of finish lines
  • Supragingival
  • At the crest of gingiva
  • Subgingival.
 
Supragingival
Advantages
  • Easy to prepare accurately without trauma
  • Gingival retraction is not required
  • Seating of the restoration can be evaluated
  • Can place finish line on enamel
  • More biocompatible with surrounding tissues
  • Good contour can be achieved
  • Easy oral hygiene maintenance
  • Easy evaluation of margins during recall
  • Can clean luting agent after luting easily.
 
Subgingival
Indications
  • Extensive restorations, caries, cervical erosion extending subgingivally
  • Proximal contact area extending to gingival crest
  • In short clinical crowns
  • For esthetics when labial metal collar needs to be hidden
  • In root sensitivity, fracture subgingivally
  • When axial contour modification is indicated
  • In young patients with high-risk caries
  • For additional retention and resistance form.
Disadvantages
  • Difficult to access and finish
  • Difficult to evaluate fit and cementation procedure
  • Periodontal health can be affected if oral hygiene is not maintained.
 
Configuration
  • Ease of preparation without overextension
  • Should be able to identify finish line in the impression and on the die
  • A good and clear boundary to which the wax pattern can be finished
  • For wax pattern to be handled without distortion and for strength of restoration
  • Conservation of tooth structure
  • Studies by El-Ebrashi1 showed that shoulder with rounded internal line angle and chamfer showed less stress concentrations. Shoulder with bevel and featheredge showed the maximum stress concentration.43
 
Marginal Fit
  • An ideal marginal fit is smooth and even
  • A sliding joint has better marginal fit.
 
Mathematical equation (Cosine rule) (Rosner 1963)
  • To calculate the marginal gaps
  • The closer the hypotenuse of the right triangle approaches the adjacent leg of the triangle, the smaller the gap between the bevel of the wall of the cavity and the casting. This relation is in the un-cemented state
  • When a luting agent is used, the fit depends on the film thickness of the luting agent used.
 
Reference
  1. El-Ebrashi MK, Craig RG, Peyton FA. Experimental stress analysis of dental restorations. Part III. The concept of geometry of proximal margins. J Prosthet Dent. 1969;22:333.
8. Illustrate with diagrams, the tooth preparation on right upper central incisor for receiving porcelain jacket crown. Add a note on the advantages, indications and contraindications of all-porcelain jacket crown.
 
Steps in Tooth Preparation for All-ceramic Restorations
  1. Placement of depth-orientation grooves.
  2. Incisal reduction.
  3. Facial reduction.
  4. Lingual reduction.
  5. Axial reduction.
  6. Lingual axial reduction.
  7. Marginal development and refinement.
Armamentarium
  1. Mouth mirror.
  2. Periodontal probe.
  3. Explorer.
  4. Chisels and hatchets.
  5. High- and low-speed handpieces.
  6. Thin tapering fissure diamonds.
  7. Narrow round-tipped tapered diamonds, regular and coarse grit.
  8. Flat-end tapered diamond, regular grit.
  9. Football-shaped diamond.
  10. Finishing stones and carbides.
Placing of depth orientation grooves (Fig. 8)
  1. Three depth orientation grooves, 1.0 mm deep, are placed:
    One in the middle of the facial wall and one each in the mesiofacial and distofacial transitional line angles in the incisal edge.44
    zoom view
    Fig. 8: Placing of depth orientation grooves
    zoom view
    Fig. 9: Incisal reduction
  2. Two more depth orientation grooves of 2.0 mm depth are placed on the incisal half.
  3. 2 mm deep grooves are placed on the incisal edge for incisal reduction.
 
Incisal reduction
 
Instrument used
Flat-end tapered diamond.
The completed incisal reduction should provide 1.5–2 mm of clearance for porcelain in all excursive movements of the mandible (Fig. 9).
Facial reduction
Done in two stages:
 
First step: Incisal half reduction (Fig. 10)
  1. A coarse, flat-ended diamond or No. 700 carbide bur is used to plane away tooth structure between the depth of orientation grooves, on the incisal half, at a 45° angle to the long axis of the tooth in a normal occlusal relationship.
  2. The reduction is done parallel to the original contour of the tooth to provide uniform porcelain thickness and good esthetics.
Second step
  1. Gingival portion of the labial surface is reduced with a flat-end tapered diamond in a flat plane perpendicular to the long axis of the tooth to a depth of 1.0 mm (Fig. 11).
  2. The reduction is carried out with cervical component parallel to the proposed path of withdrawal.
  3. This reduction extends till the labioproximal line angles.
  4. The reduction is done on half of the facial surface at a time.
 
Lingual reduction (Fig. 12)
Depth orientation grooves of 0.8 mm depth are placed.
Football-shaped diamond/Small wheel-diamond is used for lingual reduction.
Reduction is carried out until a clearance of 1 mm in all mandibular excursive movements are obtained.45
zoom view
Fig. 10: Incisal half reduction
zoom view
Fig. 11: Labial surface reduction
zoom view
Fig. 12: Lingual reduction
zoom view
Fig. 13: Lingual axial reduction
 
Axial reduction
A thin tapered fissure diamond is used to break the contact point with the adjacent tooth. While breaking the contact point, the adjacent tooth should not be abraded.
The mesial and distal areas are first reduced to a 2°–5° taper without establishing a shoulder at this time.
 
Lingual–axial reduction (Fig. 13)
Instrument used
  • Flat-end tapered diamond
  • The same path of withdrawal as that of the facial preparation is followed with lingual axial reduction
  • A depth groove is placed in the middle of the cingulum wall
  • The preparation of 2°–5° taper is done from the centre of the cingulum wall until the lingual shoulder meets the facial shoulder
  • A 0.75 mm cingulum shoulder is placed with a flat-ended tapered diamond.
 
Marginal development and refinement
 
Instruments used
  • An end-cutting bur held perpendicular to the shoulder can be used for lowering margins46
  • A sharp chisel is used to remove undermined enamel and finishing the shoulder
  • The axial walls are smoothed and all the sharp line angles and point angles are rounded. An acceptable emergence profile needs to be created for good esthetics.
 
Variation in margin preparation
  • Instead of shoulder preparation an heavy chamfer can also be prepared
  • For subgingival margin placement, a gingival retraction is done before the preparation for good access and less trauma to gingival tissue
  • Initial preparation is followed by refinement with chisels
  • The completed shoulder should be 1 mm wide, smooth, continuous and without any irregularities.
All-ceramic Crowns
Advantages
  • Superior esthetics
  • Good translucency as to that of natural tooth
  • Good biocompatibility
  • Can select the appropriate shade for luting agent.
Disadvantages
  • Reduced strength of the restoration if metal reinforcing substructure is not given
  • Significant tooth reduction on proximal and lingual aspects
  • Less conservative than metal-ceramic crown
  • Difficulties in obtaining a well-fitting margin
  • The success of the restoration depends on proper preparation design
  • An extensively damaged tooth cannot be restored with an all-ceramic crown
  • Cannot be used as retainers in long-span FPD
  • Large cross-sectional dimension connectors need to be incorporated for all-ceramic restorations to have bulk of material
  • Due to large connectors, impingement on the interdental papilla can lead to periodontal failure
  • Wear on the functional surfaces of opposing natural teeth.
Indication
Where a high-esthetic requirement exists with sound tooth structure present.
Contraindications
  • Where a more conservative restoration can be used
  • Not recommended for molar teeth
  • Where increased occlusal loads are present
  • When adequate support cannot be provided
  • When an even shoulder width cannot be prepared.
9. Define a crown and mention the advantages, disadvantages and indications of a full metal crown. Describe in detail, with diagrams, the step-by-step procedure in the preparation of a full-metal crown on a mandibular first molar.47
Definition
An artificial crown is a fixed restoration of the entire coronal part of a natural tooth that restores anatomy, function and esthetics, usually of metal, porcelain, synthetic resin, or combinations.
 
Full-metal Crowns
 
Advantages
  • Good strength
  • Better retention
  • Greater resistance form than a partial-coverage restoration
  • Conservative tooth preparation
  • Less chance of pulpal injury
  • Can easily modify axial tooth contour
  • Convenient development of contact areas
  • Can be given for caries damaged tooth after core build up
  • Can be given for endodontically treated teeth
  • Ideal retainer for restoring craniofacial anomalies
  • Can withstand occlusal loads
  • Occlusal plane modifications are easily facilitated with supraerupted teeth
  • Ideal retainer for long-span FPD and short clinical crowns
  • Embrasure areas can be enhanced in periodontally compromised dentitions
  • Buccal flutes developed for preferred contours
  • Provide desirable guide planes for RPDs
  • Only restoration that permits modifications to shape survey lines, guide planes and occlusal rests.
 
Disadvantages
  • Not esthetic
  • Cannot be used as anterior retainers
  • Not as biocompatible as ceramic restorations
  • Chances of over contouring the restoration (overcountering can result in periodontal problem)
  • Distortion of metal margins is common
  • Uniform gingival finish lines are difficult to achieve
  • Postcementation gingival caries is difficult to detect
  • Good laboratory implementation is required for good fit of the restoration
  • Margins are difficult to locate in the die
  • After cementation, electric vitality testing of the abutment tooth cannot be done
  • Restricted to maxillary molars, mandibular molars and premolars.
 
Indications
  • For restored posterior tooth in nonesthetic zone unable to withstand normal occlusal loads48
  • For a retainer requiring maximum retention
  • For short clinical crowns
  • For extensively damaged or fractured teeth
  • Complete gold veneer crowns can be prepared on both vital and pulpless teeth
  • Ideal retainer for long-span FPD
  • For endodontically treated teeth
  • Retainers for RPD.
 
Preparation of a Full-metal-crown on a Mandibular First Molar
Metals that can be used
  • Gold alloys
  • Base-metal alloys.
 
Armamentarium
  • High- and low-speed contra-angles
  • Tapered carbide bur used for occlusal guiding grooves, additional retentive features
  • Round-end tapered diamond regular (0.8 mm) used for occlusal reduction, axial reduction and chamfer preparation
  • Round-end tapered diamond fine grit for finishing
  • Utility wax and wax caliper for verification of occlusal clearance.
 
Steps in full metal crown preparation
  1. Placement of occlusal guiding grooves.
  2. Occlusal reduction.
  3. Placement of functional cusp bevel.
  4. Placement of axial alignment grooves.
  5. Axial reduction.
  6. Margin placement.
  7. Finishing of preparation.
Placement of occlusal guiding grooves
  1. A tapered carbide bur is used to place the guiding grooves of approximately 1 mm depth in the central, mesial, and distal fossae.
  2. These grooves are connected along the length of the central groove extending into the mesial and distal marginal ridge for occlusal reduction.
  3. Guiding grooves are also placed in the buccal and lingual grooves and in each triangular ridge extending from the cusp tip to the centre of its base.
Occlusal reduction
  1. One half is reduced and then the other half is reduced.
  2. Occlusal reduction should follow the anatomic configuration for a conservative tooth preparation.
  3. The occlusal reduction follows the depth orientation grooves (Fig. 14).
  4. Functional cusps (buccal cusps) on mandibular molars are “stamp cusps’ and require a “two-plane” reduction with minimum clearance of 1.5 mm.49
    zoom view
    Fig. 14: Occlusal reduction
    zoom view
    Fig. 15: Placement of functional cusp bevel
  5. For non-functional cusp a minimum of 0.6–1 mm reduction is required.
  6. Reduce the inclined planes between the depth guides in the developmental grooves and those along the triangular ridges.
  7. Reduce the marginal ridge by 1.5 mm.
  8. The occlusal surface is smoothened to duplicate the tooth anatomy.
  9. Occlusal reduction is completed when necessary clearance during excursions of the mandible, especially at the ceramic/metal junction is achieved.
  10. Evaluate occlusal clearance with wax bite and measure with wax gauge.
 
Placement of functional cusp bevel (Fig. 15)
  1. A functional cusp bevel is placed to protect tooth during function by ensuring adequate thickness of metal, as this area contacts with the opposing tooth.
  2. Functional cusp bevel ensures ideal restoration contour, maximum durability and conservation of tooth structure.
  3. A functional cusp bevel is placed 45° to the long axis of teeth on the functional palatal cusp for maxillary molar and buccal cusp for mandibular molar.
 
Placement of axial alignment grooves
  1. Three depth orientation grooves are placed:
    • On each buccal and lingual wall with round end tapered diamond
    • In the centre of the wall
    • On each mesial and distal transitional line angles.
  2. A round end tapered diamond with a taper of 6° forms an identical taper on the preparation wall.
  3. Gingivally:
    Only one half of the tip of diamond should penetrate.
  4. Facial and lingual reduction (Fig. 16):
    • The facial and lingual surface reduction is done with taper of 2°–5°. For maxillary molar a two-plane reduction is common on the facial surface
    • During the facial and lingual reduction, the taper established is in relation to the path of insertion50
      zoom view
      Fig. 16: Facial and lingual reduction
      zoom view
      Fig. 17: Axial reduction
    • The sharp line angles created after the proximal, facial, and lingual surfaces are rounded
    • Vertical seating grooves on the buccal cusp fossa for mandibular and palatal cusp fossa for maxillary, improve retention.
Axial reduction (Fig. 17)
  1. Break the contact points with a thin tapering fissure bur without abrading the adjacent tooth.
  2. In tight contact point areas a metal matrix band can be placed.
  3. Axial reduction forms the retention and resistance form.
  4. The islands of tooth structure between the alignment grooves are removed while the chamfer margin is placed with a round end tapered diamond.
  5. The preparation is done on one half at a time.
  6. A 5° convergence on the mesial surface of the tooth is required. Gingival finish line is not made at this time.
  7. The distal surface is prepared next.
Margin placement
  1. The width of the chamfer margin should be 0.5 mm.
  2. Chamfer margin must be smooth and continuous mesiodistally and ideally located supragingivally.
  3. Unsupported enamel should be removed with chisel.
  4. The tip of the bur should not exceed the depth of one-half the diameter of the bur.
Finishing the preparation (Fig. 18)
Finish the preparation by refining the line angles and point angles.
Smoothen sharp angles or surface irregularities with the diamond and, finally with a cuttle fish disc.
10. Describe in detail, with diagrams, the step-by-step preparation of a maxillary canine to receive a three quarter crown. Explain and justify different impression techniques for fixed partial denture.
Definition
Partial veneer crown is an extracoronal metal restoration that covers only part of the clinical crown. Partial veneer crowns include all tooth surfaces except the buccal or labial wall in the preparation.51
zoom view
Fig. 18: Finishing the preparation
Steps in Preparation of Partial Veneer Crowns are
  1. Incisal reduction.
  2. Lingual reduction.
  3. Interproximal reduction.
  4. Proximal box or groove placement.
  5. Incisal offset placement.
  6. Facial bevel.
  7. Finishing the preparation.
Armamentarium
  1. High- and low-speed contra-angle handpiece.
  2. Burs as listed below (Table 1).
  3. Utility wax and wax gauge to evaluate lingual reduction.
Table 1   Uses of various burs in preparation of partial veneer crown
Burs used
Used for
  • Round-ended tapered diamond
  • Incisal reduction, axial reduction
  • Football-shaped diamond
  • Lingual reduction
  • 169L carbide bur
  • Interproximal reduction
  • Narrow chamfer diamond
  • Chamfer margin
  • 167 carbide bur
  • Proximal groove
  • Flame-shaped diamond
  • Flare for proximal extensions
  • Inverted cone carbide bur
  • Incisal groove
  • Fine, flame-shaped diamond bur
  • Facial bevel
  • Carbide finishing bur
  • Finishing preparation
  • No. 1/2 round bur
  • Pilot hole
  • Hatchet instrument
  • For contact breaking
52
zoom view
Fig. 19: Lingual reduction
zoom view
Fig. 20: Interproximal reduction
 
Incisal reduction
  • Round-ended tapered diamond is used
  • Reduce the incisal edge 1 mm at a 45° angle to the long axis of the tooth. Remove 1.0–1.5 mm following the facial contour of the tooth.
 
Lingual reduction (Fig. 19)
Done in two steps.
  1. Lingual surface reduction
    A football-shaped diamond is used to reduce the lingual surface in two planes, with a slight ridge along the centre of the lingual surface incisogingivally. A clearance of at least 0.7–1 mm is required with the opposing tooth.
  2. Lingual–gingival reduction
    A round-ended tapered diamond is used to achieve a chamfer of 0.5 mm deep at the cervical finish line. The chamfer is extended to include the lingual line angles.
 
Interproximal reduction (Fig. 20)
Done in three steps.
  1. The proximal surface is reduced with a 169L carbide bur from the lingual to the facial surface with the contact point intact. The facial line angles must remain intact for good esthetic results.
  2. A light chamfer finish line is made on the proximal surface with a narrow chamfer diamond. This chamfer should merge with the lingual chamfer.
  3. The contact with adjacent tooth is broken with a hatchet instrument from the facial surface, to form labial proximal extensions. The flare of proximal extensions is finished with a flame-shaped diamond.
 
Proximal grooves (Fig. 21)
A 167 carbide bur is used for groove placement at the proper alignment. The proximal grooves are placed parallel to the incisal two thirds of the facial surface (169L carbide bur).
These grooves resist lingual displacement and should be a minimum of 3 mm long with 0.5 mm of the gingival finish line. The facial and lingual walls of the grooves should have a 2–5 degree incisal divergence.53
zoom view
Fig. 21: Proximal grooves
zoom view
Fig. 22: Incisal groove and facial bevel
The lingual wall of the proximal grooves should have a 2–5 degree incisal convergence with the lingual gingival wall.
The facial wall of the groove should be continuous with the proximal flare to add bulk to the facial margin.
 
Incisal groove (Fig. 22)
Inverted cone carbide bur is used.
A 0.5–1 mm groove is prepared within the dentin and is made parallel to the DEJ connecting the proximal grooves. The groove is not placed at the expense of the incisal edge.
 
Facial bevel
Fine, flame-shaped diamond bur is used.
A narrow bevel <0.5 mm is prepared on the labioincisal finish line at right angles to the incisal two thirds of the facial surface.
 
Finishing the preparation
A carbide finishing bur is used.
All the sharp and point angles are rounded to ensure continuity of all finish lines.
 
Cingulum modification if needed for additional retention
  1. After paralleling a 170 bur to the long axis on the proximal grooves, a ledge is prepared in the cingulum.
  2. A pilot hole is cut in the ledge with a No. 1/2 round bur.
Different Impression Techniques
  1. Stock Tray/Putty wash (single mix and double mix).
  2. Custom tray (single mix).
  3. Closed bite/Double arch/Dual quad tray/Triple arch.
  4. Copper band.
  5. Reversible hydrocolloid (laminate and wet field technique).
  6. Matrix system.54
 
Stock Tray/Putty wash
Polyvinyl siloxane is used.
 
Materials used
Reversible, irreversible hydrocolloids and elastomeric impression materials.
For accuracy, elastomeric impression materials are the commonly used.
In this technique, a single impression with polyvinyl siloxanes or a double mix with medium and heavy-bodied elastomers can be done.
 
Single mix technique
Advantages of single mix
  • Less time required
  • No need to fabricate a custom tray
  • Metal stock trays are rigid and chances of distortion are less.
 
Disadvantages
  • Each time the tray is used it needs to be sterilized
  • More impression material is required.
Technique
  1. After adjusting the chair position, a proper stock tray with the correct border extensions, and tray shape and size depending on the patient's arch shape and size is selected.
  2. Apply tray adhesive on the inside and rim of the stock tray.
  3. Manipulation of impression material:
    Mix the high-viscosity putty impression material and roll putty into elongated cylinder.
    Place the putty in the stock tray and cover with polyethylene sheet.
  4. Seating of tray:
    Insert and seat the tray with a rocking type motion.
    Seat the tray in the mouth without movement till initial set occurs (approximately 2 minutes).
    For stock tray (putty wash) single mixing technique, the unset high-viscosity impression material should already be in the tray, and the preparations syringed with low-viscosity impression material.
  5. Setting of material and tray removal:
    Remove from the mouth with minimal sideward movement and ensure the material is set using a fingernail test (material rebounds completely).
  6. Trimming the impression:
    After removing the spacer, the excess impression material is removed with a sharp knife.
Double mix technique (PVS PS CS)
  1. After a stock tray is selected, tray adhesive is applied; the impression putty is mixed and placed in tray.
  2. A polyethylene sheet is used to cover the putty material and impression is seated in the patient's mouth.55
  3. After the complete set of the impression is ensured by fingernail testing, the tray is removed.
  4. Relieving the tray:
    A sharp hand instrument is used to remove uniform amount of impression material from the tissue surface.
  5. After gingival retraction evaluate tissue displacement, check the finish line area(s), and leave cord(s) in place for 8–12 minutes.
  6. Manipulation of light-body material:
    After measuring the arch length of tray, one times the length of the tray low-viscosity elastomer is dispensed, after trimming the tip of the syringe.
  7. Making final impression:
    • A mixing pad, (6 by 8 inches) or an automatic gun dispensing system is used
    • The low-viscosity impression material is mixed with a circular motion combining the two strands, then a figure eight motion to blend and flatten the mixture onto the mixing pad (approximate mixing time less than 1 minute).
  8. Loading the impression material:
    • The syringe is loaded by holding it at a slight angle while scraping the pad
    • Screw on the tip, and insert the plunger
    • While the plunger is inserted into the syringe, the cord is removed
    • After evaluating retraction site for seepage, hemorrhage, or debris, first syringe inaccessible areas. (E.g., distal lingual finish line)
    • The syringe is positioned so the elastomer is ahead of the tip's orifice.
  9. Tray Insertion:
    • Insert the low-viscosity impression material into the tray slightly less than the depth of the external borders
    • Seat the tray from posterior to anterior, allowing the excess to extrude anteriorly
    • Seat the tray firmly in position
    • The tray should not be moved while the material is setting.
  10. Final impression:
    • After the final set is over, tray is removed
    • Rinse impression with ambient water, and dry with short, small bursts of compressed air
    • Retraction cord (s) remaining in the impression material are removed carefully.
  11. Evaluate set impression:
    • The area 0.5 mm beyond visible finish line should be visible. There should be no show-through in any areas of the impression, except at tissue stops
    • There should be no shiny smooth areas, no voids present
    • Review for tears. There should be no thin areas leaving the finish line unsupported.
 
