Implant Design

The first and most commonly used type of design is the endosteal (called endosseous) implant, a device placed into an alveolar and/or basal bone of the mandible or maxilla that usually transected only one cortical plate. These implants were formed in many different shapes, such as root-form cylindrical cones or screws or thin plates called plate or blade forms, and they were used in all areas of the mouth. One example of an endosteal implant was called the blade implant (Figure 20-1), which was developed independently in 1967 by two groups led initially by Roberts (1970) and Linkow (1968). Endosteal blade implants consisted of thin plates placed into bone; they were most commonly used for narrow anatomic structures such as posterior edentulous areas after significant resorption of bone. Because of various issues with blade implants, their application in more recent implantology has decreased. Another example of an endosteal implant was the ramus frame implant, a horseshoe-shaped stainless steel device inserted into the ascending rami (bilateral) and the anterior symphysis area of the mandible. As with the blade implants, the number of applications have been limited. The most popular endosteal implant has been the root-form (Figure 20-2), which was designed initially to mimic the shape of tooth roots for directional load distribution as well as for positioning in bone. In longitudinal studies, the root-form implant has the most documentation of the endosteal implants, although several surgical stages may be needed for completion.

The second implant design was the subperiosteal implant (Figure 20-3), which employed an implant substructure and superstructure. The custom-cast frame was placed directly beneath the periosteum overlying and fitting along the bony cortex. This implant was first developed by Dahl (1943) and refined by Berman (1950), who used a direct bone impression technique. These devices were used to restore partially dentate or completely edentulous jaws when there was inadequate bone for endosseous implants. Use of the subperiosteal implant has been limited because of numerous considerations, including the difficulty of retrieval.

The third design was the transosteal implant (Figure 20-4), which combined subperiosteal and endosteal components. This type of implant penetrated both cortical plates and passes through the full thickness of the alveolar bone. Use of the transosteal implant has been restricted to the anterior area of the mandible and provides support for tissue-borne overdentures. The concept of transosseous implants was first conceived in Germany in the early 1930s; early examples were made of a cobalt alloy. Small (1968) developed the mandibular staple implant made of a titanium alloy, which was modified by Bosker (1983), who produced the transmandibular implant (TMI) made of a gold alloy. Other names for transosteal implants have included staple bone implant, mandibular staple implant, and transmandibular implant.

The fourth implant design was the epithelial implant, which was inserted into the oral mucosa. This type was associated with a simple surgical technique where the mucosa was used as an attachment site for metal inserts placed into an acrylic denture. Several disadvantages were associated with the epithelial implant, most notably a painful healing process and a requirement for continuous wear. The current use of “mucosal inserts” or epithelial implants is very limited.

Some oral implantologists also include the endodontic stabilizer as an anatomy-specific application system. These smooth or threaded pins (or screws) were placed through endodontically treated teeth with the implant extending into the bone. They were constructed from alloys or ceramics made of alumina or zirconia. Reported difficulties included sealing the transition zone from tooth to bone and the limited strength of small-diameter pins or screws.

From a historical and applications perspective, these systems were reviewed in the early 1970s by Natiella et al. (1972) and subsequently in each decade by researchers who participated in professional society–based consensus conferences. To summarize the various biomaterials and designs tested for dental implants, examples of devices received for examination prior to 1990 are shown in Figures 20-5 through 20-7.

Implant Properties

Implant biomaterials can also be classified according to their composition and their physical, mechanical, chemical, electrical, and biological properties. These classifications often include ranked comparisons of properties such as elastic moduli, tensile strength, and ductility to determine optimal clinical applications (Table 20-1). These properties are used to aid in the design and fabrication of the prosthesis. For example, the elastic modulus of the implant is inversely related to the strain transferred across the implant-tissue interface during loading of the implant; that is, the greater the elastic modulus of an implant, the greater is the stress in the implant and the lower is the stress transferred to bone.

