EXPLAIN THE IMPORTANCE OF DIAGNOSTIC AND TREATMENT PLANNING PHASES IN RPD
Diagnosis and Treatment Planning
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.
Oral Examination Includes Evaluating
- Carious lesions and defective restorations.
- Testing for pulp vitality.
- Test for sensitivity to percussion.
- Mobility.
- Pocket depth, inflammation and amount of attached gingiva of the abutment teeth.
- Any tori, exostoses, sharp or prominent bony areas, soft or hard tissue undercuts and enlarged tuberosities.
Other Diagnostic Aids Required
- Radiographs of the abutment teeth and the residual ridge areas.
- Mounted casts: To evaluate for the presence of extruded teeth, malposed teeth, reduced interarch space and unfavourable occlusal plane.
Evaluation of Caries and Existing Restorations
- All caries lesions need to be restored with intracoronal restoration.
- Extruded tooth above the occlusal plane need to be restored with an extra coronal restoration to improve the occlusal plane.
- If rest seat need to be prepared a cast metallic restoration is preferred as restoration.
Evaluation of Pulp
- An electric pulp tester and thermal tests are used to detect pulpal necrosis or pulpitis.
- Endodontically treated tooth, if used as an abutment, should be evaluated for the success of endodontic treatment.
Evaluation of Sensitivity to Percussion
Sensitivity can be due to
- Tooth movement caused by a prosthesis or the occlusion.
- A tooth or restoration in traumatic occlusion.
- Periapical or pulpal abscess.
- Acute pulpitis.
- Gingivitis or periodontitis.
- Cracked tooth syndrome.
Evaluation of Mobile Teeth
A mobile tooth used as an abutment tooth will have a poor prognosis unless mobility is eliminated.
Causes of mobility
- Trauma from occlusion (reversible).
- Inflammatory changes in the periodontal ligament (reversible).
- Loss of alveolar bone support (not reversible).
Abutment teeth can be splinted to decrease mobility only if
- The combined splinted teeth with crowns provide an abutment with greater total periodontal support.
- Splinting can be done if two or three retainable teeth are widely placed.
Evaluation of Periodontium
Findings requiring periodontal treatment are
- Pocket depth in excess of 1 mm with furcation involvement.
- Gingivitis.
- Marginal exudate.
- Less than 2 mm of attached gingiva.
- High frenal attachment.
Periodontal treatments include
- Root scaling, root planning with good home oral hygiene procedures.
- Gingivectomy, periodontal flap procedures and free gingival grafts.
Evaluation of Oral Mucosa
Ulceration, swelling, or colour change as red or white lesions.
Common tissue reactions in prosthesis wearing patients
- Palatal papillary hyperplasia.
- Inflammatory papillary hyperplasia.
- Epulis fissuratum.
- Denture stomatitis.
- Trauma from occlusion.
- Soft tissue displacement.
Evaluation of Soft Tissue Abnormalities
- High frenal attachment of labial frenum
- Hypertrophic lingual frenum
- Vestibular extension or ridge augmentation procedures can be done.
- Xerostomia.
Evaluation of Hard Tissue Abnormalities
Torus Palatinus: A major connector can be selected and designed to circumvent the torus.
Torus Mandibularis: Occurring bilaterally on the lingual surface of the mandible, which needs to be surgically removed before construction of a removable partial denture.
The maxillary tuberosities: Need to be surgically corrected if support and stability of the prosthesis is affected.
Mandibular tuberosity: The mandibular lingual tuberosity is a bony protuberance at the distal end of the mylohyoid ridge in the third molar area. This should be surgically reduced.
Evaluation of Space for Mandibular Major Connector
A minimum of 7 to 8 mm of space should be available for a lingual bar major connector and if less than 7 mm of space is available a lingual plate major connector should be used.
A periodontal probe with millimetre markings is used to measure the space in the lingual aspect. The millimetre depth is read at the point where the probe contacts the gingival margin.
Evaluation of Radiographic Survey
- Abutment tooth is evaluated.
- Residual ridge is evaluated.
- Other factors as radiolucencies, radio opacities, root tips, impacted tooth, endodontically treated tooth are evaluated.
Radiographic Evaluation of Abutment Teeth
- Root length, size and form.
- Crown/Root ratio: at least 1:1 is required for an abutment tooth.
- Lamina Dura: Should be checked for absence, discontinuity, partial loss or thickening of lamina dura.
- Periodontal Ligament Space: A widening of the periodontal ligament space with a thickening of the lamina dura indicates occlusal trauma and heavy function.Clinical mobility of the tooth may also be present with the above findings.
- Bone Index Areas: Bone index areas have “a positive bone factor” which includes a supportive trabecular pattern, heavy cortical layer, dense lamina dura, normal bone height and a normal periodontal ligament space.
Evaluation of Mounted Diagnostic Casts
- Insufficient interarch distance,
- Irregular occlusal plane,
- Extruded or malposed teeth and
- Unfavourable maxillomandibular relationships are all better visualised on mounted diagnostic cast.
1. Interarch Distance
A decrease in interarch distance occurs when tooth unopposed by occlusion tend to over erupt over a period of time carrying the alveolar process with it. Surgical correction can be done.
2. Occlusal Plane
Occlusal plane irregularity if mild can be corrected by enameloplasty.
In extrusion greater than 2 mm the placement of an extra-coronal cast metallic restoration is indicated.
3. Traumatic Vertical Overlap
Akerly classification:
- Type I —The mandibular incisors extrude and impinge into the palate.
- Type II—The mandibular incisors impinge into the gingival sulci of the mandibular incisors.
- Type III—Both maxillary and mandibular incisors incline lingually with impingement of the gingival tissues of each arch.
- Type IV—The mandibular incisors move or extrude into the abraded lingual surfaces of the maxillary anterior teeth.
Clinical symptoms: Abrasion, mobility, migration of the teeth, inflammation and ulceration of the gingiva and palatal mucosa.
Can be corrected by: Orthodontic and orthognathic combined surgical procedures.
Lingual plates that extend on to the lingual surfaces of the maxillary anterior teeth.
4. Malrelation of jaws
Maxillary and mandibular osteotomy procedures are useful to correct these problems.
5. Tipped or malposed teeth
Limited orthodontic procedures for minor tooth movement can be used to upright the tipped tooth to allow the placement of an artificial tooth.
6. Occlusion
A discrepancy between centric jaw relation and maximum occlusal contact, or centric occlusion should be evaluated and corrected by selective grinding or coronal reshaping of teeth to, produce simultaneous occlusal contacts.
Prosthesis need to be constructed at centric jaw relation when:
- Absence of posterior tooth contacts.
- Situation in which all posterior tooth contacts are to be restored with cast restorations.
- Only a few remaining posterior contacts.
- Minimum alveolar support for all the remaining teeth corrected by minimum occlusal equilibration.
- Anterior slide from centric jaw relation and symptoms of traumatic occlusion of the anterior teeth.
Treatment planning
Phase I
- Collection and evaluation of the diagnostic data.
- Treatment to control pain or infection.
- Biopsy or referral of patient.
- Development of a treatment plan.
- Education and motivation of patient.
Phase II
- Removal of deep caries and placement of temporary restorations.
- Extirpation of inflamed or necrotic pulp tissues.
- Removal of non-retainable teeth.
- Periodontal treatment.
- Construction of interim prosthesis for function or aesthetics.
- Occlusal equilibration.
- Reinforcement of education and motivation of patient.
Phase III
- Preprosthetic surgical procedures.
- Definitive endodontic procedures.
- Definitive restoration of teeth, such as cast metallic restorations.
- Fixed partial denture construction.
- Reinforcement of education and motivation of patient.
Phase IV
- Construction of removable partial denture.
- Reinforcement of education and motivation of patient.
Phase V
- Post insertion care.
- Periodic recall.
- Reinforcement of education and motivation of patient.
Length of appointment: The length of this appointment can vary greatly, depending on the complexity of the problems presented by the patient.
LIST VARIOUS CLASSIFICATIONS OF RPD AND THE BASIS FOR CLASSIFYING PARTIALLY EDENTULOUS SITUATION. ENUMERATE MERITS OF KENNEDY'S CLASSIFICATION
Various Classifications
Cummer 1921, Kennedy 1923, Bailyn 1928, Neurohr 1939, Mauk 1942, Wild 1949, Godfrey 1951, Friedman 1953, Beckett 1953, Craddock 1954, Swenson and Terkla 1955, Skinner 1957, Austin 1957, Walt et al 1958, Applegate-Kennedy 1960, Terkla and Laney 1963, William Avent 1966, Fiset 1973, Costa 1974 and Osborne and Lammie 1974
Requirements of an acceptable method of classification
- It should permit immediate visualization of the type of partially edentulous arch being considered.
- It should permit immediate differentiation between the tooth-borne and tooth- tissue supported removable partial denture.
- It should be universally acceptable.
Support classification
- Tooth-borne.
- Mucosa-borne.
- Tooth and mucosa-borne.
Kennedy's classification
A world wide accepted classification is that devised by Edward Kennedy in 1923.
When used in conjunction with a support classification, it helps to give a clear classification understanding about the type 11of denture under consideration during a discussion on partial dentures.
The Kennedy classification is based on the relationship of the saddles to the natural teeth.
Kennedy's classification has four main groups with modifications except for Class IV
Class I: | Bilateral free-end edentulous spaces posterior to the natural teeth. |
Class II: | Unilateral free-end edentulous space posterior to the natural teeth. |
Class III: | A bounded unilateral edentulous space having natural teeth at each end. |
Class IV: | A bounded edentulous space anterior to the natural teeth. |
Modifications
All classes, except Class IV, have modifications.
Each modification is an additional edentulous area.
Examples of modifications:
- An additional edentulous area in Class I would be designated as Class I modification 1.
- If two additional edentulous areas are present it could be designated as Class I modification 2.
- A unilateral saddle with one additional edentulous area is Class II modification 1.
- A unilateral bounded edentulous area with three additional edentulous areas is Class III, modification 3.
Class IV has no modifications.
Criteria for the Kennedy's classification
- The most posterior edentulous area determines the class.
- The size of the modification is not important.
- If a third molar is missing, and not to be replaced, it is not considered in determining the class.
Merits of Kennedy's classification
- Allows visualization of partially edentulous arch.
- Differentiates between tooth supported and tooth tissue supported.
- Type of design can be decided.
- Is universally accepted.
- Aids in discussing, identifying and planning the design.
- Easy to apply the system to any situation.
- Widely used system.
- Forms the basis for two other systems as Applegate Kennedy and Swenson's system.
Diagrams
DISCUSS VARIOUS CLASSIFICATIONS OF PARTIALLY EDENTULOUS RIDGE. EXPLAIN KENNEDY APPLEGATE'S MODIFICATION
Several methods of classification of partially edentulous arches have been proposed and are in use today. There are over 65,000 13possible combinations of teeth and edentulous spaces in opposing arches.
No single method of classification can describe all the combinations except the most basic types.
Kennedy's method of classification is the most widely accepted classification of partially edentulous arches today.
A few other classifications
Cummers classification (1921)
First classification to be recognised by the dental profession.
He classified partial dentures than edentulous spaces.
Four types:
1. Diagonal: | 2 retainers diagonally opposite one another. |
2. Diametric: | 2 retainers diametrically opposite one another. |
3. Unilateral: | 2 or more direct retainers on same side. |
4. Multilateral: | 3 direct retainers on either side. |
Charles. W. Bailyn (1928)
He emphasized the importance of support for partial dentures.
“A”—Anterior restorations where there were saddle areas anterior to the bicuspids.
“P”—Posterior restorations where there were saddle areas posterior to the canines.
Classification
Class I: | Bounded saddle (Tooth supported). |
Class II: | Free end saddle (Mucosa supported). |
Class III: | Bounded saddle (More than three tooth missing). |
Example: If all posterior teeth from canines till molars are missing with lateral incisors then the classification is AI P III.14
Neurohrs Classification
Class I:
Variation I:
- Posterior missing, Anterior all teeth present.
- Posterior missing, some anterior teeth missing.
Variation II:
- Anterior missing, All posteriors present.
- Anterior missing, some posteriors missing.
Class II: Division I has variation 1 and 2.
Division II has variation 1 and 2.
This classification is tooth tissue bearing.
Class III is edentulous state.
Friedmans system
- A—Anterior space.
- B—Bounded posterior space.
- C—Cantilever situation or a posterior free end saddle.
Osborne and Lammie
Class I: | Mucosa borne. |
Class II: | Tooth borne. |
Class III: | Combination of mucosa borne and tissue borne. |
Requirements of an acceptable method of classification
- It should permit immediate visualization of the type of partially edentulous arch being considered.
- It should permit immediate differentiation between the tooth-borne and the tooth-and tissue-supported removable partial denture.
- It should be universally acceptable.
Kennedy's classification
The Kennedy method of classification was originally proposed by Dr. Edward Kennedy in 1923 and attempts to classify the 15partially edentulous arch suggesting principles of design for a given situation.
Kennedy divided partially edentulous arches into four main types.
Class I
Bilateral edentulous areas located posterior to the remaining natural teeth.
Class II
A unilateral edentulous area located posterior to the remaining natural teeth.
Class III
A unilateral edentulous area with natural teeth remaining both anterior and posterior to it.
Class IV
A single, but bilateral (crossing the midline), edentulous area located anterior to the remaining natural teeth.
Modifications
All classes, except Class IV, have modifications.
Each modification is an additional edentulous area.
Examples of Modifications
- An additional edentulous area in Class I would be designated as Class I modification 1.
- If two additional edentulous areas are present it could be designated as Class I modification 2.
- Class II modification 1: a unilateral edentulous area with one additional saddle.
- Class III, modification 3 would be a unilateral bounded edentulous area with three additional edentulous areas.
- Class IV has no modifications.
Criteria for the Kennedy's classification
- The most posterior edentulous area determines the class.
- The size of the modification is not important.
- If a third molar is missing, and not to be replaced, it is not considered in determining the class.
Merits of Kennedy's classification
- Allows visualization of partially edentulous arch.
- Differentiates between tooth supported and tooth tissue supported.
- Type of design can be decided.
- Is universally accepted.
- Aids in discussing, identifying and planning the design.
- Easy to apply the system to any situation.
- Widely used system.
- Forms the basis for two other systems as Applegate Kennedy and Swenson's system.
Applegate's rules for applying the Kennedy classification
Applegate provided the following eight rules for the easy application of the Kennedy method:
Rule 1. | Classification should follow rather than precede any extractions of teeth that might alter the original classification. |
Rule 2. | If a third molar is missing and not to be replaced, it is not considered in the classification. |
Rule 3. | If a third molar is present and is to be used as an abutment, it is considered in the classification. |
Rule 4. | If a second molar is missing and is not to be replaced, it is not considered in the classification. |
Rule 5. | The classification is always determined by the most posterior edentulous area or areas. |
Rule 6. | Edentulous areas other than those determining the classification are referred to as modifications and are designated by their number. |
Rule 7. | The extent of the modification is not considered, only the number of additional edentulous areas. |
Rule 8. | There can be no modification areas in Class IV arches. (Another edentulous area lying posterior to the “single bilateral area crossing the midline” would determine the classification.) |
Applegate added two more classes to Kennedy's existing classification
Class V
This is an edentulous area, bounded anteriorly and posteriorly by natural teeth, in which the anterior abutment is not suitable for support.
Class VI
This is an edentulous situation in which the abutment tooth is capable of total support. (As in young patients.)
DEFINE MAJOR CONNECTOR. EXPLAIN IN DETAIL ITS REQUIREMENTS. DESCRIBE MAXILLARY MAJOR CONNECTORS IN DETAIL
Definition
A major connector is the unit of the partial denture that connects the parts of the prosthesis located on one side of the arch with those on the opposite side. It is that unit of the partial denture to which all other parts are directly or indirectly attached.
Requirements of major connector
- Major connectors must be rigid to transfer forces uniformly over the entire supporting structures.
- Prevents movement of the denture base by its rigidity.
- Aids the other components of the partial denture to function effectively.
- They should not be placed on excessively movable tissues.
- Should be placed 4 mm away from gingival margin in mandibular and 6 mm away from gingival margin in maxillary.
- Should not be placed on bony and soft tissue prominences.
- Relief should be provided beneath a major connector if necessary.
- The borders of the palatal connector should be 6 mm away from gingival margins and should be parallel to their mean curve.
- The anterior and posterior borders should cross midline at right angles.
- Minor connectors must cross gingival tissues, at nearly a right angle while joining the major connector.
- An anterior palatal strap or the anterior border of a palatal plate should be located as far posteriorly to avoid interference with the tongue.
- The anterior border of such palatal major connectors should follow the valleys between the rugae.
- Should maintain oral health.
- All borders should taper towards soft tissue.
- Should be made from an alloy compatible with oral tissues
- Thickness of metal should be uniform through out.
- Finished borders should curve gently.
- Metal should not be highly polished on the tissue surface.
- All borders on soft tissue should be beaded fading out near gingival margins.
- Should not interfere and irritate the tongue.
- Should not alter the natural contour of the lingual surface of the mandibular alveolar ridge or palatal vault.
- Should not impinge on oral tissues during function.
- Should not retain or trap food particles.
- Aids in support, retention and stability.
Types of maxillary major connectors
- Single posterior palatal bar.
- Single palatal strap.
- Anterior posterior palatal bar (Double palatal bar).
- U-shaped palatal connector (Horse-shoe).
- Combination anterior and posterior palatal strap-type connector (Closed horse shoe shaped).
- Palatal plate-type connector (Complete palate).
Single palatal bar
- Is a narrow half oval shaped bar with the maximum thickness at the centre.
- Gently curves along the palatal contour with the junctions smoothly joining with the denture base.
Disadvantages
- To maintain rigidity it should possess bulk causing tongue interference.
- Decreased vertical support to prosthesis.
- Cannot replace more than two teeth on each side of the arch.
- Cannot be used in distal extension bases.
Advantages
Design for interim partial denture.
Single palatal strap
- It is a wide, thin band of metal that crosses the palate. The minimum thickness of Palatal strap is 8 mm.
- Depending on the edentulous space the palatal strap can be made wider.
- Can be used for unilaterally edentulous situation.
Advantages
- Greater resistance to displacing forces than a palatal bar.
- Better patient comfort.
Disadvantages
- Less rigidity compared to complete coverage major connector.
- Papillary hyperplasia can occur.
Anterior Posterior Palatal Bar (Double Palatal Bar)
- It consists of a flat narrow anterior bar positioned in the valleys of rugae.
- It also consists of a half oval posterior bar. The anterior and posterior bars are joined by flat longitudinal elements in the lateral slopes of palate.
Advantages
- It is rigid.
- Can replace anterior and posterior abutments.
- Less palatal coverage which is comfortable to the patient.
- It is an ideal design for patients with large palatal tori.
Disadvantages
- Less support.
- Can be used only when there is good periodontal support.
- Cannot be used in high palatal vaults.
- Tongue interferences are present.
U-shaped Palatal Connector (Horseshoe)
- It is a thin band of metal along the lingual surfaces of tooth extending to the palatal slopes. Thickness of the plate should be minimum 6 to 8 mm.
- The borders can either extend from gingival margin to palatal slopes or from lingual aspect of the tooth to palatal slopes. The palatal borders should end at the junction of horizontal and vertical slopes of the palate.
Advantages
- Ideal for replacement of missing anterior tooth.
- This design provides some vertical support and indirect retention.
- Can be designed in case of palatal tori.
Disadvantages
- Cannot be given in distal extension partial denture as it flexes on loading.
- Difficulty in speech and patient discomfort can occur.
Anterior and posterior palatal strap-type connector (Closed horse shoe shaped)
- It consists of an anterior strap, which can be placed on the lingual aspect of tooth, or it should be placed 6 mm away from gingival margin, if anterior tooth are not replaced.
- It also consists of a posterior strap, which should be placed posteriorly but without contacting the soft palate.
Advantages
- It is rigid and can replace anterior and posterior tooth.
- Can be given in cases of palatal tori.
Disadvantage
Speech problems and tongue interference can occur.
Palatal Plate-type Connector (Complete coverage)
- The anterior border should either extend to the cingula of the tooth surface or be kept 6 mm short of gingival margin.
- The posterior border should extend to the junction of soft and hard palate.
Types
- All acrylic resin.
- Combination of metal and acrylic (The anterior rugae area in metal and posteriorly acrylic).
- All cast metal.
Advantages
- Good rigidity and support.
- Better retention.
- Maximum tissue coverage.
- Can be given for bilateral distal extension.
- Better stability in flat or flabby ridges.
- Can be designed in obturator patients.
- Design for transitional dentures (All acrylic).
- Natural sensation during eating and drinking as in “All metal”.
- Can be relined later.
Disadvantages
- Large area of tissue coverage can cause tissue reactions as hyperplasia.
- Speech difficulties can occur.
DESCRIBE MANDIBULAR MAJOR CONNECTORS. ADD A NOTE ON THE ADVANTAGES AND DISADVANTAGES OF EACH MANDIBULAR MAJOR CONNECTOR.
Definition
A major connector is the unit of the partial denture that connects the parts of the prosthesis located on one side of the arch with those on the opposite side. It is that unit of the partial denture to which all other parts are directly or indirectly attached.