Custom Tray: PVS PE PS, CS
 
Advantages
  • Less impression material required compared to stock tray
  • Sterilization is not a problem56
  • Less chances of impression material getting distorted due to curing shrinkage
  • Precuring of the tray material is not required.
 
Disadvantages
  • Time consuming due to tray fabrication
  • The tray must be used after complete curing to prevent further distortion
  • Monomer sensitivity during tray fabrication for some personnel.
 
Technique
Fabrication of tray
(Wax adaptation.)
  • The diagnostic cast is soaked in slurry water for 10 minutes and then painted with a layer of tinfoil substitute to prevent the resin from adhering to the cast
  • Outline of the tray extensions is marked on the cast and two sheets of base plate wax are adapted to the cast
  • Excess wax is trimmed and a thin tinfoil (or polyethylene) sheet is placed over the wax to protect resin from wax during the exothermic cure.
Placement of tissue stops
Four widely spaced hard tissue stops of 3 mm/2 mm are placed on non-functional cusps.
Manipulation of tray material
  • The right proportion of monomer (liquid) and polymer (powder) are mixed and in dough stage, it is flattened to approximately 4 mm thick
  • The flattened putty is adapted to the tin-foiled cast; excess material is trimmed off and handle is formed with excess resin.
Removal of tray from the cast
After the resin material sets (approximately 15 minutes), the tray is lifted from the cast.
The wax spacer is removed (all wax needs to be removed).
Finishing the tray
Tray is trimmed and polished. Gingival retraction is carried out and tray adhesive is applied.
Making final impression
  • A mixing pad, (6 by 8 inches) or an automatic gun dispensing system is used
  • The low-viscosity impression material is mixed with a circular motion combining the two strands, and then it is blended and flattened onto the mixing pad. (In less than l minute).
Loading the impression material
  • The syringe is loaded by holding it at a slight angle while scraping the pad
  • Screw on the tip, and insert the plunger
  • While the plunger is inserted into the syringe, the cord is removed
  • After evaluating retraction site for seepage, hemorrhage, or debris, first syringe inaccessible areas (e.g., distal–ingual finish line)
  • The syringe is positioned so the elastomer is ahead of the tip's orifice.57
Tray Insertion
The low-viscosity impression material is placed into the tray and the tray is seated firmly in position until the material sets.
Final impression
After the final set is over, tray is removed; the impression is rinsed with water and dried with compressed air.
 
Closed–bite Double–arch Method
Synonyms: Dual quad tray, double arch, triple tray, Accu-bite, closed-mouth impression. Materials used are polyvinyl siloxane and polyether.
Minimum conditions
  1. The articulator must have a vertical dimension holding stop, such as an incisal pin to maintain vertical dimension.
  2. There should be sufficient space distal to the terminal tooth for tray approximation.
Advantages
  • A functional impression is achieved
  • Less elastomeric impression material is needed
  • The physical deformation of the mandible during opening is minimized
  • Less gagging may occur.
Disadvantages
  • Tray is not rigid
  • It is not a functionally generated technique, so it is limited to one casting per quadrant
  • The distribution of the impression material is not uniform.
Procedure
  • After evaluating the fit of the tray in patient's mouth, the tray is positioned accurately with the trays crossbar distal to last tooth in arch
  • The patient is asked to close mouth to observe the complete bilateral closure and the patient's comfort.
 
Making final impression
  • A mixing pad, (6 by 8 inches) or an automatic gun dispensing system is used
  • The low-viscosity impression material is mixed with a circular motion combining the two strands then a figure of eight motion to blend and flatten the mixture onto the mixing pad (in less than 1 minute).
 
Loading the impression material
  • The syringe is loaded by holding it at a slight angle while scraping the pad
  • Screw on the tip and insert the plunger
  • While the plunger is inserted into the syringe, the cord is removed
  • After evaluating retraction site for seepage, hemorrhage, or debris, first syringe inaccessible areas (e.g., distal-lingual finish line).58
 
Tray insertion
  • Insert the low-viscosity impression material into the tray slightly less than the depth of the external borders
  • Seat the tray from posterior to anterior, allowing the excess to extrude anteriorly
  • Seat the tray firmly in position
  • For quadrant trays, position the crossbar distal to the last tooth in that arch.
 
Closed-mouth technique
Instruct patient to slowly close mouth and evaluate the interdigitation on the opposite arch.
 
Tray removal
  • When the patient opens the mouth, the impression adheres to one arch
  • Tray is removed by placing a finger on either side of the tray
  • The handle is not used to remove tray
  • Residual impression material in sulcus or interproximal areas is removed
  • Rinse impression with water, and dry with small bursts of compressed air
  • Retraction cord(s) remaining in the impression material are removed carefully.
 
Copper Band
Copper tube impressions are made when there are multiple preparations with vague margins.
 
Fitting copper band to preparation
  • A copper band of adequate diameter is adapted to the prepared tooth
  • This adapted band is annealed by heating in flame and quenching in alcohol
  • The finish line area is marked with a sharp explorer tip
  • The marked area is cut with scissors and smoothened with carborundum stone.
 
Checking the fit
  • The copper band should extend at least 1 mm beyond finish line with a slight gap between finish line and copper band
  • Orientation holes are placed in top one-fifth of facial surface of tube.
 
Making of compound plug
  • Red stick compound is heated over a Bunsen burner flame
  • Once the appropriate temperature is reached the warm compound mass is inserted on top one-third of copper tube
  • The copper band is oriented and seated till the compound touches the occlusal surface of band.
 
Removing the impression
  • The impression is cooled in water
  • A towel clamp is used to remove copper band from mouth.59
 
Relieving for final impression
  • A slow-speed hand piece with No. 6 or 8 carbide bur is used to remove 0.2 mm of compound from the impressed occlusal surface creating a space for the heavy body polyvinyl siloxane
  • A relief vent is placed with a long shank No. 6 round carbide bur, through the center of the compound plug.
 
Making impression (PVS)
  • More relief vents are placed with a sharp No 4 or No 6 round carbide 2–3 mm above the bottom of the copper tube. These holes help to retain the polyvinyl siloxane impression material and provide a suitable space at the finish line area
  • Some areas in the internal surface are mildly coated with adhesive
  • The prepared tooth is cleaned and isolated.
 
Final impression
  • Either a automatic gun or a syringe loaded with heavy-viscosity, polyvinyl-siloxane impression is injected into the copper band, filling the space completely from the compound to copper band edge
  • After loading the impression material in the copper band the band is positioned and seated properly.
Removing the final impression
After the final set is completed a towel clamp is used to remove impression by grasping on top one-fifth of impression.
Laminate technique/Agar–alginate combination technique
  • The agar material is syringed around the prepared tooth area and the alginate is used as tray material for the rest of the arch
  • Alginate sets by chemical reaction and agar sets when it comes in contact with cool alginate.
Wet Technique
Area to be recorded are flooded with warm water and immediately syringe material is injected and while still the syringe material is liquid the tray material is loaded and seated. The pressure of the viscous tray material pushes fluid syringe material around areas to be recorded.
Matrix System1
Uses three types of impression consistencies as initially a quick setting matrix forming material is used in a clear tray. This matrix impression is retrieved and trimmed to hold a high-viscosity polyether to record the prepared teeth surfaces and then a third less viscous tray material is used which encloses the matrix with light-body material and rest of the arch.
 
Reference
  1. Livaditis GJ. The matrix impression system for fixed prosthodontics. J Prosthot Dent. 1998;79(2):208-16.60
11. Name the component parts of a bridge. Define and classify pontics. Add a note on selection of pontic design in anterior and posterior teeth. What are the requirements of a pontic?
Parts of a Bridge
  • Retainers—Are attached to the prepared abutments
  • Connectors—Pontic is connected to the retainer by a connector
  • Pontic—Artificial tooth suspended from the abutment tooth.
Pontic
Definition
An artificial tooth in a fixed partial denture that replaces a missing natural tooth, restores its function and usually fills the space previously occupied by the clinical crown (GPT).
 
Classification
  1. According to the shape of the surface contacting the ridge.
  2. According to the material used in construction of pontic.
  3. According to the type of fabrication.
  4. According to mucosal-contacting or non-mucosal-contacting.
 
According to the shape of the surface contacting the ridge
  1. Spheroidal/Egg shaped/Bullet shaped.
  2. Conical.
  3. Saddle.
  4. Modified ridge lap.
  5. Sanitary/Hygienic.
  6. Modified sanitary.
 
According to the material used in construction of pontic
  1. Metal pontics.
  2. Metal–ceramic pontics.
  3. Aluminous core-porcelains.
  4. Resin-veneered porcelain.
 
According to the type of fabrication:
  1. Prefabricated:
    • Trupontic
    • Pin facing
    • Interchangeable
    • Modified pin facing
    • Reverse pin facing
    • Harmony facing.
  2. Custom made.
According to mucosal-contacting or non-mucosal-contacting
  1. Mucosal contacting:
    • Saddle
    • Modified saddle
    • Ridge lap61
    • Modified ridge lap
    • Conical
    • Ovate.
  2. Nonmucosal contacting
    • Sanitary/hygienic/perel
    • Modified sanitary.
  3. According to materials used
    • All metal
    • Metal porcelain
    • Resin/facings.
Requirements of A Pontic
  • To provide good esthetics
  • To restore function
  • Should preserve residual ridge
  • Should be biocompatible
  • Should be comfortable for the patient
  • Should be able to maintain good oral hygiene
  • It should stabilize adjacent and opposing teeth.
Pontic Design
Saddle/Ridge lap design (Fig. 23)
Definition
A ridge lap is one in which contact extends beyond the midline of the edentulous ridge or that forms a sharp angle at the lingogingival aspect of the tissue contact. It forms a large concave contact with the ridge obliterating the facial, lingual and proximal embrasures.
Disadvantages
  • Difficult to maintain oral hygiene
  • Can cause tissue inflammation.
Modified ridge lap (Fig. 24)
Lingual surface:
Slight deflective contour.
Facial surface:
Slight faciolingual concavity.
Ridge contact:
Only on crest of ridge facially (contacting area of pontic is convex).
Used:
In maxillary and mandibular esthetic zone areas.
zoom view
Fig. 23: Saddle/Ridge lap design
zoom view
Fig. 24: Modified ridge lap
62
zoom view
Fig. 25: Hygienic pontic
 
Hygienic pontic (Fig. 25)
No contact with ridge, easily cleansable.
Uses
Mandibular molar and nonesthetic zones.
Design
  • Occlusogingival thickness should be a minimum of 3 mm and with space between the pontic and ridge for easy cleaning
  • Its an all convex design mesiodistally and faciolingually (Fish-belly design).
 
Modified sanitary pontic (Fig. 26)
Design
Concave mesiodistally and convex faciolingually (hyperbolic paraboloid design).
If in esthetic zone, visible area is veneered with porcelain.
 
Spheroidal/Egg-shaped/Bullet-shaped pontic (Fig. 27A)
Design
This pontic is convex from all directions with only one point of contact at the center of the ridge.
Use
Mandibular posterior tooth.
Modified spheroidal pontic (Fig. 27B)
Design
This pontic is convex from all directions with the only point of contact on the buccal surfaces.
Use
Mandibular posterior tooth.
 
Conical Pontic (Fig. 28)
Design
Rounded and cleansable with a small tip.63
zoom view
Fig. 26: Modified sanitary pontic
zoom view
Figs 27A and B: (A) Spheroidal pontic; (B) Modified spheroidal pontic
zoom view
Fig. 28: Conical pontic
zoom view
Fig. 29: Ovate pontic
Use
In nonesthetic zone in thin mandibular ridges.
 
Ovate pontic: Term coined by Dewey and Zugsmith in 1933 (Fig. 29).
Definition
A pontic that is shaped on its tissue surface like an egg in two dimensions, partially submerged in a surgically prepared soft tissue depression to enhance the illusion that a natural tooth is emerging from gingival tissues (GPT).64
Design
The round tissue contacting area is set into concavity of ridge.
Indications
  • For esthetic zone, immediately after extraction (temporary)
  • In broad flat ridges which is surgically prepared.
Modified Ovate Pontic1
  • Coined by Liu in 2003
  • Modification include moving the height of contour at the tissue surface from the center of base more labially.
Advantages
  • More esthetic
  • Ease of cleaning
  • Effective air seal
  • Minimizes black triangle
  • Little or no ridge augmentation.
 
Prefabricated pontics
Trupontic
Was widely used earlier.
Design
The facing consists of a horizontal tubular slot, which runs from the centre to the lingual aspect. This slot along with wide proximal bevels provides retention for the facing.
Disadvantage
In case of less occlusogingival, height fabrication of facing is difficult.
Interchangeable Facing (Fig. 30)
Design
Consists of a vertical slot running down the flat lingual surface, which is retained by a lug that engages the retention slot.
zoom view
Figs 30A and B: Interchangeable facing
65
zoom view
Figs 31A and B: Harmony Facing: (A) Anterior tooth; (B) Maxillary posterior tooth
zoom view
Fig. 32: Modified pin facing
zoom view
Fig. 33: Harmony reverse pin facing
January Pontic
Design
  • It is a rounded blunt porcelain with a slot running out to one side, which is oriented towards the lingual during fabrication of the pontic
  • After it is ground to fit into the edentulous space it is reglazed.
Harmony Facing (Fig. 31)
Design
This facing consist of an un-contoured porcelain gingival surface and two retentive pins.
Indication
For maximum esthetics in anterior tooth.
Modified Pin Facing (Fig. 32)
This facing is made by adding porcelain to the lingual gingival area of a pin facing.
Harmony Reverse Pin Facing (Fig. 33)
Design
  • Porcelain denture teeth modified as pontic facings66
  • Multiple pinholes 2 mm deep are made with drill press in the lingual surface of reverse pin facing
  • The pins come out of the backing providing retention.
Indication
Deep overbite situations.
Pontic Selection
Factors determining pontic selection
  • Type of retainers
  • Esthetics
  • Occlusal gingival height
  • Mesiodistal width of the edentulous area
  • Ridge resorption and contour.
 
Type of retainers
Depending on the type of retainer used, the selection of pontic varies.
  • In case of porcelain-bonded-to-metal retainers, the same type of pontics is used
  • If partial veneer retainers are used, prefabricated facings with the same metal as for the retainer is used
  • In maxillary anteriors and posteriors and mandibular anteriors—Modified ridge lap is pontic of choice
  • In mandibular posterior region—Spheroidal or hygienic pontic is preferred.
Esthetics
For maxillary anterior tooth modified ridge lap is the design of choice. Depending on the location of pontic placement, the selection of pontic varies.
 
Occlusogingival height and mesiodistal width
The amount of space available for a pontic alters the choice of design. When there is limited space for a pontic an acrylic pontic with facing is the choice.
Ridge resorption and contour
Classification of ridge deformity (Siebert)
Class I
Less faciolingual width, normal apicocoronal height.
Class II
Decreased ridge height with normal faciolingual width.
Class III
Loss of ridge height and width.
  • Class I defects can be corrected by subepithelial or submucosal connective tissue grafts
  • Class II defects and Class III defects can be corrected by onlay grafts
  • If there is severe bone resorption, placement of fixed restoration will hamper esthetics
  • In such cases, surgical ridge augmentation is done or a roll of soft tissue is created labial to the pontic site.
 
Reference
  1. Chiun-Lin SL. Use of a modified ovate pontic in areas of ridge defects: a report of two cases. J Esthet Rests Dent. 2004;16(5):273–81.67
12. What are the biological, mechanical and esthetic considerations in designing a pontic? Add a note on fabrication of pontics.
Pontic Design
Pontics are fixed partial denture components that replace missing teeth and restore function and appearance compatible with continued oral health and comfort.
Prerequisites
  • Proper diagnosis and treatment planning phase
  • Proper diagnostic waxing procedures
  • Pontic space.
Biologic Considerations
Pontic design needs to maintain and preserve:
  • Residual ridge
  • Abutment
  • Opposing teeth
  • Supporting tissues.
Factors affecting biologic considerations are:
  • Pontic ridge contact
  • Removal of dental plaque
  • Direction of occlusal forces
  • Surfaces of pontic.
 
Pontic ridge contact
Pressure-free contact between the pontic and the underlying tissues is indicated to prevent ulceration and inflammation of the soft tissues. Passive contact should be present only on keratinized attached tissue.
 
Dental plaque
Dental plaque releases toxins that cause tissue inflammation and calculus formation.
This occurs in the area between the tissue surface of the pontic and ridge.
Non-mucosal contacts have better cleansing space than mucosal contacts—Spheroidal and sanitary design.
Factors influencing plaque accumulation:
  • Design of the pontic
  • Materials used in its fabrication
  • Oral hygiene measures.
 
Design
Mandibular posteriors
  • An egg-shaped or bullet-shaped pontic is easiest to keep clean
  • It should be as convex as possible with one point contact at the center of the residual ridge.
Maxillary posterior and all anterior pontics
The lingual surface is still made convex and the embrasures are exaggerated on the lingual side to facilitate cleaning.68
The tissue surface of the pontic must contact the ridge passively along the faciocervical line-angle and on attached keratinized tissue.
 
Pontic materials
Should possess good biocompatibility, rigidity, strength and longevity.
Materials which can be used include:
  • Porcelain
  • Metal
  • Gold
  • Glazed porcelain.
Occlusal contacts should not fall on the junction between metal and porcelain during centric or eccentric tooth contacts. Glazed porcelain is the most biocompatible of the above pontic materials. Well-polished gold is smoother, less prone to corrosion, and less retentive of plaque than an unpolished or porous casting.
Occlusal forces
  • If the buccolingual width of the pontic is reduced by 30% the amount of occlusal forces transferred to the abutment teeth is reduced
  • Other forces, such as biting on a hard object, parafunctional activities, load the abutment teeth even if the occlusal table is narrowed.
Pontic surfaces
  • Tissue surface
  • Occlusal surface
  • Buccal surface
  • Lingual surface
  • Interproximal surface.
 
Tissue Surface
The tissue surface of the pontic should have only minimal passive contact with the ridge. Pontic should not be placed on movable mucosa and should not blanch the tissue.
Occlusal Surface
  • The occlusal table should be reduced to limit forces transferred on to the abutment teeth
  • It should be placed within the neutral zone concept and provide a stable vertical dimension support
  • The maxillary buccal cusps and mandibular lingual cusps should not be altered. Altering of pontic is done when there is lack of space or to create a favorable occlusal relationship.
 
Buccal and Lingual Surfaces
  • The pontic contours of these surfaces are determined by esthetic, functional and hygienic requirements
  • Esthetically, the pontic needs to be contoured facially with proper axial alignment and length
  • Embrasures on the lingual side are wider than the buccal or facial side.
Interproximal surface
Should be contoured physiologically to maintain oral hygiene. For proper contouring vertical clearance should be adequate.69
Mechanical Considerations
Factors influencing mechanical aspects:
  • Improper choice of materials
  • Poor frame work design
  • Poor tooth preparation
  • Poor occlusion.
Pontic materials
  • Metal
  • Porcelain
  • Metal and porcelain
  • Resin-veneered pontics
  • Bis-GMA resin.
 
Resin-veneered pontics
Resin-veneered pontics wear and discolor easily as water absorption and thermal fluctuations cause leakage at the metal–resin interface.
Advantages
  • Easy to manipulate and repair
  • Do not require high-melting range alloys.
Microfilled composites have better physical properties than resin veneers.
 
Prefabricated porcelain pontics
As slotted facings, long pin facings. Harmony facings and reverse pin facings were used earlier.
 
Metal–ceramic pontics
Metalceramic pontics are strong, easy to clean and natural appearing.
Causes of failure in metal-ceramic facings:
  • Uniform thickness of porcelain is required for longer durability
  • The metal surfaces to be veneered must be smooth and free of pits
  • Sharp angles on the veneering surface increases stress concentrations causing mechanical failure
  • Metal–porcelain junction must be placed at least 1.5 mm away from the centric contact junction.
Esthetic Considerations
Pontics should copy the form, contours, gingival margin, incisal edge, gingival and incisal embrasures and color of the adjacent teeth.
 
Factors affecting esthetics
  • Alveolar bone resorption and remodelling
  • Incorrect visual perception
  • Abnormal mesiodistal width.
 
In excessive bone loss cases
  • Recontouring the gingival half of the labial surface70
  • Pink porcelain to simulate the gingival tissues
  • Ridge augmentation procedures using hydroxylapatite
  • RPD is better than an FPD.
 
Abnormal mesiodistal width
  • Can be corrected by orthodontic treatment
  • Space can be adjusted between retainers.
 
Pontic Fabrication
Available materials
  • Metal–ceramic pontics
  • Resin-veneered
  • All metal
  • Unidirectional reinforced composite.
 
