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Minimal Preparation for Elective Porcelain Veneers

April 1, 2004
by Ross W. Nash, DDS


Many patients today are requesting porcelain veneers for elective cosmetic improvement of their smiles. Because porcelain can be etched and bonded to tooth structure with an adhesive attachment that exceeds the strength of the material or the tooth itself, this modality is viable for many clinical situations. The lamination process that occurs between the tooth and ceramic material results in a strengthening effect to the porcelain. My experience tells me that these restorations can last for more than ten years in most cases and for longer in many. One drawback is that when removal is needed or desired, they must be cut off using a diamond bur and more tooth structure is inevitably removed.

PHILOSOPHIES OF PREPARATION

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In the last 20 years, I have observed three philosophies of preparation for elective porcelain veneers. One is that of no preparation at all. In this philosophy, an impression is taken of the unprepared teeth and a model is poured for use in fabrication of the veneers. The veneers fabricated on the model are very thin and are not considered the restorative material, but rather a durable, esthetic and stain resistant covering for the underlying restorative material, a composite resin. Color modification of the porcelain is often accomplished with tints used on the inside of the veneer during the placement process. The enamel is etched as well as the inner surface of the porcelain and the bond is extremely strong.

Another philosophy is that of some minimal preparation of 0.3 to 0.5 millimeters allowing for a thicker layer of porcelain which can have most of the color built within. If the veneer is to extend over the incisal edge, 1.0 mm of reduction is needed. A moderate chamfer margin at the height of the tissue is all that is required unless a dark tooth color requires preparation just into the gingival sulcus. Only an untinted or slightly colored luting composite is needed since most of the coloration is in the porcelain itself. Stacked porcelain such as that used for porcelain fused to metal is used and is built up in layers. An opacious layer can be placed first to cover dark color and more translucent shades can be overlaid for a vital appearance. In this philosophy, the porcelain is considered the restorative material and the composite resin which bonds it to the tooth is used only as a luting agent.

The third philosophy involves use of a pressed ceramic rather than stacked porcelain. Preparation requirements for these materials usually involve more tooth reduction. A lost wax technique like that used for cast metal is used for this modality and the veneer begins as a monochromatic ceramic. For thin veneers, color modification must be performed with surface stains or from underneath with composite resin tints. I prefer the color to be added to the surface without stains. This requires a thicker layer of porcelain so that it can be cut back so that colored porcelain can be baked to the surface. Most ceramists that I know desire a full millimeter of room for this process. Therefore, a minimum of 1.0mm of axial reduction is needed if the tooth surface is to be at the same facio-lingual position as it was prior to preparation. Incisal preparation requires 1.5mm of reduction for this modality.

In the proper circumstances, all of these philosophies can be viable. I prefer, however, to maintain as much natural tooth structure as possible. Certainly, the most predictable and long-term bond is to etched enamel and minimal preparation allows the patient more options for the future.

CLINICAL CASE

The following case illustrates a technique that utilizes preparation somewhere between no preparation and minimal preparation. No definitive depth cuts for precise porcelain thickness were used here, but rather only slight reduction was used to allow for addition of porcelain without creating over-contoured restorations.

Figure 1 shows the patient’s smile before treatment. She was concerned about the eroded areas on the facial aspect of her maxillary central incisors and the incisal chipping. She desired a lighter color and wanted the right canine tooth to appear straight without orthodontic treatment. She felt that her teeth were too “flat” and wanted a broader smile.

All of these concerns could be addressed with porcelain veneers and she chose this modality of treatment. Because I wanted to help her preserve most of her natural tooth structure while achieving her cosmetic goals, I recommended stacked porcelain veneers with very little tooth reduction. Since her anterior teeth exhibited a very flat contour and her premolars were slightly tipped to the lingual (Fig. 2), additional facial bulk was desirable.