Types of Mandibular Major Connectors
- Lingual bar.
- Lingual bar with continuous bar retainer (Kennedy Bar).
- Linguoplate.
- Labial bar.
Lingual Bar
Shape and size: A 6-gauge half-pear-shaped bar (5 mm) in width located above moving tissues but as far below the gingival tissues as possible.
Borders: Has two borders: Superior and inferior.
- The superior border should be tapered to the tissues above, with its greatest bulk at the lower border. It is flat on the tissue side and having the greatest bulk in the inferior third.
- Inferior border of the lingual bar is slightly rounded so that it will not impinge on the lingual tissue when the denture bases rotate inferiorly under occlusal loads. The major connector should not have sharp margins.
Location: The inferior border should be located so that it does not impinge on the tissues in the floor of the mouth during swallowing, speaking, and other normal functions.
The location should not interfere with the resting tongue and cause trapping of food.
The superior border should be located 3 mm away from gingival margins of teeth.
25Minimum space required: At least 8 mm of vertical space between the floor of mouth and gingival margin is required.
Measuring the height of floor:
- The first method is to measure the height of the floor with a periodontal probe in relation to the lingual gingival margins of adjacent teeth with the tip of the patient's tongue lightly touching the vermilion border of the upper lip. (More accurate).
- The second method is to use an individualized impression tray with its lingual borders 3 mm short of the elevated floor of the mouth and then use an impression material that will be accurately moulded as the patient licks the lips.
The inferior border of the lingual bar can be located at the height of the lingual sulcus of the cast resulting from such an impression.
Advantages
- Simplicity.
- Minimal tissue contact.
- Less plaque accumulation.
Disadvantages
- Needs precise lab work.
- Cannot be placed when there is less than 8 mm of lingual space.
Modifications
- The thickness can be altered depending on the clinical situation to acquire more rigidity. This is accomplished by underlying the ready-made form with a sheet of 24-gauge casting wax.
- Sublingual bar:
The bar shape is same as that of a lingual bar.
- Sublingual bar can be used along with a lingual plate if the lingual frenum does not interfere.
- Can be used when an anterior lingual undercut exist where lingual bar cannot be used.
Contraindications
- When lingual tori exists.
- In cases of high lingual frenal attachment.
- High elevation of the floor of the mouth during functional movements.
Continuous bar retainer (Kennedy's bar) or Double lingual bar
- A continuous bar retainer is located on or slightly above the cingula of the anterior teeth along with a lingual bar. The upper bar is also pear shaped and is 2 to 3 mm high with 1 mm thickness. The bar should dip into the contact points of teeth downward till upper limits of cingula.
- The two bars are joined by minor connector located in interproximal spaces usually between canine and premolar.
Indications
- When a linguoplate cannot be given or if requires excessive blockout of interproximal undercuts then a continuous bar can be used.
- In cases of wide diastemae between the lower anterior teeth.
Advantages
- Enhance indirect retention.
- Provides horizontal stabilization to the prosthesis.
- Marginal gingiva receives natural stimulation.
- Can be used in large interproximal embrasures.
Disadvantages
- Tongue annoyance.
- Entrapment of food.
- Proper fit is difficult to achieve with upper bar of double bar.
Modifications (Step back design): The upper bar can be step backed in between each tooth as in cases of diastema, hence aesthetically pleasing.
Linguoplate
Shape
- It is a pear shaped lingual bar with a thin solid piece of metal extending upward from superior border of bar to the lingual surfaces of teeth.
- A Linguoplate should be contoured to follow the contours of the teeth and the embrasures.
- The upper border should follow the natural curvature of the supracingular surfaces of the teeth also covering interproximal spaces to the contact points (Scalloped appearance).
- All gingival crevices and deep embrasures must be blocked out parallel to the path of placement to avoid gingival irritation and wedging effect between the teeth.
- The Linguoplate should have a terminal rest at each end (commonly cingulum rest on canines or mesial fossae rest on first premolars).
- Material of choice-Chrome cobalt alloy.
Modification: Cut back or step back design to hide metal between wide spaced teeth without compromising the rigidity of the connector. Here the upper bar drops gingivally along the marginal ridge of tooth to cross gingiva to the other tooth and rises up the marginal ridge to the contact point.
Indications
- When the lingual frenum is high or the space available for a lingual bar is limited.
- In Class I situations in which the residual ridges have undergone excessive vertical resorption. Here Linguoplate will use remaining teeth to resist horizontal rotations.
- For stabilizing periodontally weakened teeth. (A continuous bar retainer).
- When the future replacement of one or more incisor teeth will be facilitated by the addition of retention loops to an existing Linguoplate.
- In wide diastemae.
Advantages
- Can be used in Class 1 situation when indirect retention is required.
- In periodontally weak tooth lingual plate acts as splint.
- Can be used with some modification to prevent supra eruption of mandibular anterior teeth.
- Of all the mandibular major connectors lingual plate has maximum retention, support and stability.
Disadvantages
- Irritation of soft tissues.
- Caries prone.
Labial Bar
Shape
- It is half pear shaped similar to lingual bar extending on to the labial surface and some time to the facial surface of the posterior tooth.
- Relief is required below the bar.
Indications
- Extreme lingual inclination of the remaining lower premolar and incisor teeth. This should be rectified by mouth preparation and restoration before planning for a labial bar.
- Large mandibular tori.
Modification: Swing-Lock design, which consists of a labial or buccal bar that is connected to the major connector by a hinge on one end and a latch at the other end.
Support is by multiple rests on the remaining natural teeth. Stabilization and reciprocation by linguoplate contacting the remaining teeth and by the labial bar with its retentive struts.
Retention is by bar-type retentive clasp arms projecting from the labial or buccal bar.
Use of the Swing-Lock
- Unfavourable tooth contours.
- Unfavourable soft tissue contours.
- Teeth with questionable prognoses.
Contraindications
- Poor oral hygiene.
- Presence of a shallow buccal or labial vestibule.
- High frenal attachment.
Disadvantages
- Bulk of connector distorts lower lip.
- Patient discomfort.
Design of mandibular major connector
- Outline of the basal seat areas on the diagnostic cast.
- Outline of the inferior border of the major connector.
- Outline of the superior border of the major connector.
- Unification by minor connector.
EXPLAIN IN DETAIL VARIOUS TYPES OF MINOR CONNECTORS. ADD A NOTE ON THE FUNCTIONS OF MINOR CONNECTOR.
Minor Connectors
Definition: Minor Connector is that component that joins other units of the prosthesis such as clasps, rests, indirect retainers and denture bases to the major connector.
Types of minor connector
- Joins the clasp assembly to the major connector.
- Joins indirect retainers or auxiliary rests to major connector.
- Joins the denture base to the major connector.
- Serves as an approach arm for a vertical projection or bar-type clasp.
I. Minor connectors that join clasp assembly to the major connector
- They should be rigid, as they support the retentive clasp and occlusal rest.
- To have rigidity they need to have sufficient bulk.
Location: Minor connectors that support clasp assemblies are located on proximal surfaces of teeth adjacent to edentulous areas or in the embrasure between two teeth.
Shape: Broad buccolingually and thin mesiodistally. The thickest portion buccolingually, should be at the lingual line angle of the tooth and taper evenly to its thinnest point at the buccal line angle of the tooth.
II. Minor connectors that join indirect retainers or auxiliary rests to major connector
- They arise from auxiliary rests. They should join the major connector at right angle.
- The joining junction should be a gentle curve.
Location: In the embrasure between teeth to prevent tongue annoyance.
III. Minor connectors that join denture base to major connector
Types of denture base
- Latticework construction
- Mesh construction
- Bead, wire, or nail-head minor connectors.
1. Open latticework construction:
Consists of two longitudinal struts with smaller struts connecting the two longitudinal struts.
Maxillary design: This consists of two metal struts, 12 and 16 gauge thick.
The first extends longitudinally along the edentulous buccal ridge in maxillary arch.
The border of the major connector acts as the second strut.
Mandibular design: In the mandibular arch one strut should be positioned buccal to the crest of the ridge and the other lingual to the crest of the ridge.
Smaller struts: 16 gauge thick smaller struts, connect the two struts and form the latticework.
They run over the crest of the ridge and are positioned so that there is no interference to arrangement of the artificial teeth.
Number of small struts: Commonly one cross strut between each teeth to be replaced.
Advantages
- Strongest attachment of the acrylic resin denture base to the removable partial denture.
- Easy to reline.
- For multiple teeth replacement.
Mesh Construction
It is a thin sheet of metal with multiple small holes that extends over the crest of the residual ridge to the same buccal, lingual and posterior limits, as does the latticework minor connector.
Indication: When multiple teeth are to be replaced.
Disadvantages
- Difficult for the acrylic resin to flow through the small holes.
- Mesh type is a weak attachment.
Relief
- After the wax forms of the struts are positioned on the refractory cast, a relief space is provided over the edentulous ridges for both the latticework and the mesh minor connectors.
- This relief is provided to create a space between the struts and the underlying ridge around which the acrylic resin denture base will be retained.
Tissue stop: In distal extension partial dentures using latticework or mesh retention, the framework needs to be stabilized during the acrylic resin packing and processing.
As the acrylic resin requires some pressure to be forced through the latticework and mesh minor connectors, the framework can displace or distort, hence a tissue stop is required to support the terminal portion of the minor connector.
Method
- Tissue stop is made by removing 2 square mm of the relief wax beneath the latticework.
- The wax is removed from the point where the posterior end of the minor connector crosses the centre of the ridge.
- During waxing of the framework, this depression is waxed as a projection of the latticework or mesh.
- After the framework has been cast this projection will contact the edentulous ridge of the cast and will prevent the framework from being distorted during acrylic resin packing procedures.
Bead, wire, or nailhead retention:
- The bead, wire, or nailhead minor connector is used with metal denture bases.
- No relief is provided beneath the minor connector as the acrylic resin mechanically bonds with the nail head or bead of the minor connector.
Mechanical retention:
- Can be made by placing beads of acrylic resin polymer on the waxed denture base and investing, and casting these beads.
- Wires or form of nailheads that project from the metal base can be used.
Disadvantages
- It is difficult to adjust the metal base.
- Cannot be relined in case of ridge resorption.
- Weakest of the three types of attachment.
- Can be only on tooth-supported, well-healed ridges.
- Cannot use when interarch space is limited.
- No strength to withstand the forces of occlusion.
Advantages
- Hygienic because of better soft tissue response to metal.
- Function successfully as better fit than acrylic denture base.
Attachment of minor connector to major connector
- Should be joined to major connector with adequate bulk to withstand occlusal forces and to prevent from breakage.
- The acrylic resin processed around the latticework and mesh minor connector should join the major connector in a smooth, even joint.
- To prevent acrylic resin from being thinned in order to produce a smooth joint, a space for a butt joint is placed in the design.
- The butt joint aids the acrylic resin to blend evenly with the major connector.
Finish lines/Butt joints
As acrylic resin is processed around the latticework and minor connectors, space for these butt joints are made on both internal and external surfaces of the major connector.
In the case of nail head minor connector, the acrylic resin is processed only on the external surface, so only a single finish line is required.
Types
- External Finish lines
- Internal Finish lines.
External finish lines
- If they occur on the outer aspect of the major connector.
- External finish line must be sharp and definite with a slight undercut to retain acrylic resin to the major connector.
- The angle, the finish lines form with the major connector should be less than 90 degrees.
Method of placement: Formed by placement of the wax during the waxing procedure and by carving the wax.
Location
- Should extend onto the proximal surfaces of the teeth adjacent to the edentulous space.
- Should begin at the lingual extent of the rest seat and continue down the lingual aspect of the minor connector on the proximal surface of the tooth.
Internal finish lines
Are on the tissue side of the major connector.
Method of placement
- Formed from the relief wax used over the edentulous ridges on the master cast before duplication on which the framework will be waxed.
- A 24 to 26 gauge relief wax, placed under latticework or mesh minor connectors creates space for acrylic.
- The same relief waxes margins become the internal finish line.
- The ledge created by the margin of the wax must be sharp and definite.
Minor connectors that serve as approach arm for vertical projection or bar-type clasp
- Not required to be rigid unlike other minor connectors.
- It supports a gingivally approaching direct retainer.
- It should have a smooth, even taper from its origin to its tip.
- It must not cross a soft tissue undercut.
Functions of minor connector
- Primary function is to join all the other components as clasps, rests, indirect retainers and denture bases to the major connector.
- Aids in efficient functioning of all components.
- Helps to retain rests in their rest seats, this serves to transfer forces occurring against the prosthesis down the long axis of the abutment teeth.
- Minor connector aids in uniform distributing of the stresses hence, it should be rigid.
- The minor connector distributes forces, on the edentulous ridge to the ridge and the remaining teeth by transferring it to several other minor connectors that serve as attachments for clasps, rests, or indirect retainers.
- Preservation of bone.
- In distal extension base the minor connector is part of the saddle hence, holds the artificial tooth in proper alignment.
- Aids in retention, stability and support in a denture.
DEFINE A DIRECT RETAINER. LIST THE FUNCTIONS OF A CIRCUMFERENTIAL CLASP AND VARIOUS PARTS AS RELATED TO FUNCTION. DESCRIBE THE VARIOUS TYPES OF RETAINERS.
Definition
A direct retainer is that component that engages an abutment tooth and in doing so resists dislodging forces applied to a removable partial denture.
Classification
- Intracoronal retainers.
- Extra coronal retainers.
- Circumferential or Aker's clasp
- Vertical projection or Roach clasp
Parts of circumferential clasp
- Rest.
- Body.
- Shoulder.
- Reciprocal arm.
- Retentive clasp arm.
- Retentive terminal.
- Minor connector.
Functions of clasps
- Retention,
- Stability,
- Support,
- Reciprocation,
- Encirclement,
- Passivity.
Parts of extra coronal direct retainers and specific function
- Rest: Function is vertical support
- Minor connector: Function is stabilization by acting through guide planes placed on the proximal surfaces of abutment tooth extending from the prepared marginal ridge to the middle and gingival third of abutment tooth.
- Clasp arms: Functions as stabilizing, retentive and reciprocating units.
- Stabilization is attained by rigid portion of retentive arm and reciprocal arm along with the minor connector.
- Reciprocation is attained by reciprocal arm
- Retention is attained by retentive arm especially the retentive terminal.
The reciprocal clasp arm has three functions:
- Reciprocation against the action of the retentive arm. During placement and removal, reciprocation is most needed as the retentive arm flexes over the height of contour.True reciprocation during placement and removal is possible only if guide planes are placed.
- The reciprocal clasp arm should resist against horizontal forces so rigid clasp arms, rigid minor connectors and a rigid major connector should be used.
- If reciprocal clasp arm rests on a suprabulge surface of an abutment tooth lying anterior to the fulcrum line it acts as an indirect retainer.
Figure 18: Parts of circumferential clasp: A. Minor connector, B. Retentive arm, C. Occlusal rest, D. Reciprocal arm
Types of clasps
- Circumferential clasp which approaches the retentive undercut from an occlusal direction. (Occlusally approaching).
- The bar clasp arm, which approaches the retentive undercut from a cervical direction. (Gingivally approaching).
Different types of circumferential clasps:
- Cast circumferential clasp.
- Simple circlet clasp.
- Reverse circlet clasp.
- Multiple circlet clasp.
- Embrasure or modified crib clasp.
- Ring clasp.
- Fish hook or Hairpin clasp
- Onlay clasp.
- Combination clasp.
Different types of bar clasps:
- T Clasp.
- Modified T Clasp
- Y Clasp
- I Clasp and I Bar.
Circumferential clasps
Cast circumferential clasp
It is the method of choice for tooth supported partial dentures. The retentive arm should originate above occlusal half with the retentive terminal placed below height of contour and pointing occlusally.
Advantages
- Good support, retention and reciprocation.
- Easy to repair.
- Less food retention compared to bar clasp.
Disadvantages
- Large amount of tooth surface covered that can cause decalcification and caries.
- Minimum flexibility and cannot be used in distal extension bases.
- Interferes with normal food flow pattern.
- Not aesthetic.
Simple circlet clasp
Choice for tooth supported partial denture. The clasp approaches undercut from edentulous area and engages undercut opposite to edentulous space.
Advantages
- Good support, stability, encirclement and very good passivity.
- The most widely used clasp.
Disadvantages
- Large amount of tooth surface covered that can cause decalcification and caries.
- Minimum flexibility and cannot be used in distal extension bases.
- Interferes with normal food flow pattern.
Reverse circlet clasp
Used when undercuts are present near the edentulous space. Ideally bar clasp is the choice but in situations as soft tissue undercuts where bar clasp cannot be used reverse clasp is used.
Advantage
Disadvantages
- Difficult to obtain occlusal clearance where there is tight occlusion between upper and lower teeth.
- Can cause food entrapment if occlusal rest is not well prepared.
- Wedging effect as it is placed between two teeth.
- Not aesthetic.
Multiple circlet clasp
- Consists of two opposing simple circlet clasps joined at the terminal end of reciprocal arms.
- Used as a splinting effect of weak tooth.
Disadvantages
- Large amount of tooth surface covered that can cause decalcification and caries.
- Minimum flexibility and cannot be used in distal extension bases.
- Interferes with normal food flow pattern.
- Not aesthetic.
Embrasure clasp
Consists of two simple circlet clasps joined at the body. The clasp crosses the marginal ridge and facial surface of both the teeth and engage undercuts on the opposite sides of the respective teeth. Used on side where there is no edentulous 42space. Embrasure clasps should have two retentive clasp arms and two reciprocal clasp arms, either bilaterally or diagonally opposed.
Disadvantages
- Occlusal rest should be prepared on both the teeth.
- Difficult to get occlusal clearance.
- Large amount of tooth surface covered that can cause decalcification and caries.
- Minimum flexibility and cannot be used in distal extension bases.
- Interferes with normal food flow pattern.
- Not aesthetic.
Indication
When spacing is present between two teeth.
Ring clasp
Ring clasp, encircles nearly all of a tooth from its point of origin. It is used when a proximal undercut cannot be approached by other means as in a distobuccal or distolingual undercut.
The ring-type clasp should be used on protected abutments as it covers such a large area of tooth surface.
Disadvantages
- Not esthetic.
- Large amount of tooth surface covered that can cause decalcification and caries.
- Minimum flexibility and cannot be used in distal extension bases.
- Interferes with normal food flow pattern.
Fish hook or Hairpin clasp
It is a simple circlet clasp in which the retentive arm crosses the facial surface of the tooth and loops back in a hairpin turn to engage the undercut below its point of origin. The upper arm acts as a minor connector and is rigid; the lower arm is tapered and is flexible.
Disadvantages
- Only can be used in tooth with adequate occlusogingival height.
- There tends to be food accumulation between the upper and lower clasp arms.
- The upper arm can cause occlusal interference in tight occlusion of upper and lower arch.
- Caries prone.
- Not aesthetic.
- Large amount of tooth surface covered that can cause decalcification and caries.
- Minimum flexibility and cannot be used in distal extension bases.
- Interferes with normal food flow pattern.
Onlay clasp
- It covers the entire occlusal surface with buccal and lingual clasp arms.
- Should be made in patients with good oral hygiene.
- Occlusal surface should be of gold or acrylic.
Indication: To correct occlusal plane discrepancies as in tooth tipping or rotation.
Combination clasp
Consists of a wrought wire retentive terminal and cast reciprocal arm. The wrought wire can be incorporated during wax up or can be soldered later on.
Indications
- In distal extension partial denture with mesiobuccal undercut.
- Can be used in deeper undercuts due to its flexibility.
Disadvantages
- Requires additional work during laboratory fabrication.
- Chances of breakage.
- Easily distorted during normal handling.
- Less bracing and stabilization compared to other circumferential clasp.
Advantages
- Flexibility.
- Adjustability.
- Aesthetic advantage over other retentive circumferential clasp arms.
- Minimum of tooth surface is covered compared to a cast clasp arm.
- Fatigue failures are less compared to a cast, half-round retentive arm.
Bar clasp or Roach clasp
The bar clasp arm has been classified by the shape of the retentive terminal as T, modified T, I, Y, or almost any letter clasp arm.
They originate from the framework or base and approach the undercut from a gingival direction.
Flexibility is adjusted depending on the taper and length of the approach arm.
Advantage: It is more esthetic.
Disadvantages: Causes food entrapment.
Less bracing an stabilization compared to C- Clasp.
Rules of use
- Approach arm should not impinge on soft tissues.
- Minor connector attached to bar clasp should be rigid.
- Approach arm should be uniformly tapered and should cross the gingival margin at 90 degrees.
- Approach arm should extend to the height of contour.
- The bar clasp is used only if the retentive area is adjacent to edentulous.
- Retentive terminal of bar clasp should point toward the occlusal surface.
- Bar clasp should be placed as low on tooth as possible.
I Bar is a part of the RPI system (rest, proximal plate, I-bar).
- This clasp assembly consists of a mesio-occlusal rest with the minor connector placed into the mesiolingual embrasure.
- A distal guiding plane, extending from the marginal ridge to the junction of the middle and gingival thirds of the 46abutment, is prepared to receive a proximal plate. The proximal plate, in conjunction with the minor connector supporting the rest, provides the stabilizing and reciprocal aspects of the clasp assembly.
