The tissues involved in implant procedures 45

B

Fig. 2-26. A, When a tooth is lost, alveolar bone resorption usually occurs, leading to the drifting, twisting, and exfoliation of other teeth in both arches. This causes further resorption, encouraged by food pocket areas and plunger cusp situations. B, Advanced alveoclasia showing bone loss around the exposed roots of remaining teeth. C, As even more teeth are lost, even further resorption occurs. Of particular interest here is the structure of the ridge that has developed in the maxilla as a result of resorption. Whereas resorption in the mandible reduces the crest to a broad flat ridge of dense cortical bone, resorption in the maxillae thins the bone, following the contour of the sinus, to a knife-edge ridge.

process shows a moderate degree of loss, and the compact lamella at the surface is fairly strong (Fig. 2-26, A). If, however, the sockets are bordered with teeth that have no antagonists or if several teeth are lost, disuse atrophy is more extensive and the loss of bone at the alveolar ridge is much greater. In fact, sometimes there is almost a total disappearance of the alveolar process. Reconstruction of the alveolar bone in the edentulous area is also complicated by the drifting or tipping of the remaining adjacent teeth into the space. This drifting, of course, pro-motes poor occlusion, which in turn promotes more atrophy and resorption (Fig. 2-26, B and C) .

In the upper jaw in the region of the maxillary sinus, localized disuse atrophy presents an even more complicated picture when teeth are lost. Here osteo

porosis is at least in part caused by a hollowing out of the primary bone scar from within as the maxillary sinus extends downward. This affects the teeth bordering the extraction, leaving them with only a relatively thin compact plate of bone.

The loss of almost all of the teeth results in extensive disuse atrophy. If nothing is done to re-place or substitute the normal tensions in the alveolar bone, the entire alveolar process is gradually resorbed. The atrophy may even involve parts of the maxillary and mandibular bones. In the maxilla, the subnasal parts may be completely lost. The palate and floor of the maxillary sinus may be reduced to paper-thin plates of bone containing defects. In the mandible, the loss may be so extensive that the mental foramen lies at the surface of the thin body. In ex-