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Clinical Feature
25 Years of Veneering: What Have We Learned?
Reviewing options to correct perceived flaws in smiles
Thomas D. Larson, D.D.S., M.S.D.
Abstract
This is a review of the research completed over the 25 years in which dentists have been veneering teeth. Recommendations are made relative to the tooth preparation design for composite resin veneers and porcelain veneers. Material differences in a variety of applications are discussed together with recommended applications, indications and contraindications. From the research that has been reviewed, a series of best practices in the application of veneering materials and techniques is listed.
Introduction
While commonplace now, 25 years ago, placing any veneer on a tooth was seen as a dramatic departure from accepted treatment. The first veneers were composed of materials that quickly proved unsatisfactory. Methyl methacrylate, large particle composites, pre-formed shell veneering systems (Mastique, Caulk), all came and went rapidly in an attempt to meet a growing desire among patients to look better by correcting a perceived flaw in their smiles.1 Patients with tetracycline staining of their teeth saw this as a better alternative to crown placement because it conserved tooth structure and usually was less expensive.
Eventually better materials and light curing made the placement of a direct resin veneer possible and more commonplace. In 1975, veneering with porcelain became a possibility when bonding to the porcelain was developed and tested.2 Subsequent development lead to the bonding of porcelain to etched enamel in 1983.3,4,5
This article will review the research accomplished in the last 25 years that describes the best outcomes in tooth preparation and design, material choices and preparations and clinical techniques and applications.
Tooth Preparation
Three forms of tooth preparation for veneers have developed over the last 25 years. Which is chosen depends on the material to be bonded to the tooth, the type and color of discoloration the operator is attempting to cover and specifically what type of porcelains are being used.
When direct composite veneers are being done, zero to minimal (0.5-0.75mm depth reduction) tooth preparation, contained primarily in enamel, has become most popular.6,7,8,9 This technique is modified depending on the amount and type of discoloration, with gray to purple tetracycline stains requiring the greatest reductions. Diastema closures typically require no tooth preparation for restoration with composite resins.
At first some porcelain veneers were touted as needing no tooth preparation. In fact, DenMat still advertises the CerinateTM porcelain as not requiring a tooth preparation.10,11 However, the most satisfactory results usually occur with some type of metered preparation design, removing less at the facio-gingival margin (0.5-0.75mm) and more at the facio-incisal (1-1.2mm).
Both types of preparation design for direct and indirect veneers are most satisfactorily finished with a chamfered margin gingivally and interproximally.10,11 This permits sufficient bulk for the dentist to mask underlying discoloration with the veneers and provides sufficient room for the technician to develop color/translucency for porcelain veneers. It is also helpful to the technician to have a well-defined finish line for porcelain. Unless discoloration would create a need to do so, the gingival margin typically ends at the crest of the gingival margin of tissue. If discoloration must be covered, then the margin design must usually go about 0.5-1.0mm subgingivally. The most favorable response from the gingival is achieved with the margins at the crest of the gingival tissue.10,11
Initially all preparations were ended on enamel because dentin bonding 25 years ago was not predictable. With dramatic improvement in dentin bonding, there is greater acceptance of margin designs that end on dentin. This factor alone broadens the various clinical situations in which veneers can be successfully placed. However, the clinician should realize that even using the best materials and techniques, some margins ending on cementum will leak and could affect the longevity of the restoration.12,13,14,15 This risk should be evaluated relative to the caries risk for the patient.
The design of the incisal margin has varied over time, but as materials have improved and research has been published, what emerges is that for direct composite veneers, no incisal reduction is required unless the veneer is being used to provide anterior guidance. If a veneer is used for anterior guidance, some resistance form to the preparation is required to prevent chipping. In that event, the incisal should be reduced about 1mm and the enamel beveled on the lingual cavosurface. The width of the bevel is determined by the retention required to resist the force applied in the anterior guidance of the restoration.
Preparation of the incisal margin for porcelain veneers is required and should provide no more than 2mm reduction or thickness of porcelain. Over-reduction of the incisal can cause fracture of the porcelain if the porcelain thickness is greater than 2mm.
