their profound influence on implant potential. Unless the mandibular alveolar ridge is severely traumatized, it tends to resorb more slowly than does the maxillary ridge under similar conditions. Initially in most sites, particularly the posterior ones, the ridge is stronger than its maxillary counterpart due to the affects of nearby muscle attachments. When a tooth is lost, muscle pull tends to help maintain alveolar bone as the stresses are diverted toward trajectories. The strength of these trajectories is exemplified by the shape of a severely resorbed alveolar ridge. The alveolar crest (17) sinks between the mylohyoid (18) and the external oblique (19) ridges, which are maintained by muscle pull. Anteriorly in cases of extreme resorption, the genial tubercles (20) may be higher than the alveolar crest (21) proper.

The changing relationships between the muscle-maintained ridges and the alveolar crest cause numerous problems in de-signing a stable conventional denture. With a convenient, mound-shaped ridge lost, the denture is difficult to stabilize. It can, and often does, batter the residual ridge into ulceration and resorption. Corrective surgery to reposition the muscle attachments and re-shape the ridge may be the only solution when using a conventional appliance.

Implants offer alternatives. The implantologist can possibly utilize the muscle-maintained ridges as potential bladevent implant sites, depending upon the presence or absence of concavities and the location of the mandibular canal. The bladevent may be off set-lingually in the residual crest, but form a relationship to the maxillae that is nearer to natural than that possible with a conventional appliance. Because the alveolar crest tends to migrate outward posteriorly as the maxillary crest moves inward, offsetting a blade-vent lingually may prove distinctly advantageous from a prosthodontic point of view.

When the cancellous alveolar bone has extensively resorbed in the mandible, the thick compact bone forming the mandibular "shell" remains. Furthermore, a generous layer of compact bone reforms over the crest. Compact bone is exceedingly stable; it does not resorb under surface pressure. This characteristic, plus the firm anchorage permitted by its distinct horse-shoe shape, make the mandible ideal for a subperiosteal implant.

In summary, whereas maxillary adaptations to stress minimally retard the loss of a distinct bony dental arch, mandibular trajectories encourage the maintenance of the bony ridge. These factors help make the mandible a more favorable implant site.

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