the author by placing wires through the- mandibular canal from its inception at the mandibular foramen on the lingual surface of the ramus to its exit at the mental foramen and placing another wire over the center of the existing residual crest and then radio-graphing the mandible with occlusal films revealed in over two hundred specimens that the canal while in the body of the mandible is located several millimeters buccal to the center of the residual crest.

In those situations, however, where very little or no bone exists above the canal (3) and a severe undercut exists beneath the mylohyoid ridge, (4) trying to create a groove lingual to the canal may cause a perforation of the bur or bladevent itself (5) or result in fracturing off the mylohyoid ridge (6) .

In extremely knife-edge ridges with little bone height above the canal and deep undercuts beneath the mylohyoid ridge (4) contra-indicating bladevent insertions would be a wise choice. In these cases, the sliding cable or sliding ramus implant or the five piece symphyseal-rami system (7) shows great potential.

The Totally Edentulous Mandible

When a radiograph indicates that a good amount of bone (1) remains above the mandibular canal, appropriate designs are selected by placing the potential implants or a transparent sheet bearing implant outlines over the radiograph.., Most manufacturers supply special mandibular designs, shaped to accommodate most variations. A double-posted design is desirable, with the body of

* Implants International—N.Y.

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