 
Metal–ceramic pontics
Steps in metal–ceramic pontics
  1. Wax pattern formation.
  2. Cut-back design.
  3. Investing and casting.
  4. Metal preparation.
  5. Application of porcelain veneer.
Wax pattern formation
  1. Inlay wax is softened and shaped to the desired pontic shape, by waxing up the internal, proximal, and axial surfaces of the retainers.
    The basic pontic shape can either be waxed up, or an impression of the provisional restoration can be made and duplicated in wax or a prefabricated pontic shape can be used.
  2. After the pontic is waxed up it is connected to retainers.
Cut-back design
  1. After outlining the area to be veneered with porcelain, make depth cuts in the wax pattern and complete the cut-back as far as access will allow with the units connected. The porcelain–metal junction should be placed as lingual as possible for good esthetics.
  2. Separate each retainer and complete the cut back till there is a distinct 90° porcelain–metal junction.
After cutting back reflow and finalize the margins. Do each retainer one at a time and join the entire unit back.
Variation
In some cases the gingival surface of the pontic is cut back in the metal.
Investing and casting
Sprue the units and invest and cast.71
Metal preparation
  • The casting is recovered from the investment, the sprue is cut off and the remaining investment material in the casting is removed by sand blasting with aluminum oxide
  • The gingival surface of the pontic is finished without over reducing.
Application of porcelain veneer
  1. A separating agent needs to be applied on the residual ridge of the cast to prevent porcelain powder from sticking to the stone.
  2. The metal is prepared and opaquer is applied.
  3. Cervical porcelain is applied to the gingival surface of the pontic with the castings seated on the master cast.
  4. Porcelain is built up layer-by-layer with the appropriate cervical body and incisal shades and condensed.
  5. After condensing, section between the units with a thin razor blade to prevent porcelain from pulling away from the framework due to firing shrinkage.
  6. A second application of porcelain is done to correct deficiencies caused by firing shrinkage (additions are needed proximally and gingivally on the pontic).
Contouring gingival surface
  1. After application of a porcelain separating liquid to the stone ridge, the desired tissue contact is achieved and gingival surface is contoured (as convex as possible).
  2. The porcelain on the tissue surface is made as smooth as possible.
  3. Pontic is ready for try-in, staining, glazing, finishing and polishing.
  4. The metal framework should be highly polished and gingival embrasures well contoured.
Alternate fabrication technique
In case of partial-coverage retainers made of conventional gold alloys, the retainers must be soldered to the pontic after porcelain application, final staining and glazing.
 
Resin-veneered pontics
  • Resins have low abrasion resistance and bonding between the resin and the metal framework is poor
  • New resins have better physical properties than the older ones.
    • Waxing and cut-back are similar to those for metal–ceramic restorations
    • Mechanical undercuts to retain acrylic resin can be formed by retention grooves, acrylic beads to the wax pattern, wax loops, or using an electrolytic etchant
    • Acrylic beads are placed on the entire metal surface to be veneered. The cast metal is also air abraded with an aluminum oxide
    • An opaquer is applied to the metal to mask the metal color and body shade of resin is added with a modelling instrument and polymerized under pressure in a heated water bath. A light-curing resin can be used as an alternative
    • The body resin core is ground to the desired shape, before adding the incisal shade
    • Incisal resin is applied and polymerized
    • The finished restoration is polished.72
 
All-metal pontics
  • A wax pattern is made of the desired contour
  • The completed wax pattern is finished, sprued and casted
  • Casting is retrieved, finished and polished.
 
Unidirectional reinforced composite
  • An impression is made after tooth preparation, and a cast is poured
  • After application of a special separating medium, glass fibers are placed on the groove in the cast
  • The pontic is built in indirect composite resin, finished, polished and cemented on to the prepared teeth.
13. Describe different types of provisional restorations. Describe various techniques used for making provisional restoration for anterior and posterior tooth preparation.
Definition
A fixed or removable prosthesis designed to protect enhance esthetics, stabilization/or function for a limited period of time after which it is replaced by a definitive prosthesis.
Also known as interim, transitional, temporary and treatment restorations
 
Classification
  • Custom made
  • Prefabricated.
 
Custom made
  • Cast metal (precious–gold scrap and nonprecious—Ni-Cr)
  • Heat-polymerized resin treatment restorations (custom)
  • Visible light-cure composite (Mortan et al. 1984).
 
Prefabricated
  • Aluminum shell, (Brotman—1952) copper band temporization (Amesterdam—1950)
  • Preformed metal crowns
  • Cellulose acetate forms/Vinyl polyethyl methacrylate crowns (Snap and Trim—1960)
  • Prefabricated polycarbonate forms (Charles et al. 1973)
  • External surface forms include polycarbonate cellulose acetate, aluminum tin–silver and nickel–chromium.
 
Depending on the period of use as
  • Short-term (up to 2 weeks, e.g.—polycarbonate or aluminum crowns)
  • Medium-term (>2 weeks, e.g.—resin-based provisionals)
  • Long-term (for months, e.g.—cast-metal crowns).
 
Cast-metal treatment restorations
Indications
  • Patients with maladies difficult to diagnose
  • Patients with gross maxillomandibular discrepancies73
  • As a healing matrix for medically compromised patients
  • For maintenance of vertical dimension
  • For duplication of the pretreatment canine function.
Variation
Preformed metal crowns can be modified at the gingival and then cast.
 
Heat-polymerized resins
  • A wax pattern with the desired shape is made on the mounted casts
  • Wax patters are flasked, dewaxed and packed with heat-cured acrylic resin and cured
  • If the provisional restoration was fabricated on mock tooth preparations then the crown needs to be relined with resin before cementation.
Variation
Plastic denture teeth can be trimmed and lined with tooth-colored self-curing acrylic on the lingual aspect of the crown to fabricate esthetically pleasing, functional treatment restorations.
 
Visible light-cured resin
  • Based on urethane dimethacrylate (Provipont DC, Kristall)
  • Good mechanical properties
  • More shades available
  • No residual monomer.
Disadvantages
  • Stains
  • Expensive
  • Lacks good marginal fit.
 
Aluminum shell crowns (aluminum and tin–silver)
Availability
In anatomic tooth forms and cylindrical shells resembling a tin can.
Indication
Only to be used in premolars and molars.
Technique
  • A shell of suitable diameter is selected and festooned to adapt to the preparation and height of the gingival crest
  • A resin mixture is placed within the shell for patients with a reduced interocclusal distance to enhance retention
  • The shell is then removed, trimmed for adequate occlusal relationships, and seated with a sedative cementing media.
Disadvantages
  • Less marginal strength and proximal contacts
  • Copper band temporisation is not biocompatible and has poor occlusion.74
 
Preformed metal crowns
  • Nickel–chromium shells are used primarily for children with extensively damaged primary teeth
  • Nickel–chromium shells are not lined with resin but are trimmed and adapted with contouring pliers and luted with a high-strength cement
  • Nickel–chromium alloy is very hard and thus can be used for longer-term provisional restorations.
Indications
  • For the posterior teeth
  • In pedodontics for fractured teeth.
Advantages
  • Improved occlusal and axial surfaces
  • Better biocompatibility than aluminum shells.
 
Cellulose acetate crown forms
  • Cellulose acetate is a thin (0.2–0.3 mm), soft, and transparent material available in all tooth types and sizes
  • Shades are dependent on the autopolymerizing resin
  • Mould guide for different sizes and shapes are available.
Procedure
  1. A selected crown form is trimmed and festooned to fit the preparation.
  2. The selected crown is filled with any of the resins (polymethyl methacrylate, vinyl polyethyl methacrylate or epimine).
  3. Commonly used resin is polymethyl methacrylate.
  4. The resin filled crown is gently pressed upon the lubricated preparation while the excess is removed.
  5. The crown matrix is repeatedly removed and reseated to minimize distortion and to ensure removal after the resin is set.
  6. The cellulose shell is peeled off after the material has set.
  7. The occlusion is adjusted and the treatment restoration is trimmed and polished.
 
Prefabricated polycarbonate crowns
Indications
Can be used for anterior teeth (incisor, canine and premolar).
Requirements of provisional restoration
Biologic—Pulp protection, periodontal health, occlusal stability and prevent enamel fracture.
Mechanical—Resist functional load, resist removal and maintain interabutment alignment.
Esthetic—Restore tooth contour, color translucency and texture.
Techniques in Fabrication of Provisional Restorations
  • Shells— custom, such as beaded acrylic and milled crown and proprietary, such as plastic shells—polycarbonate and metal shells, such as aluminium and stainless steel75
  • Matrices can be formed by impression, vacuum-formed thermoplastic and proprietary celluloid by either direct or indirect technique
  • Direct syringing.
Provisional restorations can be done in the following modes
  • Indirect
  • Direct
  • Indirect–direct.
 
Indirect procedure
In this technique, the provisional restoration is fabricated outside the mouth. Impression is made of the prepared teeth and is poured in quick-setting gypsum.
 
Advantages
  • No contact of free monomer with the prepared tooth or gingiva
  • Risk of pulpal damage to tooth is less
  • The marginal fit is better for this technique than direct technique
  • More comfortable for the patient with less chair side time.
 
Direct technique
 
Autopolymerizing resin–alginate impression technique
  • An alginate impression of the teeth is made before tooth reduction and impression is stored in a damp environment
  • After tooth preparations are completed, acrylic resin is mixed and placed in the corresponding section of the alginate impression
  • A separating agent is applied on the tooth preparations and the alginate impression filled with the resin and is then replaced in the mouth
  • Impression is removed when the resin reaches a doughy stage, and ones the rigid stage is almost reached the temporary crown is removed from the alginate impression
  • After evaluating occlusion in the mouth crown is finished, polished and luted with an appropriate luting agent.
 
Vacuum-formed plastic template technique
  • Transparent sheets in cellulose acetate or polypropylene in various sizes and thicknesses are available. Polypropylene or Omnivac is adapted over the unprepared cast using a thermal vacuum machine
  • A suitable shade of autopolymerizing tooth-colored acrylic resin is selected and filled in the template and seated onto the lubricated tooth preparations
  • The matrix is removed and reseated
  • The restoration is copiously irrigated with cold water
  • After the resin has completely set, the matrix is removed, the crown trimmed to the correct finish line with a convex tissue surface and luted with a temporary cement.
Disadvantage
Can cause pulpal injury if the polymerisation temperature is not controlled.76
 
Post crown technique
  • For endodontically treated teeth
  • A wire (paper clip) or a nonprecious metal post is adapted to the canal
  • The selected polycarbonate crown form is then filled with an acrylic resin and placed over the post, including a portion of the radicular surface of the tooth
  • The crown form must be removed and reseated repeatedly to prevent resin from locking into undercuts
  • After the resin has set, the crown is removed with the temporary post set within the resin
  • The entire assembly of post extension and crown form is cemented with a weak adhesive.
 
Indirect–direct procedure
  1. After selecting an appropriate preformed crown with the gingival margin adjusted to approximate the gingival termination, tooth-colored acrylic is mixed and placed inside the crown.
  2. The filled polycarbonate crown is seated onto the lubricated tooth preparation on model; the seated crown is reseated repeatedly to prevent excess resin from setting into proximal undercuts.
  3. After the resin has set, the excess resin is removed, the emergence profile and fit of the gingival margin is evaluated.
  4. The crown is polished and is cemented with a temporary sedative luting agent.
 
Advantages
  • Chairside time is reduced
  • Less heat is generated in the mouth
  • Contact between the resin monomer and soft tissues is minimized.
14. Describe ideal requirements of luting agents and various luting agents used in crown and bridge cementing. Explain the procedure and care to be taken to lute porcelain jacket crowns.
Ideal requirements of luting agents are as follows:
  • Should be tissue compatible
  • Should be resistant to disintegration in the oral cavity
  • Should provide a durable bond between dissimilar materials
  • Should possess favorable compressive and tensile strengths
  • Should have sufficient fracture toughness
  • Should be able to wet the tooth and the restoration
  • Should form a complete seal
  • Should demonstrate adequate working and setting times.
Various luting agents are as follows:
  • Zinc phosphate cement
  • Zinc polycarboxylate cement
  • Glass ionomer cement
  • Resin modified glass or hybrid ionomers77
  • Zinc oxide-eugenol with and without ortho ethoxybenzoic acid (EBA)
  • Silico-phosphate cement
  • Unfilled resins—hydroxyethyl methacrylates and 4 methacryl-ethyl trimellitic anhydride (4-META).
 
Zinc phosphate cement
Advantages
  • Prolonged success rate
  • Good compressive strength (9000–20000 psi)
  • Adequate tensile strength (720 psi)
  • It is a basic cement for comparison
  • Reasonable working time
  • Excess material is removed easily.
 
Composition
Powder
  • Zinc oxide and magnesium oxide in the ratio of 9–1
  • Water content is 33%.
Liquid
Phosphoric acid buffered by aluminum (50%) and zinc salts (traces).
 
Disadvantages
  • Pulp irritation
  • Solubility
  • Poor seal between the cement and the dentin
  • Not an ideal cement when preparation is close to pulp.
Manipulation
  • A cooled thick glass slab is used for mixing
  • Small increments of powder is incorporated into the cement liquid and mixed with a wide circular motion on about one-half of the glass slab
  • When the mixture follows the spatula, it is ready for use as a luting medium.
 
Zinc polycarboxylate cement
(Introduced in 1968).
 
Advantages
  • Good biocompatibility
  • Adhesive to tooth structure
  • Exhibit thinning with increased shear rate (less film thickness)
  • Less pulpal irritation as polyacrylic acid molecules are large and cannot penetrate the dentinal tubule
  • Cleaning the surfaces of metal with an airborne abrasive increases the adhesion of polycarboxylate cement to the metal.78
 
Disadvantages
  • Working time is short (2.5 minutes)
  • Residual zinc polycarboxylate is more difficult to remove
  • It provides less retention.
 
Indication
On retentive preparations (e.g. in children with large pulp chambers).
 
Properties
  • The compressive strength is at least one-half that of zinc phosphate cement, whereas the tensile strength is similar to zinc phosphate cement
  • Solubility of carboxylate cements is about the same as zinc phosphate
  • The film thickness of the polycarboxylate cements is approximately ±20 um.
 
Glass ionomer cement
  • Invented by Wilson and Kent
  • Put to clinical use by McLean and Wilson.
 
Advantages
  • Adhesive to enamel and dentin
  • Good biocompatibility
  • Anticariogenic effect (releases fluoride)
  • Set cement is translucent, more esthetic when used with the porcelain labial margin technique
  • The mechanical properties are almost similar to zinc phosphate.
 
Disadvantage
Is susceptible to moisture contamination.
 
Composition
Glass particles and polyacrylic or polymaleic acid—Polyhyrogel + Silica gel.
 
Reaction
When the powder and liquid are mixed, the aluminium and calcium are displaced forming a alumino-silicate network, which degrades to form hydrated siliceous gel.
 
Manipulation
  • Measured powder is divided into two equal parts and mixed with a plastic spatula
  • The first increment is rapidly incorporated in 10 seconds and the second increment incorporated and mixed for a further 10 seconds
  • The mixing is completed when the cement can be lifted off the slab with the spatula.
To reduce belated sensitivity following can be done:
  • Rapid incorporation of powder and liquid (10–20 seconds)
  • Slight hydration of the tooth before cementation by placing a drop of water on the tooth during mixing, which is gently blown off just before placing the prosthesis on the tooth
  • Allowing the cement to set hard to the touch, plus 1 minute before removing the excess79
  • Placing glass ionomer cement varnish on the margins of the restoration after removing the excess cement.
Resin modified glass or hybrid ionomers (Fuji II LC and Vitremer)1
Differences from conventional resin and glass ionomer cement (GIC) are as follows:
  1. First reaction is an acid/base setting reaction as in GIC.
  2. Second reaction is that the limited quantity of a monomer present will polymerize by light cure (as in resin-based luting agents).
  3. Polymer content is very minimal which will not interfere with the normal acid/base setting reaction and ion exchange adhesion with tooth structure as in GIC.
  4. A third setting reaction occurs in which the remaining monomer will still polymerize chemically.
 
Zinc oxide eugenol (ZOE) with and without orthoethoxybenzoic acid (EBA)
Quartz and alumina have also been substituted in the mix.
 
Zinc oxide-eugenol
 
Advantages
  • Reinforced ZOE cement is extremely biocompatible
  • Provides an excellent seal.
 
Disadvantages
  • Physical properties (such as compressive strength, solubility and film thickness) are inferior to other cements
  • Eugenol leakage can occur
  • Can be only used in restorations with good retention form.
 
EBA cement
  • The 2-ethoxybenzoic acid (EBA) replaces a portion of the eugenol in conventional ZOE cement, which improves compressive strength without affecting its resistance to deformation
  • The EBA cement has a short working time and excess material is difficult to remove
  • The compressive strength of the reinforced zinc oxide eugenol cements is approximately one-half that of zinc phosphate, whereas the tensile strength is nearly identical to zinc phosphate. The values are similar to the averages for polycarboxylate cements.
 
Advantages
  • Palliative effect on the dental pulp
  • Ease of placement in a moist environment.
 
Silico-phosphate cement
It is a mixture of zinc phosphate and silicate cements.
 
Advantages
  • Physical properties in the range of zinc phosphate cement
  • Exhibits lower solubility80
  • Set material is translucent, useful with porcelain margin crowns
  • Some anti-cariogenic properties.
Disadvantage
Low pH and consequent potential for pulpal irritation.
Unfilled resins
  • Used since1950s
  • Less biocompatible than other cements.
Based on polymerisation method:
  • Chemical-cure
  • Light-cure or dual-cure.
Metal restorations
:
Chemically cured system is used
Ceramics
:
A light or dual-cure is used
Resin bonded
:
Resin cement bonded onto etched enamel surfaces.
Indications
For luting:
  • Porcelain veneers
  • Porcelain inlays and on-lays
  • Resin-bonded prostheses
  • Post and core reinforcements (calcium hydroxide placed in deep preparation followed by etched glass ionomer liner, before placement of resin cement).
Advantages
  • Insoluble in oral fluids
  • Good strength properties
  • Easy to use.
Disadvantages
  • Bonds to tooth structure
  • Is not cariostatic
  • Leaks at dentinal margins
  • Film thickness of resin cements is greater.
Recent development
  • Organophosphonates, hydroxyethyl methacrylate (HEMA)
  • 4 methacryl-ethyl trimellitic anhydride (4-META).
Other Cements
  • Red and black copper cements: Excessively irritating to the pulp
  • Zinc phosphate cement mixed by adding water: Is weaker and more soluble than conventional cements
  • Cyanoacrylate cements: Just in the beginning stages.
Procedure to Lute Porcelain Jacket Crowns
Glass ionomer is the luting agent of choice. Other luting agents that can be used are resins and zinc phosphate.81
 
Cleaning tooth surfaces
  • All preparation surfaces are checked for any remaining provisional luting agent
  • The luting agent shade is selected depending on the adjacent tooth shade.
 
Preparing restoration
  • The casting is cleaned ultrasonically and any remnants of polishing agents are removed.
 
Preparation for cementation
  • Isolate the area with cotton rolls and place the saliva evacuator.
 
Manipulation of glass ionomer
  • Powder-liquid ratio: 1.25 part of powder to 1 part of liquid
  • After shaking the powder bottle, 2 level scoops of powder and 8 drops of liquid are dispensed in a paper mixing pad
  • Reduced temperature while mixing increases working time and allows addition of more powder
  • The measured powder is divided into two equal parts and mixed with a plastic spatula. The first part is rapidly incorporated in 10 seconds and the second part incorporated and mixed for a further 10 seconds
  • Mixing time: 45 seconds
  • Working time: 3 minutes from start of mix
  • Luting consistency:
The cement lifts off the slab with the spatula and can be pulled into a thread of 20 mm in length before snapping back onto the slab. The consistency is more viscous than zinc phosphate, but the material thins out with seating pressure.
 
Application of cement
  • A thin coat is applied to the clean internal surface of the restoration
  • Ask patient to bite on wooden stick
  • Dry the tooth with a light blast of air and seat the restoration into place.
 
Seating the restoration
  • Final seating is seated firmly with a rocking, dynamic seating force
  • After the casting is seated, the margins are verified for complete seating.
 
Removing excess cement
  • After the cement has set (7 minutes from start of mix) remove excess cement with an explorer
  • Early removal of cement exposes margins to moisture, increasing its solubility
  • A margin seal can be applied to prevent moisture contamination
  • Dental floss can be used to remove residual cement interproximally and from the gingival sulcus.
 
Evaluation of occlusion
  • An articulating paper or Mylar strip is used to check the occlusion once more.82
 
Instructions to patient
  • As the cements take at least 24 hours for their final set, patients are instructed not to chew on that side for a day or two
  • Oral hygiene instructions are also given.
 
Resin luting agents
A light- or dual-cure can be used for porcelain jacket crowns when retention form is compromised.
 
Reference
  1. Nicholson JW, Anstice M. The development of modified glass ionomer cements for dentistry. Trends in Polymer Science. 1994;2:272–6.
15. Classify adhesive bridges/resin-bonded bridges. Cite the advantages, disadvantages, indications, contraindications and preparation steps. Add a note on types of bonding to metal and tooth.
Definition
Resin-bonded bridges are fixed partial dentures, which are cemented onto the abutments using special resins.
First described by Rochette in 1973 (to splint mandibular anterior teethusing cast gold bar).1
 
Introduction
  • Howe and Denehy introduced the first form of resin-bonded FPD (RBFPD)2
  • Livaditis proposed preparation of proximal and lingual surfaces to create a path of insertion, along with occlusal rest seat preparation3
  • Livaditis and Thompson later introduced the concept of electrolytically etched non-precious metal4
  • Other developments happened in regard to bonding techniques and etching which in turn depended on factors, such as type of casting alloy, type, time and concentration of acid etchant, acid and electrical current density
  • Silicoating improved the longevity and bonding of metal. This involves the fusion of a 0.5μ thin layer of silica to the metal fitting surface which bonds with a silane coupling agent.
 
