bone loss, and bone loss is associated with an increasing proximity of an implant site to the mandibular canal. In terms of the amount of bone separating the alveolar crest from the mandibular canal after tooth loss, the individual whose canal lies well below (13) the roots of the teeth has an obvious advantage over one whose canal was so high (14) that it appeared superimposed over the roots. The more bone above the canal, the taller the bladevent body and consequently the greater the bone to implant contact.

In addition to a reduction in bone height between the alveolar crest and the mandibular canal, bone loss also leads to a shift in the relationship of the crest to the canal. Initially when teeth are present the crest (15) in the molar region is lingual to the canal (16), with the distance between the two points increasing as the canal courses forward. Variations in the initial distances are typical. The person with a thick mandibular body, usually a male, may have the canal only slightly buccal to the alveolar crest (17) or markedly buccal to the buccal surface of the roots (18), and there-fore a "considerable" distance from the crest.

After tooth loss, the relationships between the crest and canal alter as the alveolar bone resorbs. In the mandible, particularly noteworthy in the molar region, the alveolar "crest" (19) eventually resorbs thus shifting bucally. This shift brings the crest and canal in closer vertical alignment. However, no matter how extensive the alveolar bone loss, occlusal radiographic studies of edentulous mandibles with a wire inserted in the canal and another laid over the center of the crest, do not reveal that the alveolar crest moves buccal to the mandibular canal. It always remains at least slightly lingual.

These relationships must be considered in making a bladevent channel. Angling a bladevent may avoid the canal, or moving lingually away from the crest and perhaps onto the mylohyoid ridge may prove more advantageous.

The mandibular canal is a normal anatomic space and, as such, has cortical layers of bone of varying thickness at its margins. These cortical layers and the canal itself produce a wide variety of appearances radiographically. Usually the canal appears as a narrow, dark ribbon with distinct white borders, but either the upper or lower border may be indistinct or not even apparent, or the canal may appear only faintly as a gray band. The mandibular canal is more accurately viewed in periapical projections, rather than in the panoramic radiographs that are otherwise so instructive to the implantologist. Therefore, periapical views are rec-

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