A long tapered chamfer ended diamond bur was used to contour the facial surfaces of the ten maxillary teeth from second premolar to second premolar. A subtle, but definitive chamfer was prepared at the height of the tissue. The chamfer was taken slightly into the interproximal areas to allow the veneers to cover all the visible aspects of the teeth. The mesial of the right canine was more aggressively prepared to allow for the illusion of straightening.

No incisal reduction was performed, but the facial preparation was carried to the incisal edge. Figure 3 shows the finished preparations from the facial view and the prepared teeth can be seen from the incisal in Figure 4.

Full arch final impressions were taken with a polyvinyl siloxane impression material and an occlusal registration was made. No retraction cord was needed since all margins were supra-gingival.

Provisional veneers were fabricated. The enamel on the facial surfaces of the prepared teeth were spot etched with 37 percent phosphoric acid gel for fifteen seconds (Fig. 5) then rinsed and dried. A vacuum formed shim which was made over a stone model made by duplicating a preoperative wax up of the desired end result was filled with a microfilled composite resin and placed over the prepared and spot etched teeth (Fig. 6). A curing light was used to harden the composite resin material and the shim was removed from the patient’s teeth. Excess composite resin was removed with a small carbide finishing bur in a high speed handpiece and the temporary veneers were polished with composite resin finishing and polishing cups. Care was taken to make sure the patient could floss using a floss threader so the tissue would be healthy at the deliver appointment. The provisional splinted veneers can be seen in Figure 7.

At the dental laboratory, refractory stone models of the prepared teeth were made from the impressions and thin stacked-porcelain veneers were fabricated using a “Hollywood white” shade requested by the patient. The glazed facial surfaces are illustrated in Figure 8 and the etched internal surfaces are shown in Figure 9.

At the delivery appointment, the patient was anesthetized and the provionals veneers were removed by sectioning them with a twelve fluted carbide-finishing bur and lightly prying them off with a heavy spoon excavator. The porcelain veneers were tried in using a drop of water as a try-in medium and the fit and esthetics were evaluated and approved. Retraction cord was placed (Fig. 10) to prevent gingival fluids from contaminating the teeth during the bonding process.

The prepared surfaces of the teeth were acid etched for fifteen seconds with 37 percent phosphoric acid gel (Fig. 11) then thoroughly rinsed and left slightly moist for the wet bonding procedure. A dentin and enamel bonding agent was applied liberally using a brush (Fig. 12) and dried with an air syringe to remove the solvent carrier and residual water (Fig. 13). The internal aspect of the veneers were treated with silane for thirty seconds and air dried to increase the bond between the ceramic and composite resin luting agent. A curing composite resin luting agent was placed inside the veneers and the veneers were placed on the prepared teeth. Excess luting composite was removed with a brush and a light-curing unit was used to spot cure each veneer in place before additional excess was removed.

Floss was gently placed
into the interproximal areas to remove uncured resin. Final curing was accomplished by using the curing light on the facial and lingual surfaces of each tooth. Carbide-finishing burs were used to remove excess cured composite resin at the margins and aluminum oxide polishing strips were used to smooth these areas. Slight occlusal adjustment was accomplished with carbide finishing burs. Polishing cups and points were used to bring all surfaces to a smooth finish.

THE FINISHED CASE

The finished case can be seen from the facial retracted view in Figure 14. The incisal view (Fig. 15) shows the excellent marginal adaptation of the bonded porcelain veneers. Figure 16 shows a four-year post-operative image and an incisal four-year follow up view is shown in Figure 17. Note that there had been little tissue change and marginal integrity of the veneers was still excellent.

The patient’s new smile can be seen in the close-up view in Figure 18 and her full-face view in Figure 19. In Figures 20 & 21, you can see a professional portrait taken after she received her new smile.

Dr Nash is Clinical Instructor at the Medical College of Georgia School of Dentistry and a Fellow in the American Academy of Cosmetic Dentistry. He maintains a private practice in Charlotte, NC. Dr. Nash is President and co-founder of the Nash Institute for Dental Learning in Charlotte, NC.

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