- The l-bar should be located in the gingival third of the buccal or labial surface of the abutment in 0.01 inch-undercut.
ENUMERATE VARIOUS FACTORS AFFECTING RETENTION IN DIRECT RETAINERS. ADD A NOTE ON GINGIVALLY APPROACHING CLASPS AND RPI SYSTEM.
Factors affecting retention
- Flexibility of the clasp arm,
- The depth that the retentive terminal extends into the undercut,
- The amount of clasp arm that extends below the height of contour,
- The amount of retention used should always be the minimum necessary to resist reasonable dislodging forces.The retentive undercut has three dimensions:
- The buccolingual depth of the undercut expressed in thousandths of an inch. The less sharp this angle, the greater distance needed between the height of contour and the retentive terminal to achieve the same amount of retention.
- The distance between the survey line and the tip of the retentive clasp. This affects the clasp arm length, and in turn influences the flexibility of the clasp.
- Mesio-distal length of the clasp arm below the height of contour. The longer this measurement, the more flexible the clasp will be and the more important is the buccolingual dimension of the retentive undercut.The most variable factor in determining retention for a removable partial denture is clasp flexibility.
Flexibility is determined by
- The length of the clasp,
- The diameter of the clasp arm,
- Its taper, the cross-sectional form and
- The material from which the clasp is made.
Length
The greater the length of the clasp arm, the greater will be its flexibility. By doubling the length of a clasp arm, its flexibility is increased five times. Flexibility should be just adequate enough to provide retention.
Diameter
Flexibility is inversely proportional to the diameter of the clasp arm. A uniform taper in both thickness and width is essential for both the approach arm of a bar clasp and the retentive arm of a cast clasp. A clasp should be half as thick at the tip as at the origin.
Cross-sectional form
A round clasp has the ability to flex in all spatial planes, whereas a half-round clasp flexes only in a single plane.
Material
Chrome alloys have a higher modulas of elasticity than do gold alloys and are therefore less flexible.
Example: Chrome alloy clasps are placed in undercut of 0.01 inch. Gold alloys are placed in 0.015 inch.
As internal structure of wrought wire has greater ability to flex than the crystalline structure of cast alloy; a greater depth of undercut is required for a wrought wire clasp than for a cast clasp. Eg: 0.020 inch.
Details of bar or vertical projection clasps
- Vertical projection clasps approach retentive undercut from a gingival direction. Hence, called as push type of retention.
- The flexibility of the bar clasp can be controlled by the taper and length of the approach arm. The greater the length and taper, the more the flexibility of the clasp.
- As the bar clasp is gingivally approaching, it is more esthetic than a circumferential clasp.
Disadvantages
- Tendency to collect food debris.
- Increased flexibility of the retentive arm, decreases bracing and stabilization. Additional stabilizing units need to be added to attain bracing and stabilization.
Rules for use
- The approach arm of the bar clasp must not impinge on the soft tissue it crosses.
- The minor connector that attaches the occlusal rest to the framework should be strong and rigid.
- The approach arm must always be tapered uniformly.
- The approach arm must never be designed to bridge a soft tissue undercut.
- The approach arm should cross the gingival margin at a 90-degree angle.
- The bar retentive clasp is used only when the retentive undercut is adjacent to the edentulous area from which the approach arm originates.
- The reciprocal arm is positioned above the height of contour. The retentive terminal tip must point toward the occlusal surface.
- The bar clasp should also be placed as low on the tooth as possible.
Types of Bar Clasps
T Clasp
- Used often along with cast circumferential reciprocal arm.
- The retentive terminal and its opposing terminal project laterally from the approach arm to form a T.
- Both terminals should point toward the occlusal surface of the abutment tooth.
- The retentive terminal must cross the height of contour to engage the retentive undercut, while the other terminal is placed on the suprabulge of the tooth.
- The approach arm contacts the tooth only at the height of contour.
Uses
- In distal extension ridge with distobuccal undercut.
- It reduces the torque to abutment tooth.
- Can also be used for tooth-supported partial denture if the retentive undercut is located adjacent to the edentulous space.
Disadvantages
- The T clasp can never be used when soft tissue undercut is present.
- If used causes food retention and irritation to the lips and cheeks.
- Never to be used when the height of contour is close to the occlusal surface of an abutment tooth.
- Not to be used in mesiobuccal undercuts.
Modified T Clasp
The modified T clasp is a clasp in which only one terminal is present (only the retentive terminal). The nonretentive (usually mesial) finger is omitted.
Uses: Used on canines or premolars for esthetic.
Disadvantage
- 180-degree coverage is not present which can compromise other functions of clasp such as bracing and reciprocation.
- Can never be used when soft tissue undercut is present.
- If used causes food retention and irritation to the lips and cheeks.
- Never to be used when the height of contour is close to the occlusal surface of an abutment tooth.
- Not to be used in mesiobuccal undercuts.
Y Clasp
This is commonly used when the height of contour on the facial surface of the abutment tooth is high on the mesial and distal line angles but low on the centre of the facial surface.
The rest of the rules are same as T-clasp.
If recontouring can help to bring the survey line to the middle, a T-clasp can be used.
I Clasp and I Bar
Use: Used on the distobuccal surface of maxillary canines.
Disadvantage: Encirclement and horizontal stabilization are compromised.
RPI
The clasp system includes the three elements (Kratochvil's system): mesial rest, proximal plate and I bar.
Krol's criteria
- Rest preparations are less extensive in the RPI system. The mesial rest prepared on molars and canines are often circular concave depressions prepared in the mesial marginal ridge.
- Proximal plate is diminished in all directions and I bar retentive tips are placed mesial to the mesiodistal height of contour.
- Occlusal force on extension base disengages proximal plate into gingival concavity and I-bar disengages into interproximal embrasure.
Tipped abutments and tissue impingement are treated with a further modification, the RPA clasp (rest, proximal plate and Akers clasp).
When the Akers clasp arm is used, careful attention is paid to relieving all undercuts except at the retentive tip.
The requirements of a partial denture clasp system (vertical support, horizontal stabilization, retention, reciprocation and passivity) are all met by the I-bar system.
DEFINE REST AND REST SEAT. GIVE ITS LOCATION. DESCRIBE VARIOUS TYPES OF RESTS AND THEIR FUNCTION.
Definition
Rest is a rigid extension of a fixed or removable partial denture, which contacts a remaining tooth or teeth to dissipate vertical or horizontal forces.
Any unit of a partial denture that rests on a tooth surface to provide vertical support is called a rest.
Depending on the location it is classified as
- Occlusal rest.
- Cingulum rest/ lingual rest.
- Incisal rest.
Depending on the function it is classified as
- Primary rest.
- Secondary or Auxiliary rest.
Functions of rest
- Primary function is to provide vertical support for the partial denture.
- Additional functions are:
- Maintains components in their planned positions.
- Maintains established occlusal relationships.
- Prevents impingement of soft tissues.
- Directs and distributes occlusal loads to abutment teeth.
- Prevents cervical movement of partial denture.
Occlusal rest and rest seat
Shape
Triangular shape with the apex toward the centre of the occlusal surface.
Base of the triangular shape at the marginal ridge should be at least 2.5 mm for both molars and premolars.
Floor of the occlusal rest seat should be concave or spoon shaped.
Thickness
At the marginal ridge thickness should be minimum 1.5 mm.
Preparation should be within enamel.
Angulation
The angle formed by the occlusal rest and the vertical minor connector from which it originates should be less than 90 degrees.53
Method of preparation
Occlusal rest seats in sound enamel can be prepared with diamond points of the size of Nos. 6 and 8 round burs or with carbide burs.
The larger diamond is used to prepare the marginal ridge and to establish the outline form of the rest seat. The smaller diamond point deepens the floor of the occlusal rest seat making it spoon shaped.
The unsupported enamel rods are planed by round bur of suitable size revolving at moderate speed.
Finally an abrasive rubber point is used to polish the preparation.
Function
As a shallow ball-and-socket joint, hence able to resist horizontal stresses to the abutment tooth.
Types
- Interproximal occlusal rest seats.
- Internal occlusal rests.
Interproximal occlusal rest seats
Rest seats that are prepared as adjoining occlusal rest seats with preparations extending farther lingually.54
Advantages
- Avoids interproximal wedging by the framework.
- Prevents food entrapments.
Rules for preparation
- Contact points of abutment teeth should be preserved.
- Sufficient tooth structure removed to allow for adequate bulk.
- Shaped so that occlusion will not be altered.
- Sufficient space must be created to avoid interference with placement of rests.
Internal occlusal rests
Used in totally tooth-supported situations.
Enhances occlusal support and horizontal stabilization.
Occlusal support is derived from the floor of the rest seat and horizontal stabilization from the near-vertical walls.
Preparation: Rest should be parallel to the path of placement.
Tapered occlusally and slightly dovetailed to prevent dislodgement proximally.
Advantages
- Aesthetically pleasing.
- Can locate rest seat in a favourable position in relation to the horizontal axis of the abutment.
- Retention is provided by a lingual clasp arm.
- Use of a machined mandrel made of a chromium-cobalt alloy can be waxed into the crown or inlay pattern, invested and casted to a position parallel to the path of placement.
Rules for rest seat preparations
- Should be prepared in enamel.
- For effective functioning of rest guide plane preparations should be done.
- The preparation of occlusal rest seats must follow proximal preparation, never precede it.
- A fluoride gel should be applied to abutment teeth following enamel recontouring after the impressions are made.
- Occlusal rest seat preparations in existing restorations are treated the same as preparations on tooth.
- When a primary rest seat preparation is not adequate in function, a secondary occlusal rest should be used on the opposite side of the tooth to prevent slipping of the primary rest.
- Occlusal rest seats in new restorations should be placed in the wax pattern.
- Occlusal rest seats in crown and inlays are generally made somewhat larger and deeper than those in enamel.
- Occlusal rest seats supporting tooth-borne dentures are made slightly deeper than those supporting distal extension bases.
Cingulum rests on canines
- Sufficient space must be present or created to avoid interference with placement of rests.
- A lingual rest is preferable to an incisal rest because it is placed nearer the horizontal axis of rotation (tipping) of the abutment.
- Lingual rests are more aesthetically acceptable than are incisal rests.
- Should be placed in enamel.
Shape of rest seat
A slightly rounded V.
Location
On the lingual surface at the junction of the gingival and middle one third of the tooth.
Method of preparation
- Preparation is started with an inverted cone-shaped diamond stone and progressing to smaller, tapered stones with round ends to complete the preparation.
- All line angles should be eliminated.
- Shaped, abrasive rubber polishing points, with pumice, is used for polishing.
Rules for preparation
- Prepared after finalizing the path of placement.
- The floor of the rest seat should be toward the cingulum.
- No enamel undercut, should be present.
- Ideally, it is best to place the rest seat in a cast restoration.
- The lingual rest can also be placed on the lingual surface of a cast veneer crown, a three-quarter crown, or inlay.
- Cast chromium-cobalt alloy rest seat attached to lingual surfaces of anterior teeth by composite resin cements have being tried.
Incisal rests and rest seats
- Incisal rests are placed at the incisal angles of anterior teeth and on prepared rest seats.
- Incisal rests are placed on enamel.
- Incisal rests are used as auxiliary rests or as indirect retainers.
- Can be placed in mandibular canines also.
Shape
Incisal rest seat is a rounded notch at an incisal angle or on an incisal edge, with the deepest portion of the preparation apical to the incisal edge. The rounded notch is bevelled labially and lingually and the lingual enamel is shaped to accommodate minor connector connecting the rest to the framework.
Size
Function
- As auxiliary rest.
- As indirect retainer to prevent rotational movement of the denture base.
Primary rest
It is a rest seat prepared to support the clasp assembly and act as a vertical support in preventing the displacement of the denture in cervical direction.
Secondary or auxiliary rest
Function as indirect retainer in distal extension partial dentures. The indirect retainers are placed opposite to fulcrum line anteriorly.
Factors influencing effectiveness of indirect retainers
- Depends on effective function of the direct retainers.
- To assess the position of placement of indirect retainer three areas must be considered:
- Length of the distal extension base,
- Location of the fulcrum line,
- At what distance the indirect retainer should be placed.
- Rigidity of the connectors supporting the indirect retainer.
- Abutment tooth health and position.
Auxiliary functions of indirect retainers
- Reduces the anteroposterior movement on the principal abutments.
- Aids in horizontal stabilization when made parallel to the path of placement.
- Splinting effect when used in the anterior teeth as supporting indirect retainers.
- Aids in supporting major connector from settling into the tissues.
- First visual indications for the need to reline in distal extension base partial denture.
Forms of indirect retainers
1. Auxiliary occlusal rest
In Class I-Bilateral rests on the mesial marginal ridge of the first premolars.
In Class II partial dentures- Placed on the marginal ridge of the first premolar tooth on the opposite side of the arch from the distal extension base.
2. Canine extensions from occlusal rests
A finger extension from a premolar rest is placed on the prepared lingual slope of the adjacent canine tooth. Applicable when first premolar serves as a primary abutment.
3. Canine rests
When mesial marginal ridge of the first premolar is too close to the fulcrum line an adjacent canine tooth is used.
4. Continuous bar retainers and linguoplate
In Class I and Class II partial dentures a continuous bar retainer or linguoplate may extend the effectiveness of the indirect retainer.59
EXPLAIN THE VARIOUS IMPRESSION TECHNIQUES USED IN THE FABRICATION OF RPD. DESCRIBE THE FUNCTIONAL IMPRESSION TECHNIQUE USING KORECTA WAX.
The various impression techniques are
- Single pressure free impression technique
- Selective pressure impression technique
- Physiologic or functional impression technique
Different Physiologic Methods (Dual Impression Technique) are
- McLean's method.
- Hindel's method.
- Functional relining method.
- Fluid wax method.
I. Single pressure free impressions
Are commonly made in association with tooth supported and some tooth tissue supported situations.
Impression materials used
- Irreversible hydrocolloid (Alginate).
- Reversible hydrocolloid (Agar).
- Polysulfide.
- Silicone rubber.
II. Selective pressure impression technique
- The physiologic impressions produce generalized displacement of the mucosa, whereas selective pressure directs the occlusal load to selected areas.
- Selective pressure impression technique equalizes the support between the abutment teeth and the soft tissue.
- Selective pressure technique helps to direct forces to the portions of the ridge capable of withstanding the force. This is obtained by providing relief in the impression tray with an acrylic bur in selected areas and permitting the impression tray to just allow space for impression material in other areas.
- The relief areas (as the crest of the ridge in mandibular, incisive papillae and median palatine raphe in maxillary) will be the least displaced, while at the areas the tray contacts, the tissues will have maximum displacement (such as buccal shelf area in mandibular, slopes of the ridge and posterior palatal space).
- Fluid wax functional impression technique and the selected pressure technique impressions displaces edentulous ridge in certain areas as selected and the master cast is altered to accommodate the altered ridge impression. Hence, this technique is referred to as the altered cast impression technique or the corrected cast impression technique.
Impressions materials used for selective pressure technique
Zinc Oxide-Eugenol Paste
- Zinc oxide-eugenol paste is of intermediate viscosity. Its main advantage is that it requires less time to make impression.
- Zinc oxide-eugenol paste is considered the impression material of choice if gross undercuts are not present.
Rubber base materials
- Polysulfide and silicone rubber base impression materials can be used.
- They are slightly more viscous than zinc oxide-eugenol paste; this viscosity can be altered by using higher percentage of light-bodied material in the mix.
- Requires more time to make the impression.
- Cannot be corrected by addition.
- An adhesive is required to ensure that the rubber adheres to the tray.
- Indicated for patients with bony undercuts in the edentulous ridge.
- Several holes needed to prevent excessive displacement of soft tissue. The holes will also prevent air traps.
III. Physiologic Impressions (Dual Impression Techniques)
McLean's physiologic impression
This technique is used in distal extension partial dentures (as in Kennedy's Class I and II situations) to record the tissues of the residual ridge that support a distal extension denture base in its functional form. This needs a dual impression technique.
Procedure
- Custom impression tray is constructed over a preliminary cast of the arch.
- Border moulding is carried out on the edentulous portion of the tray borders and a functional impression of the distal extension ridge is made with zinc oxide eugenol impression paste.
- Hydrocolloid impression is made with a stock tray placed over the first impression, which is positioned in its functional position with finger pressure.
Disadvantage
Finger pressure cannot produce the same functional displacement of the tissue that biting force produces.
Hindel's method
The technique is same as McLean's method except that Hindel modified the stock tray for the second impression, which was provided with holes on both sides in the molar region so that finger pressure could be applied through the tray as the hydrocolloid impression was made.
Differences from McLean's technique
- Hindel's technique was an anatomic impression of the ridge at rest made with a free-flowing zinc oxide-eugenol paste.
- Functional loading is achieved by finger pressure on the preliminary impression through the holes in the stock tray while making the second impression.
Disadvantages
- The displaced or functional form technique can cause interruption of blood circulation with adverse soft tissue reaction and resorption of the underlying bone.
- When the patient's teeth came together, the artificial teeth contact first and the remaining natural teeth contact only after the mucosa has been displaced. This is uncomfortable to many patients.
Functional relining method
In this technique the secondary impression is made after the framework is constructed so it is referred as functional reline. This technique adds a new surface to the tissue side of the denture base.
This can be done before insertion or later if excessive resorption is present.
Procedure
- A soft metal spacer (Ash No. 7 metal) is adapted over the ridge on the cast before processing the metal denture base. After processing the metal is removed, leaving an even space between the base and the edentulous ridge.
- Border moulding is done and impression is made with a low-fusing modelling plastic placed over the tissue surface of the denture base. The modelling plastic is tempered in a water bath and seated in the patient's mouth until an accurate impression of the ridge is made.
- The entire procedure is done with the patient's mouth in a partially open position.
- Final impression:After the application of modelling plastic is complete 1mm of modelling plastic is scraped away uniformly from all over the crest of ridge.Final impression is made with a free-flowing zinc oxide-eugenol impression paste. In case of excessive undercuts a elastomeric impression material is used.
Disadvantages: Occlusal discrepancies are present after the new denture base is processed.
Cannot establish previous occlusal contacts.
Fails to maintain the correct relationship between the framework and the abutment teeth during the impression procedure.
Fluid wax functional impression
The term fluid wax denotes waxes that have the ability to flow at mouth temperature.
Types of waxes that can be used:
- Iowa Wax, developed by Dr. Smith.
- Korecta Wax No. 4, developed by Drs. O.C. and S.G. Applegate.
- Korecta wax No.4 has better flow properties than Iowa wax.
Objectives of this technique:
- To obtain maximum extension of the peripheral borders of the denture base.
- To record the stress-bearing areas of the ridge in their functional form and remaining tooth in their anatomic form.
Uses of fluid wax technique
- Used to make a reline impression for existing partial denture.
Procedure
- Impression tray is made, attached to the framework after verifying fit of framework.
- Peripheral extensions of tray are corrected after seating the tray in the mouth. The tray should be 1 or 2 mm short of the movable tissue.The posterior extension of the tray should end at two-third the coverage of retromolar pad.
- Border moulding the impression tray:The mandibular distal extension tray is border moulded in two steps:
- From the anterior extent of the buccal flange to the most posterior extent of the tray and
- The remainder of the lingual and distal lingual flange.
- Relieving trayAs no relief was provided between the ridge and the tray during the fabrication of denture base, 1 to 2 mm of the tray is relieved for impression procedure.
- Impression procedure
- The fluid wax impression is made with the open mouth technique.
- The impression wax is melted in a water bath maintained at 51 to 54 degree C within a container.
- The wax is painted on to the tissue side of the impression tray with a brush.
- Each time the tray is placed into the patient's mouth, it must remain in place for 5 minutes to allow the wax to flow and to prevent build-up of pressure, which can result in distortion.
- After 5 minutes the tray is removed and the wax examined for glossy surface indicating adequate contact.
- Making the impression bordersThe peripheral extension of the impression tray should be short by 2 mm to develop a proper border seal with tissue movements done by the patient.
- Buccal and distobuccal borders in mandibular impressions Are obtained by asking the patient to open the mouth wide, which will activate the buccinator muscle and Pterygo-mandibular raphe and produce the desired border.
- Lingual extension for a mandibular impressionTongue is thrust into the cheek opposite the side of the arch being border moulded by the patient.
- Distolingual extension
- The patient presses the tongue forward against the lingual surfaces of the anterior teeth.
- After the entire borders are copied satisfactorily the impression is replaced in the mouth for a final time for 12 minutes to ensure complete flow of wax and to release any pressure present.
- New cast is poured immediately to prevent wax distortion.
Advantage: Can produce an accurate impression if properly done.
Disadvantages
- Time consuming.
- Can cause excessive tissue displacement if not done accurately.
WHAT IS THE IMPORTANCE OF DISTAL EXTENSION IMPRESSION PROCEDURE IN PARTIAL DENTURE? LIST FACTORS INFLUENCING SUPPORT OF DISTAL EXTENSION BASE. EXPLAIN IN DETAIL FLUID WAX IMPRESSION TECHNIQUE
Importance of distal extension impression procedure
- If pressure free single impressions are used to fabricate distal extension partial denture than excess pressure is exerted onto the abutment tooth.
- A dual impression technique equalizes forces on the edentulous ridge and abutment tooth.
