Written by Marvin A. Fier, DDS, FASDA Thursday, 31 October 2002 19:00
Porcelain-fused-to-metal crowns and fixed bridges are some of the most commonly used restorations in dentistry today. However, as wonderful a material as porcelain is, it can and does occasionally fracture. How the problem is solved will depend on many factors. The purpose of this article is to describe the various factors that should be taken into account, and utilizing a case study, outline a method for repair with a PFM overcasting and state-of-the-art adhesion.
From a technical perspective, to repair fractured porcelain intraorally has become easier than ever, but it is always a challenge and its success is often unpredictable. There are several factors to be considered in determining how to treat the fractured porcelain. The size of the area of fractured porcelain will help determine the method of repair. Whether the fracture is on an individual crown or on a bridge, and what opposes the fractured porcelain intraorally, should be factors in deciding how to treat the problem. The type of occlusal stress on the involved area and where the fracture is located in the arch should also be considered in deciding on a method of repair. If the area in question is on an individual crown, it is not as involved to make a single new crown for the patient as would be the case if the fractured area is part of a bridge.
If composite or natural tooth structure opposes the fractured porcelain and the area in question is not very large, an intraoral repair can be the first choice. With sandbasting of the fracture site, appropriate primers and adhesives, and meticulous technique, the addition of composite can work well. The problem with this technique is that it is often difficult to match the color and texture to the surrounding intact porcelain. In addition, the bonding between resin and porcelain is susceptible to marginal leakage, which may ultimately cause an aesthetic failure.1
In addition to intraoral repair, another option is to remove the restoration with the fractured porcelain and repair or redo the porcelain in the laboratory. This technique will necessitate temporization of the crowned tooth or bridge. In many cases the restoration cannot be removed and must be cut off and replaced. Unfortunately, the clinician does not always discover this until attempted removal is underway.
In situations where a well-functioning full-arch bridge sustains a porcelain fracture, a clinician might be reluctant to attempt to remove the bridge. There is a good chance that the rest of the intact porcelain could break in the attempted removal. If the fractured porcelain on the bridge is opposed by porcelain on the opposite arch, a composite repair is not ideal because the composite repair will wear more quickly than the opposing porcelain. However, sometimes our patients do not want to risk removing the fractured crown or bridge, and will opt for the composite repair.
From the patient’s perspective, the length of time he or she has worn a crown or bridge, the length of the bridge span, the replacement cost for a new crown or bridge, and the ability to afford a replacement will all influence their reaction to the fracture. In those cases where you want to remove a bridge, and the patient knows they may be headed for a replacement bridge, I’m sure you’ve heard the question, “Is there anything else you can do?” What other options, if any, exist to deal with these difficult (and, fortunately, rare) situations?
This article describes a case in which the patient fractured porcelain (on more than one occasion) on the lower right lateral incisor of a full-arch implant-supported porcelain-fused-to-gold fixed bridge. She has been wearing the prostheses successfully for years. It should be noted that opposing the fractured prosthesis was a maxillary full-arch fixed porcelain-fused-to-gold bridge.
The person involved is a long-standing patient in the practice. She wears an upper full-arch fixed PFM bridge. I reconstructed her lower dentition with an implant-supported porcelain-fused-to-gold full-arch prosthesis in March 1995. Three years later, in April 1998, she fractured a large piece of porcelain from the facial and incisal of tooth No. 26 while biting on a lollipop. We decided to try a composite repair, and I repaired the fracture successfully. In August of 2000, she presented with No. 26 fractured again. In an attempted self-repair prior to her coming to see me, she used glue to reattach the fragment.
What were our options? We could attempt to repair the fractured porcelain with composite again. Another option was to try to tap off the prosthesis and send it to the laboratory for either a repair or stripping and baking of new porcelain. Since this was an implant-supported bridge, it was likely I could tap it off because it was originally cemented onto the implant abutments with Temp-Bond (Kerr). A third option, which the patient did not particularly like, was to make a new lower reconstruction. A fourth option was to construct some type of crown or veneer and bond it to the substructure.
In view of the opposing porcelain occlusion and the fact that we had tried a composite repair once before, it would not be my first choice. An attempt to remove the prosthesis might cause other areas of porcelain to fracture. Even with successful removal of the reconstruction, a full-arch temporary bridge would need to be created to be worn while the repair to the PFM bridge was being attempted.
