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from the oral cavity (2), or discontinuous (3,4) with a dehiscence of the sinus membrane. Such a ridge is totally inappropriate for a bladevent. There is simply no bone under or behind the sinus in which to insert an endosteal implant.

The other implant type, the subperiosteal, is lodged in or braced against variations in the bony landscape. However, the bone must be hard and compact. The implant cannot be placed against bone under or flanking the sinus buccally. Edentulous subsinus bone is fragile, and typically lacks a cortical plate (5). It is unsuitable for weight-bearing. Under pressure, it will resorb (6), shrinking away from the implant, the stability of which depends largely upon a close fit to bone. The bone flanking the sinus, al-though hard and compact and little affected by evenly distributed surface pressure, may be eggshell-thin. A strong, quick tipping of an implant strut against it could pierce it.

The characteristics of bone under and flanking the sinus have made the area the most difficult in implantology. Attempts to cantilever post-bearing struts over the ridge tend to be unsuccessful because the posts can be forced against the crest during mastication. Struts terminating on crestal bone or on thin, compact bone can be rolled against the bone and damaged if the implant loosens slightly or is unbalanced. Implant designs with segments travelling considerable distances upward from the dental arch onto solid bone elsewhere in the maxillary complex usually must cross the sinus, even if they terminate on the zygomatic arch (7) or some other nondental structure. Those struts crossing the sinus (8) can be thrust hard enough against its thin wall (9) to fracture or puncture it. Furthermore, distance means more implant material, and more material means more weight, as does increasing the bulk of struts crossing the sinus to help guard against fracturing. Gravity works against a maxillary appliance, and keeping the implant light-weight is a desirable goal.

In addition to balanced seating against solid bone, a subperiosteal implant gains additional security by a tight sealing of its buried portions (10) to bone by the regrowth of fibers from the mucoperiosteum (11) to bone (12). This binding must be firm because gravity works against a maxillary subperiosteal implant and its restoration. Therefore, all parts of the implant must be placed under dense, connective tissue that will tightly reseal to bone.

The palatal peripheral strut must remain in the zone of attached gingiva (13), stopping short of the recess in the hard palate

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