Subperiosteal implants 539

In other cases partial dehiscence results in the exposure of only the neurovascular bundle exiting from the foramina (Fig. 12-15). The mental foramina on both sides of the arch may also be unilaterally or bilaterally dehiscent. In these not too common situations, the operator must be careful to make the incision quite a bit more lingually than usual to prevent severing the exposed vessels. When the nerves in the inferior alveolar canal are exposed as a result of a dehiscency of the canal itself, the incision must be made lingual to the ridge crest to avoid the vessels. Roentgenograms help predict when these situations will occur. Fig. 12-16, a Panorex, reveals a bilateral mandibular canal dehiscence of the mental foramina on both sides. Fig. 12-17

shows two unilateral dehiscences of the mental foramina.

Making the impression

Aside from the surgery, the most important first step toward a successful subperiosteal implant is making an accurate impression including each and every imperative anatomic landmark. These landmarks are both external oblique ridges, both neurovascular bundles exiting the mental foramina, both mylohyoid ridges, the genial tubercles, and the symphysis. The importance of these landmarks cannot be stressed enough, for without the dense bone in these areas to support the metallic framework, no implant could ever be successful.

C

A

B

D

Fig. 12-18. A, The rubber impression was taken of the exposed bone that included the external oblique ridges, symphysis, genial tubercles, mylohyoid ridges, the neurovascular bundles exiting the mental foramina. B, The implant (silicone putty) impression is taken of the exposed mandible in heavy-bodied material. C, A light wash material is used for finer details. D, A silicone wash bite is taken directly over the heavy-bodied tray while it is still in the mouth.