CHAPTER 14 Mandibular Subperiosteal Implant Surgery

CAVEATS

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).

COMPLETELY EDENTULOUS DESIGNS

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.

FIGURE 14-2. Severe mandibular atrophy is managed well with a subperiosteal implant. Dehiscence of the neurovascular bundle can be a significant complication.

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.

Procedure

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).

FIGURE 14-3. Absence of vestibular morphology and knife-edged ridges are aspects of an atrophied ridge that work well with a mandibular subperiosteal implant.

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.

Incision

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.

FIGURE 14-4. Surgery begins with a crestal incision. It should be made from retromolar pad to retromolar pad. An anterior vertical relieving incision is required.

Reflection

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:

1. The external oblique ridges and beyond, inferiorly to the angle, anteriorly to the inferior border for the full width of the ramus.

2. The mental foramina, superior rims only (the neurovascular bundles should not be dissected from their fibrous sheaths).

3. The symphysis to the inferior border of the mandible for a width from canine to canine.

4. The genial tubercles, superior surfaces only (the genioglossus muscle attachments must not be released).

5. The lingual cortex from the anterior ends of the mylohyoid ridges forward to the genial tubercles; the reflections are carried down lingually to the inferior borders in the canine areas. Often finger dissection is the simplest and least traumatic means of performing this procedure. The implant periphery is extended into these areas to compensate for the potential facial structural weakness caused by the scallops designed to circumvent the mental foramina. The mylohyoid muscles are not elevated from their attachments.

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 14-5. Suturing the lingual flaps to each other across the dorsum of the tongue facilitates complete exposure of host bone. The genial tubercles, mylohyoid muscles, external oblique ridges, mental foramina, and symphysis should be seen.

Impression Making

1. The EZ Tray material is heated to 178° F in an electric water bath. When it is soft, one cake is removed and compressed over the bone immediately after the surgical assistant removes the sponges. It is molded so that it extends to all significant peripheral regions (Fig. 14-6). The molded piece (tray) then is irrigated until it hardens.

2. An extra piece of material is fashioned into a handle and attached to the tray. Drying is not necessary, because the material is quite cohesive. The tray then is removed; if it is shy in any area, it is augmented with newly softened material until the entire planned bony host site is covered. The surgical assistant should use a beaver tail (Henahan) retractor beneath one buccal flap, and the surgeon should retract the other, to perform enough rehearsal seatings to ensure efficient, rapid tray placement at the time of the final impression. If manipulation of the buccal flaps presents a problem, several 2-0 black silk sutures are passed through the flaps. The ends of the sutures can be tied in loops after the needle is removed, and they can be used as retractor handles. The wound is repacked with moistened sponges while adhesive is applied to the tray.

3. Impression material (Surgident’s Neoplex regular) is mixed thoroughly, the tray is loaded with the material, and the implant is seated per rehearsal (Fig. 14-7). It is held firmly in position for 8 minutes. A valuable handling characteristic of this material is that it need not be used until after it passes through its sticky or stringy stages. A puttylike consistency is proper for manipulation.

4. The impression is removed and examined for the following landmarks:

• The mylohyoid muscle, in the form of ruffled borders

• The genial tubercles, as two indentations

• The symphysis, which is smooth and curved inwardly as it represents the inferior border of the mandible

• The mental foramina, which may be seen only as semilunar indications of their superior borders, along with the fan-shaped representations of the neurovascular bundles

• The external oblique ridges, which should be clearly demarcated with extensions beneath them down to the inferior borders, when possible (Fig. 14-8)

FIGURE 14-6. Shaping thermolabile EZ Tray segments directly on the bone aids impression making, because the segments can be seated separately if necessary.

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.

Surgical Jaw Relationship: Centric Recording

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 14-9. After the impression has been made, vertical and centric relationships are established with an Optosil-lined wax rim resting directly on the bony surface of the mandible.

Closure

1. The flaps are coapted and compressed firmly with a surgical sponge grasped with the thumb and forefinger.

2. The buccal flap is grasped at the right posterior end of the incision with a Gerald forceps. Closure is performed using 3-0 black silk suture on a FS 2 or C-13 cutting needle (Fig. 14-10) in a continuous box-lock suture (see Chapter 6). The routine postoperative regimen is used (see Appendix G). The patient’s original mandibular denture may be adapted for use as a transitional appliance after it is lined with a tissue conditioner, such as Coe-Comfort or Viscogel.

FIGURE 14-10. Closure is accomplished with a box-lock suture.

Implant Design

1. The model is boxed and poured in pink Vel Mix stone.

2. After the Vel Mix has set, the cast is separated and checked for the landmarks listed previously (Fig. 14-11). It then is articulated with the countercast.

3. The designated places for primary, secondary, and peripheral struts are marked with a sharp pencil. An X is used to mark the position of permucosal cervices or posts. The peripheral struts are placed to the fullest extent of dissection, but undercuts are respected. Undercuts can be established by surveying the cast before design. Only primary struts (those that bear the abutments) should cross the ridge crest. Each must have only one cervix. The secondary struts should be plentiful so as to incorporate strength and rigidity, but they must never be placed less than 7 mm apart, otherwise strong periosteal reattachment to bone is discouraged.

4. Three screw holes are placed in each casting and delineated as small circles. The casting, which is waxed slightly below some of the undercuts, still may not achieve primary retention. If this is the case and the implant fails to snap into position, one to three screws are required. One hole is made just lateral to the symphysis (which is too dense to allow entry directly into the midline), and the other two holes, if needed, are made facing anteriorly (to allow direct screwdriver use) entering the lateral oblique ridges.

5. To establish cervical lengths and the height of the bar over the mucosa, 3 mm are added to the tissue thickness measurements. Sufficient space is allowed for cleaning, placement of retention clips or devices, and some implant settling and soft tissue hypertrophy (Fig. 14-12). The laboratory should be instructed to articulate the models and wax and cast an implant of surgical Vitallium in accordance with the outlined design. Bar configurations to accommodate internal clips, Hader clips, O-rings, or other retentive devices must be requested at the time of casting (see Chapter 26). Individual abutments, rather than a bar, may be used if the patient’s prosthetic requirements are best suited by this approach. In such cases, a sturdier infrastructure must be designed to substitute for the rigidity offered by the Brookdale bar.