The design of the lingual margin has varied over time and by clinician. Research was completed which provides engineering guidance in terms of the best designs to use. Porcelain technicians will tell you a butt margin is far preferable to a chamfered lingual margin, yet some clinicians favor the chamfered margin design because it provides a reliable seating mechanism for the try-in and bonding of the porcelain veneer. Magne has done significant research to optimize the preparation design with the engineering principles to give the best outcome in resisting stresses inherent in the technique and materials. Conclusions from his research and from others concur that the best incisal design is a 2mm incisal reduction with a butt joint (no lingual chamfer). This design will minimize incisal chipping, and will make the fitting, bonding and fabrication of the porcelain veneer easier. This design minimizes the stress to the tooth and the porcelain.16
Research has also shown that replacing missing tooth structure with a bonded composite to optimize the incisal preparation design in more severely fractured anterior teeth will improve the outcome in terms of fracture resistance as compared to not building up the fractured tooth and ending up with too great a thickness of porcelain.17,18 In contrast to this finding, other studies show that bonding to previously placed composites affects the bond strength and can increase incidence of chipping or fracture.19,20 The theory underlying this finding relates to the water sorption of composite resins and its effect on free monomer. As composites age, they absorb water soluble stains. Eventually these stains are seen as a generalized discoloration of the material. This water sorption will affect availability of free monomer to which you can subsequently bond. Generally, the older the composite, the less likely you are to be able to bond to it.
The best practice is to replace any composite material contained within the porcelain veneer or composite veneer preparation outline if the existing composite age is greater
than about two to three years. Repair strength studies show bond strengths after that time begin to fall below 50 percent of the tensile strength of the material being studied.19,20
How do you minimize chipping? All veneer designs depend on adequate support to minimize chipping. Bonding to tooth structure provides some of the support. Making sure there is no more than 2mm of thickness of porcelain will also minimize fractures, especially for diastema closures. Make sure there is adequate anterior guidance in lateral movement for the patient either naturally or by planning it in the restorations. Group function places too much stress on veneers and can increase the incidence of chipping.
Inadequate bonding can also cause chipping of the porcelain.6 Make sure each bonding step is followed precisely as the manufacturer directs. For both porcelain veneers and indirect composite veneers, it is essential that the material is etched with a hydrofluoric acid for one minute before drying, making sure the inside of the veneer has not become contaminated with saliva, blood or oils from either the handpiece, water spray or by handling. Chipping can also occur if the preparation design included sharp angles rather than rounded. Research is quite conclusive in demonstrating the concentration of forces to areas of the preparation that are sharp. Such a concentration of force will cause
the porcelain to develop a fracture that will eventually propagate and lead to chipping and loss of a portion of the porcelain.6,7,8
Where should the interproximal margins end? Many clinicians will favor not going through the contact of the anterior tooth and being sure the preparation margin is ended right at the proximal contact.6 Exceptions are made when losing diastemas, including old restorations that extend lingually past the proximal line angle, and when a tooth is being repositioned by veneering to alter either the inciso-apical tilt, the incisal edge placement or the mesio-distal alignment of the facial and proximal surfaces. In all cases, clinicians are urged to prepare the tooth, especially near the gingival margin interproximally, so that the original tooth structure does not show. Porcelain technicians prefer either not through contact or generously through contact to make it possible for them to finish a sound porcelain margin.