- Dual impression copies edentulous ridge in functional form and tooth in anatomic form.
- Distributes load over large area.
- Defines the peripheral extension of denture base accurately.
Factors influencing support of distal extension base
- Quality of soft tissue covering ridge:A firm tightly attached mucosa can withstand better stress and render more support to denture than a flabby displaceable tissue.
- Quality of bone supporting denture base:Cortical bone can resist vertical forces better than cancellous bone. The areas of denture bearing areas covered by cortical bone is utilised as primary stress bearing areas of that arch.
- Design of partial dentures:An accurately designed partial denture with direct and indirect retainers placed in the right areas can give better stability, support and retention.
- Amount of tissue coverage by denture base:Denture bases covering maximum surface area have uniform distribution of load.
- Occlusal forces:A partial denture opposing natural teeth is subjected to more occlusal forces than opposing a complete denture or RPD.To reduce the occlusal forces:
- Denture base need to have maximum coverage.
- Narrowing of food table of artificial tooth.
- Cutting efficiency can be improved on artificial tooth by adding sluiceways and grooves.
- Stress bearing areas need to be utilised in maxillary and mandibular dentures:Maxillary: Buccal slopes of the ridge and palatine shelves resist lateral and vertical displacement of the prosthesis.Mandibular: Buccal shelf area and slopes of residual ridge resist vertical and horizontal forces.
- A perfect fit of the denture base with adequate coverage and proper peripheral extensions give better support to denture.
Fluid wax functional impression
The term fluid wax denotes waxes that have the ability to flow at mouth temperature.
Types of waxes that can be used:
- Iowa Wax, developed by Dr. Smith.
- Korecta Wax No. 4, developed by Drs. O.C. and S.G. Applegate.
- Korecta wax No.4 has better flow properties than Iowa wax.
Objective of this technique
- To obtain maximum extension of the peripheral borders of the denture base.
- To record the stress-bearing areas of the ridge in their functional form and remaining tooth in their anatomic form.
Uses of fluid wax technique
- Used to make a reline impression for existing partial denture.
- To correct the distal extension edentulous ridge portion as in altered cast technique.
Procedure
- Impression tray is made, attached to the framework after verifying fit of framework.
- Peripheral extensions of tray are corrected after seating the tray in the mouth. The tray should be 1 or 2 mm short of the movable tissue.The posterior extension of the tray should end at two-thirds coverage of the retromolar pad.
- Border moulding the impression tray:The mandibular distal extension tray is border moulded in two steps:
- From the anterior extent of the buccal flange to the most posterior extent of the tray and
- The remainder of the lingual and distal lingual flange.
- Relieving tray:As no relief was provided between the ridge and the tray during the fabrication of denture base, 1 to 2 mm of the tray is relieved for impression procedure.
- Impression procedure:
- The fluid wax impression is made with the open mouth technique.
- The impression wax is melted in a water bath maintained at 51 to 54 degree C within a container.
- The wax is painted on to the tissue side of the impression tray with a brush.
- Each time the tray is placed into the patients mouth, it must remain in place for 5 minutes to allow the wax to flow and to prevent build-up of pressure, which can result in distortion.
- After 5 minutes the tray is removed and the wax examined for glossy surface indicating adequate contact.
- Making the impression borders:The peripheral extension of the impression tray should be short by 2 mm to develop a proper border seal with tissue movements done by the patient.
- Buccal and distobuccal borders in mandibular impressions:Are obtained by asking the patient to open the mouth wide, which will activate the buccinator muscle and pterygo-mandibular raphe and produce the desired border.
- Lingual extension for a mandibular impression:Tongue is thrust into the cheek opposite the side of the arch being border moulded by the patient.
- Distolingual extension:
- The patient presses the tongue forward against the lingual surfaces of the anterior teeth.
- After the entire borders are copied satisfactorily the impression is replaced in the mouth for a final time for 12 minutes to ensure complete flow of wax and to release any pressure present.
- New cast is poured immediately to prevent wax distortion.
Advantage: Can produce an accurate impression if properly done.
Disadvantages
- Time consuming.
- Can cause excessive tissue displacement if not done accurately.
Preparing original cast and pouring corrected cast:
- Corrected cast impression can be done in 20 to 30 minutes.
- The edentulous areas are outlined on the cast.
- These outlined areas are removed with handsaw.
- Knife-edged stone on lathe is used to make longitudinal retention grooves on cut surface of cast to provide mechanical retention for new portion of cast to be poured.
- Framework is seated on sectioned cast and secured with modelling plastic. Impression must not contact cast.
- Utility wax is used to bead impression to form land area, 2 to 4 mm below peripheral margins of impression and extending outward 3 to 4 mm from impression.
- Boxing wax is added around beading wax to confine dental stone.
- Original cast is soaked in slurry water for 10 minutes and dentrite stone is poured into the beaded and boxed portion.
- After the stone sets the altered cast is retrieved with the edentulous area recorded in functional form.
EXPLAIN IMPORTANCE OF MOUTH PREPARATION IN CAST PARTIAL DENTURE DESIGNING AND ADD A NOTE ON VARIOUS PROCEDURES AND ITS SIGNIFICANCE IN DESIGNING
Mouth preparation in RPD
Mouth preparation is part of the treatment planning phase carried out to enhance the success of the removable partial denture.
The order of procedures in mouth preparation
- Surgical procedures.
- Periodontal procedures.
- Endodontic procedures.
- Orthodontic realignment, if necessary.
- Treatment of abused tissues.
- Preparation of guide planes and rest seats.
- Prosthetic rehabilitation.
Surgical procedures
- Extractions
- Any tooth detrimental to the design of the partial denture.
- Residual roots tips should be extracted with minimum bone loss.
- Impacted teeth.
- Malposed teeth which is grossly extruded or drifted which cannot be corrected by restorations.
- Cyst and odontogenic tumoursConfirm with roentgenogram and investigated by biopsy.
- Exostoses and toriIf causing hindrance to the design of the removable partial denture a surgical removal is done with air-turbine handpiece at lower speeds (20,000 to 50,000 rpm). To prevent thermal damage to the bone adequate water irrigation is needed.
- Hyperplastic tissueSurgical excision by scalpel, curette, or electrosurgery.
- Muscle attachments and freni:Near to the alveolar crest should be corrected, if not can compromise retention of the removable partial denture.Common muscle attachments that can cause this problem are mylohyoid, buccinator, mentalis and genioglossus muscles.The maxillary labial and mandibular lingual freni are the most commonest of frenum interferences.
- Bony spines and knife-edge ridges.
- Polyps, papilloma and traumatic hemangiomas
- Hyperkeratoses, erythroplasia and ulcerations.All abnormal, white, red, or ulcerative lesions should be sent for biopsy.
- Dentofacial deformity.Surgical correction of a jaw deformity can be made in horizontal, sagittal, or frontal planes.
- Osseointegrated devices:Endosteal implants or fixtures constructed from titanium are placed under controlled oral surgical procedures if required for partial denture.
Periodontal procedures
The periodontal procedures follow surgical procedures.
Objectives of periodontal therapy:
- Removal of all etiologic factors causing periodontal disease.
- Elimination of all pockets.
- Creating physiologic gingival and osseous architecture.
- Establishment of functional occlusal relationships.
- Plaque control program and definitive maintenance schedule.
Evaluation of abutment tooth for periodontal health:
- Evaluating pocket depth especially in the abutment tooth.
- Check bleeding on probing, for sulcular health.
- The extent and pattern of bone loss.
- The degree of mobility present.
- Any traumatic occlusion.
- Diagnostic aids used are: Periodontal probe, mouth mirror, curved explorers, furcation probes, diagnostic casts and roentgenograms.
- Evaluating Pocket depth:Periodontal probe is used circumferentially around each tooth and depths are recorded in distobuccal, mesial, mesiobuccal, distolingual, lingual and mesiolingual aspects of each tooth.
Treatment planning is divided into three phases:
First Phase | Initial disease control therapy. |
Second Phase | Definitive periodontal surgery. |
Third Phase | Recall maintenance. |
First phase
Initial disease control therapy.
This phase constitutes reducing local etiologic factors by:
- Oral hygiene instructions.
- Scaling.
- Root planing and polishing.
- Endodontics.
- Occlusal adjustment and
- Temporary splinting.
- Scaling and root planing: The use of ultrasonic instrumentation for calculus removal followed by root planing with sharp periodontal curettes is recommended which will result in complete calculus removal and root surface decontamination.
- Overhanging margins (of amalgam alloy and inlay restorations): Overhanging crown margins, and open contacts leading to food impaction should be corrected before definitive prosthetic treatment.
- Traumatic cuspal interferences: Should be eliminated using grinding procedure to establish a positive planned intercuspal position that coincides with centric relation.
Interferences in working and non-working sides should be removed.
Second phase
Definitive periodontal surgery
After initial therapy is completed, the patient is re-evaluated for the definitive surgical phase.
Surgical procedures are indicated when:
- Pocket elimination was not achieved by scaling and root planing.
- Gingival recession.
- Lack of attached gingiva.
- Furcation involvement.
- Underlying osseous defects.
Pocket elimination
Can be achieved by:
- Shrinkage.
- Surgical excision.
- Gingivectomy.
- New attachment procedures.
Gingivectomy
Indications:
- Supra-bony pockets of fibrotic tissue exist.
- Absence of deformities in the underlying bony tissue with pocket.
- Pocket depth confined to the band of attached gingiva.
Contraindications
- If osseous deformities are present.
- If pocket depth traverses or approximates the mucogingival junction.
- In gross gingival recession.
The gingivectomy technique is carried out with appropriate cutting instruments.
Periodontal flap: Two commonest used flaps techniques are:
- Modified Widman Flap.
- Full-thickness mucoperiosteal flap.
- Reconstructive surgical procedures.
Modified Widman Flap: This is a more conservative surgical procedure than full-thickness flap.
Indications
- When periodontal pockets are 4 to 6 mm in depth.
- When osseous loss is primarily of the horizontal type.
Full-thickness flap: This can be replaced in its pre-surgical position, or can be apically replaced.
Indications
- When surgical area has pockets of 6 mm or more with osseous defects.
- In case crown lengthening is required.
Reconstructive surgical procedures: Commonly used procedures are:
- Laterally replaced flap.
- Edentulous area pedicle flap.
- Double papillae repositioned flap and
- Free gingival graft.
Indications
- In establishing a zone of attached gingiva on abutment teeth.
- In cases with less than 1 mm of attached gingiva.
Third phase
- Recall maintenance.
- Includes reinforcement of plaque control measures.
- Debridement of all root surfaces of supragingival and subgingival calculus and plaque by the dentist.
- A 3 to 4 month recall system.
Advantages of periodontal therapy
- Elimination of periodontal disease enhances the long-term success of dental treatment.
- Periodontium free of disease enhances restorative corrections.
- Elimination of periodontal pockets establishes a normal gingival contour on the tooth surface.
- Coronal contours and gingival margins of restorations can be placed with accuracy.
- Aids in evaluating a tooth before a final decision is made to include them in the partial denture design.
- Can analyse the degree of cooperation to be expected from the patient during the restorative phase.
- Preservation of tooth, which could have otherwise failed.
- Crown lengthening procedures enable the tooth to distribute forces more uniformly.
Prosthodontic procedures
Mouth preparation is done to suit a simple design.
- Mucosa covering the denture-bearing areas has inflammation and irritation.
- Distortion of normal anatomical structures is present.
- Burning sensation is present in any denture bearing area or other areas related to prosthesis placement.
- In cases of ill fitting or poorly occluding removable partial dentures.
The fabrication of new prosthesis should wait till the oral tissues can be returned to a healthy state.
Steps in treatment procedure
- A good home care program, such as rinsing the mouth three times a day with a prescribed saline solution.
- Use of tissue conditioning materials:They have to be lined on existing dentures after:
- Eliminating deflective or interfering occlusal contacts of old dentures.
- Extending denture bases to proper form to enhance support, retention and stability.
- Relieving the tissue side of denture bases at least 2 mm to provide space for an even thickness.
- Applying the correct amount of material to provide support and a cushioning effect following the manufacturer's directions.
- Three or four changes of the conditioning material are adequate, for the distorted tissues to assume their normal form.
Abutment teeth
- The abutment teeth are evaluated for the proposed path of placement, the areas of teeth to be altered and tooth contours to be changed and the location of rest seats.
- The rest seat preparations are carried out in the concerned abutment teeth as designed on the diagnostic cast.
ENUMERATE FACTORS AFFECTING DESIGN OF RPD AND ADD A NOTE ON DESIGN ASPECTS IN KENNEDY'S CLASS I SITUATIONS
Factors influencing design
- A choice between FPD and RPD is made depending on the number of tooth missing, periodontal situation and patient choice.If RPD is the line of treatment planned, then evaluate the:
- Occlusal relationship of remaining teeth.
- Orientation of the occlusal plane.
- Space available for restoration of missing teeth.
- Arch integrity.
- Need for abutment restorations.
- Response of oral structures to previous stress, periodontal condition of the remaining teeth, the amount of abutment support remaining and the need for splinting, etc. should be evaluated.
- Type of major connector to be used.
- Whether the denture is tooth supported, tissue supported or tooth tissue supported.If it is a distal extension denture than following has to be assessed:
- Need for indirect retention.
- Clasp designs that will best minimize the forces applied to the abutment teeth during function.
- Type of base material used.
- Secondary impression method to be used.
- Materials to be used, both for the framework and for the bases.
Design aspects in Kennedy's Class I situation
Class I: (Bilateral distal extension)
In Class I removable partial dentures, abutment teeth are situated anterior to edentulous area and hence need support from soft tissue and the remaining teeth.
78Diagnosis and treatment planning includes abutment tooth evaluation and edentulous ridge evaluation.
Abutment tooth evaluation:
- Periodontal health.
- Crown and root morphologies and ratio.
- Bone index area.
- Location of the tooth in the arch.
- Relationship of the tooth to other support units.
- The opposing dentition.
Edentulous ridge area evaluation:
- The quality of the residual ridge.
- Denture base coverage.
- Type and accuracy of the impression registration.
- Fit of denture base.
- The design of partial denture framework.
- The occlusal load.
Ideal impression technique
Any physiologic impression technique or selective pressure technique.
Important factors in the impression for distal extension partial dentures:
- The material should record the tissues covering the primary stress-bearing areas in their supporting form.
- Tissues within the basal seat area other than primary stress-bearing areas must be recorded in their anatomic form and
- The total area covered by the impression should be maximum, for uniform load distribution.
Essentials of partial denture design
- Locating rest seat on principal abutment tooth. (Support areas).
- Connecting the tooth and tissue support units by designing and locating major and minor connectors.
- Designing direct and indirect retainers to:
- Avoid direct transmission of tipping or torquing forces to the abutment.
- Correctly position clasp assembly in definitive location on abutment tooth surfaces.
- Provide retention against dislodging forces and be compatible with undercut location, tissue contour and esthetic desires of the patient.
- Connecting the retention units to the support units.
- Designing outline and joining the edentulous area to the already established design components.
Components of partial denture design
- Major connector.
- Minor connector.
- Rests.
- Direct retainers.
- Reciprocal or stabilizing components (as parts of a direct retainer assembly).
- Indirect retainers (if the prosthesis has one or more distal extension bases).
- One or more bases, each supporting one to several replacement teeth.
Principles of designing for Class I removable partial dentures:
- Support areas as rest seats.
- Major and minor connectors: Commonest mandibular major connector in situations lower anterior teeth are weakened by periodontal disease with less lingual tissue space is linguoplate.Commonest maxillary major connector is the full coverage palatal major connector. It is preferred because of its rigidity and greater stability without tissue damage.
- Direct retainers retain the prosthesis against reasonable dislodging forces without damage to the abutment teeth and to aid in resisting any tendency of the denture to be displaced in a horizontal plane.
- The circumferential and bar-type clasp retainers are mechanically effective and are more economically constructed than are intracoronal retainers.
Direct retention
Objectives of direct retention:
- Restoration of function and appearance.
- Preservation of the health.
- Integrity of all the oral structures that remain.
- Aid in close adaptation and fit of denture base against multiple, properly prepared guide planes.
Selection of clasp
Objectives
- Clasp should have good stabilizing qualities.
- Should remain passive until activated by functional stress.
- Should permit minor amount of movement of the base without transmitting a torque to the abutment tooth.
- Should be able to control stress transmitted to abutment tooth.
Design aspect
The most posterior teeth on each side of arch should be clasped.
- For distobuccal retention use T-bar.
- For mesiobuccal retention use 18 gauge wrought wire clasp or reverse circumferential clasp.
Depending on the amount of undercut
- Cast cobalt-chromium clasps for 0.010 inch of retentive undercut. (Not ideal to be used on mesiobuccal undercut on the posterior abutment tooth).
- Wrought wire clasps for 0.020 inches of retentive undercut. In case of excessive bone loss double clasping can be designed.
Ideal design of direct retainers
- A Class I prosthesis requires only two retentive clasp arms one on each terminal tooth.
- If a distobuccal undercut is present, the vertical projection clasp is preferred.
- If a mesiobuccal undercut is present, a wrought wire clasp is indicated. (A cast circumferential type clasp should not be used).
- The reciprocal or bracing arm must be rigid.
- Stabilizing componentsAre those rigid components that assist in stabilizing the denture against horizontal movement, hence distributing stresses equally to all supporting teeth without overworking any one tooth. The minor connectors that join the rests and the clasp assemblies to the major connector serve as stabilizing components.
- Guiding planes:The functions of guiding plane surfaces are as follows:
- To provide for one path of placement and removal of the restoration.
- To provide retention against dislodgement of the restoration when the dislodging force is directed other than parallel to the path of removal and also to provide stabilization against horizontal rotation of the denture.
- To eliminate gross food traps between abutment teeth and components of the denture.
Indirect retainers
An indirect retainer must be placed as far anteriorly from the fulcrum line as adequate tooth support permits.82
Either a canine or premolar tooth should be used for the support of an indirect retainer.
An incisal rest or a lingual rest may be used on an anterior tooth, provided a definite seat can be obtained either in sound enamel or on a cast restoration.
MENTION AND JUSTIFY THE COMPONENTS INDICATED FOR KENNEDY'S CLASS I AND II RPD. HOW DOES IT DIFFER FROM CLASS III DESIGN?
Difference between Class I, II and Class III line of treatment.
- The Class I type and the distal extension of the Class II type are mostly tissue supported.Class III type is tooth supported.
- Class I and II require dual impression technique, whereas Class III only needs pressure free single impression technique.
- Class I and Class II need indirect retention.
- As Class I and Class II are mostly tissue supported may need a base material that can be relined to compensate for tissue changes.As Class III is tooth supported, tissue changes are unlikely hence metal bases can be used.
- Type of direct retention for Class I and II need to be passive and should not flex except when engaging the undercut area of the tooth for resisting a vertical dislodging force and also able to dissipate stresses without harming the abutment tooth (Eg: combination clasp).The amount of stress transferred to the supporting edentulous ridge(s) and the abutment teeth will be dependent on:
- The direction and magnitude of the force,
- The length of the denture base lever arm(s),
- The quality of resistance (support from the edentulous ridges and remaining natural teeth),
- The design.
Principles of design for Class I removable partial dentures
(Refer Figure 29)
- Support areas as rest seats
- Major and minor connectors: Commonest mandibular major connector in situations lower anterior teeth are weakened by periodontal disease with less lingual tissue space is Linguoplate.Commonest maxillary major connector is the full coverage palatal major connector. It is preferred because of its rigidity and greater stability without tissue damage.
- Direct retainers retain the prosthesis against reasonable dislodging forces without damage to the abutment teeth and to aid in resisting any tendency of the denture to be displaced in a horizontal plane.
- The circumferential and bar-type clasp retainers are mechanically effective and are more economically constructed than are intracoronal retainers.
Direct retention
Objectives of direct retention:
- Restoration of function and appearance.
- Preservation of the health.
- Integrity of all the oral structures that remain.
- Aid in close adaptation and fit of denture base against multiple, properly prepared guide planes.
Selection of clasp
Objectives
- Clasp should have good stabilizing qualities.
- Should remain passive until activated by functional stress.
- Should permit minor amount of movement of the base without transmitting a torque to the abutment tooth.
- Should be able to control stress transmitted to abutment tooth.
Design aspect
- For distobuccal retention use T-bar.
- For mesiobuccal retention use 18 gauge wrought wire clasp or reverse circumferential clasp.
Depending on the amount of undercut
- Cast cobalt-chromium clasps for 0.010 inch of retentive undercut. (Not ideal to be used on mesiobuccal undercut on the posterior abutment tooth).
- Wrought wire clasps for 0.020 inches of retentive undercut.
In case of excessive bone loss double clasping can be designed.
Ideal design of direct retainers
- A Class I prosthesis requires only two retentive clasp arms one on each terminal tooth.
- If a distobuccal undercut is present, the vertical projection clasp is preferred.
- If a mesiobuccal undercut is present, a wrought wire clasp is indicated. (A cast circumferential type clasp should not be used).
- The reciprocal or bracing arm must be rigid.
- Stabilizing components:Are those rigid components that assist in stabilizing the denture against horizontal movement hence distributing stresses equally to all supporting teeth without overworking any one tooth. The minor connectors that join the rests and the clasp assemblies to the major connector serve as stabilizing components.
