Porcelain Veneer Preparations: to Prep Or Not to Prep Edward A

76 INSIDE DENTISTRY—MAY 2006 LAB Laboratory perspectives from the inside out. taLk Porcelain Veneer Preparations: To Prep or Not to Prep Edward A. McLaren, DDS, MDC Edward A. McLaren, DDS, MDC In recent years, bonded porcelain whether or not the margins were placed possible to obtain the desired result. The Director, Center for Esthetic Dentistry restorations have geometrically expand- in enamel as the primary causes of failure. same is true for bonded porcelain (porce- Founder and Director ed in their clinical use. Increased patient Bonding mechanisms and materials lain veneers). Ideally, none or only a Master Dental Ceramist Residency Program demands for esthetics coupled with the have improved over the years with bond- minimal amount of tooth structure desire by the profession for conservative ing failure minimized because of these should be removed. One decision that Adjunct Associate Professor treatments have fueled this expansion. improvements. The main mode of failure always needs to be considered is whether The University of California, Los Angeles Early concerns about the fragile nature with bonded porcelain is fracture and adjunctive orthodontics should be com- School of Dentistry of the thin shells of ceramic have been bond failure, which are interrelated. Many pleted to place the teeth in the ideal posi- allayed as multiple clinical reports have different causes can contribute to ceramic tion so that there is minimal or no Private Practice limited to Prosthodontics and documented good to excellent clinical failure, including preparation design and reduction for bonded porcelain. In the Esthetic Dentistry success. Because of this documented proper tooth reduction. Confusion exists authors’ experience, if the teeth require Los Angeles, California success, porcelain veneers have been over the many different preparation bonded porcelain anyway for reasons of used for more restorative situations and designs, specifically whether or not to form correction, color alteration, or not for just esthetic enhancement. One remove tooth structure, how much tooth replacement of failed restorations, and In the case of a translucent material exhaustive study that followed veneers structure to remove, and margin design the amount of healthy tooth structure such as a porcelain veneer, the desired for up to 15 years reported total failures and placement. This article is intended removal will be similar even if ortho- color or shade change needs to be con- of only 7%, equaling approximately a to give rational guidelines on prepara- dontics is done, then bonded porcelain sidered. Generally speaking, a veneer 1% chance of failure per year.1 Of the tion parameters for porcelain veneers, without adjunctive orthodontics should requires a minimum of 0.2 mm to (ide- reported failures, more than half were particularly the issue of “to prep or not be accomplished. But if orthodontics ally) 0.3 mm of thickness for each shade fractures. Several other reports docu- to prep” and, if prepping, “how much.” significantly reduces the need for healthy change. For example, to go from an A3 to menting the success or failure of porce- The ultimate goal in any dental treat- tooth structure removal, specifically, if A0 requires 3 shade group changes and lain veneers cite margin placement and ment should be to be as conservative as the preparation has to cross the denti- would need a minimum of a 0.6-mm to noenamel junction, then orthodontics (ideally) 0.9-mm-thick veneer. In the should be completed. laboratory, it is very difficult to fabricate a veneer less than 0.3-mm thick. Research PREPARATION done at the University of California, Los GUIDELINES Angeles and the University of Oregon on FOR PORCELAIN VENEERS 0.3-mm-thick veneers showed some Preparations are always dictated 3- cracking on cement polymerization when dimensionally by how the final restora- they wrapped around a corner as in the tion is placed within the frame of the incisal edge (unpublished data). This face, lips, and gingiva. This is determined data shows the veneers should be slightly by smile design with patient input and thicker (at least 0.5 mm) if they wrap needs to be verified functionally. The cli- interproximally or over the incisal edge. nician should work backward and Veneer thickness, which relates to tooth Figure 1 Preoperative condition in which tooth Figure 2 Postoperative view of a veneer which remove tooth structure based on the spe- reduction, is largely determined by the No. 10 is in slight lingual version and the patient was fabricated with no preparation to alter contour. cific material requirements for space (ie, amount of shade change desired and desires a fuller contour. thickness of the restorative material). tooth position for esthetics. Figure 3 Building porcelain on a refractory Figure 4 Facial and interproximal view of the Figure 5 Facial and interproximal view of the Figure 6 A case that