A lingual anterior undercut, which usually results from disease, is much rarer than a labial undercut. In such a situation, the socket axis should bisect the narrowest portion of the ridge (17), moving it more labially than is otherwise recommended.

Two narrow blades may be preferable to one wide double-posted design in an area with a deep or an irregular concavity. Each blade can be inserted to its best advantage (18,19). Two deep and narrow grooves might be the best options for each site.

No matter which option is selected, a thumb or finger bracing the concavity is recommended while drilling the socket and seating the implant (20). The area may not only be shallower, but also considerably weaker.

Problem Ridge:

Uneven Height

Variations in bone height are readily detected radiographically. Many prosthodontic problems arising from unequal height are more properly compensated in the prosthesis. Surgically the difficulties depend principally upon where adequate height and width remain, and the length of the edentulous span.

Usually the anterior ridge is at least as tall as the posterior, and in most cases it is taller (1). However, traumatic loss of the incisors or canines, or uneven distribution of masticatory forces can cause excessive labial resorption of the anterior portion of the mandible (2).

The loss of height may be limited to one area. For example, the vestiges of a single extraction site may produce a pronounced dip (3) in the crest. This feature is often accompanied by lateral recession of the walls of the socket, particularly the labial one

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