If the prosthesis was removed successfully, a laboratory repair of the fractured area would be unpredictable because the prosthesis had been in the mouth for over 5 years. Repairing porcelain that has been exposed to the oral environment for more than 2 weeks can be a technician’s worst nightmare. We expect porcelain restorations to be sealed on the surface, but this is rarely the case. Microcracks and small processing voids exist. Any occlusal adjustments we make render the porcelain surface open. Moisture from the mouth and other organic materials infiltrate any unsealed areas, and these openings allow gas bubbles to form if the porcelain is not dried out carefully prior to attempting a repair. Even with meticulous laboratory technique, there is a chance the remaining porcelain will break apart in the porcelain furnace. (Personal communication with multiple ceramists.)
In addition, the thermal history of the metal frame in the oven (when the reconstruction was first completed) could influence the number of bakes and the integrity of the full-arch metal framework in the lab oven during the attempted porcelain repair or replacement of all the porcelain. Finally, if it were necessary to replace all the porcelain on the old frame, the successful occlusal scheme would need to be replicated.
No matter what method was used to attempt to save this bridge, there were significant problems. When discussing the possibility of a new prosthesis, the patient asked me “Isn’t there something else we can do?”
|Figure 1. Unit-Built bridge design.|
When I began clinical practice, I worked with the late Maurice Saklad, a dentist who introduced me to a technique he used occasionally. It was called the “Unit-Built Bridge.” It is a technique in which a solid metal frame was constructed, and individual modified porcelain jacket crowns were placed over the frame (Figure 1). This allowed a technician to create a bridge that looked very natural because there was physical separation between the individual crowns.
There were several problems with this technique. First, the porcelain jacket crowns were not very strong (compared with today’s all-porcelain crowns). There was also a major design problem in that each porcelain jacket had a facial and lingual surface, but no real proximal surfaces. The proximal porcelain was v-shaped so the crown could slide onto the underlying frame (Figure 1). The lack of interproximal walls on the porcelain jacket led to compromised retention and the absence of a resistance form when the crown was placed on the underlying substructure. In addition, the method for attaching the porcelain jackets to the frame was conventional cementation, rather than bonding. The numerous problems with this method of bridge construction left much to be desired. Yet, the concept of the unit-built bridge seemed to offer a method that might allow for preserving the existing reconstruction.
Various techniques for repairing PFM fixed partial dentures with overcastings have been described by several authors.2-4 The technique used in this case is a modification of the original “Unit-Built” technique. Instead of using a porcelain jacket to fit over the frame, an overcasting was made and porcelain fused to it in a fashion similar to a traditional PFM. Although the same design problems exist with the PFM overcasting as with a porcelain jacket, the modified PFM is far superior to the modified porcelain jacket due to the strength of the metal coping.
After consultation with one of my ceramists (Robert Renza, MDT, New City, NY), we decided to create a PFM crown using a nonprecious metal. By fabricating the crown with a nonprecious alloy, the coping could be cast very thin and still be strong. In our limited space situation, this was desirable. The crown would then be bonded onto the existing bridge frame. With the new adhesive systems available today (as compared with the conventional cements used with the older Unit-Built technique), I thought this method of saving our patient’s reconstruction had a chance to work.
When resin cements are to be used on precious or semi-precious metals, tin plating is advisable to increase the bond strength to the metal surfaces.5 The actual process of tin plating is not difficult, but achieving the right amount of tin can be. The ideal amount of tin deposited on the surface should be between 0.2 to 0.5 µm. With a depth greater than 0.5 µm, the bond strength of resin cement to metal begins to decline.6
To eliminate the need for tin plating precious and semi-precious alloys, a metal primer, Alloy Primer (Kuraray), was developed. This product contains VBATDT, a newer monomer, which adheres to precious and semi-precious alloys and co-polymerizes with resins. With the advent of this alloy primer, tin plating with its technique sensitivity and the possibility of overplating are eliminated.
|Figure 2. Emergency temporary crown.||Figure 3. Roughened understructure.|
The patient was placed into an emergency slot in the schedule so a temporary could be made quickly (Figure 2), because the glue reattachment she had tried did not work very well. At a subsequent visit, the remaining porcelain and metal were roughened up with a coarse chamfer diamond (Brasseler USA), and the metal framework was prepared, ending in a finishing line (Figure 3). A full-arch impression was made using Impregum Penta Soft (3M ESPE). A bite registration using Regisil (DENTSPLY Caulk) and a cast of the opposing arch were also sent to the lab.
