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Written by Ross. W. Nash, DDS Thursday, 01 May 2003 00:00
I think that porcelain veneers are one of the finest aesthetic treatments that have been developed by our profession. Because of the lamination effect of bonding the ceramic material to the tooth structure, enough strength is developed to allow for function and durability. A ceramic material on its own is brittle by nature. However, when laminated to an underlying strong structure, it is markedly strengthened. That is why porcelain fused to metal works for crowns and bridges. The brittle porcelain is laminated to the metal understructure that provides it strength. This might be compared to a ceramic floor tile. A strong man may be able to break a piece of tile with his hands, but once it is laminated to the floor, it rarely breaks because it is supported all along its undersurface. Supported, a ceramic material can be very strong. The fact that we can now bond these materials to natural tooth structure allows us to eliminate the metal. The tooth becomes the underlying support structure, and the lamination results in a strong restoration.
THREE PREPARATION PHILOSOPHIES
I have watched a steady increase in the use of bonded porcelain in the last 20 years. There are 3 schools of thought on preparation design that I have seen develop in that time.
In this philosophy, the porcelain is not considered the restorative material, but rather is an aesthetic, durable, and stain-resistant surface covering for the underlying restorative material, which is composite resin. An impression is taken of the unprepared teeth, and a stone model is produced. On this model, thin porcelain shells are fabricated. When they are bonded to place, deficient areas in the tooth such as those caused by removing tooth decay are filled in with the composite. The porcelain veneer acts as a matrix for the composite resin. If no deficient areas of the tooth are present, it simply becomes a new surface for the underlying tooth. With this technique, some increase in thickness of the tooth must be acceptable to the dentist and patient. Color must sometimes be developed with tints and opaquers applied to the inner surface of the veneers during the bonding process.
The second philosophy involves minimal tooth preparation to allow for a thin layer of porcelain; no increase in the facial-lingual dimension of the tooth is created. In this technique, the ceramic is considered the restorative material, and the composite resin is used only to bond the porcelain to the tooth. The veneers are built either on a refractory cast, where the cast goes into the oven with the porcelain and is cut away after the porcelain is baked, or on a swedged foil, which is teased off the model with the porcelain intact and placed in the oven. After the porcelain is baked, the foil is peeled from the inner surface. The ceramic material can be built up in increments—allowing for addition of an opaque layer if needed to cover dark color—and followed by more translucent layers for aesthetics. A gingivo-incisal blend of color can be developed as well as actual incisal translucency.
All of the color can be built into the restoration itself, and an untinted or slightly tinted luting composite is all that is needed for most restorations. The ceramic material requires only 0.3 to 0.5 mm of thickness, so only a small amount of tooth removal is necessary if the facial position of the tooth is where the restored tooth is desired to be. If the tooth is lingually inclined, no facial reduction may be needed. If the tooth is facially inclined, it must be reduced back to the arch form desired before depth cuts are placed for reduction. If the laminate includes the incisal edge, only 1 mm of reduction is required there. A gentle chamfer margin is all that is needed.
The third philosophy involves the use of pressed ceramic rather than stacked porcelain. Pressed ceramics generally require more tooth reduction because of the need for more thickness. Most manufacturers recommend a minimum of 0.6 mm of thickness, but most ceramists I know want 1 mm for these products. The fabrication process involves waxing up the restorations to full contour, spruing, investing, and burning out just as in the fabrication of metallic restorations. A molten glass is forced into the mold under pressure, allowing for excellent fit and an increase in compressive and flexural strength because of increased density from the pressing process. However, the restorations are monochromatic because they are made in one piece. To add a gingival/incisal blend of color and incisal translucency, they must be surface stained or cut back so porcelain can be baked to the surface. I have found that the cut-back procedure results in more lifelike aesthetics; therefore, space is needed for this process. Incisal thickness of 1.5 mm is recommended, and a rounded shoulder margin is needed.
Which technique do I think is best? I use them all at times, with the general rule of saving as much natural tooth structure as I can while providing the desired aesthetics and functional requirements. If I have the room for a pressed ceramic after necessary tooth reduction, I will probably choose it for its fit and strength. However, I do not reduce for pressed ceramics when the condition of the tooth allows me to keep more natural tooth structure. In these cases, I use stacked porcelain for its conservative preparation requirements, its ability to block out underlying dark color with an opacious layer, and its vital appearance. At times, only a translucent layer of porcelain is needed to “resurface” a tooth. In these cases, I may not prepare the tooth at all except to define a margin and develop a draw path. Such is the case in the following example, in which I had the laboratory produce what I call “contact lens” veneers (first described by Dr. Mark Friedman).
|Figure 1. Old direct composite resin veneers on maxillary central incisors.||Figure 2. Incisal view of old composite veneers.|
This young lady presented with direct composite veneers that had been in place for more than 10 years. The patient reported that the composite veneers had been placed to close spaces between her central incisors and between her lateral and central incisors (Figure 1). In Figure 2, the incisal view of the direct veneers can be seen. The patient wished to replace the discolored veneers with new ones.