Classification
  1. Rochette bridge.
  2. Maryland bridge.
  3. Virginia bridge.
  4. Cast-mesh fixed partial dentures.
Advantages
  • Minimal tooth reduction—Conservation of tooth structure
  • Preparation is confined to enamel
  • No pulpal trauma
  • Anesthesia not required83
  • More biocompatible
  • Supragingival margin placement
  • Less periodontal trauma
  • Less chair side time
  • Does not require cast alterations or removable die preparation
  • Reduced cost.
Disadvantages
  • Good patient selection is a must
  • Technique sensitive
  • Laboratory errors cannot be corrected easily
  • Chances of over contouring leads to plaque accumulation
  • Over contouring can result in patient discomfort
  • Can only replace one tooth
  • Cannot be prepared on thin tooth
  • Debonding can occur due to faulty preparation or improper luting technique
  • Difficulty in isolation during bonding procedures
  • Cannot be done on tooth with small clinical crowns.
Indications
  • For abutments with sufficient enamel
  • Splinting periodontally compromised teeth
  • For medically compromised patients
  • As a temporary restoration
  • Young patient
  • Large pulp chambers
  • Post ortho-retention.
Contraindications
  • Sensitivity to base-metal alloys
  • Insufficient occlusal clearance
  • Deep vertical overbite
  • Thin teeth
  • Loss of tooth structure
  • Parafunctional habits
  • Short clinical crowns
  • Narrow embrasures
  • Mobile tooth
  • Defective enamel
  • In extensive restorations.
 
Rochette Bridge (1973) (Fig. 34)
  • It is a wing-like retainer with six perforations to provide mechanical undercuts for resin cement
  • Etched retainers are coated with pyrolized silane and bonded with resin cements.84
 
Disadvantage
The resin is exposed to oral fluids and external stress, which leads to abrasion and marginal leakage.
 
Variation
Nonperforated retainers can be used.
 
Maryland Bridge (Fig. 35)
(Livaditis and Thompson from University of Maryland School of Dentistry)
Here a nonperforated retainer was etched for mechanical retention to form microporosities present and was bonded by resin cement.
Virginia Bridges
(Moon and Hudgins)
  • Use particle-roughened retainers
  • The retainer wax patterns are sprinkled with salt crystals prior to resin fabrication
  • The salt crystals get incorporated onto the tissue surface of the resin pattern
  • During dewaxing the salt crystals dissolve leaving voids in the resin pattern for mechanical retention
  • Air abrasion with aluminium oxide improves retention.
zoom view
Fig. 34: Rochette bridge
zoom view
Fig. 35: Maryland bridge
85
zoom view
Fig. 36: Cast-mesh fixed partial denture
 
Advantages
  • Surface treatment of the retainer is not necessary
  • Noble metal alloys also can be used as retainers.
 
Cast-mesh Fixed Partial Denture (Fig. 36)
A nylon mesh is placed on the tissue surface of the retainer wax pattern before fabricating the wax pattern.
Disadvantages
  • The nylon mesh adaptation to the cast is not good
  • The wax may flow in between the mesh locking all the undercuts.
Steps in the Fabrication/Preparation of A Resin-bonded Fixed Partial Denture
  1. Preparation of abutment teeth.
  2. Fabricating the provisional restoration.
  3. Design of the restoration.
  4. Bonding.
 
Preparation of abutment teeth
  • Single path of insertion
  • The incisal finish line is kept 2 mm short of the incisal edge to avoid any esthetic impairment of incisal edge translucency
  • A reduction of 0.5 mm palatally is ensured for adequate metal thickness
  • The gingival finish line ends 1 mm supragingivally for optimal gingival health
  • Proximal undercuts must be removed
  • Interproximally, the finish line ends at the centre of the contact area or proximal grooves are given for maximum wrap effect
  • Rest seats as cingulum rest should provide good resistance form and is prepared with flat-ended tapered diamond bur
  • Definite and distinct margins should be present.
Design of Anterior Resin-bonded Fixed Partial Dentures
  • A single path of insertion in the incisogingival direction along the proximal surface of the abutment86
  • Lingual clearance (0.8 mm–1.0 mm) should be provided
  • Cingulum rest seat should be prepared to act as a vertical stop
  • A supragingival finish line (1 mm above the crest of tissue)
  • An additional retention in proximal facial extensions.
 
Cantilever Anterior Designs
Canine is used as abutment to replace the lateral incisor.
 
Design of Posterior Resin-bonded Fixed Partial Dentures
  • The occlusal rest (for resistance to gingival displacement)
  • The retentive surface (for resistance to occlusal displacement)
  • The proximal wrap (for resistance to torquing forces).
 
Occlusal Rest Seat
Should be spoon-shaped and placed on the proximal marginal ridge of the abutment adjacent to the edentulous area.
 
Retentive Surface
Proximal and lingual axial walls should be reduced to about 1 mm above the gingival margin.
 
Proximal wrap:
The alloy framework should be designed to engage at least 180° of tooth structure with a knife-edge margin (1 mm supragingivally).
 
Types of material used
  • Gold alloy earlier
  • Presently Ni–Cr alloy
  • Zirconium
  • Fiber-reinforced composites.
 
Types of bonding to provide retention
  • Mechanical
  • Chemical.
 
Mechanical Bonding
Subtypes
Macroscopic retention
By mechanical locks as in Rochette's (six perforations in metal), mesh and water-soluble salt crystal.
Microscopic retention
By electrolytic etching (Maryland bridge).
 
Chemical bonding
Chemical bonding of resin:
  1. Chemical etching.
  2. Tin-plating.87
 
Mechanical etching techniques
 
Electrochemical etching
Results in microscopic porosity.
Non-beryllium Nickel–chromium alloys
Two stages:
  1. Retainer is immersed in 35% nitric acid under a current of 250 milliamps per square centimeter for 5 minutes.
  2. Retainer is cleaned by immersing in 18% HCl in an ultrasonic cleaner for 10 minutes.
Beryllium-containing Nickel–chromium alloys
  1. Retainer is immersed in 10% H2SO4 under a current of 300 milliamps per square centimeter.
  2. Retainer is cleaned by immersing in 18% HCl in an ultrasonic cleaner for 10 minutes.
Other technique (Mc Laughlin technique)
  1. Retainer is etched with a mixture of HCl and H2SO4 in a beaker.
  2. The beaker with the retainer is directly placed in an ultrasonic cleanser for 99 seconds under an electrical field.
 
Nonelectrochemical
Roughening by aluminium oxide-air abrasion.
 
Chemical Bonding of Resin
 
Chemical etching
A gel consisting of nitric and hydrochloric acids is applied to the internal surface of the metal framework for approximately 25 minutes.
 
Tin plating
Precious alloys can be plated with tin and used as retainers to increase adhesiveness of resins thereby increasing bond strengths.
 
Conclusion
  • Ketabi et al. observed a survival of 69% after 13 years of 74 RBFPDs. The fifteen failures were due to loss of retention, caries and fracture of porcelain5
  • Djemal et al.6 studied 832 RBFPDs and found survival of seven years and 10 months
  • RBFPDs still debond frequently but showed an increase in survival rate after five years as 87.7%.
 
References
  1. Rochette AL. Attachment of a splint to enamel of lower anterior teeth. J Prosthet Dent. 1973;30:418–23.
  2. Howe DF, Denehy GE. Anterior fixed partial dentures utilising the acidetch technique and a cast metal framework. J Prosthet Dent. 1977;37:28-31.
  3. Livaditis GJ. Cast metal resin-bonded retainers for posterior teeth. J Am Dent Assoc. 1980;101:926-9.88
  4. Livaditis GJ, Thompson VP. Etched castings: an improved retentive mechanism for resin-bonded retainers. J Prosthet Dent. 1982;47:52-8.
  5. Ketabi AR, Kaus T, Herdach F, Groten M, Axmann-Kromar D, Weber H. Thirteen-year follow-up study of resin-bonded fixed partial dentures. Quintessence Int. 2004;35:407-10.
  6. Djemal S, Setchell D, King P, Wickens J. Long-term survival characteristics of 832 resin-retained bridges and splints provided in post-graduate teaching hospital between 1978 and 1993. J Oral Rehabil. 1999;26:302-20.
16. What are the requirements of dies? Describe materials used in preparation of dies and few techniques used for preparing dies?
Requirements of Dies
  • It must reproduce all surfaces of the prepared teeth accurately without any bubbles or voids
  • The remaining unprepared tooth structure 1 mm cervical to the finish line should be readily discernible on the die
  • Should be dimensionally stable
  • Strong and resistant to abrasion
  • Should be easily sectionable and easy to trim
  • Should be compatible with the separating agent used
  • Should color contrasts with the wax used
  • Should be easily wettable by the inlay wax
  • Must be compatible with the impression material.
Classification
Metallic
  • Amalgam
  • Metal sprayed
  • Electroplated.
Nonmetallic
  • Gypsum
  • Epoxy resin
  • Silicophosphate cement
  • Polymer
  • Ceramic.
Others
  • Divestment.
Gypsum (Type IV-Velmix die stone and Type V-Suprastone)
 
Disadvantages
  • Decreased accuracy
  • Decreased resistance to abrasion.89
 
Advantages
  • Inexpensive
  • Easy to use
  • Produces consistent results.
Variation of technique
Impregnate the surface of the die with a low-viscosity resin (cyano-acrylate) to improve abrasion resistance.
Epoxy Resin
Advantages
  • Good strength
  • Good abrasion resistance
  • Can be cured at room temperature
  • No complicated equipment required
  • Dimensionally stable
  • Retainers adapt better when made on epoxy dies
  • Silicone and polyether are compatible with epoxy.
Disadvantages
  • More expensive than gypsum
  • Polymerization shrinkage
  • Polysulfide and hydrocolloid are not compatible.
Variation
Epoxy resin, which is heat-treated after setting, is also available.
Electroplated die
Advantage
Good abrasion resistance.
Disadvantages
  • Silicone and polyether impressions are difficult to electroplate
  • Polysulfide can be silver plated, but cannot be copper plated
  • Silver plating uses cyanide solution, which is extremely toxic.
Technique
  • Finely powdered silver or graphite is brushed on to the impressions to make them conduct electricity
  • The impression is than placed in the electroplating bath
  • A layer of pure metal is deposited on the impression which is supported with type IV stone or resin.90
 
Methods of Die Preparation
  1. Working cast with separate die.
  2. Divestment technique.
  3. Working cast with a removable die.
 
Working cast with separate die
 
Impression
Two impressions are made, one for the sectional cast and another for full arch cast and poured in hard densite stones.
 
Pouring dental stone for full-arch impression
Dental stone is filled by gently vibrating, until the preparation area fills and then the entire area of impression. Additional base can be added after the initial set has occurred.
 
Pouring dental stone for sectional impression
The preparation area is filled without air entrapment and stone is built up to a height approximately 1 inch over the preparation for handle on the die.
 
Die preparation
After the cast is removed, separating agent is applied on the prepared teeth. A handle, larger in diameter than the preparation, octagonal in shape and parallel to the long axis of the tooth, is formed on the die. Finally prepared die should be smooth and free of ridges with the finish line highlighted with a sharp red pencil.
 
Divestment technique
A die is formed from a refractory material on which the restoration is waxed, kept for burnout and cast directly against the die.
 
Indications
For intricate patterns that are not readily removable from the die.
 
Disadvantages
  • Does not perfectly fit as retainers made of other techniques
  • The die is destroyed in the casting process and a second cast and die needs to be made for finishing purposes.
 
Working cast with removable dies (Fig. 37)
  1. Dowel pin technique.
  2. Curved dowel pin.
  3. Di-lok tray.
  4. Pindex system.
 
Requirements of removable dies
  • Must return to their exact original positions
  • Must remain stable when inverted
  • Must be easily mountable on an articulator.91
zoom view
Fig. 37: Cast with removable die
 
Dowel Pin Technique
Synonyms
  • Straight pin technique
  • Tapered pin technique.
Advantages
  • Least amount of inaccuracy in a horizontal direction
  • Less vertical deviation.
Procedure
  • An impression is made and the straight dowel pin is centred directly over the preparation
  • Bobby pins are placed across the impression buccolingually with the dowel pins placed between the arms of a bobby pin and both are stabilised by straight pins with sticky wax.
Pouring the impression
  • Die stone is poured in the impression by filling the prepared teeth and covering the knurled end of the dowel pin
  • Paper clips are added to other areas to provide retention for the second pour of stone.
Removing bobby pins and straight pins
  • After the first pour sets, the straight and bobby pins are removed
  • The tip of each dowel is covered by soft wax
  • A V-shaped buccolingual orientation groove is placed on each die
  • The area around each die is lubricated.
Removing cast
  • After the cast sets, its removed and trimmed92
  • The utility wax is uncovered and a jeweller's blade is used to cut through mesial and distal side of each die
  • The die is loosened from the cast by gently tapping on the end of dowel
  • The cuts should taper towards each other slightly from occlusal to gingival.
Mounting the casts
  • After the die is removed and ditched, utility wax is placed back into the wells around the tips of the dowels
  • The casts are mounted.
 
Curved dowel pin
Procedure
  • A position bar is used to orient the curved dowel into the impression of the prepared tooth
  • The position bar is oriented faciolingually till the dowel tip extends 1.1–2.0 mm into the impression with the tail pointing facially.
Pins placed
  • One pin in the facial aspect—Straight pin
  • Second pin in the lingual aspect
  • Pins are placed for all prepared tooth and pontic region
  • A dowel pin is also placed near the centre of each segment of unprepared tooth
  • The dowel pin's head should be parallel to the long axis of the tooth.
Pouring of dental stone
Die stone is poured into the impression until it covers the heads, and 1.0–2.0 mm of the thicker hexagonal bodies of the dowels.
Removal of positional pins
  • After the die stone has set the two straight pins and the positioning bar is slided off each dowel
  • A 2.0 mm deep hole is cut on either side of each dowel
  • Lubricate the exposed parts of the dowels
  • Boxing wax is placed around the impression and dental stone is filled till the dowels are covered by at least 2.0 mm, except for the tips.
Sawing the cast
  • After the stone sets, remove the boxing wax and place vertical saw cuts on either side of each die
  • Separate each segment by gently tapping on the protruding tail of the dowel.
Preparing working cast
  • A horseshoe-shaped working cast with the base trimmed flat is made
  • Holes are drilled with drill press in the bottom of the cast directly under the center of each prepared tooth, pontic area and segment containing unprepared teeth
  • A curved dowel is tried into the prepared holes till the head seats completely and luted with cyanoacrylate cement93
    zoom view
    Fig. 38: Removable dies: Di-lok tray
  • After the cement has hardened, a thin layer of petrolatum is applied and second pour is done.
 
Di-Lok tray (Fig. 38)
Requires a specially articulated tray with internal orienting grooves and notches to reassemble sectioned master cast accurately.
Procedure
  • Evaluate the fit of the tray on articulator.
Pouring the impression
  • The entire arch impression is poured with die stone to a height of approximately one inch. The U-shaped cast with an open lingual area must be trimmed to fit the di-lok tray
  • After the cast is allowed to dry, trim the lingual side of the cast on an arbor band and try the cast in the di-lok tray.
Placing the cast in the tray
  • After horizontal grooves are cut in the base of the cast, the tray is filled three quarter with dental stone
  • Slurry water soaked cast is seated into the tray by jiggling it slightly till the cervical lines of the teeth are about 4 mm above the edge of the tray.
Removing cast from tray
  • After the stone sets, the cast is removed from the tray by lifting back up, and then slide the buccal facing forward and tap on the base of the cast with a laboratory knife
  • The removed cast is sawed with a taper between the prepared tooth and the adjacent tooth.
Preparation of removable die
  • Remove all dies with finger pressure and trim the excess stone gingival to the finish line
  • Mark the finish line with a red pencil.94
zoom view
Fig. 39: Pindex system: Pin placement
Disadvantage
  • Large size of the tray makes articulation difficult.
 
Pindex System (Fig. 39)
Preparing the master cast
  • The final impression is poured in stone
  • After the stone sets, the retrieved cast is trimmed to a horseshoe shaped form
  • The buccolingual width of the base of the horseshoe is 13–18 mm, and thickness 15 mm between the gingival margins of the preparations and the inferior border.
Pin Placement
  • After the under surface of the cast is trimmed perfectly flat, drill two holes for each removable section as far apart to provide space for the pins and sleeves
  • The pins are cemented into holes using low viscosity cyanoacrylate cement
  • The short index pins are cemented in the lingual holes
  • Long pins cemented into the buccal holes
  • The white sleeves are placed over the long pins and the gray sleeves over the short pins.
The Secondary Base
  • A strip of carding wax is placed over the extensions of the long pins and the gray sleeves
  • The master cast is boxed or placed in a rubber base former and a secondary base is poured with cast stone.
Removable Dies
  • After the secondary base sets, remove the master cast and trim the secondary base on the model trimmer
  • Dies are sectioned with a die saw and trimmed carefully to expose the finish line (Fig. 40)
  • Clean the stone grindings from the dies, pins, base and pin holes
  • Reseat the dies
  • The master cast is mounted.95
zoom view
Fig. 40: Removable dies: Pindex system
17. Classify ceramics. Enumerate its advantages and disadvantages. Explain the mechanism of bonding and describe the laboratory steps involved in fabrication of different ceramics.
 
Classification
Depending on use
Type 1
For denture teeth
(Powders of feldspar, clay and quartz).
Type 2
For ceramometal applications
(Potassium feldspar and glass).
Type 3
  • For all-porcelain restorations
  • Porcelain jacket crowns, veneers and inlays
    (Feldspathic dental porcelain with increased amounts of aluminum oxide).
 
Depending on fusion temperature
  • High fusing (1288–1371°C)
  • Medium fusing (1093–1260°C)
  • Low-fusing (871–1066°C)
  • Ultra-low fusing (Below 850°C).
 
Based on application
  • Core porcelain: The basis of porcelain jacket crown must have good mechanical properties.
  • Dentine or body porcelain: More translucent than the above, this largely governs the shapes and color of the restoration. Enamel porcelain forms the outer part of the crown and is translucent.96
 
All ceramic restorations
Types
  • Conventional powder
    Slurry ceramics
    E.g.: Optec HSP, Duceram LFC.
  • Castable ceramics
    E.g.: Picon, Dicor Plus.
  • Machinable ceramics/Machined Densely Sintered Ceramics
    E.g.: Cerec vitablocs Mark I, Mark II, Dicor MGC, Celay.
  • Pressable ceramics
    E.g.: IPS empress, OpTEC
  • Infiltrated ceramics
    E.g.: INCERAM.
Advantages
  • Highly esthetic, can match the tooth color in translucency, color and intensity
  • Special types of porcelain are available that can simulate all colors including gingival tones
  • Life-like porcelain is achieved by glazing porcelain
  • Glazed porcelain in contact with tissue surface is more biocompatible than metal
  • All ceramic restoration has less failure rates
  • Porcelain fused to metal prepared from foil copings uses thin metal copings (0.2–0.3 mm), which can increase bulk of porcelain.
Disadvantages
  • If there is difference in thermal matching between metal and porcelain, the porcelain fails at the margin
  • More reduction of tooth structure to accommodate bulk of porcelain
  • Brittle in nature and if surface glaze is removed cracks propagate leading to failure of porcelain
  • Porcelain restorations do not give as close a marginal adaptation as metal marginal finish
  • All Porcelain has less tensile and shear strength, hence ideal to make a metal coping
  • Porcelain shrinks during firing and if porosities are present the overall strength and translucency diminishes
  • Ceramics and glasses have tensile strengths that are much lower than their compressive strengths.
  • Strength of ceramics reduces to 50% by sandblasting or by etching with hydrofluoric acid solution for 1 minute
  • If tensile stresses or sub-microcracks are present on the surface, in presence of moisture may lead to stress corrosion
  • Presence of a chemical environment decreases strength of porcelain.
Enamel–Metal Bond
  1. Mechanical bond
    Porcelain wets the surface of metal.97
  2. Compressive stresses
    Compressive stresses set up during cooling of the sintered porcelain veneer, aid in bonding of porcelain.
  3. Chemical Bonding.
 
Mechanical Bond
Microscopic irregularities on metal surface are filled with porcelain; retention of the porcelain veneer by mechanical interlocking is achieved.
 
Increasing mechanical bond strength
  • Grinding or sand blasting
  • Roughening by oxidation
  • Electrochemical corrosion of the metal by molten glass
  • Selective oxidation of grain boundaries
  • Etching by acid in pre-treatment for fusing.
 
Compressive Stresses
  • A very small degree of thermal mismatch between metal and porcelain leaves the porcelain in a state of compression
  • This increases the bonding between metal and porcelain.
 
Chemical Bond
True chemical bonding results from electron transfer between the oxygen of the glassy phase of porcelain and an oxidised metal surface.
 
Increasing chemical bond
  • By addition of tin, indium, iron to a noble metal casting alloy
  • In nickel–chromium alloy there is an indirect bond between glass and metal through an undissolved oxide formed on the metal surface.
 
Fabrication of a Ceramic Restoration
Condensation
Fine porcelain powder is mixed with water and condensed into the desired form.
Dense condensation provides
  • Lower firing shrinkage
  • Less porosity in the fired porcelain.
 
 
Methods of condensation
  1. Vibration:
    • Mild vibration to densely pack the wet powder upon the underlying framework
    • The excess water is blotted with a clean tissue.
  2. Spatulation:
    • A small spatula is used to apply and smooth the wet porcelain.
  3. Brush technique:
    • Dry porcelain powder is added to the surface with brush to absorb the water
    • Porcelain must never be allowed to dry out until condensation is complete.98
 
Firing
  • The thermochemical reaction is complete during fritting
  • Firing is done to fuse the particles of powder together, a process called sintering.
 
