CHAPTER 23 Root Form Implant Prosthodontics: Single-Tooth Implant Restorations
Patients should always be informed of all treatment options for replacing missing single teeth. The following are indications that should be considered when an implant-supported crown is used to replace a single missing tooth.
The teeth adjacent to the edentulous span may be minimally restored or unrestored. Recent restorations placed in or on them may be satisfactory, and using these teeth as abutments for a fixed prosthesis then would be unnecessarily invasive. On the other hand, the adjacent teeth may be compromised and have a poor prognosis but might not require immediate removal. If bone loss or active disease is noted nearby, however, the surgeon must be convinced that the planned host site will not be compromised.
A single-tooth edentulous area requires a provisional restoration to maintain esthetics and function and to prevent movement of adjacent or opposing teeth during osseointegration (see Chapter 21). The provisional restoration can be a removable partial denture (flipper) or a fixed prosthesis. The latter requires a pontic supported by adjacent teeth; this should be done only when the teeth require significant restoration. In such cases, a pontic may be cantilevered from one adjacent tooth with a full-coverage acrylic/composite restoration or supported bilaterally. However, when such peripheral care is required, the need for an implant should be reconsidered. The patient should be discouraged from choosing removable appliances, because their tissue-borne saddles may cause ischemia or in some other manner injure the implant host site.
When conventional, two-stage implants are placed, the surgeon may elect to make an impression of the implant after it has been seated. This allows fabrication of a master cast with an implant analog in position. Provisional or even final restorations can be fabricated on this master cast and will be available for insertion when the implant is exposed (second-stage surgery). This allows the soft tissues to heal in an ideal contour as influenced by the restoration. In addition, treatment time is shortened. Biomet-3i provides a design that facilitates this procedure.
After insertion of the implant, an impression coping is seated. This coping is accommodated by the implant’s hexagonal (or other) antirotational characteristic (Fig. 23-1, A). If the implant comes with a carrier attached to it, the carrier can act as an impression coping to register the timing of the hexagonal or other antirotational characteristic.
FIGURE 23-1. A, An impression of the implant is made at the time of implant placement. B, The transfer coping is completely seated on the head of the implant. C, A stock tray is modified for an open tray impression technique. D, Upon removal of the impression tray, the square impression coping remains in the impression. E, A provisional or even a final restoration can be inserted at the time of second-stage surgery.
The coping, which is tapered or has the square-locking design, has a center screw, which is turned clockwise until it engages the implant. Before full tightening, the coping is turned until it drops into a nonrotational mode by nestling over the hex. The center screw then is tightened completely (Fig. 23-1, B). Using a paralleling technique, a radiograph is taken to verify complete seating of the impression coping.
A polyether or similar semirigid impression material should be used in a standard stock tray. If a square-locking impression coping is chosen, an open style impression tray is required. Tapered copings, on the other hand, can be accommodated by conventional trays.
Open trays require a window directly over the copings. This is achieved by modifying a stock tray. When the tray is seated, the center retaining screw of the coping must protrude through the opening. A square of pink base-plate wax is heated and placed over the opening. It is sealed with sticky wax, closing the tray over the coping (Fig. 23-1, C).
The tray is seated in the patient’s mouth while the wax is still warm. This allows the center retaining screw to leave an indentation in it. After the proper adhesive is used, an elastomeric impression is made using a syringe, followed by seating of the tray.
When the impression material has set, the pink base-plate wax is peeled back. The center screw, which is visible, is turned counterclockwise until it completely disengages from the implant. The impression tray now may be removed. The coping will come away with the impression (Fig. 23-1, D). The appropriate implant analog is secured to the square-locking impression coping in the impression material, and a stone cast is poured.
If a nonlocking, tapered impression coping is used in a closed tray technique, it will remain attached to the implant upon removal of the impression tray. It is removed from the implant, and the proper implant analog is attached to it. The coping is reseated with the analog in the impression. To prevent errors in orientation, these tapered copings must be placed accurately. The copings have flat sides or similar identifying characteristics, so that the implant analog in its model occupies a position that exactly replicates the posture of the dental implant that has just been placed surgically. A stone cast is then placed.
The healing screw is placed in the implant and tightened. It should not be allowed to rotate. If it does, it has not been placed to its full depth, and such movement will require a new impression. If placement is satisfactory, suturing can be completed.
The master cast made from either of these impressions allows the dental surgeon to select and complete an abutment; also, a provisional or final restoration can be made to ideal tooth contours during the osseointegration period. The abutment and restoration can be inserted at second-stage surgery with the expectation that the soft tissues will heal to their outlines (Fig. 23-1, E).
Two types of abutments are available (see Chapter 22); those that receive screw-retained restorations, and those that receive cement-retained restorations. Abutments must always be detachable from these implants. Most abutments are themselves screw retained, although a few other designs are fastened by a Morse taper (or cold weld). These do not require antirotational devices or a cementing medium, and they often are retrievable.
The type of abutment is selected before implantation, because it may affect the positioning of the implants. Screw-retained crowns can be fabricated using prefabricated abutments, which are screwed into the implant. These abutments are supplied with copings, to which the complete crowns are cast. Center screws pass through these crowns and engage them to the abutments.
UCLA-type crowns also are made to be screw retained. These crowns are seated on the implant and are cast abutment-to-crown as a single entity (see Chapter 22). The unit is affixed to the implant with a center screw (Fig. 23-2).