- Guiding planes:The functions of guiding plane surfaces are as follows:
- To provide for one path of placement and removal of the restoration.
- To provide retention against dislodgement of the restoration when the dislodging force is directed other than parallel to the path of removal and also to provide stabilization against horizontal rotation of the denture.
- To eliminate gross food traps between abutment teeth and components of the denture.
Indirect retainers
- An indirect retainer must be placed as far anteriorly from the fulcrum line as adequate tooth support permits.
- Either a canine or premolar tooth should be used for the support of an indirect retainer.
- An incisal rest or a lingual rest may be used on an anterior tooth, provided a definite seat can be obtained either in sound enamel or on a cast restoration.
Occlusion
A centric relation record needs to be taken, as posterior tooth contacts are absent.
Tooth selection
- Smaller and teeth that are narrower buccolingually.
- Teeth should be positioned over the crest of the mandibular ridge when possible.
- Teeth modified if necessary to produce sharp cutting edges and ample escape ways.
Denture base
- The base should be designed with broad coverage so that the occlusal stresses can be distributed over as wide an area of support as possible.
- The extension of the borders must not interfere with functional movements of the surrounding tissues.
- A selective pressure impression should record the residual ridge in a functional form with altered cast technique.
Principles of design for Class II removable partial dentures
Ideal design
- A Class II prosthesis should usually have three retentive clasp arms.
- The distal extension side should be designed with the same considerations as for a Class I prosthesis.
- The tooth-supported, or modification, side should have two retentive clasp arms one as far posterior and one 86as far anterior as tooth contours and aesthetics permit. If a modification space exists on the dentulous side, the adjacent tooth should be clasped with simple circlet clasps into a distobuccal undercut for the posterior abutment and mesiobuccal undercut for the anterior abutment tooth.
- Bracing arms should be rigid. Lingual plating can be substituted.
- Rests
- Occlusal rests along with direct retainers.
- Positive rest seats along with indirect retention.
- Rest is prepared for maximum possible support for the prosthesis.
- Rest seats should be prepared to direct stresses along the long axis of the teeth.
- Rests should be placed next to the edentulous space generally.
- Indirect retention
- To neutralize unseating or rotational forces.
- Should be located as far anterior to the fulcrum line as possible.
- Two indirect retainers should be used in a Class I design, whereas one placed on the side opposite to distal extension base is enough for Class II design.
The indirect retainers with positive rest seats to direct forces along the long axis of the tooth.
Depending on the span of edentulous ridge all the principles of Class I distal extension is applicable to Class II.
Principles of design for Class III removable partial dentures
Edentulous areas are bounded anteriorly and posteriorly by abutments. They may be unilateral or bilateral.
- If no modification space exists, the teeth adjacent to the edentulous space should be clasped as for the dentulous side of a Class II, modification 1 arch. On the side where no space exists, clasping should be done with one clasp as far anteriorly as possible and one posteriorly positioned.
- If a modification space does exist on the opposite side of the arch, all four abutment teeth adjacent to the edentulous spaces should be clasped with the simplest type of clasp available.
- If one or both of the posterior abutment teeth are weak due to loss of bone, it may be advisable not to place retentive clasps on them but to place an occlusal rest for vertical support and nonretentive clasp arms for bracing against lateral movement.
- Direct retention
- Retention can be achieved with much less potential harmful effect on the abutment teeth than with the Class I or II arch.
- The position of the retentive undercut on abutment teeth is not critical.
- Clasps
- The quadrilateral positioning of direct retainers.
- The type of clasp selected is not critical.
- Bracing arms must be rigid.
- OcclusionCentric occlusion and centric relation should coincide with no interceptive contacts and with all eccentric movements in harmony with, the remaining natural teeth.
DISCUSS THE PROBLEMS THAT ARE ENCOUNTERED IN MAKING RPD FOR A KENNEDY'S CLASS I MANDIBULAR SITUATION. DESCRIBE THE STEPS IN THE DESIGN OF A KENNEDY'S CLASS I MANDIBULAR SITUATION.
Problems encountered in Kennedy's Class I situation of the mandibular arch
- The tissue coverage is limited in the mandibular arch. The cast partial denture cannot cover the entire denture base areas as a complete denture can; hence the support offered by denture base is minimal compared to maxillary partial denture.
- In Class I situation of the mandibular arch the commonest teeth that are remaining are the incisors, canines and, occasionally, the first premolars. It is difficult to select and place direct retainers without transmitting stress to the abutment tooth.
- If only the lateral incisors are remaining, the partial denture becomes more tissue supported with less stability.
- The direct retention depends on the periodontal situation of the remaining teeth. Weak teeth need to be splinted before taking support from them.
- The amount of lingual tissue space present decides the selection of the type of major connector. In cases of high frenal attachments another alternative like labial bar has to be designed.
- Bony spicules, knife-edge ridge with excessive resorption complicates the design aspect.
- If mandibular tori exist, surgical correction needs to be done before the partial denture is made.
- Enlargement of tongue, abnormal tongue movements complicate the partial denture design.
- In case there is flabby displaceable tissue a pressure free impression is preferred. In case the flabby tissue can be excised its better to surgically correct and then proceed with partial denture treatment.
- If lingually inclined anterior tooth cannot be corrected by altering or restorations, then a labial bar major connector is the choice.
Design principles
Class I. Bilateral edentulous areas located posterior to the remaining natural teeth In Class I removable partial dentures, abutment teeth are situated anterior to edentulous area and hence need support from soft tissue and the remaining teeth.
Principles of designing for Class I removable partial dentures:
- Support areas as rest seats (Cingulum rest or mesial fossae rest on first premolars).
- Major and minor connectors: Commonest mandibular major connector in situations lower anterior teeth are weakened by periodontal disease with less lingual tissue space is linguoplate.
- Direct retainers retain the prosthesis against reasonable dislodging forces without damage to the abutment teeth and to aid in resisting any tendency of the denture to be displaced in a horizontal plane.
The circumferential and bar-type clasp retainers are mechanically effective.
The bar type clasp is a clasp of choice for aesthetics.
Direct retention
Objectives
- Restoration of function and appearance.
- Preservation of the health.
- Integrity of all the oral structures that remain and
- Aid in close adaptation and fit of denture base against multiple properly prepared guide planes.
Selection of clasp
Objectives
- Clasp should have good stabilizing qualities.
- Should remain passive until activated by functional stress.
- Should permit minor amount of movement of the base without transmitting a torque to the abutment tooth.
Stabilizing components
Are those rigid components that assist in stabilizing the denture against horizontal movement, hence distributing stresses equally to all supporting teeth without overworking any one tooth. The minor connectors that join the rests and the clasp assemblies to the major connector serve as stabilizing components.
Guiding planes.
The functions of guiding plane surfaces are as follows:
- To provide for one path of placement and removal of the restoration.
- To provide retention against dislodgement of the restoration when the dislodging force is directed other than parallel to the path of removal and also to provide stabilization against horizontal rotation of the denture.
- Should be able to control stress transmitted to abutment tooth.
- To eliminate gross food traps between abutment teeth and components of the denture.
Kennedy's Class I (Mandibular arch with shallow ridge):
- Incorporate maximum denture base coverage (Lingual plate).
- Reduce the buccolingual width of the denture teeth.
- Posterior-most teeth are not replaced if there is no opposing natural teeth occlusion.
- Placing adequate direct and indirect retainers on the remaining tooth.
Components and their function
- Resistance to anterior movement of the saddles is provided by the distal surfaces of the canines, supported by incisors.
- The resistance to posterior movement is achieved by carrying the distal extension of the saddles as high up the ascending rami as possible.
- In addition, flexible clasps are fitted to the canines with bracing action, to provide additional resistance to posterior movement.
- In free-end saddle movement is prevented by clasps and saddle along with maximum coverage of major connector. In the lower, the retromolar pad area if covered by denture base will also provide resistance to those forces moving the denture.
Rotational displacement
In the case of the free-end saddle any force displacing the saddle will tend to cause rotation about tips of the direct retaining arms and these points, when joined by an imaginary line, form a rotational axis.
To prevent this rotation an indirect retainer is placed opposite to the fulcrum line.
For the indirect retainer to be effective the distance between indirect retainer and the fulcrum axis should be equal.92
Forces in distal extension base
Occlusal/vertical forces:
- These forces need to be distributed uniformly and this is achieved by rests as occlusal, incisal or cingulum rest in tooth supported.
- In distal extension bases the major connector and rest seats help distribute the load uniformly.
Horizontal lateral forces:
- Lateral forces are applied to the teeth and the mucous membranes when mandible moves from side to side with teeth in contact, thus causing the denture to move.
- Component distributing this force uniformly is the bracing arms placed on or above the survey lines on the lingual or palatal surfaces of the teeth.
- The bracing arm is rigid enough to resist lateral forces.
- It functions by disengaging from the tooth when the denture is withdrawn.
Indirect retainers
- An indirect retainer must be placed as far anteriorly from the fulcrum line as possible to prevent torquing or rotation of the denture.
- A canine or premolar tooth is commonly used for placing indirect retainer in the form of cingulum rest or occlusal rest.
- An incisal rest or a lingual rest as for lingual plate can also be used as indirect retention in cases of distal extension bases.
Designing steps in Class I
Stage 1: Classification of the support for each saddle
Reducing the occlusal load by:
- Special impression techniques.
- By the placing of occlusal rests away from the saddle so that more load is transmitted on the mucosa compared with the periodontium.
- The use of stress-breaking devices.
- Distributing the load widely.
- Maximum coverage of denture base.
- Placing occlusal rests in their correct positions to support the saddles.
Occlusal rests functions
- Resist vertical forces of occlusion.
- It transmits horizontal forces to the teeth.
- Prevents food entrapment.
- They may act as indirect retainers.
Stage II: The provision of bracing arms
- Resisting horizontal forces takes place via the bracing arms/reciprocating arms.
- Placed on all the abutment teeth on or above the survey lines and are joined to the occlusal rests.
- Bracing also occurs in an anteroposterior and lateral direction from connectors which are placed on the teeth.
Stage III: Direct retention
A Class I prosthesis requires only two retentive clasp arms one on each terminal tooth.
- If a distobuccal undercut is present, the vertical projection clasp is preferred.
- If a mesiobuccal undercut is present, a wrought wire clasp is indicated. (A cast circumferential type clasp should not be used).
- The reciprocal or bracing arm must be rigid.
Aesthetics: Gingivally approaching clasp is to be preferred for aesthetic reasons.
Stage IV: Indirect retention
- An indirect retainer must be placed as far anteriorly from the fulcrum line as adequate tooth support permits.
- Either a canine or premolar tooth should be used for the support of an indirect retainer.
- An incisal rest or a lingual rest may be used on an anterior tooth, provided a definite seat can be obtained either in sound enamel or on a cast restoration.
Stage V: Connectors
Commonest mandibular major connector in situations lower anterior teeth are weakened by periodontal disease with less lingual tissue space is linguoplate.
Minor connectors These are small struts which join rests and clasps to the major connector. They support the denture against the occlusal forces and transmit loads to the natural teeth.
DEFINE SURVEYOR AND SURVEYING. LIST AIMS AND OBJECTIVES OF SURVEYING. ADD A NOTE ON FACTORS AFFECTING PATH OF INSERTION
Definition
Surveyor: Is a parallelometer, an instrument used to determine the relative parallelism of surfaces of teeth or other areas on a cast of the jaws.
Surveying: An analysis and comparison of the prominence of intraoral contours associated with the fabrication of a prosthesis.
It is the study of parallelism or lack of parallelism to select path of insertion so that restorations encounter least tissue/tooth interference and will provide adequate balanced retention.
Uses of surveyor
- Surveying the diagnostic cast.
- Recontouring abutment teeth on the diagnostic cast.
- Contouring wax patterns.
- Aids in placing guide planes.
- Measuring depth of undercut on abutment tooth.
- Surveying ceramic veneer crowns.
- Placing intracoronal retainers.
- Placing internal rests.
- Machining cast restorations.
- Surveying and blocking out the master cast.
Aims and objectives of surveying
- To determine the path of placement and path of removal.Path of placement: It is the direction in which a restoration moves from the point of initial contact of its rigid parts with the supporting teeth to its terminal resting position, with rests seated and the denture base in contact with the tissues.Path of removal: It is the direction of restoration movement from its terminal resting position to the last contact of its rigid parts with the supporting teeth.
- To survey proximal tooth surfaces adjacent to edentulous areas for the placement of guide planes to achieve a parallel path of insertion.
- To locate and measure undercut areas of the abutment teeth for direct retention.
- To determine soft and hard tissue areas of interference.
- To determine the path of placement and to locate placement of retainers and artificial teeth aesthetically.
- To evaluate and chart mouth preparation procedures as preparation of proximal tooth surfaces for guiding planes, reduce excessive tooth contours, and to eliminate interference in hard and soft tissues.The areas to be relieved are outlined in red on the diagnostic cast. A trial procedure is done on diagnostic cast with the aid of surveyor so that mouth preparations can be done accurately.
- To mark the height of contour on abutment teeth, by which the exact position of the retentive terminal can be identified. The location of height of contour can also help in the accurate placement of rigid connectors, nonretentive reciprocal and stabilizing arms.
- Helps in tripoding the cast, thus the same path of insertion can be achieved every time. This can be used for future reference also. Tripoding is done by locating three dots or parallel lines on the cast, in the horizontal plane in relation to the vertical arm of the surveyor.
- Contouring wax patterns: Is done during the phase of mouth preparation to maintain the proposed path of placement, which can be copied on to the cast restorations for abutment teeth.
- Guiding planes on all proximal surfaces of wax patterns adjacent to edentulous areas can be made parallel to the proposed path of placement.
- Surveying ceramic veneer crowns:Ceramic veneer crowns wax patterns are surveyed for locating the path of placement, if extra coronal direct retainers have to be placed on them.Before the final glaze is accomplished, the abutment crowns should be returned to the surveyor to ensure the correct contour of the veneered portions or to locate those areas that need recontouring.
- Placement of intracoronal retainers (internal attachments)Factors in placement of attachments:
- A path of placement is selected in relation to the abutment teeth that will avoid areas of interference.
- A trial recesses is prepared on the diagnostic cast for estimating the proximity of the recess to the pulp. A resin guide of the recesses is prepared to aid the preparation in the mouth.
- Recesses can be carved in wax patterns, to place internal attachment, or recesses can be prepared in castings with the hand-piece holder.
- Keyway portion of the attachment is placed in the casting before investing and soldering parallel to the other keyways.
- Placement of internal rest seats.The surveyor may be used with a handpiece attached to the vertical arm by means of a handpiece holder. Internal rest seats may be carved in the wax patterns or rest seat may be cut in the cast restoration with the handpiece. Preparing rest seat in the wax pattern is the best.An internal rest provides a positive occlusal support to prevent the rotational movement of denture base. It also provides horizontal stabilization.97Type of rest seat:In distal extension bases: Ball-and-socket, spoon-shaped occlusal, or non-interlocking rest is prepared.In tooth borne situations: Dovetailed or interlocking internal rest can be prepared.Tapered fissure burs are used to form the vertical walls and small round burs are used to cut recesses in the rest seat.
- Machining cast restorations:With handpiece holder attached, axial surfaces of cast and ceramic restorations can be refined by machining with a suitable cylindrical carborundum point. This aids in placing guide planes exactly parallel to each other.
- Surveying the master cast:
Objectives
- To select the path of placement following mouth preparations of guiding planes, retention, non-interference and aesthetics.
- To measure retentive areas and to locate retentive clasp terminals in proportion to the flexibility of the clasp arm being used.
- To locate areas of undesirable undercut which need to be blocked out.
- To contour blockout material parallel to the path of placement before duplication.
Surveying achieves
- A prosthesis that can be easily placed and removed by the patient.
- A prosthesis that is well retained.
- A prosthesis that is aesthetic.
Factors affecting path of insertion
- Retentive undercuts.
- Interferences in maxilla and mandible.
- Guiding planes.
- Aesthetics.
Retentive undercuts
Retentive undercuts should be present on the abutment teeth at horizontal tilt. (Position in which the occlusal surfaces of teeth are parallel to the surveying table).
If retentive undercuts are not present then it should be created by
- Tilting cast.
- Full crowns.
Ideal retentive under cuts: 0.010 inch either mesiobuccal or distobuccal (cast chrome alloy can be used in these retentive undercuts for 0.020 inch undercut (8 gauge wrought wire combination clasp can be used).
Modification of retentive undercut: A tilt can be given to the cast to achieve better retention.
Interferences
In maxilla
- Torus palatinus:Alter the design of major connector or go for surgical correction if needed.
- Bony undercuts:Common area is buccal to posterior teeth, anterior edentulous ridge.Tilt the cast to alter undercut if mild, else surgical intervention is required.
- Tipping of teeth:Buccal/LingualClasp design needs to be altered, one can tilt the cast if tipping is mild, in excessive tipping restorations need to be placed.
In mandible
- Lingual tori:Surgical correction required.
- Lingually inclined teeth:Labial bar major connector design or cast restorations on the inclined tooth to be placed.
- Undercuts:Area lingual to retromolar pad, mylohyoid ridge area.Relieving or surgical recontouring may be necessary.
Aesthetics
- Clasp arms should not be visible.
- Artificial tooth to be positioned properly.
Factors in aesthetics
- Locating the accurate placement of clasp after selecting the ideal tilt, if survey line is high.
- Positioning natural tooth after correcting all existing malocclusion and space closures by orthodontic movement, disking proximal surfaces or placing restorations.
- If osseous defects are present ridge augmentation procedures should be carried out before replacing tooth.
Guide planes
- Prepared on the proximal aspect of the abutment tooth adjacent to edentulous area.
- Should be in intimate contact with the minor connector.
- Help protect weak tooth.
- Can be prepared on enamel surface or wax patterns.
EXPLAIN THE STEP BY STEP PROCEDURE OF SURVEYING MASTER CAST IN RPD
Step-by-step procedures in surveying
- Surveying a diagnostic cast
- Final path of placement.
- Outlining design on diagnostic cast.
- Tripoding the cast.
- Surveying master cast.
- Measuring retentive under cuts.
- Out lining design on master cast with graphite pencil.
Surveying a diagnostic cast
- Placement of cast: Cast is positioned on the surveyor table with the occlusal surfaces of the teeth parallel to the platform.
- Relative parallelism of proximal tooth surfaces are determined:
- If parallelism is present in horizontal plane it is not needed to tilt cast to achieve parallelism.
- If proximal surfaces are not in a parallel relation an anteroposterior tilt or lateral tilt of the cast is required.
- In case tilting of the cast has not achieved the required parallelism guiding planes have to be prepared.
- If the analysing rod contacts proximal surface at the marginal ridge only, recontouring can establish a guiding plane. (Recontouring areas are outlined in red).
- If the analysing rod contacts proximal surface at the cervical area only of the abutment tooth a cast restoration is the only means of establishing a guiding plane.
- Retentive areas:
- The surveyor blade is made to contact buccal and lingual surfaces of abutment teeth to evaluate the amount of retention existing below their height of convexity.
- Light source passed between the surveyor blade and the apical portion of the tooth surface confirms the amount of retentive undercut.
- If no sufficient under cut is present in the abutment tooth then a lateral tilt can be selected.
- While selecting a lateral tilt it should not disturb the earlier established anteroposterior tilt.
- The final tilt selected should maintain parallel guiding planes and also provide for acceptable retention.
- Interference:Mandibular arch:
- To check lingual surfaces for bony prominences and lingually inclined premolar teeth if lingual bar major connector is the choice.
- If bilateral interference is present surgery or recontouring of lingual tooth surfaces, is required.
- If it is only unilateral, a change in the lateral tilt can avoid the interference. This tilt should be in harmony with retentive undercuts and guide planes.
- If not then the interference should be removed by means of contouring or restorations to the tilted tooth.
Maxillary arch:- Interferences are usually found on buccally inclined posterior teeth and bony areas on the buccal aspect of edentulous spaces.
- Other areas are interference on the surfaces of abutment teeth that will be crossed by minor connectors and clasp arms as distal line angles of premolar abutment teeth and the mesial line angles of molar abutments.
- These interferences can either be blocked out, altered by tilt, clasp design can be selected depending on the interference, recontoured or cast restorations can be placed on the tooth with interference.
- Nonretentive and stabilizing clasp arms are best located between the middle third and gingival third of the crown rather than the occlusal third.
- Aesthetics:The path of placement determines the location of clasps and position of artificial teeth.Depending on aesthetics the clasp designs can be modified as:
- Gingivally approaching bar clasp.
- Slightly placing the circumferential clasp arms cervically.
- Tapered wrought-wire retentive clasp arms.
Final path of placement
The final path of placement is the anteroposterior and lateral position of the cast, in relation to the vertical arm of the surveyor, which satisfies the guiding planes, retention, interference and aesthetics.
Marking the proposed design on diagnostic cast
The components selected are outlined on the diagnostic cast as major connectors, minor connectors, location of rest seat, direct retainers, indirect retainers and saddles.
Mouth preparations
- Areas to be altered as undercuts.
- Areas of remaining teeth to be modified for:
- Guide planes.
- Preparation of rest seats.