Method
  • The condensed porcelain mass is placed in a preheated furnace (approximately 650°C) for 5 minutes
  • This removes the remaining water.
 
Stages in Firing
A low bisque (low biscuit) firing
In which the glass grains have softened and have started to flow.
A medium bisque (medium biscuit) firing
The glass grains have flowed to the extent that the powder particles exhibit complete cohesion.
In high bisque (high biscuit) firing
  • The shrinkage is complete and the mass exhibits a smoother surface
  • Lesser the firing cycles higher will be the strength and esthetics.
 
Glazing
Air-fired porcelain cannot be polished.
 
Application of over glaze/self glaze
  • Self-glazing, which was previously fired to a high bisque, is heated rapidly (10–15 minutes) to its fusion temperature
  • It is maintained at that temperature for approximately 5 minutes before it is cooled
  • Glass grains flow over the surface to form a vitreous layer
  • Glazed porcelain is much stronger and reduces crack propagation than the unglazed variety.
 
Cooling
  • Because of the low thermal conductivity of the porcelain, the differential between the thermal dimensional change of the outside and inside can introduce stresses, which embrittle the porcelain
  • The ideal cooling method for a porcelain restoration from its firing temperature to room temperature is controversial.
Other Types of Porcelain Fabrications
 
Cast coping
 
Requirements
  • To be fusible to alloys, the porcelain has to be sufficiently low fusing
  • It has to have a coefficient of thermal expansion that is considerably higher than ordinary porcelain
  • The alloy should be sag resistant at the firing temperatures of porcelain99
  • It should be rigid to prevent fracture of porcelain
  • There should be no pigmentation reaction produced between the porcelain and the alloy
  • Further oxide formed should be soluble in porcelain and provide good wetting of the metal or metal oxide for attachment.
 
Bonded platinum foil coping
  • Makes use of tin oxide coatings on platinum foil
  • The esthetics is improved with a thin platinum foil, which allows more room for porcelain
  • The bonded foil reduces subsurface porosity and micro cracks in the porcelain and increases the strength of the unit.
 
Swaged gold alloy foil coping
  • Renaissance, by the Williams Gold refining company, is a laminated gold-alloy foil that is delivered to the use in a fluted shape
  • This foil shape is swaged onto the die and flame sintered to form a coping
  • An interfacial alloy powder is applied and fired, and then coping is veneered with porcelain.
 
All-ceramic Materials
Conventional powder–slurry ceramics
 
 
Available as
  • Powders to which water is added to produce a slurry
  • Available in various shades and translucencies, with characterizing stains and glazes.
 
Manufacturing
  • Leucite crystals are dispersed in a glassy matrix by controlling their nucleation and crystal growth
  • The leucite and glassy matrix fuse together during the baking process
  • The build-up and contouring of the crown can be done by using the powder–slurry technique on semi permeable die material.
 
Advantages
  • Greater strength than conventional feldspathic porcelain due its leucite content
  • Does not require a core as with aluminous porcelain jacket crowns
  • The body and incisal porcelains can be pigmented to provide desired shade and translucency
  • Does not require special processing equipment
  • These restorations fit accurately.
 
Disadvantage
Their high leucite content can cause increased wear to opposing teeth.
 
Other types
Duceram LF
  • Hydrothermal low-fusing ceramic
  • Composed of an amorphous glass containing hydroxyl ions.100
Advantages
  • Greater density, higher flexural strength, greater fracture resistance and lower hardness than feldspathic porcelain
  • No special laboratory techniques or equipment required.
Uses
For fabrication of ceramic inlays, veneers and full-contour crowns.
Procedure
Base layer:
Duceram metal–ceramic (a leucite-containing porcelain) is placed on a refractory die using powder–slurry techniques and baked at 930°C.
Over base layer:
Duceram LFC is applied using the powder–slurry technique and baked at a relatively low temperature (660°C).
Castable ceramic systems
Available as
  • Solid ceramic ingots, which are cast using lost-wax and centrifugal casting technique
  • Only one shade is available
  • Staining is done to obtain proper shading of the final restoration.
 
Picon
A polycrystalline glass–ceramic material, in which glass is heat-treated under controlled crystallization.
Procedure
  • When solid ceramic ingots are cast at 1350°C, a transparent glass crown is obtained
  • This crown is then heat-treated at 1075°C for 10 hours. “Ceramming” causes partial crystallization (55%) of tetra-silic-mica-like crystals.
Advantages
  • Easier, as it is a lost wax technique
  • The transparent crown after heat treatment at 1075°C for 10 hours forms an opaque crown, which increases fracture resistance and strength
  • Less abrasive to opposing tooth structure
  • For final restoration colorant, stains are baked on the surface of the glass–ceramic material.
 
Dicor Plus
It is a shaded feldspathic porcelain veneer applied to the Dicor substrate.
Disadvantages
  • Abrasive to opposing teeth
  • Requires a special high-temperature, electric-heated casting unit
  • Fracture of the restorations is common.101
 
Machinable ceramics
Available as
  • Ceramic ingots in various shades
  • Used in computer-aided design and computer-aided manufacturing (CAD-CAM) procedures
  • The machined restoration are stained and glazed to the desired characterization.
 
Cerec vitablocs Mark I
  • This is a feldspathic porcelain, used with the Cerec system
  • Composition, and properties are the same as feldspathic porcelain used for porcelain-fused-to-metal restorations.
 
Cerec vitablocs Mark II
This is a feldspathic porcelain of increased strength with less abrasive wear of the opposing tooth structure.
 
Dicor MGC
  • Contains fluoro-silicic mica crystals in a glass matrix
  • It has greater flexural strength than the castable Dicor and Cerec
  • Less abrasive wear of the opposing tooth structure than Cerec Mark I.
 
Celay
  • Can be used for CAD-CAM and copy-milling technique
  • Identical in physical properties to Cerec Vitablocs Mark II.
 
Disadvantage
Marginal fit is not good.
 
Pressable ceramics
Available as
  • Ceramic ingots, which are melted at high temperatures and pressed into a mould created by lost-wax technique
  • Can be made to full contour, or can be built up using feldspathic porcelain.
 
IPS Empress
  • This is a feldspathic porcelain supplied in ingot form
  • The ingots are heated and moulded under pressure to produce the restorations.
Procedure
  1. The ceramic ingot is placed under the plunger, and heated to 1150°C.
  2. The plunger presses the molten ceramic into the mould.
  3. The final shade of the crown is done by staining or veneering (cut back technique of wax pattern).
 
Optec Pressable Ceramic
This is a type of feldspathic porcelain with increased leucite content, processed by moulding under pressure and heat.102
 
Advantages of IPS and OPC
  • Produce strong, translucent, dense and etchable ceramic restorations
  • Useful in fabricating ceramic veneers.
 
Disadvantage
Require special equipment to fabricate the restorations.
 
Infiltrated ceramics
Available as
  • Powder (aluminum oxide or spinel), which is fabricated into a porous substrate, and a glass, which is infiltrated at high temperature into the porous substrate
  • The infiltrated ceramic is then veneered using conventional feldspathic porcelain technique.
 
In-Ceram
  • The spinel cores are more translucent than the aluminum oxide cores
  • The core is made from fine-grained particles that are mixed with water to form a suspension referred to as a “slip”.
Procedure
  • The slip is placed on a gypsum die and baked at 1120°C for 10 hours to produce the opaque, porous core
  • An appropriate shade of glass powder is applied to the core, which is baked again at 1100°C for four hours
  • During this process, the molten glass infiltrates the porous alumina core by capillary action.
 
Advantages
  • Extremely high flexure strength
  • The aluminum oxide or spinel crystals limit crack propagation and the glass infiltration reduces porosity
  • Provide an accurate fit.
 
Disadvantages
  • Cannot etch internal surface hence a resin cement, such as Panavia 21TC is recommended
  • Not as esthetic as other systems
  • Requires specialized equipment to fabricate a restoration.
 
Recent Advances in Metal–Ceramics
  • Pure titanium can be used as a coping and framework metal for metal ceramic restorations because of its excellent biocompatibility
  • Copy milling is used to prepare duplicate dies of graphite and to machine the outer form of a titanium crown. The graphite die is then used as a positive electrode in a spark erosion system to serve as the pattern for removal of interior portion of the crown
  • Other titanium-based products, such as Tycast are melted in a specialized casting machine and cast using the conventional lost-wax technology. Ultra low fusing porcelain is used along with this.103
18. What are the types of veneering materials? Describe their advantages, disadvantages and indications. Add a note on the recent advances in veneering materials used in fixed partial denture.
Veneers are thin facings of porcelain or resin affixed directly to teeth using a composite resin-bonding agent.
 
Types of Veneering Materials
  1. Porcelain veneers (Cerinate, Porcelite and Chameleon).
  2. Resin veneers by direct technique (Durafill, Heliosit, Silux).
  3. Resin veneers indirect technique (Dentacolor, Isosit).
  4. Composite veneers direct and indirect (Visio-Gem).
  5. Composite reinforced with glass fibers (Targis/Vectris, Variolink II System).
  6. Castable hydroxyapatite (Cast apatite).
  7. Injectable ceramics (Dicor, Cerestore).
Comparison of Veneer Systems
Direct composite veneers
  1. Require only one appointment.
  2. The shade, contour and final form can be corrected to the dentist's and patient's satisfaction.
  3. Less expensive when compared to porcelain and other laminate systems.
  4. Direct veneers can be repaired easily.
 
Indirect resin veneers
The indirect resins have better bond strength than direct resins.
 
Porcelain veneers
  1. Have good translucency.
  2. Reduced plaque adherence.
  3. They can be fabricated only by indirect technique.
  4. Their advantages are that chair side time is reduced even for placement of multiple units.
Have optimum color stability, esthetics, wear resistance, and tissue compatibility.
Porcelain Laminates
  • Porcelain laminates are thin facings constructed on refractory dies
  • The porcelain veneer's inner surface is etched with hydrofluoric acid and bonded to tooth with composite resin cement.
 
Indications
  • When patient demands high esthetics
  • For mildly stained tooth
  • Enamel defects and diastema.
 
Contraindications
  • In dark stains
  • Patients with parafunctional habits.104
 
Advantages
  • Highly esthetic
  • Good bond strength
  • Resistant to abrasion and fluid absorption
  • Good periodontal health.
 
Disadvantages
  • The fragile veneer can break
  • Loss of glaze while finishing
  • Technique sensitive
  • Expensive and extensive tooth preparation.
 
Different technique
  • Platinum foil is burnished onto the die on which porcelain is applied
  • This prevents build up of heat during firing and increases surface area for etching.
 
Steps before bonding
  1. Check for fit of veneer.
  2. Color check:
    Place laminate on tooth with glycerine and compare it with shade tab. If laminate appears darker then select a lighter shade composite luting agent.
  3. The actual composite is placed and trial checked.
  4. Clean the veneers and isolate the tooth.
  5. The etched surface of veneer is applied with silane coupling agent.
  6. The tooth is cleaned and enamel etching is done with 30–37% phosphoric acid for 15–20 seconds.
  7. The tooth is cleaned, isolated again and light activated dentin bonding agent is applied.
  8. The veneer is seated with composite cement and cured.
  9. Lingual finishing and occlusal equilibration is carried out.
 
Variations
Rochette's bridge with lingual retainers and interproximal retainers.
 
Resin Veneer Direct Technique
Indications
  • For dark stains
  • Patients with parafunctional habits.
Advantages
  • Good esthetics, if color blending can be done
  • The longevity is equally good for recent resins when compared to porcelain
  • Easy to repair.
Disadvantages
  • Can produce staining
  • Less abrasion resistant.105
 
Resin Veneer Indirect Technique
Indication
  • Mildly stained or striated teeth
  • For patients with parafunctional habits.
 
Advantages
  • Can be used in patients with parafunctional habits
  • Can be adjusted with ease.
 
Disadvantages
  • Can produce staining
  • Less abrasion resistant.
 
Indirect Composite Veneers
Indications
  • Enamel defects
  • Diastema closures.
 
Advantage
Easy preparation.
 
Disadvantage
Less bond strength.
 
Technique
  1. Intra-enamel preparation or extra enamel preparation can be done.
  2. Intra-enamel preparations are more accurate.
  3. Shade selection of body and incisal shades is done and impression is made after gingival retraction.
  4. Die is fabricated and veneer is made.
  5. At the second appointment the seating is evaluated and the veneer is bonded with a mixture of Visi-fil (75%)/Visio-bond (25%) blend.
 
Recent Advances In Veneering Materials
Reinforced composites
 
 
Encore Bridge
  • The composite superstructure is bonded with porcelain veneers
  • It is composed of 81% filled composite with a glass fiber reinforcement (Sculpture/FiberKo, Jeneric/Pentron, Inc)
  • The framework has sufficient flexure to attain a Class I mobility
  • An all-porcelain bridge will not allow flexure.
Advantage
Tooth Preparation is minimal (only on lingual surface with a small proximal extension).106
Technique
The preparation starts 1 mm from the most distal aspect of the lingual surface and extends into the proximal area.
Bonding the Framework
After verifying the fit, the framework was bonded into place with C&B-Metabond (Parkel I), after which the veneers are bonded onto the composite pontics with UltraBond (Den-Mat).
 
Targis/Vectris
A product from Ivoclar Williams.
Composition
It is a glass fiber-reinforced composite, with silanized glass fibers, a BIS-GMA matrix and an 85% ceramic-filled composite veneer.
Advantages
  • Class I flexure attainable
  • Good fracture resistance
  • Esthetic properties comparable to metal-free ceramic restorations.
 
Variolink II System
A product from Ivoclar Vivadent.
Technique
  1. After sandblasting the veneer under surfaces, they are silanated with Monobond-S.
  2. The teeth are now isolated, and the preparations are etched.
  3. The enamel is etched for 30 seconds and the dentin for 5 seconds.
  4. The etchant is thoroughly washed away and the preparations dried.
  5. Syntac dentin primer is applied for 15 seconds.
  6. Teeth are dried.
  7. Syntac dentin adhesive is then applied for 10 seconds and dried.
  8. Heliobond (Ivoclar Vivadent) is applied to tooth and the under surface of the veneer inlays and luted with Variolink II cement (has different consistencies and shades) and cured.
 
Castable hydroxyapatite
Synonym: Cast apatite.
Hydroxyapatite mixed with composite fibers is slip cast by vibration.
Studies are yet to prove its clinical use.
 
Injectable ceramics/Castable ceramics
(Dicor, Cerestore)
 
Dicor (Tetrasilicic fluoromica)
It was earlier used for FPDs, inlays and onlays. Recently, it is used for laminate.
Indication
Laminates for periodontally compromised patients.107
Contraindication
In short clinical crowns.
Advantages
  • Good strength
  • Good marginal adaptation
  • Biocompatible
  • Highly esthetic
  • Low thermal conductivity.
Disadvantage
Tooth preparation is extensive.
 
Cerestore (shrink free ceramic system)
Indication
For periodontally compromised patients.
Advantages
  • Good flexural strength
  • Highly esthetic
  • Good marginal fit.
19. Discuss in detail failures in fixed dental prosthesis.
Failure can occur as a result of an isolated incident, a progressive disease or bad planning or execution of the treatment plan.
Prevention is largely under the control of the patient assisted by the dentist in well made restorations.
 
Classification of failures (Flowchart 1)
zoom view
Flowchart 1: Classification of failures1,2
 
Biological failures
Some of the few biological failures include the following:
  • Caries
  • Pulpal degeneration
  • Endodontic
  • Periodontal
  • Tooth perforation108
  • Sub-pontic inflammation
  • Occlusal problems
  • General pathosis
  • Maintenance failure.
 
Caries
It is one of the commonest types of failure and can be due to secondary caries or caries in the abutment tooth.
Symptom
The patient may perceive this as pain, sensitivity to hot, cold or sweet substances.
Causes
Caries in teeth adjacent to fixed partial dentures due to poor proximal contact. Root caries in elderly patients or in patients undergoing radiotherapy.
 
Management
  • Small carious lesions can be removed without removing the prosthesis and restored using amalgam, composite or glass ionomer
  • Extensive caries require removal of the prosthesis followed by endodontic treatment and fabrication of new prosthesis.
 
Pulpal degeneration of abutment
Causes
Inadequate cooling during tooth preparation, abutments with old restoration, use of cements causing pulpal irritation, presence of occlusal forces.
Management
Access is made through the retainer and endodontic treatment is performed followed by post and core if required.
 
Endodontic failures
Causes
Inadequate endodontic treatment or an old root perforation.
Symptom
The patient perceives this as pain on biting or the presence of swelling.
Management
Endodontic retreatment if possible or as a last resort extraction. Generally retreatment must be done if required before the prosthesis fabrication itself.
 
Periodontal failures
Causes
Poor marginal adaptation, over contouring of retainer axial surfaces, large connectors, pontics contacting large tissue contact, prosthesis with rough surface, lack of abutment support, etc.109
Management
Effective oral hygiene measures, periodontal therapy, or surgery Proper fabrication of prosthesis and proper maintenance can avoid this problem and prevent the failure.
 
Tooth perforation
Causes
Improper placement of pins, endodontic treatment, preparation of post and core.
Management
  • If the defect occurs above the crest it can be covered up, below the crest it can be covered by periodontal surgery
  • Defects in inaccessible areas can only be treated by extraction
  • If pins or pinholes are the problems, only an endodontic treatment can save the tooth.
 
Subpontic inflammation
Causes
Excessive pressure by pontic due to improper pontic design or due to poor maintenance by the patient.
Symptom
Pain, swelling, bleeding gums or halitosis.
Management
Re-fabrication of the fixed prosthesis with modified pontic design. Reinforce hygiene instructions with stress on use of floss.
 
Occlusal problems
Causes
Interference in centric or eccentric contacts or an altered vertical dimension that can lead to pulpal damage and mobility causing unesthetic appearance.
Management
Refabrication of the prosthesis and if in a state of grade III mobility then extraction would be the only option.
 
General pathosis
Causes
Failure to diagnose a pathological change. More serious conditions must be treated before the treatment of missing teeth.
 
Maintenance failure
Causes
Failure of the dentist to prescribe a maintenance program, a recall system or inadequate motivation.110
 
Mechanical failures
 
Loss of retention
Causes
Improper cementation, poor retention and resistance form, poor fit of casting, large span length and heavy occlusal forces. This in turn can cause marginal leakage and caries.
Management
If improper cementation is the cause then recementation needs to be done. If the design is the cause then a modification of design must be done and for repeated failure of long span bridges, removable prosthesis is the only option.
 
Connector failure
Causes
Supraeruption of opposing teeth, internal porosity, incomplete casting or soldering/welding.
Management
If the cause is supraeruption, then offending tooth must be contoured to provide adequate clearance. If severe, endodontic treatment may be required. If a casting defect is detected a new prosthesis must be made.
 
Occlusal wear
Causes
Insufficient thickness of restoration which can be due to inadequate preparation of occlusal surface. Bruxism, rough porcelain surface also can cause wear of occlusal surface.
Management
Modifying the preparation followed by new prosthesis, use of night guards; If bruxism is anticipated a metal occluding surface must be given.
 
Tooth fracture
Can be crown or root fracture.
Causes
  • Crown fracture can occur due to excessive tooth preparation, endodontic treatment without post, interfering eccentric contacts, and unseating of cemented bridge
  • Root fracture can be due to a poorly fitting post, post endodontic root fracture or increased load on endodontically treated teeth.
Management
  • Small coronal fracture can be treated by a restoration
  • Extensive fractures are treated with post and cores
  • Root fractures must be extracted.111
 
Metal–ceramic fracture
Causes
Improper framework design, heavy occlusal force, distorted impression, perforation in metal, unsupported porcelain and metal porcelain incompatibility.
Management
Resin repair using composite resin or the facing can be repaired with porcelain by attaching the portion with pins. If fracture is extensive redo the prosthesis.
 
All ceramic fracture
Causes
Inadequate finish line, sharp area on prepared tooth, round preparation without resistance forms, over prepared tooth, opposing tooth contact located incisal to prepared tooth and inadequate lingual tooth preparation.
Management
Remaking the prosthesis.
 
Esthetic failure
Causes
  • Immediate esthetic failure can be due to poor shade match, poor tooth and gingival contour, poor marginal placement, framework displaying the metal and unrealistic expectations of the patients
  • Delayed esthetic failure can occur due to gingival recession, sub pontic tissue shrinkage following extraction, after periodontal surgery, unglazed porcelain can lead to black specks overtime.
 
Psychogenic failure
 
Causes
Frequent complaints can be attributed to stress and behavioral changes in the individual. These patients require counseling and should be identified in the diagnostic phase itself.
 
Conclusion
Finally to resolve the failure in an effective and economical manner, ascertain the cause and re-evaluate the treatment plan and all the factors associated with failure.
 
References
  1. Smith BGN, Howe LC. Planning and making crowns and bridges, ed4. Informa Healthcare; 2007.
  2. Rangarajan V, Padmanabhan TV. Text book of prosthodontics. Elsevier; 2013.112
IMPLANT DENTISTRY
20. Classify and discuss on various impression procedures involved in implant-supported prosthesis.
Definition
An impression is defined as the negative replica of teeth and the oral tissues.
Criteria for good impression
  • Must accurately record the soft tissue and supporting areas
  • Able to record the positioning of the implant components
  • The resiliency difference between the implant and the mucosa should also be considered in the impression technique used
  • Inaccuracies in impression can cause misfit of the prosthesis, uneven force distribution, abutment screw loosening and occlusal inaccuracies.
Classification
General classification1,2
  • Direct transfer (open-tray impression technique)
  • Indirect transfer (closed-tray impression technique).
 