failed clinically after 38 cast using a duplicate provisional as a guide for facial window preparation in which the incisal incisal wrap preparation in which the preparation months. There was significant dentin exposure contour. The preparation allows for 1 mm to 1.5 edge is not covered. is carried to varying degrees over the incisal and lingual wrapping. mm of porcelain extension beyond the edge of edge and finished on the lingual. the preparation. Figure 7 Facial and interproximal view of the Figure 8 Spot etching for provisionals. Figure 9 Provisionals after polymerization and incisal shoulder preparation. Note the rounded margin adjustment. facial-incisal line angle. Figure 10 Prepared case in which the margin Figure 11 Incisal view showing the lingual Figure 12 The finished case of the prepara- is placed more to the lingual to be able to con- position of the interproximal finish line for cases tions in Figure 11. tour the porcelain to close the diastema. of closing diastemas or widening of the tooth. TO PREP OR NOT TO PREP To prevent overcontouring and pro- there is a change in form and when there It is generally recommended that the vide for proper masking, the authors is going to be an incisal lengthening. The margins be placed circumferentially in prefer to prepare teeth in the gingival window preparation is recommended enamel. Recommendations range from third, and prepare a light chamfer for a when only color changes are desired. minimal to no preparation to a heavy definitive finish line. The one exception Most ceramists find it difficult to work chamfer. Many of the techniques for to this is if the tooth is in slight lingual with the window preparation, as it is margin design and volume of tooth version and the finish line of the porce- problematic to place incisal effects in the structure removal are dictated by manu- lain can be kept supragingivally (Figures porcelain with this design. This incisal facturers’ requirements for a specific 1 and 2). From the laboratory perspec- wrap creates more room for incisal porce- material. Although this is acceptable if tive, it is extremely beneficial for the lain, but creates a thin area of porcelain the clinical situation dictates it, many dentist to place at least a light chamfer on the lingual that could have higher times this is not the case. Frequently, finish line so the ceramist clearly knows potential for fracture (Figure 6). If form excessive tooth structure is removed so where to build the porcelain. The alteration is one of the treatment goals, that a specific material or technique can authors recommend at least a light the authors prefer the more recent be used, although another technique or chamfer of 0.3 mm gingivally and inter- incisal margin placement (incisal shoul- material would have been the more con- proximally. If there is peripheral enamel, der preparation) (Figure 7). In this servative option. The material or tech- the authors prefer to prepare the incisal preparation, instead of wrapping over nique should not be made to fit the edge to allow for 1 mm to 1.5 mm of the lingual edge, the incisal edge is pre- clinical situation but rather the best incisal porcelain (Figure 3). This creates pared for a shoulder. The facial incisal material or technique should be chosen room to internally build the incisal line angle is rounded slightly to mini- for the existing clinical situation. effects, which are present to varying mize any stress concentration in the If the porcelain cannot be supported degrees on natural teeth. ceramic. This is by far the easiest incisal by enamel, it is critical to design the Incisal margin placement has gener- finish line design. In the authors’ clinical preparation so the cemented veneer is ally followed 2 designs, the incisal wrap experience, there has been no issue with subjected to minimal or no tensile or (Figures 4 and 5) and the incisal window. debonding of porcelain veneers with this shear stresses. Laminated structures such The incisal wrap is recommended when preparation design, particularly when as porcelain/enamel or porcelain/dentin by definition are a constant strain system.2 When a stress is applied in such a BECAUSE DENTIN IS A LOWER system, the material with the highest modulus of elasticity (stiffest) absorbs MODULUS MATERIAL (MORE FLEXIBLE) most of the stress. Because dentin is a lower modulus material (more flexi- THAN PORCELAIN, IT FLEXES ble) than porcelain, it flexes more than enamel under a given load, thus subject- MORE THAN ENAMEL UNDER ing the veneered porcelain to higher tensile and shear stresses. Being brittle, A GIVEN LOAD, THUS ceramics fail at a critical strain of 0.1%;3 therefore, bonding to the more flexible SUBJECTING THE VENEERED dentin could lead to early failure. The stiffness of enamel and its ability to PORCELAIN TO HIGHER TENSILE absorb stress clearly demonstrates the need to save as much enamel as possible AND SHEAR STRESSES.

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