Dr. Paul Tipton discusses the advances in full arch implant restorations and presents a case study demonstrating how they can be applied to improve facial esthetics
For many years, the holy grail of implant prosthodontics has been a passive fit of the bridge framework onto titanium implants. The original Brånemark protocol (1981) relied heavily on this goal to ensure a long lasting restoration and longevity of the implants.
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While a passive fit may have been achieved on many traditional acrylic on gold, screw-retained restorations, several further difficulties were encountered achieving the same passive fit with a porcelain-fused-to-metal (PFM) bridge.
Jemt (1996) stated that, in fact, none of the prostheses he tested presented a completely passive fit. His study indicated that a certain biologic tolerance for misfit may be present in most restorations and, in conclusion, that an absolute passive fit was impossible to attain for a traditional screw-retained restoration.
Impression techniques
Carr (1991) and Hsu (1993) have shown that full arch impression techniques using either pickup or transfer style impression copings are also inaccurate. Many hours have been spent in sectioning frameworks from an inaccurate case, picking up these sections in the mouth prior to soldering in an attempt to achieve an adequately fitting framework (see Figure 6 in the case study).
Shiffleger (1985) showed that large one-piece castings are not accurate and that these need to be sectioned and soldered for a more accurate casting. Furthermore, as soon as porcelain is added onto the framework, Bridger (1981) showed that the framework will distort, leading to further in-
accuracies in the fit. These inaccuracies tend to be larger when they are more posterior in the arch.
Passive fit
Misch (1995) suggested that a cement-retained implant-supported prosthesis offers several advantages when compared to screw-retained, in that the superstructure may be more passively attached.
The cement-retained restoration may offer a better chance of a passive fit in some areas of the implant abutment crown interface. However, because of the distortion previously described during impression techniques, casting and then porcelain application, spacers often need to be incorporated under the framework to achieve adequate fit, leaving large cement spaces in the posterior areas of the bridge.
Alternatively, by means of a fit checker, the internal aspects of the bridge can be adjusted to achieve adequate fit with the same result of open cement margins and loss of retention.
In order to achieve retrievability of the restoration, the cemented prosthesis is usually cemented with a soft cement. Unfortunately, one of the problems of the softer cement is that of cement washout. Anecdotally, other clinicians have maintained that this can then lead to excess stress being placed on certain abutments and implants due to cement washout under the crowns on top of other abutments. Again, this leads to potential problems with overstressing of implants.
Esthetics
A further problem that has been observed with the large porcelain-fused-to-metal full arch bridge is that of less than ideal esthetics. It is a daunting task for the technician to build pink porcelain to restore the lost soft tissues combined with the need to restore functioning prosthetic teeth at the same time. A technician only has a limited number of firings available to build up this final fixed restoration prior to the porcelain becoming too translucent and losing its natural color.
Maintenance
Porcelain is a very brittle material and has the potential to fracture under parafunctional and/or impact loading. Cibirka (1992) recommended acrylic as the veneering material for a full arch bridge because of its dampening effect; however, this has been questioned by Davis (1988), as porcelain has been shown to be more beneficial under static loading. Changing from a screw-retained design of restoration with access holes in the center of the occlusal and palatal surfaces to a cement-retained restoration will increase the strength of the final porcelain-fused-to-metal bridge. However, there is still the potential for fracture or cracking during long term function. Should this occur, then it might be impossible to retrieve this from the mouth and repair the porcelain in the laboratory due to the contamination of the porcelain by saliva. This contamination makes the porcelain more liable to explode while in the furnace.
Very often, reshaping of the bridge or composite repair have been the only options to maintain the bridge long term in function, again with loss of form, function, and esthetics. The alternative, which is both time consuming and expensive, is stripping the porcelain and remaking it on the
same framework.
Case study
The female patient was referred from her GP in Brighton. She required a full makeover, including dental and facial changes (Figure 1).
As can be seen from the photos, she was suffering from advanced periodontal disease to the upper teeth and lower incisors together with an unattractive smile. She had a severe Class II division I jaw relationship with a deep fold under the lower lip. She was overclosed with the lower third of the face shorter than middle and upper thirds (Figures 2 and 3).
In conjunction with the implant surgeon, orthodontist, and oral maxillofacial surgeon, the treatment plan was to first correct the Class II jaw relationship with the combination of provisional upper restorations, orthodontics (Figure 4), and orthognathic surgery via a sagittal split procedure in the mandible to advance it to a Class I relationship. She was then to be restored with implants and crown and bridgework to an increased vertical dimension to improve the facial esthetics.
The upper teeth were extracted and bone grafting done from the hip to onlay the maxilla and inlay the sinuses. Eight upper and two lower implants were then placed (Figure 5).
In the mandible, a chin implant was placed to eliminate the fold under the lower lip. The lower jaw was then restored with bridgework on the implants and crowns on the side and posterior teeth (Figures 7 and 8).
The final result was cosmetic in that the patient made it known from the outset that she wanted a white even setup with no esthetic elements (Figures 9 and 10).
The aims and objectives of the treatment plan were all executed according to standard occlusal and prosthodontic principles.
Acknowledgments
The author would like to thank John Wibberley and Martin Fletcher from Waters Edge Dental Laboratory for their help in this case.
- Bridger DV, Nicholls JL. Distortion of ceramometal fixed partial dentures during the firing cycle. J Prosth Dent. 1981;45(5):507-514.
- Carr AB. Comparison of impression techniques for a fiveimplant mandibular model. Int J Oral Maxillofac Implants. 1991;6(4):448-455.
- Cibirka RM, Razzoog ME, Lang BR, Stohler CS. Determining the force absorption for restorative materials used in implant occlusal surfaces. J Prosth Dent. 1992;67(3):361-364.
- Davis DM, Rimrott R, Zarb GA. Studies on frameworks for osseointegrated prostheses: part 2. The effect of adding acrylic resin or porcelain to form the occlusal superstructure. Int J Oral Maxillofac Implants. 1988;3(4):275-280.
- Hsu CC, Millstein PL, Stein RS. A comparative analysis of the accuracy of implant transfer techniques. J Prosth Dent. 1993;69(6):588-593.
- Jemt T, Book K. Prosthesis misfit and marginal bone loss in edentulous implant patients. Int J Oral Maxillofac Implants. 1996;11(5):620-625.
- Misch CE. Screw-retained versus cement-retained implantsupported prostheses. Pract Periodontics Aesthet Dent. 1995;7(9):15-18.
- Schiffleger BE, Ziebert GJ, Dhuru VB, Brantley WA, Sigaroudi K. Comparison of accuracy of multiunit one-piece castings. J Prosth Dent. 1985;54(6):770-776.
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