Other systems
Virtual impression system
 
Other classifications
Are modifications of above two techniques by utilizing different—
  • Impression materials
  • Tray
  • Type of transfer—Abutment or implant level
  • In the type of derived support.
Techniques3-5
  • Direct impression transfer (open-tray impression technique)
  • It is the most accurate type of implant impression transfer in which impression transfer copings are picked with the impression. The transfer coping are disengaged through the impression trays opening after polymerization of impression and removed along with the impression (Fig. 41).
Armamentarium
  1. Custom tray/stock-tray.
  2. Tray adhesive.
  3. Modelling wax.
  4. Medium-bodied and light-bodied polyvinyl siloxane impression material.
  5. Essential implant components alike transfer coping, lab analogues, abutments, hex drivers.
Procedure
  • A custom acrylic resin tray is fabricated from the primary cast with openings in the area where the implants are located or a stock tray is perforated in the region of implant113
    zoom view
    Fig. 41: Open tray for impression
  • The healing abutments are removed and implant impression copings are placed. The approximation of components is verified with radiographs
  • The impression tray is evaluated for good adaptation, uniform impression material space and for adequate clearances for the impression coping to protrude through the openings of the tray
  • The excessive openings are sealed with the modelling wax
  • Adequate amount of tray adhesive is coated on the tissue surface of impression tray
  • Impression is made with medium body or monophase polysiloxane impression material. The tray is seated intraorally with all the impression coping pins protruding through the opening of tray
  • After complete polymerization of the impression material the excess wax or impression material on the guide pins are removed from the impression pins for the drivers to gain acces
  • The impression copings are disengaged from the implants and impression is removed from the patient's mouth (Fig. 42)
  • The impression is evaluated, disinfected and components are replaced with suitable laboratory analogues
  • Master cast is made with type IV gypsum product.
Advantages of Open Tray Technique
  • Accurate
  • The best technique for angulated implants
  • Ideal for multiple implant restorations.
Disadvantages
  • Accurate only with the use of custom trays hence, additional steps in the fabrication procedure of custom trays
  • Not suitable in patients with restricted mouth opening and in posterior edentulous regions with reduced access.
 
Closed Tray Technique
In this technique the impression copings remain in the mouth on the removal of the set impressions. After removing the set impression from the mouth the copings are transferred or oriented to the set impressions.114
zoom view
Fig. 42: Direct impression transfer technique
zoom view
Fig. 43: Indirect transfer technique
 
Procedure
  • The transfer coping is fixed to the implant after removing the healing abutments
  • The screw of transfer coping are blocked with cotton balls or modelling wax
  • Radiographs are made for verification of approximation of coping and implant
  • Suitable stock or custom tray is selected
  • A regular precaution of tray selection and adhesive application is followed
  • Light-body viscosity material is injected around the implant and the tray is loaded with putty viscosity impression material and seated intraorally
  • The set impression material is removed from the mouth and the components are approximated extra orally (Fig. 43)
  • The evaluated impression is disinfected and type IV gypsum cast is made.
 
Advantages
  • Simple and easy to follow
  • May not require custom tray
  • Is ideal in case of restricted mouth opening115
  • Employed for simple implant prosthesis as single crowns and small span FPD with parallel implant abutments.
 
Disadvantages
Reorientation of copings can incorporate errors that can lead to inaccuracy and poor fit of the prosthesis.
 
Virtual Impression System
  • This is a CAD–CAM system
  • Eliminates the inaccuracies possible in direct and indirect transfer impressions.
 
Procedure
  • An optical scanner reads and translates the codes embedded in the healing abutments
  • This data is transferred to the CAD software, the implant abutments are virtually designed and the CAM milling apparatus produces the final titanium abutments
  • The completed abutments are returned to the laboratory for the completion of definitive prosthesis using conventional technique or by CAD–CAM procedure.
 
Advantages
  • Offer options of both stock and custom abutments without their associated disadvantages
  • Are custom made for each patient and hence possess the optimal peri-implant soft tissue support
  • Require no manipulation after they are machined resulting in a more precise fit.
Modification in techniques by type of impression materials used6
  • The most ideal materials for the implant prosthetic use (in the order of preference) are addition silicone, polyether and condensation silicone (Tables 2 and 3)
  • The putty and light-body materials are used in combination
  • Ideally auto mixing devices are preferred for better material properties.
Table 2   Summary of the merits and demerits of elastomeric impression materials
Material
Merits
Demerits
Polysulfide
Long working time
High tear resistance
Modest cost
To pour impression immediately
Stretching leads to distortion
Stains clothing
Obnoxious odor
Condensation Silicon
Good working time
Clean and pleasant
High polymerization shrinkage due to volatile by product
Low tear strength
Immediate pour for accuracy
Addition silicone
Clean and pleasant
Ideally elastic
Repeated pour can be done
Stable dimensionally
Low tear strength
High cost
Polyether
Good stability
Delay pour
Shelf life: 2 year
Bitter taste
Leaches components
High cost
116
Table 3   Comparison of properties of elastic impression materials
Property
Polysulfide
Condensation
Addition
Polyether
Tear resistance
+++
++
++
++
Elasticity
+++
++++
++++
++
Accuracy
+++
++
++++
+++
Dimensional stability
++
++
++++
++++
++ -Adequate, +++ - Good, ++++ - Very good
 
Impression trays
  • Both stock and custom trays can be used
  • Conventional method of evaluating the extension of tray is followed
  • In order to accommodate the transfer copings the tray materials must be easier to modify
  • Tray material needs to be rigid.
 
Tray Adhesive
Composition—Silicone adhesive (styrene acrylonitrate) dissolved in solvent, such as ketone, ethyl acetate or chloroform and colorant.
 
Significance
  • The adhesive ensures that the impression remains firmly attached to the tray upon removal from the mouth
  • Inadvertent tray separation while removing impression from mouth can lead to distortion errors.
 
Procedure
  • A thin layer of tray adhesive is applied onto the inner surface of tray and allowed to air dry for at least five minutes, but no more than 30 minutes
  • Mix and place desired tray viscosity impression material directly into coated impression tray in accordance to the manufacturer instructions.
 
Gingival Retraction
Before impression making gingival retraction need to be done.
 
Procedure
  • Fix the desired abutment to the implant
  • Fit the appropriate retractor impression cap over the abutments and on the exposed portion of the dental implant
  • Move and verify the retractor cap to verify the snap fit without impinging adjacent teeth
  • Inject the light body material into the impression cap and place heavy-body material, in the tray, over the impression caps
  • Remove impression from the patient and inspect for accuracy. Secure the impression cap so that it has not been dislodged or so that its position has not been distorted
  • Snap the shoulder analog on the white nylon gingival retractor impression cap in the final impression. Ensure that no change of portion or distortion has occurred when snapping shoulder analog in the cap117
  • Pour the impression in type IV die stone. Place the reinforcement pin in the assembly as the impression cap/shoulder analog is filled with stone.
 
Disinfection of Impression
On intra oral removal of impression immediately rinse with tap water, dry with an air syringe and disinfect with suitable disinfectants (Table 4).
Table 4   Recommended disinfectants for elastomers
Disinfectant
Polysulfide
Silicones
Polyether
Glutaraldehyde 2%
(10 minutes soak time)
Yes
No
No
Iodophors
Yes
Yes
No
Chlorine compounds
Yes
Yes
Yes
Complex phenols
Yes
Yes
No
Phenolic glutaraldehyde
Yes
Yes
No
Repeat impression
Impression must be repeated if:
  • Visible streaks of base or catalyst material is evident
  • The tray exposure in vital areas. This can be due to inaccurate seating of the tray
  • Occurrence of voids, folds or creases in critical areas
  • Separation of the impression material from the impression tray.
 
Cast making
  • There should be no impression material present between the critical implant components, impression coping and abutment
  • The corresponding abutment or analogues is carefully threaded onto the impression
  • The soft tissue mask is injected around the implant region to simulate soft tissue. The impression is trimmed, boxed and poured with type IV gypsum
  • Once the stone has set completely, the transfer copings or corresponding analogues are removed.
 
References
  1. Misch CE. Dental Implant Prosthetics. St Louis: Elsevier Mosby; 2004.
  2. Misch, CE. 2nd. ed. Contemporary Implant Dentistry. St. Louis: Mosby.
  3. Lee H, So JS, Hochstedler JL, Ercoli C. The accuracy of implant impressions: a systematic review. J Prosthet Dent. 2008 Oct; 100(4):285-91.
  4. Burns J, Palmer R, Howe L, Wilson R. Accuracy of open tray implant impressions: an in vitro comparison of stock versus custom trays. J Prosthet Dent. 2003 Mar;89(3):250-5.
  5. Bambini F, Ginnetti L, Memè L, Pellecchia M, Selvaggio R. Comparative analysis of direct and indirect implant impression techniques an in vitro study. An in vitro study. Minerva Stomatol. 2005 Jun;54(6):395-402.
  6. Donovan TE, Chee WW. A review of contemporary impression materials and techniques. Dent Clin North Am. 2004 Apr;48(2):vi-vii, 445-70. Review.118
21. Discuss on implants for fixed prosthesis work.
 
Definition
Dental implant prosthesis is defined as any prosthesis that utilizes dental implants in part or whole for retention, support and stability.
 
Advantages of implants over conventional prosthesis (Table 5):1
  • Esthetics and functions are better
  • The bone resorption is reduced by keeping the bone functionally active
  • Occlusal and muscle functions are improved
  • Improved proprioception, increase in success rate, and enhanced function has better psychological impact on the patient
  • Prosthesis design is simpler even in distal extension situations with implant-supported FPD improving function and patient acceptance
  • Caries, periodontal break down, teeth fracture are not associated with implant-supported FPD.
Table 5   Differences between implant and conventional FPD2,3
Implant-supported FPD
Tooth-supported FPD
Implants take support from adjoining bone
Support derived from adjacent teeth
Bone resorption in partially edentulous region is reduced
Bone resorption in partially edentulous region is more than implant-supported FPD
Better psychological impact on the patient.
Tooth preparation of adjacent teeth can affect patient psychologically
Prosthesis design is possible and simpler even in distal extension situations
Prosthesis design is complicated in distal extension situations
No postoperative complaints of teeth like caries, periodontal breakdown is expected
Caries, periodontal breakdown are expected if not properly maintained
Patient comfort and ease of maintenance is more
Patient comfort and ease of maintenance is less.
Involves surgical procedure
Simplified intraoperatory procedure
Types of Fixed Prosthesis supported by Implants
Based on implant type
  • Subperiosteal implants
    • Unilateral
    • Interdental
    • Total
    • Circumferential.
  • Transosteal implant/mandibular staple/transmandibur
  • Endosteal implant.
 
Based on implant macro surface
  • Threaded, non-threaded
  • Tapered, parallel119
  • With vent holes, without vent holes
  • Hollow cylinder, solid screws
  • Root form, blade form.
 
Based on implant material
  • Metallic implants commercially pure titanium
    • Titanium alloy
    • Cobalt chromium molybdenum alloy.
  • Nonmetallic implants
    • Ceramics
    • Carbon.
 
Based on implant surface
  • Turned surface/machined surface
  • Titanium plasma spraying
  • Hydroxyappetite coating
  • Blasting
  • Acid etching
  • Combination—etching and blasting
  • Electrochemical oxidation surface
  • Ion implementation
  • Doped surface
  • Other surface modifications.
 
Based on type of prosthesis
  • Single crown
  • Cantilever prosthesis
  • Three-unit FPD
  • Multiple-unit FPD
  • Segmental or full-arch prosthesis
  • Over dentures.
 
Based on type of prosthesis materials
  • Ceramic FPD
  • Metal ceramic FPD
  • Metal FPD
  • Acrylic FPD
  • Reinforced acrylic FPD.
 
Based on prosthesis movement
  • PM0: Rigid no movement—O ring with bar
  • PM2: Hinge movement—Dolder bar
  • PM3: Apical and hinge movement—Dolder bar with spacer and clip
  • PM4: Moves in all directions
  • PM6: All range of movement—Resilient ERA.120
 
Misch classification of implant prosthesis
  • FP1
  • FP2
  • FP3
  • RP4
  • RP5.
Misch implant prosthesis classification
Misch classified five possible combinations of implant prosthesis which enlist three types for FPD and two for RPD. They are as follows:
 
FP 1
Fixed prosthesis that replaces the anatomic crown of missing natural teeth.
 
Criteria
  • The residual bone quality and quantity should be good
  • It is more seen in maxillary anterior situations
  • The material of choice is all ceramic or noble materials because of less corrosive action of noble metal when compared to base-metals
  • Soft tissue augmentation might be needed as interdental papilla might be missing in majority of the situations
  • To regenerate or improve inter-proximal position for ideal esthetics and function.
FP 2
It restores crown and a portion of the root similar to the situations exhibiting gingival recession or periodontal pockets.
  • The restoration fabricated is similar to position of adjacent teeth on occlusal or incisal surface but the margins extend towards gingival surface both facially and palatally
  • The restoration has to be fabricated with metal ceramic or all ceramic. The extended cervical borders have to be matched to gingival color.
FP 3
FPD restores crown, gingiva and a portion of soft tissue of edentulous site.
  • This situation arises due to greater bone loss or lesser availability of bone in the partially edentulous area
  • All precautions have to be followed during restoration to check for cantilever action, function and esthetics.
RP 4
Removable prosthesis where the over denture is completely supported by implant
  • Ideally 5 implants for mandible and 8 implants for maxilla are placed for supporting the restoration
  • The superstructure additionally might have attachments and majority of the prosthesis is fabricated with acrylic teeth and supported by acrylic denture base.
RP 5
Removable prosthesis in which over-denture is supported by both soft tissue and implant.
  • The prosthesis is supported with lesser implants121
  • The movement of prosthesis is controlled by obtaining support from both implant and the soft tissues
  • The major limitation of this design is bone resorption in partially edentulous area which might necessitate frequent relining of dentures (Table 6).
Table 6   Differences between screw-retained and cement-retained implants4-6
Cement-retained prosthesis
Screw-retained prosthesis
Surgical placement of implants for cement prosthesis is easier. Smaller angulations can be accommodated
Meticulous surgical planning is essential. Access hole for screw might be difficult to locate
Prosthesis fit is more passive
Less passive when compared to cement-retained prosthesis
Highly esthetic since no openings are displayed on the crown
Screw opening might be visible on crown which can alter the esthetic value
Retrievability of crown is slightly difficult when compared to screw-retained prosthesis
Retrievability of crown is easier
Retention is good but can be compromised in limited interocclusal space
Retention of prosthesis can be achieved even in minimal inter-occlusal space
Best occlusal form can be obtained
Occlusal form gets altered due to the screw access opening
Impression procedures are slightly complicated when compared to screw-retained prosthesis
Impression procedures are relatively easy
Repair of prosthesis is difficult
Repair of prosthesis is easy
Cement might extrude to mucosal tissue which might be difficult to remove
Inter proximation of machined parts, no issues of cement
Cement entering the surgical site can interfere with the healing site of immediate implants
No cement can be more ideal for healing of immediate loading implants
The expansion or alteration of peri-implant mucosa with cemented provisional is difficult
It is easy to expand the peri-implant mucosa
 
Types of restoration7,8
Types include single crown, multiple-unit FPD and fixed removable prosthesis (FRP-Hybrid prosthesis).
The biomechanical factors that can influence the treatment plan are as follows:
Prosthesis forces on implant—Utmost care is taken to avoid deflective contacts.
Angulated abutment—Greater than 25 degrees generates more nonaxial forces and crestal bone loss hence, use of straight abutments are preferred.
Cantilever prosthesis—Advocated in lower denture than maxillary denture without deflective occlusal contacts.
Occlusal forces and pattern—All nonaxial forces have to be reduced in the prosthesis design by:
  • Directing forces on occlusal contacts towards the central fossa
  • Reducing the inclination of cusps of the restored teeth122
  • Reducing the size of occlusal table
  • Any existences of parafunctional forces are relieved by protecting restorations in the form of night guards or splints.
Design of Superstructure—
  • A maximum of 4–6 µm is the permissible space between the prosthesis and abutment
  • Very minor numerical differences in force transmission exist between acrylic, metal and ceramic hence, the prosthesis design matters more
  • In case an implant and tooth are joined, a rigid connection is preferred.
Risk factors involved in the FPD designing can arise due to the following:
  • Design geometry
  • Supporting structures
  • Prosthetic factors.
 
Design Geometry
The risk factors that influence the design geometry are the number of implant, type of platform, positioning and connection of implants. The number of implants to be placed depends on the number of roots of the teeth rather than number of teeth9 (e.g. it is preferential to have two implants for mandibular molar).
The other factors that reduces risk factors in design geometry are as follows:
  • The wider implant platform to be used
  • Cantilevering of implant prosthesis to be avoided
  • Implants to be placed near to prosthesis aids in transmitting more of axial forces
  • Proper crown: implant ratio to be maintained.
 
Supporting structures
Structures are the bone support, primary stability of implant during postsurgical stage and implant dimension.
 
Prosthetic factors
Prosthetic risk factor can be reduced by the following:
  • Passive and accurate fit of the prosthesis
  • Following more established and accepted procedures for prosthesis fabrication
  • Premachined prosthetic components are more scientific and easier to use
  • Proper use and applications of implant armamentarium. Standardized instruments with a stable and predefined tightening torque aids in reducing the future hazards
  • Good overall treatment planning assessment to prevent bone component failures, ceramic fracture, screw loosening and bone resorption.
 
Conclusion
Extreme care in diagnosing, designing, surgical planning, fabrication and maintenance of restoration for good prognosis.123
 
References
  1. Layton D. A critical appraisal of the survival and complication rates of tooth-supported all-ceramic and metal-ceramic fixed dental prostheses: The application of evidence-based dentistry. Int J Prosthodont. 2011;24(5):417-27. Review.
  2. Abt E, Carr AB, Worthington HV. Interventions for replacing missing teeth: Partially absent dentition. Cochrane Database Syst Rev. 2012;2:CD003814.
  3. Salvi GE, Brägger U. Mechanical and technical risks in implant therapy. Int J Oral Maxillofac Implants. 2009;24 Suppl:69-85. Review.
  4. Shadid R, Sadaqa N. A comparison between screw- and cement-retained implant prostheses. A literature review. J Oral Implantol. 2012;38(3):298-307.
  5. Chaar MS, Att W, Strub JR. Prosthetic outcome of cement-retained implant-supported fixed dental restorations: A systematic review. J Oral Rehabil. 2011;38(9):697-711.
  6. Wittneben JG, Millen C, Brägger U. Clinical performance of screw- versus cement-retained fixed implant-supported reconstructions--a systematic review. Int J Oral Maxillofac Implants. 2014;29 Suppl:84-98.
  7. Pjetursson BE, Lang NP. Prosthetic treatment planning on the basis of scientific evidence. J Oral Rehabil. 2008;35 Suppl 1:72-9.
  8. Mamalis A, Markopoulou K, Kaloumenos K, Analitis A. Splinting osseointegrated implants and natural teeth in partially edentulous patients: A systematic review of the literature. J Oral Implantol. 2012;38(4):424-34.
  9. Hoffmann O, Zafiropoulos GG. Tooth-implant connection: A review. J Oral Implantol. 2012; 38(2):194-200.
22. Classify types of implants and discuss the role of surface topography of implants in osseointegration.
 
Definitions
The dental implant is defined as a prosthetic device made of alloplastic material(s) implanted into the oral tissues beneath the mucosal or periosteal layer on or within the bone to provide retention and support for a fixed or removable dental prosthesis; a substance that is placed upon the jaw bone to support a fixed or removable dental prosthesis.
Osseointegration—The process and resultant apparent direct connection of an exogenous material's surface and the host bone tissues, without intervening fibrous connective tissue present.
 
Classification of implants
The implants are classified on the basis of tissue placement, materials, based upon bone attachment, shape, surgical stage, upon angle of insertion and tissue - systemic responses1 (Table 7).
Classified according to the tissue engagement as:
  • Subperiosteal implants: An implant that is placed beneath the periosteum of the bone, it receives its primary bone support by resting on it (unilateral, interdental, total and circumferential)
  • Transosteal implant/mandibular staple/transmandibur: A dental implant that penetrates both cortical plates and passes through the entire thickness of the alveolar bone124
    Table 7   Classification of implants based on different parameters
    Based upon
    Types
    Placement within the tissue
    1. Subperiosteal implants
      • Unilateral
      • Interdental
      • Total
      • Circumferential
    2. Transosteal implant/mandibular staple/transmandibur
    3. Endosteal implant
    Based upon materials used
    1. Metallic implants, commercially pure titanium
      • Titanium alloy
      • Cobalt–chromium–molybdenum alloy
      • Non metallic implants
      • Ceramics
      • Carbon
    Based upon the attachment mechanism
    1. Osseointegration
    2. Fibro osseous integration
    Based upon the macro-design
    1. Thread, nonthreaded
    2. Tapered, parallel
    3. With vent holes, without vent holes
    4. Hollow cylinder, solid screws
    5. Root form, blade form
    Based upon the surgical stage
    1. One-stage implant
    2. Two-stage implant
    Based upon mode of insertion
    1. Axillary inserted (crestal approach)
    2. Laterally inserted (basal approach)
    Based upon tissue response and systemic toxicity effect of implants
    1. Biotolerant materials—Polymethyl methacrylate
    2. Bioinert materials—Titanium and aluminium oxide
    3. Bioactive materials—Glass and calcium phosphate ceramic
  • Endosteal implant: An implant that is present within the bone extending into basal bone for support (screw form, cylinder form and blade form).
Classification based on the degree of roughness, surface treatment and manufacturing process (Table 8).
 