|Figure 4. PFM overcasting at high-bisc bake stage.||Figure 5. Checking for internal high spots.|
|Figure 6. PFM overcasting lateral view.||Figure 7. PFM overcasting internal view.|
|Figure 8. PFM overcasting at final try-in.|
The PFM overcasting (Figure 4) was tried in at a high-bisc bake stage for aesthetics and fit. A pressure spot indicator (Occlude, Pascal) was used to reveal any spots where the crown was binding on the substructure. (Figure 5) When the fit and appearance were confirmed, the crown was returned to the laboratory, glazed, and returned for insertion. The design of the PFM overcasting can be seen in Figures 6 and 7. An occlusal view at the try-in stage is seen in Figure 8.
|Figure 9. Microetching the substructure.|
At the insertion visit, wax was placed on the teeth adjacent to No. 26 in order to prevent microetching the porcelain on those teeth (Figure 9). Using a Microetcher ER (Danville Engineering), the substructure was sandblasted with 50 µm aluminum oxide. It has been documented that metallic and porcelain surfaces sandblasted with 50 µm aluminum oxide particles have enhanced micromechanical retention to resin cement.7,8 Since the metal of the full-arch bridge was a precious alloy, it would require either tin plating or appropriate priming in order to enhance bonding of the resin cement. Instead of tin plating, a few drops of Alloy Primer (Kuraray) were applied to the substructure. This product is acetone based and does not require air drying. It has been shown to increase bond strengths comparable with those achieved with tin plating.9 Panavia 21 (Kuraray) was mixed and placed into the PFM overcasting. The inside surface of the crown did not require pretreatment because it was a nonprecious alloy. Panavia 21 contains MDP, a phosphate-based monomer, which provides chemical and micromechanical bonds to nonprecious metal without pretreatment.
|Figure 10. Embrasure protection and cement curing.||Figure 11. Completed case.|
The crown was seated on the substructure, and Premier Sycamore Wedges (Premier) were placed gently in the gingival embrasures of the substructure adjacent to tooth No. 26 (Figure 10). The purpose of the wedges was to prevent resin cement from getting locked into the embrasures. Oxyguard (Kuraray), a gel that prevents an oxygen-inhibited layer from forming, was placed after most of the excess resin was removed (Figure 10), thereby ensuring complete curing of the resin cement. The Oxyguard was washed off after 3 minutes. Any remaining excess cement was trimmed away, and the occlusion was checked. Cleansability of the ridge area was checked, and the patient was dismissed. Figure 11 shows the case completed.
At the time of writing this article, 2 years have passed since the procedure described was performed. I am happy to report that the technique was successful and the PFM overcasting has remained exactly as it was placed with no sign of loosening. How long will it last? No one can say, but for now the patient is extremely happy that she did not have to remake her reconstruction.
I appreciate that many manufacturers are listening to clinicians more than ever before, and developing products to address the profession’s needs. It is through ongoing communication among dentists, laboratory technicians, and dental manufacturers that we make our techniques more user-friendly and continue to improve the way we care for our patients.
1. Finger IM. Salvaging the restoration. Dent Clin North Am. 1987;31:487-503.
2. Cohen B, Weiner S. Restoration of fixed partial dentures with fractured porcelain veneers using an overcasting. J Prosthet Dent. 1989;64:390-392.
3. Gelbman J, Biber CL. Salvaging the broken porcelain-fused-to-metal partial prosthesis. Clin Prev Dent. 1986;8:22-23.
4. Dent RJ. Repair of porcelain-fused-to-metal restorations. J Prosthet Dent. 1979;41:661-664.
5. Yamashita A, Kondo Y, Fujita M. Adhesive strength of adhesive resin Panavia EX to dental alloys. J Jpn Prostho Soc. 1984;28:1023-1033.
6. Wood M, Litkowski LJ, Thompson VP, et al. Repair of porcelain/metal restoration with bonded overcasting. J Esth Dent. 1992;4:110-113.
7.Chung KH, Hwang YC. Bonding strengths of porcelain repair systems with various surface treatments. J Prosthet Dent. 1997;78:267-274.
8. Bertolotti R, Paganetti C. Adhesion monomers utilized for fixed partial denture (porcelain/metal) repair. Quintessence Int. 1990;21:59-82.
9. Tensile Bond Strength Studies, R&D Department, Kuraray Co, Ltd.
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