Extensive ridge resorption calling for the use of. a subperiosteal implant will require exposing the foramen. This must be done with great care. After ridge resorption has lowered the crest and diminished the distinct contours of the alveolus, the foramen can usually be located by palpation. With its location by palpation coordinated radiographically, the soft tissues are incised and retracted (reflected).

When bone resorption is extensive and the radiographs clearly indicate that the foramen is near the crest, the incision to reveal the ridge should be made lingual to the crest. As the scalpel passes the mental nerve on each side of the arch the incision can be made along the anterior crest, within the zone of attached gingiva, to avoid severing the neurovascular bundle.

The soft tissues should be reflected away from the crest with a blunt instrument, preferably a subperiosteal elevator, never separating from bone with a scalpel in this region. When a semilunar depression in bone is seen, the foramen has been reached. Great care must be used to probe around the tissues and free the neurovascular bundle. Because the bundle trifurcates as it leaves or soon after it leaves the foramen, all three branches must be freed. The neurovascular material is encased in elastic connective tissue, and will slip relatively freely within the tissues in which it is inserted. This gives a certain amount of freedom in handling the material.

Because the foramen is an opening in bone and because the neurovascular bundle and its branches are distinct features, special attention must be paid to the area during impression-making procedures. All impression material packed into the foramen must be removed. It is very likely that impression material will creep under the neurovascular bundle and harden. The material must be freed under the bundle before removing the impression to avoid tearing the blood vessels and nerve.

The Mylohyoid Ridge

Mylohyoid muscle attachment begins low on the lingual side of the mental symphysis, between the digastric fossa and the genial tubercle, and gradually inclines diagonally along the mandibular body toward the ramus, ending at the third molar. In very few cases, bony reinforcement of the attachment site exists as a distinct ridge along the entire length of the site. In most cases, a notice-

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