- All the areas to be modified are outlined in red on the diagnostic cast. The rest seats are shaded in red pencil.
- Trial mouth preparations as guide planes and rest seats can be done on the diagnostic cast. This helps to orient for accurate preparations in the patients mouth. (The surveyor blade can be used to do the trial preparations so that the angle at which the handpiece must be held to prepare the amount of tooth in the mouth is identified. The cut surface on the stone tooth is again outlined in red pencil).
Tripoding the cast
Objectives
- Helps to return working cast to the surveyor in the same relation as the proposed path of placement for shaping wax patterns, trimming blockout on the master cast, or locating clasp arms.
- For future reference.
Methods of tripoding
First method
- Three widely divergent dots on the tissue surface of the cast are placed with the tip of a carbon marker, having the vertical arm of the surveyor in a locked position.
- Then the dots are encircled with a coloured pencil for easy identification.
- On returning the cast to the surveyor, it may be tilted until the tip of the surveyor blade again contacts the three dots in the same plane. This orients the cast to the original position of the cast.
- Tiny pits can be formed in the cast instead of markings hence transferring this relationship to the refractory cast.
Second method:
- Cast can be scored on two sides and the dorsal aspect of the base of the cast with a sharp instrument held against the surveyor blade.
- Tilting the cast until all three lines are again parallel to the surveyor blade orients the cast to the original cast position.
Surveying the master cast
- The master cast is surveyed as a new cast; the already prepared proximal guiding planes will indicate the correct anteroposterior tilt.
- The lateral tilt will provide equal retentive areas on all principal abutments in relation to the planned clasp design.
- The type of clasp should be planned depending on the undercut area, factors of flexibility, etc.
- If any further undercuts are observed during the designing of clasp it should be blocked out.
Steps
- Tripoding is done for master cast.
- Measuring retention:Undercuts are measured with an undercut gauge, provided with the surveyor. The amount of undercut is measured in hundredths of an inch.Under gauges are of three types – 0.01, 0.02, 0.03.
- 0.01 inch – retention by cast retainers,
- 0.02 inch – retention by tapered wrought-wire,
- 0.03 is rarely used.
- The amount of retention can be verified by a source of light, which forms a triangle bounded by the surface of the abutment tooth on one side and the surveyor blade on the other side.Retention is determined by:
- The magnitude of the angle of cervical convergence.
- The depth at which the clasp terminal is placed in the angle and
- The flexibility of the clasp arm.
- Final design is drawn on the master cast:Fine, graphite crayon pencil is used to out line the design. The graphite crayon pencil does not erase off or distort during duplication procedures.
- Design transfer:Is done with specific colour codes as
- Brown for metal contours.
- Blue for resin outline.
- Red for relief areas.
- Black for the height of contour.
After the final design is drawn the dentist sends the following items to the laboratory:
- Properly prepared and articulated master cast.
- Diagnostic cast with specific design drawn on it.
- Work authorization order.
DRAW A DIAGRAM OF NEY SURVEYOR. LABEL ITS PARTS. LIST FUNCTION OF TOOLS IN SURVEYOR. DESCRIBE HOW RETENTIVE UNDERCUTS ARE LOCATED USING A MODEL SURVEYOR AND HOW THEY AFFECT THE PATH OF INSERTION
Parts of the Ney surveyor are:
- Platform on which the cast holder is moved.
- Vertical arm that supports the superstructure.
- Horizontal arm from which the surveying tool suspends.
- Surveying arm to which the special tools are attached.
- Table to which the cast is attached (Cast holder or surveying table which permits movement in horizontal planes by ball and socket action).
- Mandrel for holding special tools.
Tools in surveying
- Analysing rod or paralleling tool.
- Carbon rod or marker.
- Undercut gauges.
- Wax knife.
Analysing rod
- This tool is used to check the parallelism of surfaces of tooth.
- Check undercuts in soft and hard tissues.
- Check height of contour.
Carbon marker: A carbon marker replaces analysing rod to mark:
- Survey lines.
- Other area to be modified located by analysing rod.
Undercut gauges: Are used to identify specific amount and location of undercut on abutment tooth.
Undercut gauges are of three types – 0.01, 0.02, 0.03.
- 0.01 inch – retention by cast retainers
- 0.02 inch – retention by tapered wrought-wire.
- 0.03 is rarely used.
The amount of retention can be verified by a source of light, which forms a triangle bounded by the surface of the abutment tooth on one side and the surveyor blade on the other side.
Wax knife
- Used to eliminate or block out undercuts.
- To carve waxed up cast restorations for guide plane placement.
- Locating path of placement.
Surveying a diagnostic cast
Placement of cast
Cast is positioned on the surveyor table with the occlusal surfaces of the teeth parallel to the platform.
Relative parallelism of proximal tooth surfaces are determined
- If parallelism is present in horizontal plane, it is not needed to tilt cast to achieve parallelism.
- If proximal surfaces are not in a parallel relation, an anteroposterior tilt or lateral tilt of the cast is required.
- In case tilting of the cast has not achieved the required parallelism, guiding planes have to be prepared.
- If the analysing rod contacts proximal surface at the marginal ridge only, recontouring can establish a guiding plane. (Recontouring areas are outlined in red).
- If the analysing rod contacts proximal surface at the cervical area only of the abutment tooth a cast restoration is the only means of establishing a guiding plane.
Retentive areas
- The surveyor blade is made to contact buccal and lingual surfaces of abutment teeth to evaluate the amount of retention existing below their height of convexity. The apical angle is called the angle of cervical convergence.
- Light source passed between the surveyor blade and the apical portion of the tooth surface confirms the amount of retentive undercut.
- If no sufficient under cut is present in the abutment tooth then a lateral tilt can be selected.
- While selecting a lateral tilt it should not disturb the earlier established anteroposterior tilt.
- The final tilt selected should maintain parallel guiding planes and also provide for acceptable retention.
- The same is established on master cast.
Surveying a cast for direct retention
Factors determining the amount of clasp retention
- Size of the angle of cervical convergence.
- Placement of clasp terminal in relation to angle of cervical convergence.
- Flexibility of the clasp arm.
Factors in retention related to abutment tooth
- Height of contour cervical to which the surface converges. A line is marked with a carbon marker at the greatest circumference of abutment tooth. This line was referred to by Kennedy as the height of contour.Cummer used it as a guide in the placement of retentive and non-retentive clasps.DeVan divided the line into two as suprabulge, above the line and infrabulge, below the line.Ideal placement of clasp arms:Areas cervical to the height of contour are used for the placement of retentive clasp arms and areas occlusal to the height of contour is used for the placement of non-retentive reciprocating or stabilizing components.Flexible components as clasp tips are to be placed gingivally to the height of contour and rigid components above height of contour.If these guide lines are not achieved when the cast is placed at horizontal plane than only the cast should be tilted to find uniform retentive areas.This new path of placement should also accommodate all factors as absence of undercuts, the location of vertical minor connectors, the origin of bar clasp arms and the denture bases. The clasp should have only a passive relationship to the teeth except when a dislodging force is applied.
- Guide planes prepared aids in retention and easy placement and removal of prosthesis too. Relative uniformity of retention.
Factors affecting flexibility of clasp arms
- Length of clasp arm.
- The longer the clasp arm, the more flexible it will be.
- The length of a circumferential clasp arm is measured from the point at which a uniform taper begins.
- The retentive circumferential clasp arm should be tapered uniformly from its point of origin.
- The length of a bar clasp arm is measured from the point at which a uniform taper begins at its point of origin from a metal base.
- A bar clasp arm though longer than a circumferential clasp arm has less flexibility because of its half-round form.
- Diameter of clasp arm.
- The greater the diameter of a clasp arm, the less flexible it is if all other factors are equal.
- The taper should be uniform, for the clasp to function properly.
- Cross-sectional form of clasp arm.
- Half-round form only flexes in one direction where as the round form is flexible in all directions.
- Cast clasps are half round in form- hence less flexible and more used in tooth borne situations.
- A retentive clasp arm on an abutment adjacent to a distal extension base requires a clasp with universal flexibility.
- Material used for clasp arm. Material flexibility is proportionate to their bulk. Cast gold partial dentures need bulk to be rigid where as chromium alloys have greater rigidity with less bulk.
Reciprocal-stabilizing cast clasp arm
A reciprocal-stabilizing clasp arm should be rigid.
Its average diameter must be greater than the average diameter of the opposing retentive arm.
Criteria for selecting a given clasp design
- Proper diagnosis and treatment plan should be established.
- Depends on the classification as tooth supported or tissue supported.
- Depends on the amount of retentive undercut and if mesiobuccal or distobuccal.
- Depends on aesthetics.
- Function and limitations of clasp designs must be evaluated.
- Clasp design must be biologically and mechanically acceptable.
- Clasp design should perform the specific functions of support, stabilization, reciprocation and retention.
EXPLAIN THE THREE BASIC THEORIES BY WHICH FORCES ACTING ON RPD ARE DISTRIBUTED BETWEEN SOFT TISSUE AND TEETH
In a Class III arch which is tooth supported, a single impression can record the teeth and soft tissue. As the edentulous ridge does not offer support, it can be recorded in its anatomic form. As rotational forces do not occur, indirect retention or flexible direct retention is not required. Hence a Class III design is the simplest of design.
The problems encountered are in the Classes I and II arches and to some extent in the Class IV arch.
Many methods and impression techniques are present for equalizing support from the edentulous ridge and the remaining teeth.
These different methods gave rise to various design philosophies.
Three basic philosophies, which are supposed to distribute the forces acting on a partial denture between the soft tissue and the teeth, are:
- Stress equalization.
- Physiologic basing.
- Broad stress distribution.
Stress equalization / stress-directing
Philosophy
The resiliency of the tooth secured by periodontal ligament in an apical direction is not comparable to the greater resiliency and displacability of the mucosa covering the edentulous ridge. This difference in the amount of displacability between tooth and tissue causes the forces to be transmitted to the abutment teeth as the denture bases are displaced in function.
Rectification
This school of thought believed that the rigid connection between the denture bases and the direct retainer on the abutment teeth is damaging and if some type of stress director or stress equalizer is used it can counter this harmful effect.
Types of stress equalizers
- Movable joint between the direct retainer and the denture base.
- Flexible connection between the direct retainer and the denture base.
Movable joint: In the movable joint type falls the hinges, sleeves and cylinders and ball-and-socket devices.
They permit both vertical movement and hinge action of the distal extension base, which prevent direct transmission of tipping forces to the abutment teeth. (Dalbo attachment and the Crismani attachment).
These attachments are available as:
- Prefabricated (More lasting and accurate and are available in chrome).
- Custom made.
Flexible connection: This category includes wrought-wire connectors, divided major connectors and other flexible devices permitting movement of the distal extension base.112
Advantages
- The stress equalizer design needs only minimal direct retention.
- Equalizes forces between tooth and edentulous ridge. (Sum of its resiliency and that of the periodontal ligament is equal to the resiliency of the mucosa).
- Used in distal extension bases.
- As it prevents the tipping forces on the tooth, there is less bone resorption about the tooth.
- Reduces chances of relining or rebasing as intermittent pressure against the mucosa causes stimulating effect on the underlying bone and soft tissue which reduces chances of bone resorption.
Disadvantages
- Fragile.
- Construction is complex.
- Expensive.
- Difficult to repair.
- Some amount of lateral forces is transmitted to ridge, which can result in resorption and settling of denture.
- If thickness of metal in the hinge is used to prevent lateral movement, the prosthesis becomes bulky and annoying to the patient.
- This design has the fewest advocates.
Physiologic basing
Philosophy
(Is same as for stress director but rectification is different).
The resiliency of the tooth secured by periodontal ligament in an apical direction is not comparable to the greater resiliency and displacability of the mucosa covering the edentulous ridge.
This difference in the amount of displacability between tooth and tissue causes the forces to be transmitted to the abutment teeth as the denture bases are displaced in function.113
Rectification
The equalization is accomplished by physiologic basing, or lining, of the denture base.
Method
The physiologic basing is produced by displacing or depressing the ridge mucosa during the impression-making procedure and copying the tooth in its anatomic form, or by relining the denture base after it has been constructed.
Impression techniques as McLean's, Hindel's, fluid wax functional impression and selective pressure impression all displace the mucosa during the impression procedure and records soft tissue in its functional form and tooth in its anatomic form.
Modifications in design
The artificial teeth of a removable partial denture constructed from a tissue-displacing impression will be positioned above the plane of occlusion when the denture is not in function.
To permit movement of the partial denture from the rest position to the functioning position, the direct retainers must be reduced in number and also designed with minimal retention.
Advantages
- The intermittent base movement has a stimulating effect on the underlying bone and soft tissue, which is likely to produce less resorption and hence chances of relining and rebasing are less.
- Design is simple with minimal retention requirements.
- Lightweight prosthesis.
- Minimal maintenance and repair.
- As there is only a passive placement of the clasps, forces transmitted to the abutment tooth are less.
Disadvantages
- Stability against lateral forces is less.
- As the artificial teeth are above the occlusal plane, premature contacts between the opposing teeth and the denture teeth are present which is uncomfortable for many patients.
- Indirect retention is less.
- Broad stress distribution:
- This school of partial denture design believes that excessive forces to the remaining teeth and residual ridge can be prevented by distributing forces over as many teeth and soft tissue area as available.
Broad stress distribution
The partial denture based on this philosophy uses additional rests, indirect retainers, clasps and broad coverage denture bases.
Advantages
- Uniform distribution of forces over a wide area.
- Decreased bone resorption.
- Lateral forces are better distributed than other designs.
- Less expensive.
- Less distortion.
- Chances of breakage are less.
- Good horizontal stabilisation.
- No frequent relining.
Disadvantages
- Maximum tooth and soft tissue coverage can cause tissue irritation,
- Increased bulk,
- Less patient comfort.
LIST VARIOUS PARTS OF RPD. EXPLAIN WITH DIAGRAM AND LIST FUNCTIONS OF EACH COMPONENT IN DETAIL.
Components of a typical removable partial denture are:
- Major connector.
- Minor connector.
- Rests.
- Direct retainers.
- Indirect retainers in case of distal extension bases.
- Saddles for replacement of teeth.
Figure 37: Components of RPD, 1. Major connector, 2. Minor connector, 3. Rests, 4. Direct retainers, 5. Tooth replaced on saddle framework
Functions of each component
Major connector
A major connector is the unit of the partial denture that connects the parts of the prosthesis located on one side of the arch with those on the opposite side. It is that unit of the partial denture to which all other parts are directly or indirectly attached.
Functions of major connector
- Major connectors must be rigid to transfer forces uniformly over the entire supporting structures.
- Prevents movement of the denture base by its rigidity.
- Aids the other components of the partial denture to function effectively.
Minor connectors
The primary function of a minor connector is to join clasps, rests, indirect retainers and denture bases to the major connector.
Four types of minor connectors
- That join the clasp assembly to the major connector.
- That join indirect retainers or auxiliary rests to major connector.
- That join the denture base to the major connector.
- That serve as an approach arm for a vertical projection or bar-type clasp.
Functions of minor connector
- Distributes the stresses that occur against certain components of the partial denture to other components hence minor connectors need to be rigid.
- Distributes forces, on to the ridge and the remaining teeth.
- Aids in maintaining proper function of other components.
Stabilizing components: Minor connectors that join the rests and the clasp assemblies to the major connector.
Function:
Stabilizes the denture against horizontal movement.
Rests
Are placed in the rest seats prepared on the occlusal surface of the abutment tooth.
Types
Occlusal rest, lingual rest and incisal rest.
Functions of rest seat
- Provides vertical support for a removable partial denture.
- Maintains components in their planned positions.
- Maintains established occlusal relationships by preventing settling of the denture.
- Prevents impingement of soft tissues.
- Directs and distributes occlusal loads to abutment teeth.Can act as indirect retainer (Cingulum/Incisal rest) in distal extension partial denture.
- Joined rest in spacing of tooth can prevent food impaction.
- Thus rests serve to support the position of a partial denture and to resist movement in a cervical direction.
Factors affecting function of rest seat
- Improper preparation of rest seat:Rest seat should be triangular shape with the apex toward the centre of the occlusal surface.The rest seat depth at marginal ridge of the abutment tooth must be at least 1.5 mm thick.The angle formed by the occlusal rest and the vertical minor connector from which it originates should be less than 90 degrees so that occlusal forces can be directed along the long axis of the abutment tooth.
- Fabrication defects causing unseating of rest seats.
- If sufficient space is not created to avoid interference while placement of rests.
Internal occlusal rests
Occlusal support is derived from the floor of the rest seat.
Horizontal stabilization is derived from the near-vertical walls.
The form of the rest should be parallel to the path of placement, tapered occlusally and slightly dovetailed to prevent dislodgement proximally.
Functions
- Occlusal support and horizontal stabilization.
- Permits location of the rest seat in a more favourable position in relation to the “tipping” axis of the abutment.
- Retention is provided by a lingual clasp arm.
Direct retainers
Definition
A direct retainer is that component that engages an abutment tooth and in doing so resists dislodging forces applied to a removable partial denture in a horizontal plane.
Classification
- Intracoronal retainers.
- Extra coronal retainers.
Extra coronal retainers are classified as
- Circumferential or Aker's clasp.
- Vertical projection or Roach clasp.
Functions of clasps
- Retention.
- Stability.
- Support.
- Reciprocation.
- Encirclement.
- Passivity.
Extra coronal direct retainers
Parts and specific function:
- Rest in clasp assembly gives vertical support.
- Minor connectors function is stabilization.
- Clasp arms functions are stabilization, retention, and reciprocation.
- The reciprocal clasp arm has three functions:
- Reciprocation against the action of the retentive arm. True reciprocation during placement and removal is possible only if guide planes are placed.
- The reciprocal clasp arm resist against horizontal forces.
- Can function as indirect retainer if placed anterior to the fulcrum line.
Effectiveness of clasps depend on:
- Depth of the undercut engaged.
- Cross-sectional shape, size, length, position of clasp and frictional resistance between the enamel and the clasp arm of the clasp.
- The modulus of elasticity of the alloy.
- Angle of the incline plane of the tooth surface.
- Mobility of the tooth.
Guiding planes:
- Provides for one path of placement and removal.
- Provide retention against dislodgement of the restoration.
- Provide stabilization against horizontal rotation of the denture.
- Eliminates food entrapment between abutment teeth and components of the denture.
Indirect retainers
An indirect retainer must be placed as far anteriorly from the fulcrum line as possible to prevent torquing or rotation of the denture.
A canine or premolar tooth is commonly used for placing indirect retainer in the form of cingulum rest or occlusal rest.
An incisal rest or a lingual rest as for lingual plate can also be used as indirect retention in cases of distal extension bases.
Saddles
These are parts of the denture, which replace the lost alveolar bone and the teeth.
If the saddle is tooth-supported at each end (bounded saddles) the area of the saddle is unimportant and can be as small as is practicable.
In the case of saddles without a posterior tooth (free-end saddles) support must come from the soft tissues and the alveolar bone, together with the tooth at the anterior end of the saddle hence should be of maximum coverage.
Components need to resist various forces and support the partial denture to prevent its displacement.120
Functions of components
In a bounded saddle, antero-posterior movement is resisted by the teeth themselves.
In the case of an anterior saddle, additional resistance is also obtained by clasps passing round the teeth.
In free-end saddle it is prevented by clasps and saddle along with maximum coverage of major connector. In the lower, the retromolar pad and in the upper, the tuberosity will also provide resistance to those forces moving the denture.
Rotational displacement In the case of the free-end saddle any force displacing the saddle will tend to cause rotation about tips of the direct retaining arms and these points, when joined by an imaginary line, form a rotational axis.
To prevent this rotation an indirect retainer is placed opposite to the fulcrum line.
For the indirect retainer to be effective the distance between indirect retainer and the fulcrum axis should be equal.
WHAT ARE VARIOUS FORCES ACTING ON RPD? ENUMERATE DIFFERENT METHODS IN WHICH STRESSES ARE CONTROLLED IN RPD
Forces acting on RPD can be divided as:
- Vertical forces.
- Lateral forces.
- Anteroposterior forces.
Factors influencing the forces
- Depending on the denture such as tooth-borne, mucosa-borne or tooth-and-mucosa-borne the type of forces alter.
- A tooth-borne denture will resist all types of loads better and provide the most efficient mastication.
- Depending on the number of natural teeth, more the natural teeth the better the distribution of forces.
- Depending on the opposing arch (If it is a denture opposing a RPD or natural teeth opposing RPD). If it is natural teeth opposing RPD the load applied to the denture will be at the maximum.
- There must be sufficient space for occlusal rests to be adequately seated to transmit the loads imposed on the denture effectively to the abutment teeth. The rest seat should always be prepared on the enamel.
- If the saddle areas are extensive, the design should be altered so that the loads applied to denture are transmitted via the soft tissues.
Vertical dislodging forces are
- Gravity, in upper dentures only.
- Sticky foods, both upper and lower dentures.
- The tongue.
How vertical loads can be controlled in different designs
Kennedy's Class I Mandibular arch with shallow ridge:
- Incorporate maximum denture base coverage (Lingual plate).
- Reduce the buccolingual width of the denture teeth.
- By not replacing the posterior most teeth if no opposing natural teeth occlusion.
- Placing adequate direct and indirect retainers on the remaining tooth.
