April 1, 2006
by Ross W. Nash, DDS
During the last twenty years porcelain laminate veneers have grown rapidly as a treatment modality in dental practices across the globe. Their use has expanded from simple coverings for anterior teeth to full laminates covering all of the coronal tooth structure. There are several ways to produce the ceramic restorations for use in these processes. One is to use conventional porcelains for thin veneers. They can be fabricated on refractory dies or over swaged foil and fired in conventional porcelain ovens. High strength porcelains that allow for more functional contact and resistance to fracture can be fabricated in the same way. Core reinforced ceramics can provide opacity and extra strength when needed. Pressed ceramics allow for use of all ceramic materials with exceptional fit and high flexural and compressive strengths. CAD/CAM machinable ceramics can provide laminates through milling of prefabricate ceramic blocks.
Bonded laminates can be useful in many clinical situations. Tooth color modifications, closing spaces, correcting the appearance of mal-aligned teeth, correcting of enamel defects, repair of worn dentitions, restoration of broken down teeth, tooth position and contour modifications are only a few of the applications which enter modern treatment plans.
Functional changes can be made with the high strength materials now available. While esthetic oral rehabilitation is desired by many patients today, demand for elective procedures is also on the increase. I have been exposed to three philosophies for preparation of teeth for elective laminate veneers.
In this philosophy, an impression is taken of the unprepared teeth and thin porcelain veneers are fabricated in the lab on a model made from the impression. The porcelain is not considered to be the restorative material, but rather a thin, durable, esthetic and stain resistant surface covering for the underlying restorative material that is a composite resin. The composite resin can fill in deficient areas of the tooth while acting as a luting agent for the overlying porcelain facing. Color modification is accomplished by adding tints to the underside of the veneer at placement. Only 0.3 to 0.5mm of thickness is added to the tooth surface. Teeth that can benefit by additional contour are ideal for this technique.
While no preparation at all is the most conservative technique as far a conservation of tooth structure, the additional thickness may not be acceptable for many circumstances. Stacked ceramic materials should be at least 0.3 to 0.5mm thick for the fabrication process, so this amount of tooth removal provides room for these materials without adding additional bulk to the tooth. A gentle chamfer margin allows for precise placement with minimal finishing. In this philosophy, the laminate is considered the restorative material and it is simply luted to place with the underlying composite resin cement. With this amount of room, the technician can begin by placing an opacious layer of material (ceramic or composite resin) to cover nearly all of the underlying color and then overlay this layer with more translucent layers. Actual translucency can be developed in the incisal edges and a gingivo-incisal blend of colors can be established mimicking natural tooth structure. All of the color can be contained in the laminate allowing for use of an un-tinted luting agent to be used to bond the veneers to place. Shaded luting composites can also be used when needed.
One manufacturer of a pressed ceramic recommends grinding the pressed material to as little as 0.3mm but most require at least 0.6mm. Every ceramist I know tells me that they really need a minimum of 1.0mm of reduction to allow for cut back and layering of additional surface porcelain to produce life like results which are not monochromatic in appearance. A rounded shoulder margin is needed rather than a gentle chamfer. One new pressed ceramic on the market advocates a minimum of 0.7mm of preparation on axial surfaces rather than the full millimeter previously recommended.
Since pressed ceramics are made by using a lost wax technique like that for cast metals, room is needed for the process of waxing to full contour, sprueing, investing and burning out the wax pattern. The final pressed restoration is a single monochromatic shade. In order to achieve a gingivo-incisal blend of natural tooth colors and incisal translucency, the ceramist must either use surface stains or cut back the surface to make room for addition of surface porcelain for these effects. I find that the later technique provides the esthetic result that I require for my patients. Therefore, more aggressive preparation is needed.
When I have the room after necessary preparation due to the condition or position of the tooth, I will choose this modality. I will also use these materials when I am restoring a tooth in the posterior area of the mouth or when functional considerations require a material with higher flexural and compressive strength. However, I do not routinely use pressed ceramics for elective veneers when the materials requiring less tooth reduction can be used.
WHY CONSERVE NATURAL TOOTH STRUCTURE?
My rational for maintaining the maximum amount of natural tooth structure is that none of our restorative processes can be considered permanent. Even natural teeth deteriorate in the oral cavity due to the fatigue that occurs in this traumatic environment. Gold has the most potential for longevity, but every other restorative material has a limited life.
My clinical experience tells me that porcelain laminates can be expected to last around ten to fifteen years. When bonded ceramics are removed, a diamond bur in a high- speed hand-piece must be used and more tooth reduction is inevitable. The more tooth structure we maintain for the patient, the more options we will have for future treatment. It is simply a matter of conservation of natural tooth structure.
