Written by Stephen M. Parel, DDS, FACD Friday, 31 January 2003 19:00
Conventional fixed bridge reconstructions on multiple-unit implants have traditionally involved a technique that requires a wax-up and casting for the substructure. Veneering with either denture teeth and resin or ceramics is then accomplished for case completion. This technique, although using time-tested and reliable procedures, is often challenging for creating a precise passive fit at the metal interfaces.1-11 This is especially problematic when long-span PFM fixed partial dentures are constructed due to the distortions caused by thermocycling through the multiple firing stages.12,13
A new computerized milling procedure (Procera Implant Bridge), which creates a substructure from a single piece of titanium, offers an alternative to the conventional casting process, with improved potential for fit and eliminating distortion.
The most important clinical aspect of any procedure for creating a passively fitting implant framework is the accuracy of the impression technique. The master cast should be a replica of the implant/abutment relationships in the mouth, and should be produced using procedures that minimize distortion. Luting the impression copings with an ultra-low distortion resin (GC Pattern Resin, GC America Inc) and picking them up with a rigid open-top tray has traditionally been the procedure of choice for accuracy. Transfer techniques (removing the impression coping from the mouth and replacing it in the impression) are generally considered less precise. When anatomy permits, the use of rapid-setting plaster (Kerr No. 2, Kerr Corp), one of the oldest and most accurate impression materials in dentistry, will provide excellent reproduction of implant position and tissue detail (Figures 1 through 4). The verification of the master cast as an exact replica of the patient’s arch is highly recommended prior to proceeding with framework construction. This can be done with a simple verification jig, made from metal-reinforced resin joined to impression copings, which is very effective for evaluating master cast validity (Figure 5).
|Figure 1. Following a period of immediate loading with a provisional bridge, these five integrated implants are ready for the definitive restoration.||Figure 2. Square impression copings are attached to each abutment for a “pick-up” technique final impression using a rigid open-top tray.|
|Figure 3. Plaster impression material was used to accurately record the spatial relationships between the implant components and reproduce the soft tissue anatomy of the lower arch.||Figure 4. The definitive master cast is poured after a silicone soft tissue. Replicating material was applied around the abutments.|
|Figure 5. A verification jig is highly recommended to validate the cast as an exact replica of the mouth. If the reinforced resin bar stays passively in contact with all abutments after the single screw is tightened, then the cast should be accurate.||Figure 6. The patient is taken through the jaw relationship appointment to a wax try-in. When tooth position and jaw relationships are acceptable, the casts are sent to the laboratory for pattern fabrication.|
|Figure 7. A labial/buccal index is made from silicone putty which allows the space between the dentition and the cast to be visualized.||Figure 8. The original wax-up is essentially duplicated in resin using the core as a matrix. Temporary components are used to attach the resin bridge to the abutment analogs.|
|Figure 9. The resin pattern is reduced to the approximate shape desired for the final milled framework. This shape is for eventual resin processing. A more anatomic configuration is needed for PFM (see Figure 18).||Figure 10. At the milling facility, the master cast is scanned using a probe technique. The location of all abutments is recorded, as is the type of abutment or implant surface used.|
|Figure 11. The resin pattern is laser scanned on all surfaces for further input into the computerized milling machine. The pattern is painted white prior to scanning for easier data recording.||Figure 12. Using information from the scans, the computerized milling machine attaches a single piece of titanium from six different directions using three separate cutting heads. Mineral oil is the lubricating medium.|
|Figure 13. The “roughed-out” bar structure will require hand finishing of the nonfit surfaces for completion.||Figure 14. The metal interfaces (fit surfaces) are precisely milled, as a secondary step, to tolerances in the single-digit micron range.|
|Figure 15. The completed bar structure is surface treated for retention, after which the denture teeth are processed in resin. (Metal bar structure completed by Marotta Dental Laboratory, Farmingdale, NY.)||Figure 16. The screws should go smoothly to final seating without binding. All are tightened to a uniform torque recommended by the manufacturer.|
|Figure 17. The final radiograph indicates a precise fit at the cylinder abutment interface at all locations. These films are most diagnostic when taken at right angles to the metal interface.||Figure 18. The resin pattern for a ceramic restoration is shaped to provide metal support for the veneering material. The metal framework is then produced using the same process.|
|Figure 19. The full-arch PFM restoration fits passively, even after multiple firings. The durability and cleansability of the ceramic technique make it an attractive alternative for some patients. (Bar structure and ceramics completed by Creative Dental Designs, Waco, Tex.)||Figure 20. The panoramic survey indicates a passive fit, but is not as diagnostic as perpendicular individual films.|
Once the master cast has been verified, jaw relationship records are produced and the case proceeds through a wax try-in appointment where tooth position is evaluated and approved (Figure 6). The mounted casts with the acceptable wax-up can then be shipped to a laboratory for construction of a resin pattern for scanning (Figure 7).
A full-contour resin pattern is made by essentially duplicating the wax-up in auto-polymerizing resin (Figure 8). The resin duplicate is cut back to contours for either processing of resin teeth or firing of ceramic materials (Figure 9). The connectors to the implant abutments are temporary resin or metal cylinders that are incorporated into the pattern.
The case is then shipped to the scanning facility where computerized measurements are recorded of both the cast and the resin pattern (Figures 10 and 11). With this information entered into the computer-controlled milling machine, a solid piece of titanium is shaped by burs in three cutting heads attacking from six different directions (Figure 12). The nonengaging surfaces are “roughed in” and require hand finishing and polishing to finalize the product (Figure 13). The individual surfaces which contact the abutment analogs to provide passive fit are milled separately to tolerances in the single-digit micron range (Figure 14). The completed milled bar structure is checked again for accuracy, and is returned to the laboratory for dentition processing.
Ceramic materials are applied with conventional techniques using layering and various translucencies for a realistic effect. Processing resin and denture teeth may require the addition of retention loops through laser welding, or specific surface treatments using air abrasion technology to assure resin adherence (Figures 15 and 16).
Clinically, the fit of these restorations is excellent as long as the master cast is accurate (Figure 17). Of particular interest is the apparent absence of distortion through the various firing cycles when creating PFM restorations (Figures 18 through 20). Previous concerns of warping with gold-based long-span ceramic bridges do not appear to be a compromising factor with these one-piece titanium structures.
This new implant technology is part of the Procera product line (Nobel Biocare), and is available through a growing network of dental laboratories. It offers the possibility of a consistently excellent passive fit compared with conventional casting techniques, and essentially eliminates the labor intensive process of sectioning and soldering sometimes associated with gold alloy castings. Eliminating the distortion commonly associated with multiple firings of long-span ceramic restorations is also a significant benefit.
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Disclosure: Dr. Parel is a consultant to Nobel Biocare.
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