Significance of implant surface and osseointegration2,3
  • Enhances cellular activity and bone apposition
  • The bone-to-implant contact (BIC) is enhanced with improved effects on cell migration, proliferation, biomaterial–tissue interaction
  • In the early healing stages the implant surface properties provides ideal biological and chemical properties
  • The implant surface chemistry can modify the interactions, protein adsorption and cellular activity
  • Surface morphology with defined microstructures enhance stable anchorage, increases the wettability and osteoblast activity125
    Table 8   Classification of various implant surfaces
    Based on
    Types
    1. Degree of roughness
    1. Isotropic
    2. Anisotropic
    2. Surface treatment
    1. Additive surface
    2. Subtractive surface
    3. Modifications
    3. Manufacturing process
    1. Turned surface/machined surface
    2. Titantium plasma spraying
    3. Hydroxyapatite coating
    4. Blasting
    5. Acid etching
    6. Combination—etching and blasting
    7. Electrochemical oxidation surface
    8. Ion implementation
    9. Doped surface
    10. Other surface modifications
  • The expressions of bone-specific differentiation factors for osteoblasts are higher and blood coagulation is better on hydrophilic surfaces.
 
Implant surfaces4
Implant surface roughness is divided, depending on the dimension of the measured surface features into macro, micro- and nano-roughness.
 
Macro roughness
  • Range of millimeters to tens of microns
  • Directly relates to implant geometry, with threaded screw and macro porous surface treatments
  • The primary implant fixation and long-term mechanical stability can be improved by an appropriate macro-roughness
  • The macro-roughness enhances the mechanical interlocking between the implant surface and the surrounding bone.
 
Micro-roughness
Defined as being in the range of 1–10 μm. This range of roughness maximizes the interlocking between mineralized bone and implant surface.
 
Nano-roughness
Composed of nano-sized materials with a size range between 1 and 100 nm. They increase the process of bone growth, turnover and remodeling.
 
Various surface treatments5
Additive surface
The implant surfaces are modified with an addition of material that increases the implant surface area and enhances osseointegration. E.g. titanium plasma spraying (TPS) technique and hydroxyapatite coating (HA) technique.126
 
Subtractive surface
Indents the implant surfaces with a suitable medium and thus, increases the implant surface area. E.g. blasting with titanium oxide/aluminum oxide and acid etching.
 
Based on manufacturing process
 
Turned surface/machined surface
When the titanium rod is loaded to the machine which turns titanium into screw or cylinder.
 
Titanium plasma spraying (TPS)
Titanium powder particles are plasma sprayed through a nozzle at a temperatures of around 1500 degrees centigrade under argon gas atmosphere at a velocity 600 m/s that results in 0.04–0.06 mm diameter projections.
Advantage
Primary stability is enhanced and surface area is increased to 5–6 times.
 
Hydroxyapatite coating (HA)
HA-coated implant has a bioactive surface with increased primary stability due to the greater bone to implant contact area.
Indication
Type IV bone, fresh extraction sites and newly grafted sites.
 
Blasting with titanium oxide/aluminum oxide
The implant surface is bombarded with particles of aluminum oxide (Al2O3) or titanium oxide (TiO2) to produce a rough surface with irregular pits and depressions.
Blasting with Al2O3 particles of 25–75 µm produces surfaces with roughness values of 1.16–1.20 µm, respectively.
Limitations
Embedded remaining particles interferes with osseointegration hence, resorbable blast medium (RBM), such as hydroxyapatite and calcium phosphate are substituted.
 
Acid etching
  • A mixture of acids like HCL and H2SO4 is used. Other acids used for etching are HF, HNO3 and H2O2
  • The acid concentration, type of acids, temperature and duration of contact are determining factors in creating ideal surface microstructure.
 
Combination (SLA)
  • The combination of two subtractive techniques, such as sand-blasted large-grit acid-etched technique (SLA) is the commonest example of this category
  • This surface is produced by a large grit (250–500 µm) blasting process followed by etching with hydrochloric–sulfuric acid.
Objectives
Sand blasting creates surface roughness and acid etching cleans the surfaces producing a micro surface.127
 
Electrochemical oxidized surface
A 3–10 nm coating is made on the implant by electrochemical process which improves the corrosion, wear resistance and increases osseointegration.
 
Ion implantation
High-energy nitrogen ion or calcium ion are added on to the implant surface using higher surface energy. These treatments are usually done in medical grade instruments to increase hardness, corrosion resistance and wear resistance.
Doped surface
Coating surfaces through sol–gel technique with various types of bone growth factors or other bone-stimulating agents enhance growth of bone and osseointegration.
Disadvantages
Include disintegration of coating during implant placement, plaque retention, greater chances of failure and increased cost.
 
Other surface coatings
Nanosized hydroxyapatite coated on nanotubular Ti surface promotes osteoblast cell adhesion and deposition of osteoblast.
Oxidation of implant surfaces with chemicals and laser roughening are the other methods.
 
References
  1. Dohan Ehrenfest DM, Coelho PG, Kang BS, Sul YT, Albrektsson T. Classification of osseointegrated implant surfaces: Materials, chemistry and topography. Trends Biotechnol. 2010;28(4):198-206.
  2. Gaviria L, Salcido JP, Guda T, Ong JL. Current trends in dental implants. J Korean Assoc Oral Maxillofac Surg. 2014;40(2):50-60.
  3. Gittens RA, Scheideler L, Rupp F, Hyzy SL, Geis-Gerstorfer J, Schwartz Z, Boyan BD. A review on the wettability of dental implant surfaces II: Biological and clinical aspects. Acta Biomater. 2014;10(7):2907-18.
  4. Novaes AB Jr, de Souza SL, de Barros RR, Pereira KK, Iezzi G, Piattelli A. Influence of implant surfaces on osseointegration. Braz Dent J. 2010;21(6):471-81. Review.
  5. Anil S, Anand PS, Alghamdi H, Jansen JA (2011). Dental implant surface enhancement and osseointegration, implant dentistry–A rapidly evolving practice, Prof. Ilser Turkyilmaz (Ed.), ISBN: 978-953- 307-658-4.
23. Explain procedural options for single-tooth implant prosthesis.
Single-tooth partially edentulous space is the most frequently encountered clinical situations. The success rate of single-tooth implant depends on selection of patients, vigilant clinical procedures and follow-up protocol.
 
Indications
  • Loss of teeth due to caries
  • Endodontic failure
  • Trauma
  • Infection
  • Fracture
  • Repeated restorative failure.128
 
Contraindications
  • Reduced intertooth space—can alter the contour and form of the restoration leading to overcontouring of crown
  • Insufficient bone width, height and volume—A minimum width of 5 mm of labiopalatal thickness and 10 mm of height is essential
  • Poor periodontal health of adjacent teeth—can affect the bone density, volume and resorption of the adjacent edentulous area.
 
Advantages of single-tooth implant compared to fixed partial denture
  • It is more conservative option
  • Success of single-tooth implant restoration is greater
  • Bone resorption is controlled in edentulous region and the bone health is maintained for longer period of time when compared to the conventional prosthesis
  • It reduces the failure of abutment teeth which is commonly seen in tooth-supported prosthesis
  • Post restorative complaints of sensitivity, tooth failure, need of endodontic treatment and poor gingival health are reduced with single-implant restoration
  • Improve psychological comfort for patients.
 
Disadvantages
  • The time spent both by the dentist and the patient on the treatment is more
  • Expensive when compared to the conventional treatment modalities
  • The problems associated with implant failure can affect the patient psychology and future decisions on the treatment modalities.
 
Factors affecting Treatment plan:1,2
  • Patient age—Young patients
  • Patient need—all treatment options need to be discussed
  • Duration of treatment—3–6 months in delayed loading procedure
  • Cost—Expensive
  • Provisional restorations—In majority of situations, transitional removable partial denture is given to the patient
  • Periodontal health—The patient has to be motivated to improve and maintain the periodontal heath
  • Inter-tooth space—An implant should be placed at least 1.5–2 mm away from the adjacent teeth and should possess a minimum of 6.5 mm of mesiodistal space to achieve good esthetics and ideal function
  • Bone dimensions and tissue support—if limitations exist in sufficient bone dimensions then augmentation, grafts and other options of enhancing the dimensions have to be verified before implant placement.
 
Treatment planning
A comprehensive data on medical history, dental history, radiographs, study casts, photographs are essential for the treatment planning.3129
The dimensions of the implants are determined from diagnostic records, radiographs and in some cases the scans. The width of the implant can also be determined by bone mapping.
Procedure
  • Determine the implant dimensions with suitable radiographs and bone mapping
  • Make a surgical stent that guides in implant placement
  • Disinfect and prepare the surgical site for implant placement
  • Surgical incision is made along the crest of the edentulous area (palatal crestal line angle). A relief labial incision is placed involving interdental papilla. Full-thickness mucoperiosteoum flap is raised
  • The surgical round bur is used to perforate the center of the bone cortex and that will facilitate the guiding of other drills
  • Sequential drilling with different osteotomes is done in accordance to implant dimensions. One width shorter than width of implant dimension at same height of implant is preferred (osteotomy method for tapered-screw implants)
  • Torque the implant on to the prepared osteotomy site
  • In case of insufficient labial bone, subsequent bone grafting procedure have to done
  • Place the cover screw and close the surgical site with interrupted sutures
  • Sutures are removed after a week and evaluated for postoperative complications.
Second stage surgery
  • Access for the cover screw either by surgical incision or punch technique
  • Remove the cover screw and fix the gingival former. Wait for a period of 4–6 weeks for tissue cover or gingival cuff to be formed
  • The gingival former is removed after a period of 4 weeks and subsequent procedures for fabricating the restoration is done.
Impression transfer
  • The impression transfer can be done by direct or indirect method. The direct method is done with the impression post transfer, lab analogues are fixed to the impression post and cast is made. In indirect technique, implant abutment alterations are made intraorally and impression made like tooth-supported FPD
  • The restorations are fabricated on the cast, try-in done on patients, milled abutments are torqued to 30N and restoration cemented.
Cementation of restoration
Cementation is done after checking for occlusal contacts and if any deflective contacts are present they are removed.
Additional care in single restorations
  1. A proper chamfer or if the implant angulation and coping are placed labially, shoulder finish line is preferred.
  2. Chosing a proper abutment platform depending on the clinical situation
  3. Soft mask simulating gingiva provides the technician information of gingival margin that aids in designing the restoration.
  4. The implant body has to be placed 4–5 mm below the CE junction of adjacent teeth predicting the labial bone resorption.130
 
Evidence-based Study reports4-6
  • Based on the meta-analysis of Jung et al, the survival rate of single crown implants are 97.2% for 5 years and 95.2% for 10 years
  • The biological complications for a 5-year cumulative soft tissue complication rate of 7.1% and a cumulative complication rate for implants with bone loss >2 mm of 5.2% were calculated
  • Technical complications reached a cumulative incidence of 8.8% for screw-loosening, 4.1% for loss of retention, and 3.5% for veneer fracture after 5 years
  • The cumulative 5-year esthetic complication rate amounted to 7.1%
  • The outcomes of the meta-analysis demonstrated high implant survival rates for both the single-tooth implants and the respective single crowns after 5 and 10 years.
 
References
  1. Atieh MA, Atieh AH, Payne AG, Duncan WJ. Immediate loading with single implant crowns: A systematic review and meta-analysis. Int J Prosthodont. 2009;22(4):378-87. Review.
  2. Lewis MB, Klineberg I. Prosthodontic considerations designed to optimize outcomes for single-tooth implants. A review of the literature. Aust Dent J. 2011;56(2):181-92.
  3. Shah KC, Lum MG. Treatment planning for the single-tooth implant restoration--general considerations and the pretreatment evaluation. J Calif Dent Assoc. 2008;36(11):827-34.
  4. Benic GI, Mir-Mari J, Hämmerle CH. Loading protocols for single-implant crowns: A systematic review and meta-analysis. Int J Oral Maxillofac Implants. 2014;29 Suppl:222-38.
  5. Annibali S, Bignozzi I, Iacovazzi L, La Monaca G, Cristalli MP. Immediate, early, and late implant placement in first-molar sites: A retrospective case series. Int J Oral Maxillofac Implants. 2011;26(5):1108-22.
  6. Jung RE, Zembic A, Pjetursson BE, Zwahlen M, Thoma DS. Systematic review of the survival rate and the incidence of biological, technical, and aesthetic complications of single crowns on implants reported in longitudinal studies with a mean follow-up of 5 years. Clin Oral Implants Res. 2012;23 Suppl 6:2-21.
24. Discuss the immediate loading and progressive loading of endosseous implants.
 
Definitions
Immediate loading
The prosthesis is attached to the implant within 24 hours of the implant being placed.
 
Early loading
The prosthesis is attached to the implant within days/weeks of the implant being placed.
 
Delayed loading
The prosthesis is attached to implant after a healing period of 3–6 months (Table 9).
 
Conventional or delayed loading protocol
Was initially proposed by Brånemark where a waiting period is essential for the implant to osseointegrate and the prosthesis is fabricated after the delayed waiting time after complete integration.131
 
Advantages of the delayed protocol
  • It can be used on weaker bone in both quality and quantity. The procedure provides sufficient time for the bone to heal and implant to integrate
  • The stimulatory forces of bone metabolism aid in improving the bone density and enhance osseointegration
  • Improved bone architecture and bone forming factors around implant is achieved better with delayed loading protocol
  • Extensively documented data available on its success
  • Increased predictability of osseointegration.
The prosthodontic loading time in delayed protocol varies with the type of bone, jaw and span of the restoration. The maxilla has increased waiting time than mandible because of lesser quality of bone. The universal waiting period for different bone type are listed below:
Table 9   Type of bone and waiting period before implant loading
Type of bone
D1
D2
D3
D4
Maxilla
6 months
6 months
6–8 months
6–8 months
Mandible
3 months
3 months
4–6 months
4–6 months
Disadvantages of progressive loading
  • Time taken to fabricate the restoration. It can vary between 3 and 6 months depending on type of bone and prosthesis
  • Patient psychology can be affected due to the delay in the implant loading.
The advancement in implant surfaces and improved understanding of bone science, prosthesis and occlusion has lead to development of improved implantology protocol of immediate loading which reduces the limitations of delayed implant loading.
 
Immediate loading
Immediate loading is loading the implant immediately. This occlusal loading can be with a provisional restoration or with a definitive prosthesis. The type of loading and prosthesis depends on the clinical situations (Table 10).
 
Indications
  • Adequate bone quality and dimensions
  • Psychology needs and patient who dislike temporary removable partial denture either due to prosthesis movement or due to its supporting mechanism.
 
Contraindications
  • Inadequate bone architecture, poor bone quality and density (Type IV bone)
  • Parafunctional habits
  • Poor systemic health
  • Inadequate bone height (≥12 mm) and bone width ≥5 mm
  • Inability to achieve an adequate AP spread
  • Contraindication factors associated with generalized implantology procedures.132
 
Advantages of immediate loading
  • Minimise the requirement of angle abutments since the implants can be placed in the same position as the extracted tooth or in extraction sites
  • Immediate restoration improves patient satisfaction and psychology
  • Osseointegration is more favorable when implants are placed immediately following an extraction
  • The recession of tissue, bony receptors and the crestal bone are preserved better with immediate loading of implants
  • Stage 2 surgery of conventional protocol is eliminated
  • Countersinking of implants done in delayed protocol is avoided which reduces the chance of crestal bone loss
  • Splinting of implants aids in early healing and improves biomechanical advantages
  • The primary healing time of bone and tissues are reduced coupled with implant surgical healing in immediate loading.
Disadvantages of immediate loading
  • If the primary stability of implant is reduced it affects the success rate of procedure
  • The micro movements of implants increase chances of bone micro fracture
  • Parafunctional habits like tongue thrusting can disturb the implant site and can cause crestal bone loss eventually leading to failure
  • Entrapment of impression or surgical material can interfere in healing process.
Factors that influence the immediate-loading protocol1,2
Surface area
The implant surface area is enhanced with increase in the number, dimension, design and surface area of implants.
Nonaxial forces
The nonaxial forces are less endured by the supporting structures and it can enhance bone resorption in immediate loaded implants. The possibility of nonaxial forces is observed significantly in parafunction, cantilever and inappropriate crown/implant ration.
Occlusal consideration—Implant protected occlusal scheme reduces the damaging forces transmitted to the implants.
Implant position
Mandible is divided into three segment and maxilla into four segments.
The three mandible segments are bilateral canine region, canine and two posterior segments on either side.
In maxilla, there are four segments midline to canine on either side or posterior bilateral space present behind canine to molar on either side. An implant is also necessitated in the midline to improve the supporting mechanism.133
Table 10   Different steps in delayed and immediate-loading protocol
Delayed-loading protocol
Immediate-loading protocol
1
Diagnosis and treatment planning
Diagnosis and treatment planning
2
Selection of implant dimensions
Selection of implant dimensions
3
Stage I—Implant surgery
Implant surgery
4
Waiting period between 3 and 6 months
Subsequent prosthodontic procedures followed
5
Stage 2 surgery—Removal of cover screw, gingival former placement
Implant loaded with provisional/final restorations
6
After 4 weeks of stage 2 surgery, gingival formers removed. Implant and soft tissues reevaluated for esthetics and function
7
Choice of abutment in accordance to angulation, type of prosthesis is selected and try in done. If intraoral adjustments required, abutment is prepared
8
Impression made by direct/indirect technique and subsequent laboratory procedures done
9
The time, procedures of provisional and final restoration can slightly vary with the type of restoration, such as single crown, FPD, over denture and complete denture
Immediate loading protocol for different kinds of prosthesis designs3-5
 
Single tooth replacements
  • Ideal soft tissue, bone and implant conditions
  • Threaded- or screw-type implant design with highest pitch and depth that increases the implant surface area
  • Implant-protected occlusal scheme
  • Definitive cement is used for luting the restoration.
 
Partially edentulous area (missing 2 or more teeth)
  • Each missing tooth has to be preferentially restored with one implant
  • Maximum dimension of implant that can be accommodated with minimum dimension is 10 × 3.75 mm
  • Threaded- or screw-type implant design with highest pitch and depth that increases the implant surface area
  • No cantilever load
  • Implant-supported occlusal scheme.
Fixed prosthodontics in completely edentulous situations
  • Necessitates minimum of 8 implants in maxilla and 5 in mandible
  • Maximum implant dimension that can be accommodated should be used with of minimum 3.75 mm in width and 10 mm in length134
  • Threaded- or screw-type implant design with highest pitch and depth that increases the implant surface area
  • No cantilevers
  • Implant-supported occlusal scheme.
 
Overdenture
  • Adequate residual bone height is essential
  • Interarch space should be minimum of 12 mm
  • Minimum of four implants to be placed between two mental foramen and widely distributed in maxilla (two bilateral canine and molar regions)
  • Implants have to be splinted with bar for force distribution
  • Acrylic denture with metal framework engaging the bar will be more ideal prosthesis.
Conclusion
The immediate loading of implants has become more a standardized protocol of acceptance. The choice between immediate- and delayed-loading protocol depends on the clinical situation.
 
References
  1. De Bruyn H, Raes S, Ostman PO, Cosyn J. Immediate loading in partially and completely edentulous jaws: A review of the literature with clinical guidelines. Periodontol 2000. 2014;66(1):153-87.
  2. Atieh MA, Payne AG, Duncan WJ, de Silva RK, Cullinan MP. Immediate placement or immediate restoration/loading of single implants for molar tooth replacement: a systematic review and meta-analysis. Int J Oral Maxillofac Implants. 2010;25(2):401-15. Review.
  3. Del Fabbro M, Ceresoli V, Taschieri S, Ceci C, Testori T. Immediate loading of postextraction implants in the esthetic area: Systematic review of the literature. Clin Implant Dent Relat Res. 2013 Apr 22.
  4. Esposito M, Grusovin MG, Polyzos IP, Felice P, Worthington HV. Interventions for replacing missing teeth: Dental implants in fresh extraction sockets (immediate, immediate-delayed and delayed implants). Cochrane Database Syst Rev. 2010;(9):CD005968.
  5. Su M, Shi B, Zhu Y, Guo Y, Zhang Y, Xia H, Zhao L. Comparison of implant success rates with different loading protocols: A meta-analysis. Int J Oral Maxillofac Implants. 2014;29(2):344-52.
25. Occlusal consideration for implant-supported prosthesis.
 
Definition
Occlusion has been defined as the static relationship between the incising or masticating surfaces of the maxillary or mandibular teeth or tooth analogues.
 
Significance
  • It determines the success of implant prosthesis
  • The supporting structures of implant and the natural teeth are different, as natural teeth are suspended by periodontal ligament whereas the implant has a rigid connection to the bone (Table 11)
  • An implant-protected occlusal scheme reduces the occlusal forces on implant thus, protecting and preserving the surrounding bone.135
Table 11   Biomechanical differences between natural teeth and implant
Tooth
Implant
Bone connection
Suspended by PDL
Rigid connection with bone
Proprioception
Mechanoreceptors of PDL
Osseoperception
Tactile sensitivity
High
Low
Axial mobility
25–100 µm
3–5 µm
Movement phases
Two phases
Primary nonlinear and complex
Secondary: Linear and elastic
One phase
Linear and elastic
Fulcrum to lateral forces
Apical third of root
Crestal part of implant
Adjacent bone
More of cortical bone
More of trabecular bone
Load bearing characteristics
Shock absorption and stress distribution
Stress
Signs of overloading
PDL thickening, mobility, wear facets, fremitus, pain
Screw loosening or fracture, abutment or prosthesis fracture, bone loss and implant failure
 
Types of occlusion1,2
  • Balanced occlusion
  • Group function occlusion
  • Monoplane occlusion
  • Mutually-protected occlusion
  • Lingualized occlusion
  • Implant-protected occlusion.
Greater emphasis is placed on mutually-protected occlusion since most of the implant restorations are associated with the greater number of natural teeth.
 