THE LAMINATION EFFECT
The reason that thin layers of porcelain can be used with great success in these veneering processes is because of something called “the lamination effect”. Ceramic materials are inherently brittle in nature. But when a ceramic is laminated to an underlying strong material, it becomes quite strong itself. An example of this phenomenon is that of a piece of floor tile. A strong man can break a piece of floor tile in his hands. However, after being laminated to the floor, a floor tile rarely breaks. The tile is strengthened by lamination. This is why porcelain fused to metal works so well. The porcelain is laminated to the metal substrate providing it with the strength to work in highly stressed areas such as molars. Now that we can laminate a ceramic material to the tooth structure itself through the bonding process, the tooth becomes the supporting structure and the ceramic is strengthened.
THE “ULTRATHIN VENEER”
While my normal preparation regime prescribes 0.5mm of axial reduction, 1.0mm of incisal overlap and a chamfer margin for stacked porcelain veneers, I sometimes use what I refer to as “ultrathin” veneers for selected cases. These require only minimal tooth contouring to establish room for only an enamel layer of porcelain. No body colors are prescribed to the lab, only a translucent outer covering for the tooth. Such a case is illustrated below.
The patient’s smile in Figure 1 exhibited direct composite resin veneers placed on her two maxillary central incisors a number of years earlier. The patient desired new veneers and chose porcelain for the replacements. In Figure 2, you can see that the composite, while still in tact, has begun to stain and chip. The previous dentist chose to veneer only the two central incisors to close spaces in the midline and between the central and lateral incisors causing the centrals to look triangular in
shape. I recommended that we veneer both lateral incisors also to create normal morphology.
After removal of the old composite resin and performing minimal contouring, the underlying tooth color proved to be very attractive. Only a new surface to replace the missing enamel was needed. In Figure 3 you can see that only slight preparation was accomplished with a diamond bur leaving rounded line angles, a subtle chamfer margin at the height of the tissue and no undercuts. The incisal view in Figure 4 shows that the preparations were taken into the interproximal areas where the porcelain would close the diastemas and only just to the incisal edges.
Final impressions were taken with a polyvinyl siloxane impression material for both the prepared and opposing arches. An occlusal registration was made with a hard setting polyvinyl material and temporary veneers were fabricated using a bisacrylic provisional material and cemented to place with a flowable composite resin.
The lab was instructed to fabricate ultrathin veneers using only an enamel shade of stacked porcelain. A light shade to simulate the surrounding enemal was chosen. The underlying dentin shade would show through the new “enamel” layer to create a realistic appearance. The final veneers are shown on the working model from the facial view in Figure 5 and the incisal view in Figure 6. The translucent ultrathin veneers can be seen off of the models in Figure 7. Two of the veneers are shown tried in using a drop of water as the try-in medium in Figure 8. Note the realistic appearance.
Certainly, the veneers could be broken easily before bonding to place. A delicate and precise technique will insure that they reach their ultimate strength through the lamination process. In Figure 9 the enamel is being etched with 37% phosphoric acid for fifteen seconds. The adjacent teeth have been protected from the etching solution using dead soft metal matrix strips. The etching solution is thoroughly rinsed after fifteen seconds and the teeth are lightly dried with an air syringe. A wetting and desensitizing agent is applied with a brush and the etched teeth are left slightly moist for the wet bonding procedure to follow. A one step dual curing dentin and enamel bonding agent is liberally applied with a brush (Fig. 10) and dried with an air syringe to remove the water and solvent (Fig. 11).
The veneers were treated with silane and dried. A dual curing luting composite was mixed and applied to the inner silane-treated surfaces of the veneers. The veneers were placed and held lightly to place with a porcelain veneer stabilizer instruments (Fig. 12). When the dual curing luting composite began to gel (about three minutes after the start of the mix), the excess was gently teased away from the margins (Fig. 13). Floss was gently worked into the interproximal areas (Fig. 14) and pulled to the lingual to remove excess resin. The composite resin luting agent was then light cured (Fig. 15) for thirty seconds on the facial and lingual of each veneer. A finishing and polishing cup was all that was needed to smooth the margins (Fig. 16).
THE FINAL RESULT
The final result can be seen in Figure 17 several weeks after placement. Note the vital appearance, natural looking morphology and excellent tissue health. The patient’s new smile is shown in Figure 18.
Using “ultrathin” veneers, we were able to provide this patient with the esthetics she desired while removing very little of her natural tooth structure. While these may not be her final restorations, they should last for many years. When she needs or desires replacements, there will undoubtedly be improved materials and techniques available to her and most of her natural tooth remains intact for whatever option she chooses.
Ross W. Nash, DDS is co-founder and president of the Nash Institute for Dental Learning in Charlotte, NC, where he provides esthetic and cosmetic dental treatment for patients and continuing dental education for dentists and team members. He is a Fellow in the American Academy of Cosmetic Dentistry and a Diplomat for the American Board for Aesthetic Dentistry. Dr. Nash is a consultant to numerous dental products manufacturers. rossw email@example.com. Nash Institute at www.nashinstitute.com.
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