Caution: Remember that an implant candidate requires implants because of an abused, diseased, or neglected (a form of abuse) ridge. Problem ridges are more typical than atypical.

Concavities or undercuts are difficult to detect radiographically because they are aberrations in the labio/bucco shape of the ridge, rather than in its height. Only reflecting the tissues accurately reveals their presence and extent.

The deepness and length of the concavity and the resulting shape of the ridge around it influence the angle of the implant socket and its shape.

Ideally in a good ridge the groove is initiated slightly to the lingual of the center of the crest (9). However, when a concavity is present labially, it is imperative to use the concavity—the narrowest portion of the ridge—as the prime determinant in angling the groove. The groove should bisect the narrowest portion (10). This bisection should be potentiated by angling the socket into denser lingual bone, a desirable maneuver. Moving the socket labially (11) may cause the labial plate to fracture (12) during groove-making or implant-seating procedures, or later during mastication.

In most cases, angling the groove away from a concavity will mean that the neck of the bladevent must be acutely bent for proper alignment of the abutments, and the bone directly under the post on the lingual side of the ridge must be notched (13) to countersink the post so that the labial side of the blade is properly buried (13a).

Generally, a ridge with an undercut need not be reshaped. However, if a bony knob (14) at the crest complicates the access to the underlying narrow portion of the ridge (15), the knob can be removed, leaving the ridge at least 2.5 mm wide.

If there is enough bone below the undercut (16) and the con-cavity is very deep it may be advisable to reduce the ridge beyond the 2.5 mm minimum to a more generous dimension.

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