way (10) up the root. Sometimes the foramen lies closer to the first bicuspid than to the second, again with its height at varying levels.

The foramen is higher than the mandibular canal, and it usually appears so radiographically. However, the distance of the mental foramen from the mandibular canal is difficult to deter-mine from radiographs. It varies considerably, depending upon the precise location of the foramen (11) and the depth of the mandibular canal (12) within the mandibular body. Thus the length of the mental canal may range from a few millimeters to over one centimeter. The further the mandibular canal is from the labial surface of the mandible and the higher and more posterior the mental foramen is to the bifurcation, the longer the mental canal will be. And vice versa. Also, the mental canal (13) itself may be distinct radiographically because the shadow of the foramen (14) is superimposed.

When the site or jaw is edentulous, the alveolar crest becomes the most important reference point — particularly when implants are contemplated. Normally when the crest is high, the foramen is slightty below an imaginary horizontal midline of the mandibular body. When the teeth are lost, ridge resorption moves the crest down toward the foramen. Extensive resorption may place the foramen (15) at the crest, with the mental canal — depending upon its course — dehiscent. If the mental canal rises only slightly superiorly from the mandibular canal, (16) its contents — and perhaps those of the mandibular canal — may be exposed (17). If the canal drops sharply down towards a deep mandibular canal (18), only that superior portion (19) near the foramen may be affected.

The variations in foramen and canal location are stressed to avoid diagnostic generalizations in terms of implants. For example, the superior location of the mental foramen — even with a partially dehiscent mental canal — does not necessarily contraindicate the use of bladevents posterior (20) to it. The mandibular canal itself may be a significant distance lower. Also in addition to bone above the canal, there may be adequate bone lingual to it (21) for a bladevent.

Normally the foramen lies within some mucosal tissue, and occasionally at the border of attached gingiva. When a good amount of bone exists in the area and the ridge is being exposed, it will probably not be necessary to expose the foramen. It will lie below the range of the contemplated implant insertion.

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