the muscle is associated with a strong, prominent ridge (12), which provides horizontal reinforcement against the bending tendencies of muscles attached to the chin.

The chin is subjected to stress by several muscles. The mylohyoid, geniohyoid (depressor) (13), and digastric (depressor) (14) muscles anchor in bone on the lower inner surface of the chin, near the midline. The two lateral pterygoid muscles pull the mandible inward. In response to these forces, the chin is reinforced by massive compact bone that forms the mental protuberance. Internally the trabeculae form tracts at right angles, making a cancellous bony network (15) that extends diagonally across the midline from the lower border of the mandible to the alveolar crest.

   Between the trajectories and the compact cortical bone of the mandible, the cancellous bone is relatively free of stresses, and its trabeculae are thin, poorly organized, and the marrow spaces wide. An example important to the dentist exists in the angle of the jaw. The reasons being 1.) hematopoetic marrow is stored there. 2.) a deep submandibular fossa may add to the radiolucency. 3.) the size or diameter of the inferior alveolar canal may also contribute to the translucency. Formerly this was considered, and named, a "zone of weakness" (16) by implantologists. Here there are simply not enough bone trabeculae to engage post- or pin-type endosteal implants. These types of implants usually cannot be successfully stabilized because of the scarcity of solid bony contact. The bladevent implant, however, does contact adequate trabeculae because of its broad horizontal extension in bone. Thus of all the endosteal implant designs to date, the bladevent is the most effective in this particular area of the mandible. Even when the bladevent is used, however, the weakness and thinness of the bone trabeculae should be taken into account when making the implant socket and seating the implant.

Mandibular stress trajectories have been detailed because of

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