To properly seat the posts, it is necessary to countersink them; that is, to cut a depression in the higher — or palatal — side of the bone (29).

The 700 XL bur is used, with the water coolant particularly essential because of close proximity to the metal bladevent. Only that bone interfering with proper seating is removed (30). This leaves bone behind the post (31) as additional protection against tongue thrusts. It is neither necessary nor desirable to extensively reshape the crest to correctly seat the posts. Countersinking is adequate.

After removing bone for countersinking, the posts may not touch the crest (32) because the socket is now too shallow. The implant is removed, and the socket deepended to properly seat the implant (33).

Problem Ridge: Knife-edge

The knife-edge ridge is a common maxillary problem, particularly in a totally edentulous arch. After tooth loss the labial face of the ridge tends to move inward faster than the height diminishes. When some teeth remain, this narrowing tendency can be somewhat retarded in the edentulous span by osteogenic stimulation from teeth bordering the area. However, the longer the span, the less effective such "borrowed" stimulation.

Although the ridge may have thinned considerably from its original dimension (1), the change is usually difficult to detect superficially. Typically the soft tissues have thickened, replacing lost bone, and palpation is inadequate for determining what constitutes the bulk of the ridge. Radiographs still show a tall, bony ridge. Only reflection of the soft tissues reveals the true landscape.

The thinning of a totally edentulous ridge may have occurred around the entire arch, or be localized. When a limited edentulous span is involved, its narrowness is usually related to the span's position, its shape prior to tooth loss, its length, and tooth loss sequence. A longer span particularly anteriorly (2) tends to be narrower than a shorter span (3) due to more prolonged abuse. However, even in the longest span, the narrow ridge widens as it approaches teeth (4). In all situations the labial plate usually be-comes more concave.

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