22 Theories and techniques of oral implantology

done on both sides of the jaw so that the bundles are not impinged upon and are circumvented by the peripheral borders of the implant.

The external oblique ridges should be exposed posteriorly on both buccal aspects for the impression, while the entire symphysis should be exposed in the anterior inferior labial portion of the mandible.

A surgical tray is then made by mixing cold cure acrylic until it is of a clay-like consistency. The acrylic "clay" is molded to the exposed jawbone, with special care being taken not to burn the soft tissues. Burning may be avoided if most of the liquid has evaporated, which should have occurred if the material is clay-like or if the tray, after it has hardened sufficiently, is removed prior to heating. Burns caused by heating may be avoided by spraying the acrylic with cold water, keeping it cool. Although there is danger of burning the soft tissues or bone by making the surgical tray over the exposed bone, this method is the surest way of getting an accurate impression. One of the most important initial steps in achieving a successful subperiosteal implant is an accurate impression that includes each and every imperative anatomic landmark.

When the surgical tray has been made, it should be notched along its inside surface and treated with an adhesive agent that allows rubber to bind to it. This tray will be used to make a bone impression.

A rubber base material (Neoplex by Surgident) is usually the material chosen for a bone impression. Rubber should be mixed to an even consistency, placed into the tray, and held in the mouth for at least 10 minutes to set. The advantage of using Neoplex instead of Coeflex (Coe Laboratories), although both set at about the same speed, is that the operator can mold the Neoplex in his hand like clay. He can then set it onto the exposed bone and mold it around the various anatomic landmarks before setting the tray over it.

After the impression has hardened, it is removed and checked for accuracy (Fig. 1-58). Some opera-tors take two impressions as a routine procedure. However, one impression is preferable, because paresthesia can result if some of the rubber impression material flows underneath the neurovascular bundles and sets there, creating a great deal of pull and tension when the impression is removed. Some-times too, there can be a dehiscence of the mandibular nerve along the resorbed alveolar crest of the bone. In such a situation some of the impression material might slip underneath, creating some post-operative complications. Today, however, Input

(silicone; Vicon, Inc.) is used to make the tray and the impression.

While the mandible is still exposed, a surgical bone bite using wax should be taken. The xvax is merely heated and placed over the exposed bone of the mandible, and the patient meshes it with the teeth of the opposite arch. This allows the technician to know how high to make the four abutments that will protrude from the substructure into the oral cavity. An even more accurate method for obtaining a bone bite is to add soft wax to the acrylic bone tray after the rubber impression on the other side of the tray has hardened and been re-moved. When the wax is soft enough, it is sealed to the acrylic tray. The tray is replaced over the ex-posed mandible, using the rubber impression as a guide. The patient then bites into centric occlusion. An alginate impression of the opposing jaw must also be taken.

An accurate measurement of the soft tissue over-lying the four areas where the abutment posts or necks will protrude should also be sent to the technician. In this manner the height of the necks that join the abutments themselves with the substructure framework can accurately be determined.

After all impressions have been taken and the bone thoroughly cleaned of any debris, the soft tissues may be sutured back into place. Care must be taken in suturing to knot the threads to one side of the wound so that the knots will not press on it (Fig. 1-59). Then one of the patient's dentures can be

Fig. 1-58. A rubber base impression of the exposed bone must include the external oblique ridges, genial tubercles, symphysis, mylohyoid ridges, and nerve bundles on both sides of the arch.