Kennedy Class III type:
- The adjacent two teeth on either side of the edentulous space should be given occlusal rest to make it tooth supported. This can distribute the load uniformly. If any of the abutments are weak with bone resorption seen radiographically than the saddle need to be made mucosa borne.
- The palatal and cingular slopes of canines form an inclined plane, causing the vertical load applied through a rest to incline in an angle to the palatal surface instead of transmitting the force along its long axis. This situation also needs a mucosa borne denture.
Kennedy's Class II design with two modification spaces:
- Here the distal extension area will be mucosa-borne while the modification spaces will be tooth borne.
- When a natural tooth is loaded vertically it intrudes into the alveolus by about 20 μm but mucosa loaded vertically displaces up to 500 μm this difference is not considered in the designing aspect, the abutment teeth can fail in the long run.This different behaviour of these different tissues has a profound effect on the design of connectors between saddles of partial dentures.
What ever may be the situation a tooth-borne denture is better in resisting vertical loads than a mucosa-borne design.
In such situations mucosal borne dentures with stress breaking principle can be used.
Lateral forces
These forces or loads are imposed during lateral movements of the mandible and also during normal mastication; they vary depending on the tooth form, tooth area and the design of the occlusion.
Great lateral loads present
- When cuspal interference exists.
- When occlusion is not proper.
Lateral loads are resisted by
- Lingual and palatal surfaces of the teeth.
- Buccal surfaces, if clasps incorporating bracing elements in the functional arms are fitted.
- The lingual, palatal and buccal surfaces of the ridges.
Designing to resist lateral loads
Kennedy Class I upper with well-formed ridges, deep sulci and a high, broad palate.
- Well-developed ridges of the saddle areas could provide good resistance to lateral movement of a denture. If the remaining teeth can also efficiently made to withstand lateral loads by placing occlusal rest the design is ideal.
- A mucosa borne denture with stress breaking principle can be used.
Kennedy Class I upper with poor, atrophic ridges, shallow sulci and a broad, flat palate.
- Here the ridges are unable to withstand lateral loads so the only option is to make the denture with more of mucosa support with all the palatal surfaces of the teeth covered by the denture to brace it against lateral movement.
- Additional bracing is also obtained from the rigid parts of the clasp arms.
Kennedy Class III modification 1 case with sound tooth:
- The occlusal load is supported by rests.
- Lateral loads can be accepted by these teeth and resistance to lateral movement can be enhanced by direct retainers and reciprocals on the buccal and palatal aspects.
- The occlusal rests prepared on teeth also provide resistance to lateral loads.
- Here the design is tooth borne.
Kennedy Class III modification 1 with weak abutment teeth which are periodontally involved:
- The design should be mucosa borne with the palatal aspects of all the standing teeth covered to gain sufficient resistance.
- In case of modification spaces with isolated tooth, designing should include guiding surfaces or guide planes on the proximal and lingual aspects of the abutment teeth, which effectively prevents rotational movements of dentures.
Anteroposterior forces
These, like lateral loads, are best resisted by the remaining natural teeth, resisted by the soft tissues via a large labial flange or an extension of a lower denture on to the anterior slope of the ascending ramus.
Groups of teeth provide the most satisfactory resistance to anteroposterior loads applied to a denture.
Kennedy Class III modification 1:
- The resistance to anterior movement of this denture is provided by the distal surfaces of the canines, reinforced by the four incisors. This group of teeth are capable of resisting all the anterior loads.
- Posteriorly-directed loads are similarly resisted by the mesial surfaces of the first molars, which are buttressed by the second molars. In this case anteroposterior type of movement is well resisted.
Kennedy Class I free-end saddle mandibular dentures:
- Resistance to anterior movement of the saddles is provided by the distal surfaces of the canines, supported by incisors.
- The resistance to posterior movement is achieved by carrying the distal extension of the saddles as high up the ascending rami.
- In addition, flexible clasps are fitted to the canines with bracing action, to provide additional resistance to posterior movement.
- In Kennedy Class I maxillary dentures a well-defined tuberosity and hamular notch both resist backward movement but if either of these is absent, then buccal arms on the canines may be necessary.
- The distal surfaces of the canine teeth and the anterior slope of the palate provide adequate resistance to anterior movement.
The Kennedy Class II case:
- For resistance to posterior movement bracing is designed by carrying a rigid connector to the other side of the mouth and firmly clasping the teeth on that side.
- The canine on the side of the free-end saddle is also clasped and the denture carried round it and up its lingual surface and the posterior aspect of the saddle is carried on to the ascending ramus, adequate resistance to posterior movement can be provided.
- The carrying of sublingual bar, or lingual plate, to the opposite side of the mouth provides more resistance to lateral movements.
Kennedy Class IV cases:
- Posterior movement is resisted by a labial flange on the mesial surfaces of the upper canines while the anterior movement of the denture can be resisted by the slope of the palate and by carrying the denture around the palatal aspects of the canines.
- More resistance can be gained by clasping the last molar teeth, which are situated some distance from the fulcrum.
Factors which help to improve retention and there by enhance force distribution:
- The use of clasps (direct retainers).
- Flexibility of clasps.
- The use of undercuts existing naturally or by using restorations.
- Guiding surfaces (or guide planes).
- Adhesion and cohesion, increased in mucosa-borne dentures by covering as large an area as possible.
- Indirect retainers.
- Arranging teeth in proper occlusion.
Harmful effects of wrongly designed mucosa-borne dentures
- If maximum coverage is not utilised, it overloads the area which is covered.
- If insufficient lateral bracing against the natural teeth or slopes of the ridges, are present it displaces the denture and causes trauma of the mucosa by friction.
- By fitting a denture which accepts the first occlusal contact on closing, the entire load is taken on the denture but little or none on the natural teeth this overloads the mucosa and underlying alveolar bone. This result in resorption of the alveolar ridge leading to an inaccurate fit of the denture base.
- Gross cuspal interference and locking of the occlusion resulting in the denture being dragged across the mucosa.
EXPLAIN STEPS IN LABORATORY PROCEDURE FOR FRAMEWORK CONSTRUCTION OF CAST PARTIAL DENTURE
Laboratory procedures
Needed items for construction of the removable partial denture
- Properly prepared and articulated master cast.
- Diagnostic cast with specific design drawn on it.
- Work authorization order.
Transfer of design from diagnostic cast to master cast
Design transfer is to convey the outline of the proposed prosthesis from the diagnostic cast to master cast.
Re-tripoding the master cast
When the three marks are in the same horizontal plane as that of the diagnostic cast, the table is securely locked with the re-tripoded master cast.
Height of contour
A carbon marker is used to place the height of contour line on involved teeth and soft tissue areas on the re-tripoded cast.
Design transfer
This is done with specific colour codes as:
- Brown for metal contours.
- Blue for resin outline.
- Red for relief areas.
- Black for the height of contour.
Block out and Relief
Block out: Is elimination of undesirable undercut areas on the cast to be used in the fabrication of the removable partial denture.
Relief: Is the procedure of placing a sheet of wax in strategic areas on a master cast to be duplicated so that a refractory cast can be made.
Done on both hard and soft tissues to achieve desired path of insertion.
Cast preparation
Beading the outline of major connector on the cast
Beading: Is scoring a cast with a sharp instrument or bur in any desired area to provide a seal between the finished prosthesis and the soft tissue.
- The outline of the major connector will be scraped on the maxillary cast to the depth of half of the diameter of a No. 2 round bur. This ensures a positive contact against the palatal tissues and prevents food packing beneath the major connector.
- Beading is not done on the mandibular major connector.
Application of model spray on the cast
- Model spray deposits a film on the cast that protects and seals the design throughout the blockout and duplication process.
- Done after the design is transferred.
- The sprayed cast is dried for 5 minutes.
Blockout technique
Composition of block out wax:
Technique
Melted wax is applied with a spatula. Blockout wax should not be placed in clasp tip areas, guiding planes and above the contour line.
Types of block out
- Tapered / Shaped / Formed Blockout.
- Parallel Blockout.
- Arbitrary Blockout.
Tapered / Shaped / Formed blockout
- In case of tapered blockout the blade-like device used will be tapered (usually 2 to 4 degrees) or can be positioned to provide a range of taper by placing the cast on a specific area of the base.
- Tooth-tissue-borne dentures can be given a tapered blockout to allow freedom of movement in function.
Parallel blockout
- The blade surface will normally be perpendicular to the surveyor base.
- All tooth-borne partial dentures are blocked out parallel to the path.
Arbitrary blockout
- Areas of undercut not involved with the framework should be blocked out as to prevent distortion during duplication.
- Areas of gross soft tissue undercuts are blocked out with soft wax or clay.
- To save finishing time a very small amount of wax can be flown over the major connector area.
- When infrabulge clasp arms are used, soft tissue undercuts in the area of the approach arm are blocked out.
Wax contouring
- Excess wax is removed from beneath the height of contour and shaped using a warmed blade.
- The blockout wax in the clasp tip area is contoured with hand instruments to provide a slight ledge apical to the clasp tip.
- This ledge helps in the placement of the wax pattern at the correct position in the undercut area.
Relief
- A 1 mm thickness of the relief wax is placed under the retentive meshwork for acrylic resin to flow on the cast with a hot spatula. A small square wax of 2 mm is removed with a blade held at 90 degrees to the cast for a tissue stop.
- One margin of the relief wax is used to develop a sharp, definite internal finish line of the framework. The finish line must have a uniform depth of at least 1 mm and be no closer than 2 mm from the abutment teeth.
- The blade is held at 90 degrees to the surface of the cast to develop a internal finish line that is sharp with the metal-resin junction at right angles.
Laboratory procedures
Sprue guide placement
A small metal, resin, or wax tapered cylinder is placed on the master cast in the exact position where the main sprue will be placed on the refractory cast.
Duplication
Impression
- The blocked out master cast is placed on the base of a duplicating flask.
- The duplicating flask has a metal base on which the master cast is fixed. The metal base aids in cooling of the agar.
- Agar at the proper temperature is poured into the flask. This flask is cooled in a regulated cooling tank for an hour.
- The flask is then disassembled and the master cast is removed.
Refractory cast
- Gypsum-bonded investments, are poured into the mould for Type IV/ gold partial denture.
- Phosphate bonded investments are used for chrome-cobalt alloy.
- After the refractory material sets the cast is removed and placed in a drying oven at 93°C.
- When dry, the cast is trimmed to within 6 mm of the proposed design.
Beeswax dip: The cast is dried in a hot air oven at 82 to 93°C for 1 hour, dipped into beeswax at 138 to 149°C for 15 seconds and set immediately on end to allow all the excess wax to run off.
Transfer of design to the refractory cast
Placement of wax pattern design on refractory cast: For retentive clasp arms, patterns with a width/thickness 2:1 is to be used.
For reciprocal clasps, patterns that are roughly square (width / thickness ratio 1:1) can be used.
Waxing technique
- Gluing the pattern to cast with a mixture of acetone and plastic pattern scraps mixed to a watery consistency.
- This glue is painted on the design outline with a fine brush and dried for a few seconds and the needed pattern is adapted.
- The prefabricated patterns adapted on the cast should not distort while removing it from the card or while placing it.
- The pattern is placed on the refractory cast with a modified tip of eraser that is wedged shaped.
- Once the plastic patterns are placed they must be joined together with wax (blue inlay wax).
- This wax is used to seal margins of the major connector, wax the minor connectors, rests and also reinforce the areas where the retentive meshwork joins the major connector.
Spruing the framework
Types:
- Single sprue (Ticonium)
- Multiple sprues (Gold castings and chrome-cobalt alloys).
Sprue size used should be uniform.
Auxiliary sprues: Are used when the framework is large, so that sufficient molten metal reaches the area. They should be one-third to one-fourth the dimension of the major sprues. The smaller sprue should be connected to the main sprue in a gentle curve.
Investing the refractory cast
Two-part mould:
- The refractory cast is dipped in slurry water to prevent water absorption from first paint on layer.
- Then a thin 3 to 4 mm layer of investment material is painted on the waxed refractory cast to ensure that no voids are present.
- After the first layer sets, an investment ring is placed to accommodate the refractory cast.
- The second part of investment material is mixed and placed in the ring.
Mould storage:
- Some alloy systems only place the mould in the burnout furnace. (Without the ring).
- Others burn out the mould with the ring in place.
- The sprue lead, of metal or plastic, is removed and the entrance is cleared of any debris or sharp edges. The moulds are normally stored in a plastic bag to keep them from completely drying out till it is placed in the burnout furnace.
Burn out
Furnaces: Either electric or gas with electronic adjustment of time and temperature according to the alloy manufacturer's specifications.132
Casting
Induction casting:
- Alloys are melted using gas and oxygen to 1093°C. Oxyacetylene mixture is available for castings requiring higher heat.
- The casting machine is set to the manufacturer's directions to include the revolutions per minute of the casting arm and the temperature required to start the machine.
- An uncontaminated crucible with the proper amount of alloy is melted by activating the alternating current.
- Once the alloy is reached the desired temperature, casting machines is set to cast. The alloy can be viewed through the protective lens of the casting machine.
Casting recovery
After the casting is complete, the mould is removed and allowed to cool. After adequate cooling, mould is broken and casting is retrieved.
The first layer of investment is removed by sand blasting. After examining for defects, finishing procedures are done.
Finishing the framework
- Sprue Removal: With high-speed lathes and large abrasive disks, the sprues are cut off.
- Rough Finishing and Shaping: The major connector is shaped and then rubber wheeled to a “satin” finish. The entire framework is contoured to its final finish.
- Minimum finishing and polishing of rests, retentive clasp tips, guiding plane and minor connectors are done.
Fitting the frame work
Fit checking: Silicones, disclosing wax or commercial dies are used to locate improper seating of framework on the master cast or on the duplicate cast and carefully finished with fine stones.133
Common areas requiring adjustment
- The area of the clasp tip undercuts passing the height of contour.
- Rest seat areas.
- Guide planes.
Final finishing
Rag and felt wheels are used on the high-speed lathe to apply the final polish.
Ultrasonic cleaning is commonly used to remove all traces of the polishing materials.
Sectioning and re-soldering the framework: If the framework does not fit the design well it can be sectioned and soldered later. Minor connectors and some major connectors can be sectioned and soldered with precious metal solder or non-precious brazing alloys with the electro-soldering device.
Adjusting occlusion: The seating of rest seat is ensured with an opposing cast and suitable jaw relation records.
Wrought wire retentive clasp arms
Common materials used- Precious alloys as gold, palladium, platinum, silver or non-precious alloys as stainless steel, nickel-chromium, nickel-chromium-cobalt.
Gauge: 17 to 20 gauge.
Attachment of wrought wire by
- Embedding the wire in the resin of the denture base.
- Including in the wax-up of the framework.
- Attaching the clasp by soldering it to the framework.
In the finished frame work:
- The surfaces of clasps and mandibular major connectors should be highly polished and free from pits and other irregularities.
- The taper of clasps, should be uniform and free from nicks and notches.
- The major connector should be rigid enough not to bend under finger pressure.
ENUMERATE STEPS IN CLINICAL PROCEDURE FOR FITTING THE FRAMEWORK OF A CAST PARTIAL DENTURE
Clinical procedures
Objective
- To fit the framework to the teeth.
- To adjust the framework to the opposing occlusion.
Steps in fitting frame work to the teeth
Use of disclosing media: To locate small areas interfering with the fit of framework.
Types of disclosing media used:
- Jeweller's rouge dissolved in chloroform and painted on the metal.
- Commercially available disclosing waxes.
Differentiate between:
- Normal wiping away of wax.
- A wax tear caused by its sticking to the tooth surface.
- The show-through of metal caused by pressure spot.
Advantages of commercial disclosing medium:
- Easier to use than Jeweller's rouge.
- It is three-dimensional.
- It will aid in locating the interference in the fit and the problem causing it.
- By the thickness of the remaining wax the amount of unseating can be evaluated.
- Helps in evaluating the overall fit of the framework.
Procedure of application:
- A thin even coat of melted disclosing wax is placed on the entire framework surfaces that will contact the teeth with the help of a roach carver.
- The framework is placed along the planned path of insertion and gentle finger pressure is applied.
- If resistance to seating is felt the framework is re-examined for distortion of a clasp arm (especially retentive clasp tips).
Correcting discrepancies:
- The framework should be removed from the mouth without damaging the surface of the disclosing wax.
- Evaluate the thickness of wax beneath occlusal rests or indirect retainers.
- Evaluate the inner surface of the framework, for “high spots,” or areas of metal show-through.
Common areas of metal show through are:
- Above the survey line on the teeth,
- Under rests,
- At the shoulder of circumferential clasps,
- Under embrasure clasps,
- In interproximal extensions of lingual plating,
- Guiding planes.
Correction procedure:
- The located areas of interference are relieved by grinding with a No. 2 round carbide bur in the high-speed handpiece.
- After relieving the areas, the disclosing wax is removed completely by holding the framework over a small flame and the melted wax is blown away with air syringe.
- After the wax is removed fresh wax is added.
- The entire procedure should be repeated until the framework is seated and when the disclosing wax is displaced evenly.
- The time required to seat a framework clinically is approximately 15 to 20 minutes.
- The framework should seat with is a sliding action ones the perfect fit is achieved.
- If gross errors are present repeating the entire framework is better.
Fitting frame work to opposing occlusion
Correction of interferences:
- Correction should be carried out outside the mouth.
- Heatless stones or diamond instruments in the high-speed handpiece are used.
Correcting interferencesin centric occlusion:
- After the framework has been fitted to the teeth, it must be adjusted to the opposing occlusion.
- During correction of occlusal discrepancies the vertical relationship of the jaws should not alter.
- If both upper and lower frame works are present, the correction should be done one at a time.
- Observe the patients maxillary and mandibular natural tooth relationship in centric occlusion.
- After placing both the framework in the patient's mouth, the relationship of tooth in centric occlusion should be the same as without framework placement.
- One framework is positioned in the mouth, with the patient occluding in centric occlusion. The amount of occlusal interference is evaluated by observing the amount of space between the natural teeth.
- Articulating paper is placed over the teeth on one side of the arch and the patient is asked to tap the teeth together with light vertical force.
- All interferences are corrected.
- The same procedure is repeated with the articulating paper on the opposite side of the arch.
Correcting interferences in eccentric positions:
- After the centric occlusion corrections are carried out, the patient is guided in both lateral and protrusive positions.
- Articulating paper is used to locate interferences in each of these positions.
- The eccentric interferences are corrected the same way as in centric occlusion.
- Each framework is corrected separately and later checked in the mouth together.
- After correcting all interferences, a final evaluation is made using articulating paper.
WHAT ARE THE FACTORS INFLUENCING MAGNITUDE OF STRESSES TRANSMITTED TO THE ABUTMENT TEETH?
Factors influencing magnitude of stresses transmitted to abutment teeth
- Length of edentulous span.
- Quality of ridge support.
- Qualities of clasp.
- Clasp design.
- Length of clasp.
- Material used in clasp construction.
- Abutment tooth surface.
- Occlusal harmony.
Length of span
A longer edentulous span transfers greater force to the abutment teeth. This happens because the denture base tends to displace the underlying mucosa and moves the partial denture in different positions. This constant movement during function results in larger forces being transmitted to the abutment tooth.
All distal extension denture bases have large edentulous spans depending on the number of missing tooth. The fulcrum line in distal extension partial dentures pass through the posterior most abutment tooth present. If the span of the distal extension denture base is longer from the fulcrum line then more force is exerted on the abutment tooth.
Distribution of force depends on:
- Load applied to the artificial teeth,
- Length of the lever arm,
- Coverage of denture base,
- Thickness of the mucosa over the ridge,
- Type of direct retainer used depending on the retentive undercut,
- Type of artificial tooth used (Eg. the buccolingual width of tooth etc).
If the posterior abutment tooth can be preserved as vertical support or as an overdenture partial denture abutment then force distribution can be efficiently controlled than when only the canines or first premolars are left as the posterior most abutment teeth.
Quality of ridge support
Large, well-formed ridges absorb greater amounts of stress than knife-edged ridges or shallow ridges.
Ideal ridge form:
- Broad ridges with parallel sides and mucoperiosteum with 1 mm thickness can withstand greater functional loads.
- Broad ridges permit maximum coverage of denture base, hence stabilizing the denture against lateral forces.
Ideal mucosa:
- Mucoperiosteum, which is firm with 1 mm thickness, can transfer the forces to the ridge.
- Thin atrophic mucosa or soft, flabby, displaceable tissue offers less vertical support and lateral stability to the prosthesis.
- The flabby tissue causes excessive movement of the denture, resulting in transmission of stress to the abutment tooth.
Qualities of clasp
- Flexible clasp arm transmits less force to the abutment tooth.
- In distal extension partial denture a combination or wrought wire retentive clasp is the ideal direct retainer (Class I or II partial dentures).
Disadvantages of combination clasp:
- Less resistance to horizontal stresses.
- Increased flexibility can result in both lateral and vertical stresses to be transmitted to the residual ridge instead of abutment tooth.
Clasp design
Factors influencing choice of flexible clasp:
- Whether the ridge or the abutment tooth requires the most protection.
- The periodontal support of the abutment tooth.
- If the periodontal condition is good a less flexible clasp such as a vertical projection clasp can be used.
