INTRODUCTION
The goal of implant therapy is to restore an individual to normal anatomy, function, aesthetics, comfort and speech regardless of the loss of bone due to disease or injury to the stomatognathic system. Based on the concepts of osseointegration described by Branemark more than 50 years ago implant dentistry has evolved extensively and has become an integral part in dental rehabilitation.1 Though it was developed primarily to rehabilitate fully edentulous patients, and it has gradually shifted to partially edentulous patients. Today single tooth replacement by implant has become the number one indication rather than fixed prosthodontics (bridge) because of increased risk of pulpal damage, secondary caries, factures of abutment teeth and periodontal disease (Figs. 1.1A and B). Today implants are no longer placed only in areas where adequate width and height of bone are available but due to the significant advances in bone augmentation procedures implants are placed wherever prosthesis are required.
Guided-bone regeneration with or without membranes and sinus floor elevation have become standards of care to correct bone deficiencies in other parts of the oral cavity. Improved osteophylic microstructured titanium implant surfaces such as the Plus Surface (Dentsply Sirona Implants) significantly reduce treatment time because of accelerated growth of bone.2
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These advances in implant therapy have made implants more predictable and attractive to patients. This has also led to more dentists placing implants in daily practice then ever before.
WHAT IS OSSEOINTEGRATION?
Osseointegration refers to incorporation of an inanimate metallic component into living bone. A successful osseointegrated implant is one in which there is a direct connection between ordered living bone and titanium. This attachment must be able to endure conditions of loading. There is no fibrous tissue intervening between the implant and bone, hence the osseointegrated implant is more akin to an ankylosed tooth root. The success of osseointegration has been proven beyond all doubts, however successful achievement of osseointegration depends on careful planning, meticulous surgical technique and skilful prosthetic management. It demands an appreciation of bone biology and wound healing in particular.
The prepared implant site is treated correctly as a wound in which tissue damage has to be minimized. The special characteristics of titanium particularly its resistance to corrosion and its biocompatibility are important. When these above criteria are met living bone recognizes titanium as its own and not as a foreign object.
Types of Dental Implants
Implants may be classified according to their position in the bone, constituent material, and their morphologic design.
Position in the bone
Subperiosteal Implant
Subperiosteal implant is a non-osseointegrated implant which rests on the surface of the bone below the mucoperiosteum (Fig. 1.2). Some of these implants may have served the patients well for several years but even the best of case reports have not shown success rates over 10 years. Problems have included infection, exposure of framework by down growth of epithelium and damage to the bone. Removal of these implants are also often difficult.
Transosseous Implants
The most common example of a transosseous implant is the mandibular staple implant. It has a plate which fits on to the lower border of the mandible at the symphysis and has posts arising from it (Fig. 1.3). Some of these posts pass 4into the jaws and others pass transosseously into the mouth where they serve to stabilize a denture. They are used only in the mandible. Bone loss around the post has been a common problem.
Endosseous Implants
Endosseous implants are placed into the maxilla or mandible from intraoral incisions after raising the mucoperiosteum and drilling into the bone. The shapes and construction materials vary, however the most common today is a tapered microtextured screw implant (Fig. 1.4). They may be used to replace single teeth, partially edentulous jaws or totally edentulous jaws. Most claim to be osseointegrated.
Materials
The most frequently used material for dental implants today is pure titanium or a titanium alloy. However, Zirconia implants are also available commercially.
Designs
There are numerous implant designs in the market, however the most prevalent today is the solid screw, which confirms to the shape of the tooth root and has a microtextured rough surface. An example of a good design is the Ankylos C/X Implant, Xive (Dentsply Sirona Implants), Noble Implant System and Straumann Implant System. In a survey of practicing dentists, Worthington listed the following features as important in making a right choice for an implant system:
- Demonstration of reliability (over at least 5 years)
- American Dental Association Approval
- Quality of instrumentation
- Quality of Prosthodontics
- Versatility
- Reputation of the manufacturing company
- Ease of use
- Training and after sales services
- Cost to the patient
- Start up cost.
In evaluating an implant system, the clinician must inquire:
- Were animal experiments conducted before the implant system was marketed?
- Were progressive clinical trials undertaken?
- Are the results of at least 5 years long trials published in reputed journals?
- Have there been multicenter replication studies?
