132 Theories and techniques of oral implantology

was no significant bone resorption or settling. The bone tended to grow over the borders of the implant's periphery, and some immature bone was found in direct contact with a small part of the metal. In summary, there was no histologic evidence of dam-age to the adjacent tissues. The implant was well tolerated.

GENERAL CONCLUSIONS

Generalizations on the various histologic aspects of implantology are valuable guides in evaluating the success of an intervention and predicting the future of implantology. Each of the cases presented in this chapter has included details which, when synthesized, give an accurate portrayal of the implant's relationship to the living tissues that form its site. As further case studies become available, the generalizations will move into the highly documented specific. At this point in the history of implants, current cases provide significant insight into what happens to the site when an implant is inserted and what constitutes a successsful intervention. Histologic studies now clearly indicate that implants are promising tools for the stabilization of a fixed denture that is functionally and esthetically superior to conventional restoration techniques.

Bone

When an implant has been well stabilized so that it does not traumatize the bone during mastication, the bone begins to heal around it. The nature of bony repair is that the area becomes encircled with connective tissue, some of which organizes into immature bone that in turn matures. The mature bone, like bone elsewhere in the body, is continually breaking down and simultaneously building up. This is the normal, healthy pattern and it continues as long as the bone is stimulated by the pull of the false periodontal membrane around the implant.

Bone around a successful implant has been shown by histologic studies to be in a healthy state of maturity. It is characterized by numerous haversian systems interspersed with marrow spaces, the typical picture of normal alveolar bone. Those areas continually stimulated by fibers extending from the false periodontal membrane are undergoing changes, again the normal picture of healthy bone. The bone surrounds the implant, separated from it only by connective tissue. It grows so close to the implant that when a post type implant is carefully unscrewed from its site, its spirals are clearly outlined in bone. In radiographs, as has been demonstrated numerous

times, only a very narrow translucent band—the false periodontal membrane—occurs between the bone and the implant.

The nature of bone around a failing implant is quite different. In x-rays, the immaturity of the bone is obvious: the mineral deposits that harden the tissue, characterize its mature state, and are radiopaque are lacking, as indicated by the large translucent areas seen around the implant. Under the microscope, the tissue is seen to be in an almost chaotic state of flux, with those elements that serve to break down bony tissues predominating.

So far, the histologic studies on bone around an implant have been consistent. When an implant, no matter of what design, is failing from its onset, the clot formed around the site never matures into hard, healthy bone. When a previously successful implant

begins to fail for some reason   for example, trauma

caused by changes in occlusion   the bone begins to
degenerate.

Connective tissues

The connective tissue that forms around an implant is noteworthy for two reasons: its anatomy and its function. The connective tissue forms a kind of wrap around the implant. It follows the contours of the embedded portions, and its fibers become oriented along stress lines. The part of the tissue in direct contact with the implant material is smooth surfaced, except in some cases of plastic implants in which fibers extend into the implant material or in areas where they extend into large open vents of a blade type implant. Fibers extend into the bone from the surface facing bone. These fibers pull on the bone as the connective tissue is forced down with the implant during mastication. The role of this pull, or tension, on osteogenesis and osteolysis is well known. Healthy bone depends upon balanced stimulation, according to Wolff's law.

Because the connective tissue is elastic, it acts like a suspensory ligament, reducing masticatory trauma and maintaining it within tolerance limits. The degree of elasticity is determined by the density and thickness of the tissue. This explains in part why the mobility of an implant varies from the time of its insertion until the maturation of the bone around it. When first positioned, a correctly inserted implant is quite immobile. Yet a few days later it loosens, only to tighten up later. This results from changes in the connective tissue.

When first inserted, the implant is flush with the bone. Because this bone has been damaged during