CHAPTER 24 Implant Prosthodontics: Fixed and Fixed-Detachable Prosthesis Design and Fabrication
Several options are available for the patient who requests a fixed prosthesis. The appliance can be made so that it cannot be removed (cemented), or it can be fabricated so that the practitioner (but not the patient) can remove it by backing off its fixation screws; this is known as a fixed-detachable prosthesis.
When root form implants are to be used for a full arch fixed or fixed-detachable prosthesis in the mandible, the implantologist should follow some general guidelines. If usable bone is available only between the mental foramina, a minimum of five properly spaced root form implants must be inserted. A full arch fixed prosthesis in the maxilla requires a minimum of six properly spaced root form implants, because the bone is not as dense as in the mandible.
The dental surgeon must always keep in mind that the length of the implant plays a significant role in determining the amount of allowable cantilever extension. Implants 18 to 20 mm long are much more resistant to failure than those that are 8 or 10 mm long. For this reason, maximum distal extension is allowed only with longer implants (i.e., 15 mm in mandibles).
Submergible blade implants are inserted instead of root forms if anatomic conditions dictate their use. In a few cases, their abutments have attachments that are compatible with root forms of the same manufacturer.
Single-stage implants are one-piece devices that protrude through the gingival tissue. Abutments may be attached to them. The implant’s head projects into the oral cavity immediately upon insertion. Such implants are available in both blades and root forms (e.g., Zimmer’s and BioHorizon’s one-piece implant; Straumann’s one-stage design), as well as subperiosteal implants. In general, these implants are used as distal or pier abutments for fixed (cementable) bridges rather than for the detachable types. (The ITI design may be used for detachable designs.) Along with adjacent natural teeth, they also are used for coping bar overdentures.
The best design for a restoration often is one in which the superstructure occupies a position high above the tissues to allow easy access for oral hygiene (“high water” design); or, for a more esthetic result, the restoration may rest directly on or even below the gingiva. Patients often are unwilling to accept the latter design because of problems with speech, saliva, and food lodging. However, the ultimate benefits of this design justify the practitioner’s efforts to persuade the patient.
The surgeon must give special consideration to oral hygiene when planning a low-rise prosthesis (see Chapter 29). The patient’s manual dexterity plays an important role in the amount of home care that can be done, and the practitioner must consider this factor when selecting the type and design of the superstructure.
The UCLA abutment (with or without a metal collar), zirconium abutments, and similar custom-cast abutments should be used when reorientation, reangulation, or esthetic demands require that no metallic implant, collar, or transepithelial abutment (TEA) material be seen at the gingival margin. If, for example, less than 1 mm of gingival tissue overlies the implant, when the implant is uncovered, the standard TEA (1 mm or more in height) places the margin of the restoration supragingivally. In such cases, a specially designed abutment is attached to the implant, solving the cosmetic problem (Fig. 24-1).
Abutments (TEAs) for submergible implants are designed to be attached after integration has taken place. Implants are allowed to integrate for 4 to 6 months after surgery, depending on their location. If desirable, existing or new removable or fixed prostheses are used during this period; the surgeon should opt for a design that spares the operative sites from contact. If this is not possible, the ridge laps over the implant sites are relieved and relined with a soft material. (Chapter 20 presents the details of temporization.) Once the integration period has been completed, each implant is ready to receive its TEA. (The proper selection and methods of attachment of abutments are presented in Chapter 22.)
Single-stage implants (implant heads that are allowed to protrude from the time of insertion) succeed best if they are allowed to remain in a trauma-free environment for a 12-week healing period. If the surgeon places them in a position that requires coverage for esthetic reasons, the permucosal restoration should be relieved so that the implant heals free of trauma, including occlusal prematurities and parafunctional habits, such as tongue thrusting, nail biting, and bruxism. Implants that require it can be stabilized with a temporary splint, which should be fabricated before surgery. When the splint is removed, classic single tooth restorative procedures should be performed. A number of single-stage systems (e.g., Nobel Biocare’s NobelActive) can be placed to function immediately.
After the TEAs have been placed, they require a temporary prosthesis. This protects the abutment heads, provides the patient with a more stable interim prosthesis, and adds comfort. It also establishes esthetics, tooth form, and occlusal stability. The temporary prosthesis sometimes is referred to as a provisional prosthesis. Detailed instructions on the selection, conversion, and fabrication of interim prostheses are presented in Chapter 20.