The clinician should realize that it is not valid to extrapolate results from one product to another merely on the basis of some superficial morphologic resemblance. The composition of the material, its purity, its surface characteristic and its preparation are of vital importance.
BONE BIOLOGY, IMPLANT HEALING AND THE IMPLANT TISSUE INTERPHASE
Surgical Wound Healing after Osteotomy and Implant Placement
Osteophylic Phase
When an implant is first placed into bone a blood clot forms around the implant. Titanium is a light weight non-noble metal that is corrosion resistant as the result of the formation of surface oxides. The biologic inertness of this oxide leads to the implants being so biocompatible. The body does not react to the oxides on titanium as a foreign object but recognize it immunologically as self.
It is important to inflict the least damage to the bone and soft tissues while drilling and placing implants. Sharp drills and proper saline coolants are necessary to keep the temperature in the bone below 47°C for one minute.2,3 After the blood clot has formed, there is generalized inflammation due to the surgical insult. However while the inflammatory phase is still on, a more mature vascular network forms around the implant during the first 3 weeks. In addition ossification also begins during the first week by the migration of osteoblasts from the periosteum and endosteal osteoblast from the walls of the osteotomy. The trauma of placing the implant also results in necrosis of a thin layer (0.5 mm to 1 mm) of peri-implant bone. There is a critical period at around 2 weeks when bone resorption exceeds bone formation resulting in lower degree of primary stability than that achieved at the time of placement. The inflammatory or osteophylic phase lasts for one month and results in the formation of weak cell rich woven bone which chemically bonds to the oxides of the titanium implant. Following the formation of woven bone, the remodeling phase starts at the end of 4 weeks.
Remodeling Phase
Osteoclasts and osteoblasts interact in a coordinated way to replace the weak woven bone into a more load bearing lamellar bone. The remodeling phase lasts 6for about 3 months. It is influenced by micro-movement (not more than 100 μ) between the bone and implant, and good vascular supply.4 Under electron microscope, it is shown that there is an intimate contact between the bone and the implant surface oxides due to certain bone matrix proteins which act as binders.
Primary Stability
Primary stability or the initial rigid fixation of the implant to the bone is of prime importance for secondary osseointegration. An implant which is mobile at the time of placement will never achieve secondary osseointegration. By mobile we mean which moves inside the osteotomy and is not in close contact to the bone. It is different from an implant which is in close contact to the bone but only turns when you torque it (spinner). A spinner implant which has a moderately rough surface, e.g. Dentsply Sirona Implants Plus surface and Noble Biocare Implants—Anodic oxidation surface, can get subsequently osseointegrated. Most implant systems today use screw threads to gain primary stability. The presence of screw threads also help by transferring compressive forces to the surrounding cortical and cancellous bone which is favorable and leads to increased bone density.
OSSEOINTEGRATION
Osseointegration is the replacement of initial bone fixation of the implant with mature load bearing lamellar bone. The surface roughness of the implant especially the grit blasted and thermally acid etched (Dentsply Sirona Implants—Plus surface) are of immense benefit for faster and stronger laying of load bearing bone due to their osteoconductive surface.5 The rough surface also leads to increased surface area on implant which helps in obtaining greater bone to implant contact (BIC). An additional advantage of the thermal acid etching is that in addition to increasing the surface roughness of the grit-blasted surface, it also cleans and removes foreign matter from the implant surface.
REFERENCES
- Branemark PI, Adell R, Breine U, et al. Intra-osseous anchorage of dental prostheses. I: Experimental studies. Scand J Plast Reconstr Surg. 1969;3:81–100.
- Eriksson AR, Albrektsson T. Temperature threshold levels for heat induced bone tissue injury: a vital-microscopic study in the rabbit. J Prosthet Den. 1983;50:101–7.
- Eriksson AR, Albrektsson T. The effect of heat on bone regeneration: an experimental study in rabbits using the bone growth chamber. J Oral Maxillofac Surg. 1984;42:705–11.
- Szmukler-Moncler S, Piattelli A, Favero GA, Dubruille JH. Considerations preliminary to the application of early and immediate loading protocols in dental implantology. Clin Oral Implants Res. 2000;11:12–25.
- Neugebauer J, Weinlander M, Lekovic V, et al. Mechanical stability of immediately loaded implants with various surfaces and designs: a pilot study in dogs. Int J Oral Maxillofac Implants. 2009;24(6):1083–92.