Historically, dental implants have been classified according to their design. This design was in turn based on the way in which they are surgically implanted. The three types of implants commonly used for the past 40 years are the subperiosteal implant, the transosteal implant, and the endosseous implant (Table 15-1).

Endosseous Implant

Endosseous implants are by far the most common type of implant placed today. Implants are placed directly into the mandible or maxilla (Figure 15-1). A pilot hole is drilled into the alveolar or basal bone beneath (in cases in which the alveolar bone has been partially or completely resorbed), and the implant body is inserted into this site. The top of the implant is positioned so that it either protrudes slightly through the cortical plate or is flush with the surface of the bone. Typically a superstructure containing a prosthetic tooth or teeth connects to the implant body through an abutment that is screwed into the body directly through the mucosa.

Osseointegration is now formally defined as the close approximation of bone to an implant material (Figure 15-2). To achieve osseointegration, the bone must be viable, the space between the bone and implant must be less than 10 nm and contain no fibrous tissue, and the bone-implant interface must be able to survive loading by a dental prosthesis. In current practice, osseointegration is an absolute requirement for the successful implant-supported dental prosthesis. To achieve osseointegration between an implant and bone, a number of factors must be correct. The bone must be prepared in a way that does not cause necrosis or inflammation. The implant must be allowed to heal for a time without a load. Finally, the proper material must be implanted, because not all materials will promote osseointegration.

In recent years, various surface configurations have been proposed as means of improving the cohesiveness of the implant/tissue interface, maximizing load transfer, minimizing relative motion between the implant and tissue, minimizing fibrous integration and loosening, and lengthening the service life of the construct. Because of the necessity of developing a stable interface before loading, effort has been placed on developing materials and methods to accelerate tissue apposition to the implant surface. Surface-roughened implants and ceramic coatings have been implemented into clinical practice. Other, more experimental techniques include electrical stimulation, bone grafting, and the use of growth factors and other tissue engineering approaches described in Chapter 16.

The application of bioactive ceramics as implant materials was traditionally limited to their use as bone bonding and augmentation materials. There has been interest in coating titanium alloys with bioactive materials to promote an implant bone connection. Bioactive ceramic materials are more than just biocompatible. The use of the term bioactive implies that they have the ability to elicit a favorable tissue response when implanted in vivo. These ceramics form a direct chemical bond with natural tissues and are most often designed to bioresorb or biodegrade, having high solubility. Commonly implanted dental ceramics include the calcium phosphates with various calcium-to-phosphorus ratios (e.g., hydroxyapatite and tricalcium phosphate), bioactive glasses (mixtures of SiO₂, CaO, P₂O₅, and sometimes Na₂O, and MgO), and glass ceramics.

Important examples of bioactive glasses and glass ceramics include Bioglass (a glass containing a mixture of silica, phosphate, calcia, and soda); Ceravital (which has a different alkali oxide concentration compared to Bioglass); Biogran (which has a different physical conformation compared to Bioglass); and glass ceramic A-W (a glass ceramic–containing crystalline oxyapatite and fluorapatite (Ca₁₀[PO₄]₆[O,F₂]) and β-wollastonite (SiO₂CaO) in a MgO-CaO-SiO₂ glassy matrix). Additionally, many other glass and glass-ceramic compositions, based on recently developed sol-gel synthesis methods, are being developed. Calcium phosphate ceramics vary in composition, depending on processing-induced physical and chemical changes. Among this group are the apatite ceramics, and of particular interest is hydroxyapatite (HA). This is the synthetic version of the inorganic phase found in tooth and bone and is the bioactive ceramic material that has been most extensively investigated.

The impetus for using synthetic HA as a biomaterial stems from the perceived advantage of using a material similar to the mineral phase in natural tissues for replacing these materials. Because of this similarity, better tissue bonding is expected. Additional perceived advantages of HA and other bioactive ceramics include low thermal and electrical conductivity, elastic properties similar to those of bone, control of in vivo degradation rates through control of material properties, and the possibility of the ceramic functioning as a barrier to metallic corrosion products when it is coated onto a metal substrate.

However, temperature-induced phase transformations while processing HA provoke considerable changes in its in vitro dissolution behavior, and the altered structure changes the biological reaction to the material. Given the multitude of chemical compositions and structures resulting from processing bioactive ceramics and the resultant fact that pure HA is rarely used, the broader term calcium phosphate ceramics (CPCs) has been proposed in lieu of the more specific term hydroxyapatite. Each individual calcium phosphate ceramic is then defined by its own unique set of chemical and physical properties.

Although calcium phosphate ceramics are too brittle and too stiff to serve as stand-alone dental implants for prosthetic tooth replacement, there has been continuing interest in using a thin (50 to 75 μm) layer of ceramic materials to coat the surface of metallic implants. This provides the beneficial osseointegration characteristics of the ceramic combined with the high strength of the metallic alloy. Most manufacturers provide implants coated with calcium phosphate ceramic for use in sites where poor bone quality exists. A major limitation in using this concept in all clinical situations, however, has revolved around the inability to predict and maintain the bond strength of the coating to the metal. When the bioactive ceramic material resorbs in vivo, an unpredictable change occurs in the implant-bone interface, and implant micromotion and loosening may occur. This makes the long-term stability of these implants uncertain.

If successful, the ceramic coating becomes completely fused with the surrounding bone. In this case, the interface is called biointegration rather than osseointegration, and there is no intervening space between the bone and the implant (see Figure 15-2). A number of ceramic coatings have been used in this manner.

Typically, these coatings have been applied to the surface of an implant via a plasma-spray deposition process. This results in a complex mixture of HA, tricalcium phosphate, and tetracalcium phosphate in the coating, rather than a recapitulation of the starting powder mixture. Physical properties of importance to the functionality of calcium phosphate ceramics include powder particle size, particle shape, pore size, pore shape, pore-size distribution, specific surface area, phases present, crystal structure, crystal size, grain size, density, coating thickness, hardness, and surface roughness.

The long-term integrity of the ceramic coating in vivo is not known, but evidence indicates that these coatings will resorb over time. Additionally, results of ex vivo push-out tests indicate that the ceramic-metal bond fails before the ceramic-tissue bond and is the weak link in the system. Thus the weak ceramic-metal bond and the integrity of that interface over a lengthy service life of functional loading is reason for concern.