ommended when the operator is unsure of the canal's exact course in relation to the alveolar crest.

The bone between the cortical layers of the alveolar crest and the mandibular canal margin is spongy bone, and highly responsive to surface stress. Consequently, the bone separating the two points can be worn away. The canal then becomes a bony trough with its contents exposed to whatever pressures are applied to the surface of the ridge. The dehiscent situation is more common in the bicuspid region where the canal, as the mental canal, moves superiorally toward the crest and outward toward the foramen. A denture pressing on this area can be acutely painful and intolerable.

The neurovascular bundle can be locally dehiscent (20), or dehiscent along most of its entire length (21) in the mandibular body, from the bicuspid region through the molar region. A posterior dehiscency is less common than a mental nerve dehiscency, principally because the inferior dental nerve is further from the crest than the mental nerve is where it mounts superiorally to pass into the soft tissues. However, extensive periodontal disease, traumatic extraction of the molars or premolars, a poorly fitting denture that batters the ridge, or overerupted unopposed maxillary teeth can lead to a deep-dish picture of resorption in the more posterior edentulous areas. In an individual with a highly positioned canal, the likelihood of dishing producing a dehiscency is great (22).

When the mandibular or mental canal is dehiscent, the neurovascular bundle can be carefully deepened or repositioned. The original canal can be deepened with dental burs in partially or totally edentulous arches to seat a subperiosteal implant with its bars or struts on top of the ridge or a new canal can be made buccal to the original one in a totally edentulous mandible to insure a generous safety margin on the lingual side of the ridge for bladevent insertion.

The morphology of the blood vessels and nerves facilitates their repositioning. The blood vessels and nerves are ensheathed in a strong layer of connective tissue and form a distinct bundle. The size and elasticity of the neurovascular bundle make it easy to handle without tearing or otherwise harming it during surgical procedures.

Although the canal itself is considered the landmark, primarily because of the ease in identifying its course radiographically, it is the contents of this space and the possible effect on

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