After discussing her options, which included new direct composite veneers, laboratory-processed composite veneers, or porcelain veneers, she opted for porcelain laminates. One of the factors that influenced her decision was that porcelain provides a durable, stain-resistant surface that is color-stable. I recommended that she consider having me place laminates on her maxillary central and lateral incisors rather than only the central incisors, as the previous dentist had done. This was to provide her with better proportions and to eliminate the triangular appearance of her central incisors.
|Figure 3. Previous composite removed.|
One advantage of composite resin is its ease of removal. I used an 8-fluted carbide-finishing bur and was able to eliminate the composite without damaging the underlying tooth structure. After removal of the composite, it was obvious to me that the natural teeth had been barely prepared, if at all (Figure 3). The natural tooth color blended well with the surrounding teeth. It appeared me that adding a new surface covering that mimicked the patient’s enamel would be all that was needed to enhance the appearance of the teeth and close the spaces. Minimal preparation would allow the room needed for bonded stacked porcelain veneers.
|Figure 4. Central and lateral incisors prepared for “contact lens” veneers.||Figure 5. Incisal view of prepared teeth.|
No depth cuts were used, but rather the facial surface was only slightly shaped with a diamond bur while creating a gentle chamfer margin at the height of the tissue (Figure 4). The preparation was carried into the interproximal areas at the mesial and distal of the central incisors and the mesial of the lateral incisors. The prepared teeth can be seen from the incisal view in Figure 5. The corners of the prepared teeth were slightly rounded.
Impressions of the prepared teeth and the opposing arch were taken using a polyvinyl siloxane impression material, and an occlusal registration was made. Provisional veneers were fabricated using a bis-acrylic temporary crown and bridge material. They were cemented to place with an unfilled resin material after spot-etching the prepared teeth.
I called one of the master ceramists (William Kim) at the Americus Dental Laboratory Group in Queens, NY, and asked him to fabricate 4 porcelain veneers using only the enamel shade of a Vita B-1 stacked porcelain. I informed him that my preparations involved no more than 0.3 mm of tooth reduction and that the laminates would be very thin.
|Figure 6. “Contact lens” veneers on the working model.||Figure 7. Incisal view of veneers on working model.|
|Figure 8. “Contact lens” veneers off the model.||Figure 9. Veneers tried on one central and one lateral incisor.|
The final veneers can be seen on the working model from the facial view in Figure 6 and the incisal view in Figure 7. Figure 8 shows the veneers off the model. Note the extreme translucency and the beautiful surface characterization. In Figure 9, you can see 2 of the veneers tried in using a drop of water as a try-in medium. Based on the try-in, an untinted luting agent was chosen (Calebra, DENTSPLY Caulk). The veneers were thoroughly dried, and a silane coupling agent was applied to the internal etched sufaces of the restorations.
|Figure 10. Acid etching of prepared teeth.||Figure 11. Bonding resin applied.|
The adjacent unprepared teeth were protected with dead soft matrix strips, and the prepared teeth were etched for 15 seconds with 37% phosphoric acid (Figure 10). The etchant was thoroughly rinsed and left slightly moist. A wetting and desensitizing agent (Hurriseal, Beautlich Pharmaceuticals) was applied, and the excess was removed with a high-speed evacuation tip. A dual-curing one-step bonding agent (Prime&Bond NT, DENTSPLY Caulk) was liberally applied to the slightly moist surfaces (Figure 11), and lightly air-dried to remove the remaining water and solvent carrier. A glossy surface remained on the surface of the tooth.
|Figure 12. Veneers bonded with dual-cure luting composite.||Figure 13. Light-cure of luting composite.|
A dual-curing luting composite (Calebra, DENTSPLY Caulk) was mixed and applied to the inner surfaces of the veneers. The veneers were placed on the prepared teeth and held in place (Figure 12) until the “gel” state was reached (about 3 minutes after the start of the mix). The excess gelled luting composite was gently teased from the margins using a thin scaler. A visible light-curing unit was used to finish the polymerization of the material by exposing each veneer for 40 seconds (Figure 13). The margins were lightly finished with an 8-fluted carbide finishing bur and polished with an Enhance cup (DENTSPLY Caulk). No occlusal adjustment was needed.
|Figure 14. Finished veneers bonded to place.||Figure 15. Incisal view of veneers at seating.|
|Figure 16. The patient’s smile before direct composite veneers removed.||Figure 17. The patient’s new smile.|
The final restorations can be seen bonded to place in Figure 14. Figure 15 shows the incisal view directly after bonding. The patient’s smile before treatment is illustrated in Figure 16, and her new smile with the “contact lens” veneers bonded to place can be seen in Figure 17.
There are clinical situations like the one illustrated here that do not require aggressive tooth reduction to achieve the room for a bonded porcelain laminate veneer. A “contact lens” veneer can be one solution when a new translucent surface is needed to improve the aesthetics of a sound natural tooth.
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