mandible, with its well-developed stress trajectories, posterior maxillary occlusal pressures are directed toward the sinus for deflection, and eventual dissipation, upward. Thus, apical trauma directly encourages bone loss at the sinus floor.

Nondental diseases involving the sinuses also may cause them to enlarge by actively contributing to bone loss. Chronic infection or inflammation of the sinus membrane can destroy bone, particularly that on the floor of the sinus because of the tendency of fluids to collect there. Thus, patients with a tendency toward severe respiratory infections and frequent stuffiness or pain in the sinuses may be contraindicated for implantation. The symptoms for such conditions could be easily, but inaccurately, attributed to implants.

It is important to stress that sinus enlargement is not an active process; i.e., it does not cause bone loss. On the contrary, the sinus merely expands to occupy the area vacated by bone. The most obvious result of bone loss in the posterior portion of the maxilla is the expansion of the sinus anteriorly toward the canine pillar, posteriorly into the tuberosity, and inferiorly toward the alveolar ridge. This pneumatization is evident in panographic radiographs of the arch. Not as obvious is the thinning of the buccal and palatal walls.

Initially the walls contain a core of cancellous bone sandwiched between dense cortical bone. As osteogenic stimulation is reduced due to tooth loss, the cancellous bone loses volume and resorbs. In the region of the disto-buccal and lingual roots of the first molar, for example, the wall (25) is thicker when a healthy tooth is present than after tooth loss (26).

Lateral pneumatization is much less easily detected than is antero-posterior enlargement of the sinus, which is usually obvious in radiographs. It is important, however, to recognize that as the sinus expands anteriorly or posteriorly, it is probably also expanding laterally, thinning the buccal face of the maxilla, in some cases to the point where it becomes eggshell-thin. Thus, it is unwise to extend a subperiosteal implant bar or strut over the buccal face of the sinus. Although the implant probably will not cause the dense cortical bone forming the facial wall of the sinus to collapse from surface pressure, the very thin, hard bone can be punctured by a sharp thrust, such as a strong, quick grinding of the teeth.

On the buccal face of the ridge, weight-bearing parts of the implant should be restricted to those portions of the ridge — and of the maxilla — with underlying bone. Extending braces or struts

25