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Either due to extractions or tissue atrophy, the bony dental arch is discontinuous. Only the soft tissues prevent the oral cavity and the sinus from exchanging material, and the integrity of these essential tissues must be preserved. To test that the sinus membrane is intact, the patient is asked to inhale; pinch the nostrils closed, and gently force air into his nose and sinuses. An intact membrane will balloon into the defect. Even though the membrane is intact, such a fragile ridge cannot bear the weight of a subperiosteal strut or bar. The ridge must be avoided, passed over by the type of bar arising from a pterygoid extension implant.

The bony floor of the sinus may be thin for several reasons. Before tooth loss, the sinus may have dropped between the roots of the teeth. Radiographically, such roots appear to protrude into the sinus (17), with their supporting alveolar tissues (18) draped over them. If these teeth are stable, and they may well be, an eggshell-thin layer of bone separates them from the sinus membrane. However, bone loss around the apices of the roots presages further periodontal problems. The loss of teeth from such thin bone leaves very little bone remaining to either serve as an implant site or to protect the sinus cavity.

Great care is needed when extracting "draped" teeth. Radio-graphs must be scanned for the thin radiolucent line, the lamina propria, that indicates intact bone between tooth and sinus. If the line is missing or discontinuous (19), the root may directly con-tact the sinus membrane and tear a hole in it during extraction.

When a molar overerupts (20) because it has no functional antagonist, the sinus drops (21) as it follows the alveolar process. If the tooth is lost or extracted and the ridge reduced (22), very little bone remains. Such a ridge is unsuitable for a weight-bearing implant.

Bone resorption in the posterior portion of the maxilla causes bone loss at both the crest and from the sinus floor. Thus, the amount of bone available for endosteal implantation uniquely diminishes from two vertical directions, superior and inferior.

Disuse atrophy, resulting from a lack of osteogenic stimulation when the maxillary molars are lost or when the molars lose their antagonists, can be complicated by trauma, further accelerating bone loss. As a molar moves into a space vacated by one of its neighbors, it generally moves into traumatic occlusion. The bone responds by resorbing, allowing the tooth to move out of con-tact. When a maxillary molar is involved, bone near the sinus floor (23) — as well as crestal bone (24) — is afflicted. Unlike in the

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