- If the periodontal support is weak a more flexible clasp such as the combination clasp with a wrought wire retentive arm can be used so that the residual ridge can share the horizontal forces acting on the partial denture.
Factors influencing clasp design:
- A passive clasp transmits less stress to the abutment tooth.
- Clasp should be accurately placed on the abutment tooth for each of its components to function properly.
- The framework has to be fitted using a disclosing wax and should seat perfectly for the clasp to be passive.
- The retentive tip should lie in a 0.010 inch undercut.
- The reciprocal arm should contact the tooth before the retentive tip passes over the greatest bulge of the abutment tooth.
Length of clasp
- The more flexible the clasp, the less stress to the abutment tooth.
- Flexibility can be increased by lengthening the clasp.
- Doubling the length of a clasp will increase its flexibility five times.
- Clasp length can be increased by using a curved course on an abutment tooth.
Material used in clasp construction
- Chrome alloy clasps are less flexible than other clasp materials thereby greater stress is transmitted to the abutment tooth.
- To compensate for this property, clasp arms of chrome alloys are constructed with a smaller diameter.
Abutment tooth surface
- A restoration on the abutment tooth offers more frictional resistance to clasp arm movement than does the enamel surface of a tooth.
- Greater stress is exerted on a restored tooth than on natural tooth.
Occlusal harmony
- An improper occlusion generates horizontal forces destructive to both the abutment teeth and residual ridges.
- Partial denture opposing natural dentition exerts a greater force than one opposing a complete denture.
- The coverage of denture base also influences the amount of stress transmitted.
- If more load from occlusion is applied at the distal end of the denture base more stress is transmitted to the abutment tooth.
- The second premolar and first molar are the best areas for the application of masticating load.
- Artificial teeth should be arranged so that masticating force is applied in that area.
Controlling stress on abutments by design considerations
- Direct retentionClasp retention should be placed to a minimum, yet provide adequate retention to prevent dislodgement.
- Forces of adhesion and cohesionAdhesion is the attraction of saliva to the denture and the tissues and cohesion is the internal attraction of the molecules of saliva for each other.Atmospheric pressure contributes a slight amount of retention especially in maxillary complete palate major connector.
- Frictional controlGuide planes are created so that they are parallel to each other and parallel to the path of denture insertion. The frictional contact of the prosthesis against these parallel surfaces increases the retention of the denture.
- Neuromuscular controlIf the patient is able to control the action of the lips, cheeks and tongue with the denture, retention of a denture is improved.A properly contoured denture base aids the patient's neuromuscular control of the prosthesis.
- Clasp positionPosition of the retentive clasp to the height of contour is more important in retention and in controlling stress than is the number of clasps.
- Quadrilateral configuration: Indicated for Class III arches with modification space on the opposite side of the arch.A retentive clasp is positioned on each abutment tooth adjacent to the edentulous spaces, hence giving a quadrilateral configuration.
- Tripod configuration: For Class II arches with a modification space on the dentulous side. The teeth anterior and posterior to the space are clasped to form a tripod configuration.
- Bilateral configuration: Class I situation. Retentive clasp on each side of the arch on the posterior most abutments.
- Type of clasp
- Circumferential cast clasp: Originates from a distal occlusal rest on the terminal abutment tooth and engages a mesiobuccal retentive undercut.142Not be used on a distal extension removable partial denture.
- Bar clasp: Can be used on the terminal abutment tooth on a distal extension partial denture when the retentive undercut is on the distobuccal surface.It is never indicated when the tooth has a mesiobuccal undercut.
- Combination clasp: When a mesiobuccal undercut exists on an abutment tooth adjacent to a distal extension edentulous ridge and in distal extension bases.
- Splinting of abutment teethAdjacent teeth may be splinted by means of crowns to control stress transmitted to a weak abutment tooth.Splinting can be achieved by crowns, multiple clasps, lingual plating, swing-lock partial denture and cross arch stabilisation with major connectors.Splinting is also indicated when the abutment tooth has a tapered root or short roots.
- Indirect retentionAn indirect retainer is a part of the removable partial denture that prevents the direct retainer from displacement of the distal extension denture by resisting the rotational movement of the denture around the fulcrum line, established by the occlusal rests.
- OcclusionAn occlusion that is in harmony with the movements of both the temporomandibular joints and the neuromusculature will reduce the stress transferred to the abutment teeth and residual ridge.Occlusion must be coordinated with mandibular movements and the guiding influence of the remaining natural teeth. The occlusal contacts of the remaining natural teeth should be the same after the placement of removable partial denture in the mouth.
- Artificial teethThe number of teeth being replaced can also be reduced to decrease the stress.Artificial posterior teeth should possess sharp cutting surfaces.Artificial teeth should not have steep cuspal inclines.
- Maximum coverage of denture baseThe distal extension denture bases of mandibular should extend onto the retromolar pad area.The distal extension denture bases of maxillary should cover the entire tuberosity.Adaptation of the denture base to the residual ridge should be perfect.A dual impression of distal extension gives accurate precision.
- Major connectorLingual plate major connector with properly supported rests aid in the distribution of functional stresses to the remaining teeth.A complete palatal major connector of maxillary distributes forces better than other major connector designs.
- Minor connectorHelp all the partial denture components to function efficiently.
DISCUSS THE HARMFUL EFFECTS OF A WRONGLY DESIGNED RPD ON A PATIENT OF 40 YEARS
Causes of damage related to the wearing of wrongly designed partial dentures
- Plaque accumulation around partial denture.
- Direct trauma from components of the denture, from excessive functional forces.
- Denture-induced hyperplasia due to the transmission of excessive functional forces.
- Occlusal error dysfunction.
- Aggravation of existing periodontal disease.
Plaque accumulation
Causes decalcification and caries.
If plaque persists, it can lead to chronic periodontitis with the periodontal attachment destroyed, periodontal pocket and alveolar bone loss.
If plaque collects between the denture and the denture-bearing mucosa, denture stomatitis can occur.
Direct trauma from components
Examples of components that can cause trauma to tissues:
- The major connectors placed close to gingival margins,
- An active clasp arm,
- An occlusal rest that doesn't seat and
- Minimum coverage of denture base.
Symptoms
- Mobility of tooth.
- Wear of tooth.
- Periodontal changes.
- Excessive bone resorption.
Occlusal error dysfunction
- If the premature contact is on a natural tooth.
- If the saddle is overloaded, mucosal inflammation and resorption of the underlying bone occurs.
- Abnormal closing pattern as a result of faulty occlusion.
Ill-fitting prosthesis can be due to
- Improper treatment planning:The complete oral examination must include both clinical and radiographic evaluation of caries, condition of existing restorations, periodontal conditions, vitality of remaining teeth and existing occlusal relationship.
- Wrong designing:Failure to identify the tooth supported and tissue supported design principles as
- Prosthesis support.
- Impression methods required.
- Types of direct retainers best suited for each.
- Denture base material suited for each.
- Need for indirect retention.
- Eliminating the use of surveyor:It is difficult to plan mouth preparations without surveying. Mouth preparations enhance the longevity of the prosthesis providing better support, stabilization, retention and a harmonious occlusion for the partial denture.
- Eliminating guide planes when needed.
- Improper preparation of occlusal rest seats, wrong placement and lack of clearance for occlusal rest seat can result in failure of partial denture.
- Retentive areas must be identified correctly, placement of direct retainers in correct position, selecting the right direct retainer, selection of the material and amount of flexibility required for direct retainer are all important factors in the success of partial denture.
- If block out procedures are not carried out when required can also distort the framework.
- Precision in laboratory procedures is a must in partial denture which otherwise can lead to failure in the partial denture design.
- Checking the fit of framework procedure should be carried out accurately or the framework can cause destruction to the underlying bone.
Support for distal extension denture bases
If inadequate support is given due to wrong designing the partial denture tends to fail. A dual impression procedure needs to be done for the success of distal extension bases.146
Reasons for failure of clasp-retained partial dentures
- Inadequate mouth preparation and wrong planning of the design.
- Impression procedures carried out without normal tissue health.
- Incorrect use of clasp designs.
- Material of choice for clasps. (Cast clasps are rigid and if used in distal extension case can result in failure of partial denture).
- Material of choice for denture.
Materials that can be used:
- Acrylic resin.
- Gold alloys.
- Base metal alloys.
Factors to be ensured for successful clasp design
- Retentive undercuts:Gold alloy:undercut usually 0.75 mmCobalt-chromium:undercut usually 0.25 mm.Undercuts greater than 0.25 mm, wrought wire clasp is to be used.
- The retentive arm should be at some distance from the body or point of attachment.
- Usage of gingivally approaching clasps when needed as less lateral load to the abutment tooth.
- Do not adjust cobalt-chromium cast clasps.
- Clasps should taper more towards their tips as for the clasp arm to displace into a severe undercut when forces are applied.
- Clasps must be accurately positioned and accurately cast.
- Do not use rigid clasping system when distal extension partial dentures exist.
A successful partial denture will function and be comfortable to the patient.
Care should be taken from diagnosis and treatment planning to the final insertion of a partial denture.147
For longevity of partial dentures good oral hygiene measures, as well as fluoride therapy for abutment teeth has to be ensured.
Recall visits aid the dentist to check the fitness, function and oral hygiene of the patient.
Proper instructions and home care regime is a must.
DESPITE SHORTCOMINGS, ALGINATE IMPRESSION MATERIAL ENJOYS POPULARITY AMONG DENTISTS. DISCUSS. ADD A NOTE ON ITS SHORTCOMINGS AND ADVANTAGES.
Alginate is the most widely used impression material and is also the most versatile impression material. Its popularity is due to its various advantages.
Advantages
- Easy handling.
- Inexpensive yet reasonably accurate compared to other elastic impression materials.
- No additional items or equipments required.
- Cleanliness of the material.
- Can use different types depending on the need for fast setting or normal setting.
- Custom made tray is not required.
- Need no application of adhesive when perforated trays are used.
- Best for single impressions in partial dentures.
- In dual impression the functional impression can be picked up using alginate impression in a stock tray.
- Modified alginates have better surface reproduction.
- Colour coded alginates help in identifying the time required to complete an impression.
- Dust free alginates containing glycerine helps to prevent allergies.
- Disinfected alginates are also available.
All the above factors still favour alginate to be used as a material of choice in all type of impressions.148
Problems encountered with alginate impression making
- Alginate sticking to teeth.
- Voids in impression.
- Peripheral underextension.
- Alginate tears while removing impression.
- Grainy appearance.
- Alginate sets before impression is completed.
- Gag reflex while making impression.
- Alginate washed by saliva in palate.
- Alginate pulled away from the tray.
Problems encountered while pouring cast
- Cast has rough surface.
- Cast has chalky appearance.
- Cast has soft appearance.
- Breakage of cast.
- Separation of cast between first and second pours of stone.
- Voids in the cast.
- Underextension of cast.
- Inaccurate cast.
Problems encountered with alginate impression making
Alginate sticking to teeth
Reasons:
- Vigorous pumicing of teeth.Avoid impression making immediately after polishing tooth with pumice or silicone as protective coating for teeth.
- Excessively dry tooth.Before impression making for a dry field isolating the area with gauze packs can avoid over drying of tooth.
- Repeated impressions can wash away the protective coating of tooth.Use silicone as protective film before impression making.
Voids in impression
Reasons:
- Improper mix of alginate.Spatulation should be within 45 to 60 seconds by hand or 15 seconds mechanically under vacuum.
- A thick mix, loading the tray with less material or improper tray placement can cause deficiency of material in certain areas. The commonest defects occur in palate, vestibular areas and lingual aspect in mandibular.Avoid mix that is too thick or thin.Use correct water/powder ratio, measure alginate by weight, avoid using old packets of alginate that was exposed by heat or moisture contamination.
Peripheral underextension
Reasons:
- Improper mix:Less material loaded on to the tray.
- Improper selection of tray:Use tray with 5 to 7mm clearance.
- Incorrect tray placement:Centre tray with handle corresponding to mid line of face, seat tray below gingival marginal areas, select the correct tray size, which will not interfere with coronoid processes of mandible.
- Cheeks, lips, or floor of mouth trapped under tray.Check for entrapment by asking the patient to protrude the tongue.
Tearing of alginate while removing impression from patient's mouth
Reasons:
- Improper mix of alginate;Use correct water/powder.Measure alginate by weight.
-
Impression should be placed in the mouth 2 to 3 minutes after it loses its tackiness.
- Less amount of alginate:Select proper tray with good clearance.Centre tray properly.
- Use of deteriorated alginate:Store alginate in airtight containers at room temperature.
- Prolonged or insufficient spatulation:Spatulation time 45 to 60 seconds.
- Improper removal from mouth:Should be removed with a snap along long axis of tooth
Grainy appearance
Reasons:
- Prolonged or insufficient spatulation.
- Insufficient flow of material.
- Improper mixing:Use correct water/powder ratioMeasure by weight.
- Impression removed before setting time is complete.
Early setting of alginate
Reasons:
- When warm water is used to mix alginate it sets fast (Ideal temperature is 22°C)
- Contaminated mixing bowl with particles of dental stone. (Use different mixing bowls for alginate.).
- Prolonged spatulation of alginate.
- Use of deteriorated alginate.
- Fast-set alginate. Regular-set alginate has normal setting time.
Patient gags while impression is made
Distract patient's attention to other topics.
Reasons:
- Psychological.
- Too thin a mix.
- Excessive material loaded on to the tray, which can flow into patient's throat.
- Improper seating of patient.
- Over extended tray.
- Excessive pressure while making impression.
To prevent gagging:
- Instruct patient to keep eyes open and breathe through the nose at normal rate.
- Use of topical anaesthetics or cold water rinses.
- Leg lift procedure; along with use of fast-set alginate.
Alginate washed away by saliva in palate
Reasons:
- When mucous saliva is not removed from palate before impression making:Use astringent mouth rinse and cold water rinse.Wipe and isolate palate with gauze.
- Excessive palatal secretion:Use warm gauze to increase secretion followed by cold pads to constrict gland openings.
- Copious amounts of saliva:Premedicate with 15 mg of propantheline bromide 30 minutes before procedure.
Alginate pulled away from the tray
- Use rim lock perforated trays
- Can use alginate adhesives to enhance retention of alginate to tray.
Problems in relation to pouring of cast
Cast has rough surface
Reasons:
- Incompatibility between alginate and dental stone.
- Insufficient spatulation of stone.
- Saliva retained on impression.
- Water left on impression.
- Improper mix of alginate.
Chalky appearance of cast
Reasons:
- Incompatible alginate/stone combination.
- Stone slurry left on cast after trimming.
- Impression left in contact with cast for prolonged period.
Cast has a soft surface
Reasons:
- Watery stone mix – Use correct water powder ratio, measure stone by weight. In case if inverted single-stage pour technique.
- Use of contaminated stone.
- Prolonged spatulation of dental stone.
Breakage of cast
Reasons:
- Premature removal of impression from cast.
- Too much water in mix of stone.
- Use of single-stage pour technique.
- Water left in impression.
- Low compressive strength of dental stone.
- Alginate impression left in contact with cast overnight.
Separation of cast between first and second pours of stone
Reasons:
- Not placing mechanical retention for second pour.
- Failure to wet first pour before adding second pour.
Voids in surface of cast
Reasons:
- Air entrapment while mixing
- Cast poured too rapidly.
- Overvibration during pouring
Underextension of cast
Reasons:
- Cast over trimmed
- Dental stone did not flow into all peripheral areas of impression
- Underextension of alginate impression
- Prolonged setting time of stone
- Contamination of stone by heat or moisture
Inaccurate cast
Reasons:
- Syneresis or imbibition has altered the impression.
- Strains or distortion caused by movement during gelation.
- Impression removed before gelation was complete.
- Strains induced in impression during its removal from mouth.
- Non-rigid impression tray.
- Unsupported impression posteriorly.
- Washing or soaking cast in tap water caused details to be lost.
- When teeth contact tray during making of impression.
- Distortion in palate due to incorrect tray selection.
DEFINE IMMEDIATE DENTURE. CLASSIFY IMMEDIATE DENTURES. ADD A NOTE ON THEIR ADVANTAGES AND DISADVANTAGES. ENUMERATE THE CLINICAL STEPS IN RELATION TO IMMEDIATE DENTURES.
Definition
A denture constructed before the extraction of the teeth, which it replaces and fitted immediately after the teeth are extracted.
Types
Interim—Short-term prosthesis until treatment is feasible.
154Transitional in cases where the rest of the tooth are not restorable but the patient is not physiologically or psychologically fit for a permanent prosthesis.
Treatment prosthesis contains tissue conditioners or other treatment procedures to bring the tissues to normal condition.
Advantages
- The edentulous period is eliminated hence great social and psychological significance.
- A natural and functional relation ship of tooth can be obtained as the lip position, occlusal plane, vertical height and occlusion are not altered from the natural dentition.
- Facial contour and tone of facial muscles is maintained.
- No development time for unnatural mandibular movements.
- Size, shape, shade and position of teeth can be accurately reproduced.
- Speech is not hampered.
- No interference with the temporomandibular joint and its function.
- Function of mastication is maintained.
- No change in tongue shape.
- Little interference with diet.
- Resorption of alveolar bone is less.
- Healing is quickened.
Disadvantages
- In cases of deep vertical overlap, extreme rotation, tilting or migration it is difficult to achieve occlusion in the prosthesis.
- Require relining the denture after some time due to bone remodelling.
- General health may not permit multiple extractions.
- Additional expense and time.
- Only can be made for responsible patients with good oral hygiene.
- Gross oral sepsis is possible.
Types of immediate denture
- Open face design.
- Labial flange without alveolectomy.
- Labial flange with alveolectomy (Excision).
- Labial flange with alveolectomy (Incision).
Open face design
Advantages:
- Natural appearance.
- Exact reproduction of tooth position.
- Easy to insert.
- Teeth arrangement is easy.
- No interference with lip musculature.
Disadvantages:
- Poor retention and inadequate support.
- Denture has short life.
- Natural appearance is lost after bone remodelling starts.
- Difficult to rebase.
- Irregularities of anterior ridge as spiky alveolus or flabby mucosa can develop.
- Sockets and gingivae must not be traumatized during surgery.
Labial flange without alveolectomy
Advantages
- Good retention and support.
- Ease of rebasing.
- Rapid healing with smooth ridges.
- Stronger denture.
Disadvantages
- Labial fullness.
- Difficult in case of undercuts.
- Lack of space around necks of teeth.
Labial flange with alveolectomy (Excision)
Indications
- Prominent pre-maxilla, which prevents insertion of a flanged denture and osseous defects.
- Limited anterior interalveolar space and deep vertical overlap.
Contraindication: Increase in resorption following labial cortical bone removal.
Labial flange with alveolotomy (Incision)
A modification of the above where the operation of transeptal alveolectomy or intra septal alveolotomy is carried out which collapses the anterior buccal plate to eliminate the undercut.
Advantages:
- No cortical bone is removed and post-surgical resorption is reduced.
- Surgery is less traumatic than alveolectomy.
- Less interference with facial form.
Disadvantages:
- The undercut cannot always be completely eliminated.
- Bone is removed which can cause further resorption.
Post-immediate or delayed immediate dentures
Can replace posterior and anterior teeth at the same time.
In this technique trial dentures are prepared on casts with the teeth been cut off and the ridge is smoothed. After the extraction of teeth, the trial dentures are inserted in the mouth with a zinc oxide-eugenol paste or elastomeric impressions made in closed mouth position.
The dentures are processed and fitted the same day.
Clinical steps in relation to immediate partial denture
- Oral hygiene prophylaxis is carried out.
- Posterior teeth are extracted, with the exception of first four premolars.
- The extraction of the posterior teeth is carried out under local or general anaesthesia in two visits.During extraction the interdental and interradicular alveolus is smoothed with adequate suturing.
- The patient is recalled after six to eight weeks.
- Preliminary impressions are made in alginate and stock trays.
- Master impressions are made in alginate or elastomer. Or a dual impression technique is made.
- Record blocks are prepared and jaw relations are registered in retruded contact position.
- A facebow record of the maxillary cast is made.
- Shade of teeth is noted.
- The posterior teeth are set in balanced occlusion if possible.
- The partial wax dentures are tried in the mouth, the occlusion verified and the anterior teeth checked for shade, shape and form.
- Laboratory procedures for placement of teeth:
- Removing the plaster teeth and carving the cast after the partial wax dentures have been checked in the mouth.
- The alternate teeth are removed one at a time and the acrylic tooth set in position.
- The altering of cast to place tooth differs in each type:
- Open face design The plaster tooth is removed at the level of the gingival margin; the socket and the labial margins are deepened 1–2 mm leaving the margins intact.
- Labial flange without alveolectomy Sockets are not carved only the labial part of the gingival margin is scraped to allow for collapse of the soft gingival border after extraction.
- A duplicate cast is made with a clear acrylic base-plate that can act as a guide to the removal of bone.
- The dentures are processed, remounted and then finished in the laboratory.
- The patient is recalled for the extraction of the remaining anterior teeth.The finished denture is delivered with minimal adjustment.
- After three to five days the occlusion is checked and perfected by selective grinding.
- The patient is recalled in two to three months and rebase of the dentures are done if required or replacement of dentures can be done.