Balanced occlusion
Definition
It is the bilateral, simultaneous, anterior and posterior occlusal contact of teeth in centric and eccentric positions.
Indication
Mostly preferred in complete dentures to enhance the stability of prosthesis.
Advantages
Uniform distribution of forces to all the occlusal units improves stability of the prosthesis.
Limitation
Lateral forces on the nonworking side of implants causes eccentric deflective contacts.
 
Group function occlusion
A variant of balance occlusal scheme is unilateral balance or group function occlusion. The teeth on the working side will be in contact and the balancing side teeth are not in contact. It can be a choice of occlusion when canine is replaced.136
 
Monoplane occlusion
An occlusal scheme utilizing a zero degree or nonanatomic teeth commonly preferred in highly resorbed ridges. The flatter occlusal surfaces dampen the damaging forces transmitted to the resorbed and flat ridges.
 
Mutually-protected occlusion
The maxillary and mandibular anterior teeth disocclude the posterior teeth during protrusion and in lateral movement of mandible the canine on working side disocclude the other teeth. In centric, the posterior teeth prevent the anterior group of teeth from contacting.
 
Advantage
Distributes the forces.
 
Lingualized occlusion
The maxillary lingual cusp of posterior teeth is predominant teeth in contact during the movements rather than mandibular buccal cusp.
 
Advantage
This direction of arrangements helps in directing the occlusal forces towards the center of ridge and thus, reduces the lateral stress on the implants.
 
Implant-protected occlusion
Misch and Bidez in 1994 proposed the occlusal concept to reduce the force transmission to implant which lead to the better prognosis and success of implant restoration.
Factors to be considered in the implant-protected occlusion3-6 are:
  • Implant surface area
  • Implant and abutment angulation
  • Crown: implant ratio
  • Implant crown contour
  • Cusp inclination
  • Occlusal contact and its timing
  • Width of occlusal table
  • Occlusal materials
  • Protection of weaker component.
 
Implant surface area
An increase in surface area permits greater transmission of occlusal forces. This is done by increasing number of implants and splinting implants.
 
Implant and abutment angulation
The forces parallel to the long axis is better tolerated than the lateral forces.
  • If the angulation is straight there are more of compressive forces which are more acceptable to biologic structures than the tensile force
  • It is ideal to place the angulated abutment of same angulation rather than altering the angulation at the interface (e.g. for a 15° angulation of implant, better to use a 15°137 angulated abutment) which makes the abutment –implant body interface straight and aids in transmitting the forces away from the crest
  • If the angulation cannot be avoided then wider implant, ridge augmentation or additional implants can be placed.
 
Crown: implant ratio
  • Ideal crown to root ratio is 1:2
  • Any deviations from this generate cantilever and nonaxial forces to the crest of the implant.
 
Crown contour
Depends on:
  • Bone contour and morphology in edentulous areas
  • Bone resorption pattern
  • Buccolingual dimension of the implant is lesser than that of natural teeth so it is essential to contour the crown which in turn will reduce the lateral forces to the bone crest.
 
Cusp inclination
  • Increase in cuspal angulation increases the masticatory efficiency
  • Broadening the angle and widening the cuspal angulation reduces the force concentration
  • Recontouring of cusps with lesser angulation transmit lesser and favorable axial loads to the implant.
 
Occlusal contacts
  • The occlusal contact of the prosthesis with opposing teeth determines the transmission of forces to the implant
  • Occlusal contacts should exist within the diameter of the implant. Cusp fossa occlusal scheme is better than cusp marginal ridge contact which centralizes the forces of occlusion
  • Occlusal contacts has to be matched to the difference in range of movements between natural tooth (27 µm) and implant (3–5 µm) with 28 µm articulating paper until an even contact is obtained
  • Uniform occlusal contact ensures long-term success of the implant and also prevents fracture of the ceramic restorations.
 
Occlusal table
Narrow occlusal table reduces the forces and aids in easier maintenance.
 
Occlusal materials
The choice of material is metals, porcelain fused to metal (PFM), ceramics, composite and acrylics. Among these materials PFM are commonly used as permanent restorations and acrylic as provisional restorations.
 
Protection of weaker components
Weaker aspects as cantilever length, length of edentulous span, angulation of implants, poor labial or buccal bone have to be identified and the stress transmitted to these components should be reduced during prosthesis designing.138
Occlusion schemes in different types of prosthesis
  • The occlusal schemes vary with the type of implant prosthesis, number of teeth replacement, type of support and opposing dentition
  • In full-mouth rehabilitation and if full-arch prosthesis opposes the natural dentition, mutually protected or group function is ideal
  • Natural dentition opposing single complete denture a balance occlusal scheme is ideal
  • For an over denture prosthesis, a balanced occlusal scheme with lingulaised occlusion is preferential choice
  • In situations of highly resorbed ridges monoplane occlusal scheme is preferred
  • For replacement of single teeth or fewer teeth, the already existing occlusal scheme (usually a mutually protected) is chosen
  • When a segment of teeth are replaced or when the abutment teeth are periodontally compromised a group function occlusion is chosen for uniform force distribution.
 
Conclusion
The potential complications of decementation, screw loosening, abutment failure, fracture of restoration, crestal bone loss and implant failure can be avoided with an implant-protected occlusal scheme (Table 12).
Table 12   Occlusion determinants in implant-supported restoration
  • Axial inclination to be parallel to the long axis of adjacent teeth or at right angles to the occlusal plane
  • Minimum interimplant distance is 5 mm
  • Increased number of implants reduces biomechanical risk
  • Splinting of implant is preferable to reduce occlusal load
  • Ideal crown/implant ratio is 2:1 or minimum of 1:1 is essential
  • Minimum implant dimensions of 10 x 3.75 to reduce biomechanical risk
  • Restore in centric relation
  • Cusp fossa occlusal scheme in centric should be followed
  • Constricted occlusal table
  • Flatter cusps for improved implant biomechanics
  • No cantilevers
  • No heavy contacts or infra-occlusion in unavoidable cantilevered situations
  • Reduced lateral or excursive guidance on restored teeth
  • Group function or balance occlusion to reduce biomechanical risk
  • Ceramic should be adequately reinforced with metal framework
  • Use of night splint is recommended in bruxism
 
References
  1. Ben-Gal G, Lipovetsky-Adler M, Haramaty O, Sharon E, Smidt A. Existing concepts and a search for evidence: a review on implant occlusion. Compend Contin Educ Dent. 2013;34 Spec No:26-31.
  2. Koyano K, Esaki D. Occlusion on oral implants: current clinical guidelines. J Oral Rehabil. 2014 Oct 6.139
  3. Yuan JC, Sukotjo C. Occlusion for implant-supported fixed dental prostheses in partially edentulous patients: a literature review and current concepts. J Periodontal Implant Sci. 2013 Apr;43(2):51-7.
  4. Kim Y, Oh T-J, Misch CE, Wang H-L. Occlusal considerations in implant therapy: clinical guidelines with biomechanical rationale. Clin Oral Impl Res. 2005;16(1):26-35.
  5. Gross MD. Occlusion in implant dentistry. A review of the literature of prosthetic determinants and current concepts. Aust Dent J. 2008 Jun;53 Suppl 1:S60-8.
  6. Chang M, Chronopoulos V, Mattheos N. Impact of excessive occlusal load on successfully-osseointegrated dental implants: a literature review. J Investig Clin Dent. 2013 Aug;4(3):142-50.
26. Recent advances in surgical techniques.
Introduction
  • Placements of dental implants are by either one stage surgery or two stage surgery in ideal situation
  • In limitations, such as bony defects, soft tissue defect, bony imperfections and inadequate bone dimensions, advanced surgical options are done.
 
Classification of advanced surgical procedures
Regenerative procedures
  1. Bone augmentation procedures
    • Sinus elevation procedures
      • Direct sinus lift
      • Indirect sinus lift
    • Bone grafting/regeneration procedures
      • Vertical grafting
        1. Distraction osteogenesis
      • Horizontal grafting
        1. Onlay grafting
        2. Sandwich grafting
      • Guided bone regeneration procedures
  2. Gingiva augmentation procedures
    • Free gingival grafts
    • Connective tissue grafts
    • Pedicle grafts
    • Guided tissue regeneration procedures
Resective procedures
c. Bone resective procedures
d. Soft tissue resective procedures
 
Sinus elevation procedure
Definition
It is a surgical procedure that increases the amount of bone in posterior maxilla. The bone grafting material is deposited on the inferior wall or floor of sinus to increase the bone dimensions available for implants in maxillary posterior region.140
 
Indications
  • Decreased bone height in posterior maxilla
  • Implant restoration with deficient bone dimensions (length) in posterior maxilla.
 
Contraindications
  • The presence of sinus infections, inflammation and pathologic growth
  • Systemic diseases that have poor prognosis to healing
  • Psychological impairment.
 
Procedure
Based on the surgical approach a direct or indirect technique can be done.
In direct technique the sinus approach is done directly through superior, middle or inferior lateral wall approach.
In indirect technique the sinus approach is through crestal direction of the partially edentulous space.
Superior lateral approach of direct technique and crestal osteotome of indirect technique are the commonly followed technique for sinus elevation.
 
Superior Lateral wall approach (Caldwell–Luc approach)
  • The surgical approach is through the buccal surface of edentulous site to gain access to the floor of sinus raising the Schneiderian membrane
  • The space thus created between the sinus floor and the Schneiderian membrane is filled with a suitable bone grafting material to obtain the desired bone dimensions
  • Commonest postoperative complication is the tear of sinus membrane which is repaired by stitching the membrane or restored by undisturbed healing for a period of 3–6 months for auto-repair.
 
Osteotome technique
  • With the crestal approach, crestal access till the floor of sinus is obtained by implant drills or by other methods of osteotomy
  • With suitable osteotomes bone is compressed in incremental quantities through the osteotomy site towards the sinus floor elevating the Schneiderian membrane till the required amount of bone augmentation is done
  • The procedure is completed with the implant insertion on the augmented site.
 
Important points to note
The healing time associated with sinus grafting is in average of 3–6 months. Complications, such as infection, inflammation, nerve damage, graft failure, oro antral communications and implant loosening or implant failure, can occur.
Evidence-based data1,2
The meta-analysis on sinus lift procedures by Stephen S Wallace and Stuart J Froum (2003)2 concludes the following:
  • The survival rate of implants placed in augmented sinuses varied between 61.7% and 100% with the average survival rate of all interventions being 92.6%
  • Reports favored better implant survival rates in the non-grafted posterior maxilla141
  • Rough-surfaced implants have a higher survival rate than machine-surfaced implants when placed in grafted sinuses
  • Particulate grafts show a higher survival rate than those placed in sinuses that had been augmented with block grafts
  • Better prognosis is obtained when a membrane was used in covering the lateral window technique
  • No statistical difference between delayed implant placement, types of rough-surfaced implants, length of follow-up, year of publication, and the evidence level of the study.
Implant site augmentation methods3-5
Classified as
  • Horizontal augmentation techniques
  • Vertical augmentation techniques.
Minor bone deficiency areas are restored with guided bone regeneration materials with suitable alloplastic materials.
Major deficiency regions are restored by vertical or horizontal augmentation techniques in accordance to bone deficiency.
 
Horizontal Augmentation Techniques
Horizontal bone grafting is done by onlay and sandwich grafting.
Indication
Atrophy of the edentulous ridge with adequate residual height.
Contraindication
  • Excessive inclination of the alveolar ridge
  • Presence of bony undercuts prone to fracture during the space creation.
Onlay grafting
Block autogenous bone, derived from other intraoral or extraoral sites, is placed in the deficient region, secured with retrievable screws or resorbable materials.
Sandwich grafting
Space created is by splitting the bone between the cortical plates and filled with suitable augmentation materials. This technique is mainly indicated in the maxilla. It is difficult in the mandible due to the rigidity of the bone.
Vertical Augmentation Techniques
  • The vertical bone height is achieved by securing the grafts with screws, plates or by any retrievable materials
  • The vertical augmentation technique is highly sensitive and success rate is less compared to horizontal augmentation techniques
  • The success rate depends on the site vascularity.
Vertical distraction osteogenesis
Increases the vertical height of bone by the process of slow distraction. Mostly done in mandible.
Indication
Insufficient vertical height with satisfactory residual width.142
Contraindications
  • Severely deficient mandible has high risk of fracture or neural damage
  • Close proximity of maxillary sinus
  • Rigid palatal mucosa might reduce the distraction forces.
 
Soft tissue surgeries
  • Are done for better esthetics
  • Done either by resective surgery or by regenerative surgery
  • The resective surgery corrects the irregular bone or the increased thickness keratinized gingiva. It is generally a final option to be adapted after attempting the regenerative procedures in the management of implant failures.
Types of gingival graft surgeries6
  • Free gingival graft (FGG)
  • Connective tissue graft (CTG)
  • Pedicle grafts (PD) are the commonly employed.
The FGG requires a gingival tissue from a donor site whereas CTG require a part of tissue from a flap.
The pedicle graft utilizes gingival tissue close to required region.
The CTG is more efficient with encouraging results of root coverage and with lesser patient discomfort and postoperative complaints.
The FGG is conventional and standardized procedure for increasing the thickness of attached gingiva where less than 2 mm of attached tissue is present. The FGG procedure reduces the frenum or muscular tension on the graft or the flap.
 
References
  1. Chen ST, Beagle J, Jensen SS, Chiapasco M, Darby I. Consensus statements and, recommended clinical procedures regarding surgical techniques. Int J Oral Maxillofac Implants. 2009;24 Suppl:272-8.
  2. Wallace SS, Froum SJ. Effect of maxillary sinus augmentation on the survival of endosseous dental implants. A systematic review. Ann Periodontol. 2003 Dec;8(1):328-43. Review.
  3. Harris D. Advanced surgical procedures: bone augmentation. Dent Update. 1997 Oct;24(8):332-7. Review.
  4. Draenert FG, Huetzen D, Neff A, Mueller WE. Vertical bone augmentation procedures: basics and techniques in dental implantology. J Biomed Mater Res A. 2014 May; 102(5):1605-13.
  5. Nkenke E, Neukam FW. Autogenous bone harvesting and grafting in advanced jaw resorption: morbidity, resorption and implant survival. Eur J Oral Implantol. 2014 Summer;7 Suppl 2:S203-17.
  6. Kao RT, Conte G, Nishimine D, Dault S. Tissue engineering for periodontal regeneration. J Calif Dent Assoc. 2005 Mar;33(3):205-15. Review.
27. Discuss implant failures and precautions that need to be taken.
Implants have become most successful treatment modality in the treatment of partially and completely edentulous patients. Even with the success rates more than 70%, as stated by Brånemark and Misch after detailed clinical study, there exist a failure at various levels of the treatment procedure.143
 
Classification of implant failure
Two main types—Types due to etiology1 and according to types of failure2
  1. Etiology
    • Host factors
      • Prevailing medical and dental status
      • Psychology
      • Habits
    • Surgical procedure
      • Pre-surgical
        1. Infected implant sites
        2. Inadequate mineralization of grafts
      • Stage 1 surgery
        1. Inappropriate flap design
        2. Improper implant angulations
        3. Insufficient primary stability
        4. Excess pressure and overheating of bone
        5. Teeth–implant space
        6. Implant contamination
      • Stage 2 surgery
        1. Improper loading protocol
        2. Local and systemic entrusted factors
    • Implant selection
      • Implant dimensions
      • Mismatch of bone and implant type
      • Number of implants
      • Improper implant design and system
    • Prosthodontic factors
      • Increased cantilever
      • Implants–natural teeth connection
      • Prosthesis design
        1. Absence of passive fit of prosthesis
        2. Improper abutment fit interface
        3. Improper prosthesis design
        4. Improper occlusal design.
    • Periodontal factors
      • Peri–implantitis
      • Improper oral health maintenance
  2. Type of failure
    • Esthetics failure
      • Gingival esthetics failure
      • Restoration factors
      • Implant factors144
      • Osseointegration failure
      • Functional and psychological problems.
 
Etiological Factors
The implant failures can be subdivided into factors caused due to host or patient, surgical failure, improper implant selection and prosthetic factors.3
 
Host/Patient Factor
  • Prevailing medical and dental status of patient
  • Psychology
  • Habits.
 
Contraindications for implants
Pregnancy, granulocytopenia, long-term steroid use, diabetes, hemophilia, Ehler–Danlos syndrome, Marfan's syndrome, osteoradionecrosis, renal failure, organ transplants, anticoagulant therapy, Crohn's disease, fibrous dysplasia, Paget's disease and osteoporosis.
Systemic complications delay healing phase or bone repair mechanisms in the implant site.
 
Patient psychology
The patient has to be educated and motivated to maintain the oral hygiene.
 
Habits
Such as pan chewing, smoking and other parafunctional habits can cause increased forces on implants and local ischemia.
 
Surgical failures
Presurgical and surgical failure.
 
Presurgical
  • Infected implant sites—with periodontal infections, endodontic lesions, infected socket and dental pathologic conditions
  • Inadequate mineralization of graft—Implants fail to integrate in immature grafts. It is ideal to wait till all the woven bone matures to lamellar bone that can have better implant prognosis.
 
Factors that can contribute to the failure of implant in stage 1 surgery are:
  • Inappropriate flap design— Improper flap design can lead to decrease in nutrients reaching implant site and blood supply, infection and tissue ischemia. Proper flap closure ensures better healing and surgical prognosis
  • Improper implant angulations—It is advisable to have straight implant and abutment for favorable prognosis since the implants are designed for axial forces
  • Insufficient primary stability—Precision osteotomy reduces the implant mobility and facilitates faster healing
  • Excess pressure and overheating of bone—The use of excessive pressure and heat can damage the bone cell by forming a barrier layer between the implant and healthy bone causing osseointegration failure
  • Teeth–implant space—Minimum of 1 mm space is required between teeth and implant to maintain ideal bone health145
  • Implant contamination—Improper sterilization protocol during surgical procedure, broken manufacture seal and oral cavity contamination.
 
Factors causing failure in Stage 2 surgery
Improper loading protocol
Adequate healing time has to be provided before loading the implant.
Local and systemic factors
The protocol measures for stage 2 surgery as removal of cover screw with adequate tissue relief, infection free zone and adequate keratinized gingiva should be maintained for the success of implant.
Implant selection
Bone support, bone type, number, type and size of the implants play a vital role in implant success. The precautions followed in implant selection to prevent implant failure are:
  • The length and width of implant has to be sufficient to support the superstructure
  • In possible limits, increased number of implants has to be placed to support the restoration for uniform stress distribution
  • Selection of a proper implant system according to available clinical situation for better success of implant
  • Mismatch of system can compromise the implant health and success.
 
Prosthetic factors
Factors influencing the prosthetic design4 are:
  • Reduced number of implants can cause a cantilever design with undue forces on the supporting implant. A minimal of 8 mm cantilever is the permissible limit with increased number of implants
  • The preferred design is to avoid implant connection to natural teeth, if connected can cause overload and mismatched resiliency
  • A lack of passive fit of prosthesis at the abutment interface.
 
Periodontal factors
Peri-implantitis
Is the postoperative failure arising due to Gram negative bacteria. Retrograde peri-implantitis can be caused due to micro fracture of bone, infection from adjacent teeth or early loading. Preventive care has to be followed during the various procedures to avoid retrograde peri-implantitis.
 
Improper oral maintenance
Can lead to wider problems from tissue loss to bone loss which in turn affects esthetic and function.
Types of failure
Esthetic failure can be further subdivided into:
  • Gingival esthetics—papilla esthetics, contour and form of gingiva around the restoration
  • Restoration esthetics—color, emergence profile, size, form and type of restoration
  • Implant esthetics—thread exposure, angulations of implants, bone resorption146
Undue care is taken during treatment planning, surgical stage and during fabrication of restoration to avoid esthetic failures.
Lack of osseointegration can be due to:
  • Bone grafting issues
  • Surgical and prosthetic factors
  • Functional and psychological problems.
Treatment options for management of implant failure:
  1. Nonsurgical
    • Topical treatments and oral medicaments
    • Removal of occlusal interferences
    • Debridement
    • Repair of prosthesis.
  2. Surgical
    • Resective and regenerative surgery
    • Implant removal.
Appropriate procedures have to be selected depending on the clinical situation and type of failure.5 Careful evaluation procedures are done and suitable management protocol is done to correct the failures.
 
References
  1. Marcelo CG, Filié Haddad M, Gennari Filho H, Marcelo Ribeiro Villa L, Dos Santos DM, Aldiéris AP. Dental implant fractures—aetiology, treatment and case report. J Clin Diagn Res. 2014 Mar; 8(3):300-4.
  2. Chrcanovic BR, Albrektsson T, Wennerberg A. Reasons for failures of oral implants. J Oral Rehabil. 2014 Jun;41(6):443-76. Review.
  3. Zembic A, Kim S, Zwahlen M, Kelly JR. Systematic review of the survival rate and incidence of biologic, technical, and esthetic complications of single implant abutments supporting fixed prostheses. Int J Oral Maxillofac Implants. 2014;29 Suppl:99-116.
  4. Abduo J, Judge RB. Implications of implant framework misfit: a systematic review of biomechanical sequelae. Int J Oral Maxillofac Implants. 2014 May-Jun;29(3):608-21.
  5. Hsu YT, Mason SA, Wang HL. Biological implant complications and their management. J Int Acad Periodontol. 2014 Jan;16(1):9-18. Review.