CHAPTER 14 Mandibular Subperiosteal Implant Surgery
The exposed operative site requires the surgeon’s absolute respect. The bone that supports the subperiosteal implant must be treated with the highest level of care, and its response to the simple trauma of mucoperiosteal reflection is responsible for some level of resorption, as evidenced by radiographic signs of change about 3 months after insertion.
Some clinicians maintain that grooves can be cut into the crest of the ridge (i.e., “witness marks”) to allow countersinking of primary struts. However, this should never be done, because it may threaten the prognosis of the implant. Bone has plastic memory, and if it is altered by cutting a groove or mortise form to seat a component of the infrastructure, it almost immediately begins to undergo resorption. Within weeks, what might begin as an intimate metal to bone relationship can become a growing, rounded radiolucency. This phenomenon, which takes place beneath the abutments, can be the first sign of a cascade of events leading to implant failure (Fig. 14-1).
FIGURE 14-1. Witness marks are grooves cut across the crest of the ridge to allow flush seating of the primary struts of subperiosteal implants. However, these marks should never be used, because geometric patterns cut into bone do not retain their sharp line angles. This radiograph shows the pathologic downfall of an implant under which such grooves had been cut and subsequently were resorbed into rounded troughs (arrows).
Preventing injury to mental or dehiscent mandibular nerves is important. The implant design should be selected with care in advance (Fig. 14-2), and if any question of dehiscence arises, a tripodal infrastructure (discussed later in the chapter) should be planned.
The more complex impressions are made in several parts. The surgeon, therefore, should have the materials and equipment on hand to make an index so that the segments of the impression can be assembled accurately. Tissue thickness measurements are obtained so that the laboratory can construct a casting with abutments that have accurate cervical height.
The laboratory also must receive a good surgical centric recording that relates the bony mandible to the cranial base, or the implant abutments may be placed in incorrect and unusable positions. Complete mandibular implants should not be made in opposition to natural maxillary dentition.
Before beginning surgery, the surgeon should examine the intact mucosa that covers the mandible. The sublingual adnexa are palpated (i.e., sublingual glands, mylohyoid and genioglossus muscles, plicae sublinguales, Wharton’s ducts). These adnexa usually lie above the ridge crest. The vestibules often are ill defined, and a mucosal linea alba is seen at the ridge crest (Fig. 14-3).
Bilateral mandibular block anesthesia is used. Local infiltrations in the form of long buccal blocks and infiltrations deeply into the anterior area also should be used. These infiltrations create profound anesthesia, so that aggressively made flaps can be reflected fully for properly designed implants. The limit for the average adult is eight ampules of 2% lidocaine for the first hour. Subsequent injections are administered only after the solutions given at the outset have been absorbed.
After administering the anesthetic but before making an incision, the surgeon uses a sharpened periodontal millimeter probe to measure the thickness of the mucoperiosteum at the four to six potential permucosal sites (points of implant after emergence) on the linea alba. These measurements are recorded on a chart.
A Bard-Parker (BP) No. 15 blade is used to trace the linea alba, a fine white scar on the crest of the ridge that results from the trauma of past extractions and denture wearing. The incision starts at one retromolar pad and proceeds around the arch to the contralateral side (Fig. 14-4). If the radiograph shows that the mental foramina are at or near the crest of the ridge, the incisions are curved slightly lingually to avoid injuring the emerging neurovascular bundles. In addition, a vertical relieving incision is made just lateral to the labial frenulum. Incisions are made directly to bone so that they need not be retraced.
A sharp periosteal elevator is used to reflect the mucoperiosteum. Full-thickness flaps are elevated carefully. If the flaps resist reflection, a BP No. 12 blade is used, hugging the bone, to cut scar adhesions. The following structures must be exposed and visualized:
After vital anatomic structures have been exposed, the two lingual flaps are sutured to each other with 2-0 black silk sutures, which are brought across the dorsum of the tongue. If the tongue slips from beneath these shoestring ties, its lateral border is included on either side in the suturing process. These sutures are tied tightly so that the tongue, floor of the mouth, and lingual flaps are bundled compactly together in the midline and the mandible is well exposed for impression making (Fig. 14-5). The exposed bone is protected with saline-moistened sponges; this creates hemostasis and prevents dehydration of the tissues.
FIGURE 14-7. A, When undercuts are present in the rami area and a tripodal implant is planned, the tray can be made in two or three segments. B, Each tray portion is filled with an elastomeric impression material and can be seated and removed separately if no satisfactory path of insertion can be found. An index, made while the tray halves are in position, is used after removal for reassembly.
FIGURE 14-8. The completed rubber base impression should have a dull homogeneous surface, mental bundles, genial tubercles, the turned border of the symphysis, both external oblique ridges, and the ruffled border of the mylohyoid muscles.
If the impression is accurate and has a smooth texture, it is set aside while copious saline is used to irrigate and thoroughly debride the host site. If the tray is unsatisfactory, making a new impression best solves the problem; the surgeon should not try to correct the faulty impression. A panoramic radiograph should be taken postoperatively to detect fragments of residual impression material, which are radiopaque. These fragments can be removed during second-stage surgery.
The patient’s acceptable maxillary denture prosthesis or a prefabricated, properly adjusted wax rim is placed on a base plate, and a thick roll of Optosil putty or a prefabricated bite rim is used to establish centric and vertical relationships. The elastomer is pressed and molded to the bone while the patient is guided to closure in a natural mandibular position (Fig. 14-9). The Optosil is removed after it has set; the wound is reinspected and irrigated again with saline, and the retraction sutures are removed from the tongue and buccal flaps. An impression of the upper denture, which should have been made previously, serves as a countermodel.
FIGURE 14-12. Vitallium casting of the lateral ramus design of Dr. Robert James. The peripheral struts should avoid the mental foramina and extend to the symphysis. The Brookdale bar, which is highly polished, has six points of permucosal penetration, and the ramus extensions should curve laterally to allow them to exit from fixed gingivae. The bar’s height is determined by tissue thickness measurements, with additional consideration given to hygiene measures, hypertrophy of the mucosa, and manipulation of retention clips or other extension devices.
If a two-stage “immediate” 12- to 24-hour insertion is planned, regional anesthesia is more difficult to achieve. When the silk sutures are removed, the tissues fall open with ease (which is an advantage of the 1-day procedure). The host site is irrigated with care, and all residual clots and debris are removed. A Poole or Frazier plastic suction tip is used rather than a metal one. The sterilized, passivated implant (see Appendix E) is seated in the host site. It is tapped firmly into place with an orangewood stick and a mallet, with care taken that the flaps are not caught beneath the peripheral struts. Accurate adaptation and primary retention must be verified (Fig. 14-13).
FIGURE 14-13. The second-stage operation involves a soft tissue reflection that can be somewhat less aggressive than the first. Protection of the mental neurovascular bundle must be an integral part of the operation and the implant design.
If retention is not positive, the anterior or all three of the fixation screws are used to establish primary retention. With the assistant holding the implant firmly in place, the surgeon uses an Atwood 47/>