An implant with a elastic modulus comparable to that of bone should be selected to produce a more uniform stress distribution across the interface. Metals possess high strength and ductility, whereas the ceramics and carbons are brittle materials. Ductility is also important because it relates to the potential for permanent deformation of abutments or fixtures in areas of high tensile stress.

Tissue Integration

Another way of classifying implants is through interactions at the implant-to-tissue interface. Periodontal structures, which attach teeth to bone, consist of highly differentiated fibrous tissue. These fibers are replete with numerous cells and nerve endings that allow for functional force transfer, sensory function, bone formation, and tooth movements. Although this is the ideal form of integration, there are no known implant biomaterials or designs at present that can stimulate the growth of these fibers and fully mimic the function of the periodontal ligament and a natural tooth.

Historically, implant interactions occurred through fibrous connective tissue, and this was accepted as a measure of successful implant function (the pseudoligament concept). This type of interaction was reported to be very susceptible to acute or chronic inflammatory responses, which were accompanied by pain and eventual loss of the implant. Such an implant has also been called a pseudoperiodontium. Despite numerous reports of implant success, clinical studies indicate that this type of interaction is susceptible to a greater amount of progressive loosening and infection, with subsequent loss of the implant construct.

The implant-to-bone interaction (called osseointegration) is characterized by direct contact between bone and the surface of a functional implant after one year. This central theme of the Brånemark group was called “direct anchorage to bone” and has become a major attribute of dental implants. This mode is described as the direct adaptation of bone to implants without any other intermediate nonbony tissue and has been described by some as similar to tooth ankylosis, where no periodontal ligament or fibrous tissue exists. The strength of this contact has been shown to be stable, which is advantageous compared with the soft tissue interface described previously.

This type of osseous interface has been described extensively and includes a process wherein bone-producing cells migrate along the implant surface through the connective tissue scaffolding that forms adjacent to the implant surface. Integration at the implant interface is highly dependent on the implant surface’s chemistry and design. Bone apposition has been reported at higher rates when microscopic surface ridges are present. Osseous integration has also been achieved through the use of bioactive materials that stimulate the formation of bone along the surface of the implant. Another way of achieving osseous integration involves de novo bone formation, wherein a mineralized interfacial matrix and/or active growth factors are deposited along the implant’s surface. Once again, the implant’s surface topography influences the strength of its attachment to bone.

Implant-supported restorations differ from tooth-supported restorations in that the former lack a periodontal ligament, which reduces shear stress and strain, provides shock absorption, and reduces the development of dangerously high occlusal forces. These benefits of the periodontal ligament reduce the potential for inadvertent damage to the tooth or restoration. With the increased popularity of all-ceramic restorations, the effect of no periodontal ligament attachment in implant-supported restorations must be established. Ceramic materials are brittle by nature and because of inherent processing flaws, they cannot withstand any stress above their yield point, which is equivalent to their tensile strength. These stresses can be generated during occlusal loading. Implants, on the other hand, are directly anchored to the bone and cannot readily flex laterally under extremely high occlusal loads to relieve some of the excessive loading. Based on an in vitro study, Vult von Steyern et al. (2005) analyzed the fracture strength of all-ceramic prostheses on abutment teeth and implants. They concluded that implant-supported ceramic prostheses fractured at higher loads than those supported by natural teeth because of the lack of a periodontal ligament. To date, there are no clinical studies comparing the performance of ceramic FDPs supported by implants and natural teeth.

Conversely, studies have analyzed whether the type of prosthetic material can affect the longevity of an implant. Brånemark and Skalak advocated the use of acrylic for the prosthetic superstructure to act as a shock absorber and essentially dissipate the load on the implants and the bone surrounding them. To date, acrylic, gold, and ceramic are being used in implant superstructures. In the absence of a periodontal ligament, minimizing the load along the bone-implant interface is logical. However, there is no evidence to associate the use of any prosthetic material with the longevity and survival of the implants.