resorption of both the mylohyoid and external oblique ridges by rocking against them. At this stage, oral surgery far more radical and extensive than implant surgery may be employed to fit the suffering patient with conventional masticatory replacements, when a sub-periosteal implant is the best solution.

The mylohyoid ridge is principally used as an endosteal bladevent site when a bladevent cannot be used in the alveolar region due to proximity of the mandibular canal. Also, because a bladevent provides a stronger abutment than does a posterior unilateral subperiosteal implant, mylohyoid ridge implantation is attempted when a posterior unilateral site is opposed by strong natural teeth. In these situations the mylohyoid ridge's shape must permit inserting the bladevent without too closely approaching the mandibular canal, threatening a fracture, or penetrating the submandibular fossa.

Stress Adaptations

Mandibular and maxillary adaptations to mechanical stress are strikingly different. Whereas the maxillae are firmly fused to other bones of the upper face, the mandible is separate. It is the only movable bone of the face. This characteristic, and the fact that the mandible does most of the work during mastication, are reflected in morphological adaptations to pressure and tension.

The alveolar processes of the maxillary and mandibular jaws have responded to masticatory pressure in similar ways, at least initially. The alveolar bone surrounding the teeth in both arches is highly trabecular. Pressure on the teeth is transmitted as tension by the periodontal ligament, and the trabeculae are arranged along the lines of tension. They run horizontally in regular or irregular patterns between the teeth and toward the compact bone that forms the outer walls of the ridge, and extend beyond the apexes of the teeth in patterns that differ according to the arch and the tooth's position in the arch. The similarity in adaptive patterns around the teeth in both arches ends beyond the alveolus proper.

The posterior teeth in both arches have more alveolar bone laterally than do the anterior teeth, which may be covered by only a thin layer of compact bone. The amount of trabeculated bone and the thickness and regularity of the trabeculation reflect the amount, type, and direction of stress in that particular area of the dental arch.

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