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-palatal shape of the ridge, rather than in its height. Only reflecting the tissues accurately reveals their presence and extent.

The depth 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 socket is initiated in the center of the crest (9), with a slightly off-center inclination towards denser palatal bone (10). However, when a concavity is present, it is imperative to use the concavity — the narrowest portion of the ridge — as the prime determinant in angling the socket. The socket should bisect the narrowest portion (11). The bisection naturally angles the socket into denser palatal bone, a desirable maneuver. Moving the socket labially (12) may cause the labial plate to fracture (13) during socket-making or implant-seating procedures, or later during mastication.

In most cases, angling the socket 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 palatal side of the ridge must be notched (14) to counter-sink the post.

Generally a ridge with an undercut need not be reshaped. However, if a bony knob (15) at the crest complicates the access to the underlying narrow portion of the ridge (16), the knob can be removed, leaving the ridge at least 2.5 mm wide.

If there is enough bone above the undercut (17) 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.

A palatal undercut, which usually results from disease, is much rarer than a labial undercut. In such a situation, the socket

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