If the patient wears a complete denture, it should be modified at the same appointment during which the abutments are inserted. An indelible pencil is used to mark the positions of the abutments on the tissue-borne surface of the prosthesis. These areas are relieved with an acrylic bur to create space for each abutment. When the denture seats completely over the abutments, it is relined with a temporary soft relining material (e.g., Coe-Soft). The denture is now secure, stable, and comfortable.
Other interim prosthesis options are available. One logical approach is to create a fixed-detachable temporary prosthesis from a denture. The positions of the abutments are transferred to the denture using pressure indicator paste (PIP) or Thompson’s sticks, and holes are drilled completely through the acrylic base at each site. This allows the denture to be seated passively over the abutments.
Plastic waxing sleeves or temporary metal copings provided by the manufacturer are attached to each abutment by their fixation screws. If plastic waxing sleeves are used, they are scored and roughened with a bur to create mechanical retention, because acrylic does not bond to them. All areas adjacent to the metal TEAs are blocked out with periphery wax before the fenestrated denture is seated. Pink self-curing, hard relining acrylic is painted around each plastic waxing sleeve or notched metal coping to ensure reliable adhesion.
After the acrylic has set, the fixation screws are removed from each abutment, the denture and luted sleeves are removed, and final curing is completed in a pressure pot. The denture is used to reconfirm passive seating. Excess acrylic is trimmed from the undersurface to leave space around each abutment for proper oral hygiene.
All flanges to the crest of the ridge are cut away, creating a convex, highly polished undersurface. The posterior saddles of the denture are reduced distally so that no more than 15 mm of denture extends beyond the distal portion of the most posterior abutment. Flanges and soft tissue ridge lap contact are important in these areas for additional support.
In single-stage implants (e.g., blade, subperiosteal, and Zimmer’s one-piece root form implants), the abutments are attached. A working or master cast is created so that a final prosthesis can be fabricated. To make this cast and its dies, an impression of the abutments is made using any comfortable technique. First, alginate is used in a stock tray. A cast is poured, and a custom resin impression tray is prepared. Any conventional technique can be used, with judicious application of gingival retraction cord around teeth and, if necessary, around implants. Elastomeric impression materials are recommended. All dies are poured in epoxy resin for strength (Figs. 24-2 and 24-3).
FIGURE 24-2. A, A fixed prosthesis sometimes is desirable for restoration of a universal subperiosteal implant combined with natural abutments. B, This composite-veneer gold coping bridge is both functionally and esthetically acceptable as a final prosthesis. If temporary cement is used, gold copings are provided for the natural teeth.
Morse taper (cold weld) abutments include traditional root form implants and single-stage implants, such as some Straumann models and Zimmer’s Screw-Vent (which, although considered a single-stage implant, has attachable abutments). If a system with non-screw-attached abutments is used (e.g., Bicon models), they are inserted so that a classic impression procedure can be performed.
Threaded one-piece abutments without collars are rarely used. To produce a working cast for this type of abutment, the abutments, which have one flat side, are screwed into their implants. Their alignment and relationships of fit are checked clinically and with bitewing radiographs; if necessary, the alignment is corrected with cooled diamond stones.
If the alignment of the TEAs is acceptable after initial placement, each is scored with a ¼-inch round bur at the gingival margins. This level is important to record in areas of esthetic concern. The TEAs are numbered with the same bur, returned to the mouth, and packed with retraction cord. An impression is then made in the conventional manner.
The scoring shows the laboratory where to end the restorations, or the laboratory can shorten the margins and return the abutments for a new master impression. One of these strategies is necessary, because this device has no shoulders or finishing lines to indicate where the crowns must terminate. The laboratory should pour the impression in epoxy resin and then proceed with fabrication of the superstructure (Fig. 24-4).
FIGURE 24-4. A, Cemented or threaded abutments may be designed for fixed retainers to be attached with hard cement. B, A polyether (Impregum) impression is made using the classic technique for fixed prostheses. C, A master cast is made, on which the final prosthesis can be fabricated.
If the location, angulation, or emergence profile presents a problem, the solution is best worked out on a master cast rather than directly in the mouth. To accomplish this, a transfer of the relationship of the implants to their surrounding tissues and teeth is obtained; transfer copings may be used. Tapered copings have at least one flat side to ensure that they seat accurately in the impression.
After the abutments have been tightened securely to the implants and complete seating has been confirmed radiographically, the appropriate transfer copings are screwed to the abutments. As an alternative, direct impressions of the implants can be made. This requires acquisition and placement of abutment analogs for each implant. Impressions are made with either a closed or an open top tray, depending on the design of the transfer device. The procedure for doing this is as follows (Fig. 24-5):