Both faces of the knife-edge ridge are supported by the fingers as the bladevent is tapped home. The plates will spread slightly as the bladevent is inserted, making the ridge wider than it was prior to implant insertion. Evidence so far indicates that the slight spreading does not exceed the physiological limits of bone. To the contrary, the spreading appears to stimulate more rapid bone regrowth.

In the canine and cuspid areas particularly, the labial plate of bone (23) may have almost entirely resorbed over the tooth due to occlusal trauma (24). Consequently, when the tooth is lost, the remaining bone presents a peculiar picture. A knife-edge ridge exists, formed almost exclusively by the palatal plate of bone (25).

This sharp crest usually can be reduced to a favorable table. The amount of reduction depends upon the height of the ridge. When the ridge is shallow (26), the crest is reduced only until 3 mm is achieved (27). When the ridge is tall with good bone (28), it can be reduced until there is generous bone to flank the implant's shoulders labially (29) and palatally (30).

Problem Ridge: Uneven Width

A totally edentulous ridge may range in width from a knife-edge portion (1) to a site almost as wide as one that would invest a healthy tooth (2). Bone loss — and consequently narrowing of the ridge — usually occurs primarily on the labial face (3), but it may also reduce both faces (4,5). A greater tendency toward narrowness should be anticipated anteriorly because incisor and cuspid roots have narrower alveolae than posterior teeth. However, tooth loss sequence, time, and/or trauma can upset this pattern.

The same factors influence a partially edentulous arch. When more than one edentulous span exists, which is a common occur-

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