the remaining bone presents a peculiar picture. A knife-edge ridge exists, formed almost exclusively by the lingual plate of bone (26).

This sharp crest usually can be reduced to a favorable occlusal table. The amount of reduction depends upon the height of the ridge. When the ridge is shallow (27), the crest is reduced only until 3 mm of width is achieved (28). When the ridge is tall with good bone (29), it can be reduced until there is generous bone to flank the implant's shoulders labially (30) and lingually (31).

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 some-times may also reduce on 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. Also, there usually remains more alveolar bone height anteriorly than posteriorly, as posteriorly the ridge tends to flatten out or even becomes concave leaving only the mylohyoid ridge to exhibit the highest dimension.

The same factors influence a partially edentulous arch. When more than one edentulous span exists, which is a common occurrence, the width of each may differ considerably. Longer spans, tend to be narrower and more irregular than shorter spans, for obvious reasons. The presence of some teeth helps divide the ridge into areas that can be evaluated separately and approached accordingly.

Adapting to a Limited, Wide Site: A generously flared anterior area (6) can usually accept any of several standard bladevents, the choice of which depends upon matching the mesio-distal length (7) of the implant and its height (8) to the available bone. Neither shoulder of the implant should approach too closely the narrowed portion of the ridge (9). A slip could fracture the plates, and bone dieback can weaken them.

The area below the crest proper should be carefully examined for concavities and other flaws. The narrowed area should have

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