The implant site 75

occurs, bringing in all the elements needed for ossification.

The connective tissue that fills in around the osseous portions of the implant site differentiate directly into coarse fibrillar bone. The reconstruction of the alveolar bone into mature, dense osseous tissue occurs. This process is governed by changes in functional stress on the bone. Here there is a marked difference between healing of a socket left by a tooth extraction and the healing of an implant site. In the case of a tooth extraction, the area receives no stimulation because of the empty socket. This generally leads to loss of bone density in the area and to the formation of a compact lamellar bone at the surface of the bony scar. The reconstruction may also be complicated by the movement of adjacent teeth toward the empty socket. This means that not only must the empty socket heal, but the spaces vacated by the moving teeth must be replaced.

The site of an implant intervention, however, contains the implant itself. The top of this implant bears an artificial tooth that, when correctly articulated, receives the impact of its opposing tooth, thus maintaining the functional stresses on the alveolar bone. This prevents extensive loss of alveolar bone by maintaining nearly normal stress conditions.

Although stress on the implant helps the alveolar bone heal, movement of the implant leads to failure. If the implant is moving around in the clot or callus, fibrinoid will form. Fibrinoid is like fibrin, as the name implies, but it is a mass of collagen and ground substances in all stages of degradation. Fibrinoid is acellular, homogeneous, and amorphous. Mechanical disturbances cause it to split apart in the center around the implant and produce a cavity filled with mucous fluid. By osmosis fibrinoid imbibes water, and internal pressure results. This may lead to failure of the implant.

If the implant continues to move, fibrinoid de-generation continues. All bone in the area becomes converted to compact bone inside and around the cortical ends but not across the fracture site. Little vascularization can exist inside the dense cortical bone, and the bone thus fails to repair.

For this reason, it is important to immobilize an implant whose design does not give it immediate stability.

The implant "alveolus"

Radiographic evidence indicates that the bone that forms around a post type or blade implant very closely resembles that around a natural tooth in good

occlusion. The bone follows the design of the implant itself and is separated from it only by a thin layer of connective tissue. In some instances bone also grows into open spirals or vents, but these spaces are more typically occupied by connective tissue unless they are quite large.

The bone nearest the connective tissue forms a thin layer of dense bone in which the connective tissue fibers are anchored. Blood and lymph vessels and nerves pass through it and supply the connective tissues. As supply is related to demand and because there is no living tooth in the site, these vessels and nerves are probably fewer in number than those passing through the cribriform plate around a natural tooth.

Flanking the thin layer of dense bone is sup-porting alveolar bone. This contains distinct trabeculae arranged along the most advantageous routes for the distribution of stress. The trabeculae may be thick and few, or thinner and more numerous. Both patterns are effective in directing or dissipating stress. The pattern assumed in each site depends upon its location in the mandible or maxilla and the design of the implant.

The post type implant, which more nearly resembles a natural root in overall shape than other designs, is buttressed by trabeculae that radiate in the directions most appropriate to efficient stress distribution in the area. The blade, which contacts a great deal more bone than does any other type of endosseous implant, also stimulates the formation of trabecular bone. However, the true picture of its trabecular pattern in any given site is exceedingly difficult to distinguish in x-rays because of the size of the implant and the fact that its broadest face is usually perpendicular to the x-ray. However, the adjacent bone appears normal in every way, proof that the bone is not being traumatized or resorbing from a lack of functional stimulation.

The bony support around a pin implant or a triplant is different from that around a spiral-shaft implant, vent-plant, or blade. Inserting the pins initially destroys a smaller amount of bone than does inserting either a post type implant or a blade, but bone cells not in immediate contact with the implant site also die as a result of the intervention.

The connective tissues around the individual pins do not stimulate the bone to regenerate and replace itself. Thus, as the bone is healing around the implant, it lacks the functional tension that stimulates the differentiation of osteoblasts. Certainly pressure is exerted on the bone as a triplant is moved,