8

Problem Ridge:

Knife-Edge

The knife-edge ridge is often seen in the mandible particularly in a totally edentulous arch. The mandibular ridge resorbs downward and flat, leaving often the mylohyoid ridge as the highest portion of the arch. When some teeth remain, this narrowing tendency can be somewhat retarded in the edentulous span by osteogenic stimulation from teeth bordering the area. However, the longer the span, the less effective is such "borrowed" stimulation.

Although the ridge may have thinned and flattened considerably from its original dimension (1), the change is sometimes difficult to detect superficially. Typically the soft tissues in the mandible are extremely thin compared to the maxillary tissues. However, as thin as it is it still has a camouflaging effect over the underlying bone especially covering the mylohyoid ridge.

Radiographs still show a tall, bony ridge, but due to the loose attachment of the lingual tissues on the lingual side of the bone posteriorly with its gradual blending in with the floor of the mouth, it makes the mylohyoid ridge look like it is the true alveolar crest since it appears to be more buccally located than it truly is. Only reflection of the soft tissues reveals the true landscape.

The thinning of a totally edentulous ridge may have occurred around the entire arch, or be localized. When a limited edentulous span is involved, its narrowness is usually related to the span's position, its shape prior to tooth loss, its length, and tooth loss sequence. A longer span particularly anteriorly (2) tends to be narrower than a shorter span (3) due to more prolonged abuse. How-ever, even in the longest span, the narrow ridge widens as it approaches teeth (4) . In all situations the labial plate usually be-comes more concave.

A bladevent must be introduced into a ridge at least 2.5 mm wide, approximately twice the width of the bladevent's shoulder. Most knife-edge ridges can be reduced, if need be, to the desired width with a 700 XL bur (5), a heatless stone (6), or a rongeurs (7).

The crest is reduced perpendicular (8) to the vertical plane (9), to retain the labial plate and to avoid countersinking the posts—a process requiring great care because of ease in chipping the narrow, fragile wall of the socket. The crest can also be reduced perpendicular (10) to the inclination of the ridge (11) in those situations where there is extreme labial inclination of the anterior ridge. In

66