What is the optimal thickness of veneering material? For direct composite resins, opaquing materials and the esthetic requirement will determine thickness. Resin is more forgiving in this aspect than is porcelain. Thickness of porcelain in part is determined by a need to cover discoloration. However, specifically for porcelain, because newer veneering porcelains have been manufactured, it is now quite possible to maintain similar thickness regardless of the types of discoloration you may be covering. From engineering standards as determined by research, it is important to maintain an even thickness of porcelain to distribute stress evenly on the facial surface.21,22 If an area of the facial porcelain is thin, that area is subjected to increased stress and higher probability of fracture. It is also critical to maintain an even thickness of luting resin when bonding the porcelain to tooth structure. The thicker the luting agent, the more likely thermal changes will create cracking of the porcelain. Using refractory dies and die spacers seems important in producing an optimized result.6,7,8
Material Differences
The original composites used most often for veneering were microfills, primarily because of the desire to mimic the reflectivity of the enamel surface. Certain types of applications of this material created significant problems in chipping because microfills have inadequate tensile strength and elastic modulus to prevent fracture.23 Their use in diastema closures has resulted in some significant fractures unless the stress was minimal and the space being closed small. Then the technique evolved to use layered composites, with hybrid composites or large particle composites to replace missing tooth structure (as in diastema closures or Class IV restorations) layered with a microfill. That technique, while not eliminating fractures of composites, lowered the risk to a more acceptable level.24
The advantage of layering direct composites with different particle sizes is that it refracts light differently and improves the esthetic matching with tooth structure. Composite materials, especially microfills, also have a tendency to discolor over time. Over-heating the material during finishing will hasten the discoloration. However, because composites have about 3 percent water sorption, over many years they will absorb all water soluble stains and ñyellowî as they age.19,20
Most often microfill veneers can average a life span of about 13 years, assuming reasonable patient hygiene and minimal noxious oral habits.25 While microfill composite resins can mimic the reflectivity and translucency of enamel, they do show wear over time, and they will lose the original luster imparted in the finishing.26 These materials also have optical properties different from tooth structure, so that while adequate to a conversational distance in mimicking tooth structure, porcelain is far superior in optical properties. Invariably, direct composite resin veneers will be replaced because they discolor. Some of these materials also seem to have many air voids from the manufacturing process that will occasionally appear at the surface and need repair as they fill with stain.27
Indirect composite veneers offer several distinct advantages to their direct resin cousins. The indirect resin materials can be cured in an inert atmosphere using heat and pressure to accomplish:
´ a greater rate of polymerization conversion, as high as 95-98 percent, compared to about 55-60 percent for direct resin,
´ higher strength values, because of greater cross-linking and polymerization of the resin,
´ more color stability because there is much less free monomer in the resin.
Polymerization in an inert atmosphere eliminates the problem of polymerization inhibition by oxygen. Because the polymerization occurs under pressure and heat, generally all air voids are eliminated or a least decreased in the mass being polymerized.27,28,29,30 Bonding to this material is made more difficult by the more complete polymerization conversion of monomer and therefore requires pretreatment with hydrofluoric acid to etch the filler particles in the composite and provide micro retention for the bonding resin, silane to bond to the filler particles and the same luting technique as with porcelain.
The longevity of these restorations should conceivably be greater than for direct resin. However, even though stronger and more abrasion resistant than direct resin veneers, nevertheless they do abrade much more than porcelain. The probable mode of clinical failure will most likely be a combination of discoloration, abrasion and chipping.11
Porcelain veneers usually exhibit a low rate of chipping. Unlike composite veneers, they cannot be readily repaired.31,32 Porcelain veneers will not change color over time. However, if the technician has placed much of the staining on the outside of the porcelain veneer rather than building the color variations into the surface of the porcelain build-up, the clinician should expect to see a change in color due to wear of the ñsofterî layer of glazed porcelain stains. Intrinsic stains and color distribution in porcelain build-up provide superior esthetic outcomes and diminish the need to use colored luting agents. Intrinsic staining of the porcelain while building it up also mimics the essential structure of the tooth structure and the distribution of opacity and translucency.
The veneer itself will not change color over time and is impervious to water soluble stains. The margin is susceptible to marginal staining from any water soluble stains (tea, coffee, cola, red wines, fruits, berries etc.) because the luting agent is a resin based product, and all resin based products eventually discolor. Marginal adaptation and luting technique play a large role in the extent and timing of marginal staining of porcelain, as does patient hygiene. The better the fit of the porcelain, the less marginal staining occurs. Luting a porcelain veneer requires leaving an excess of luting agent at the margin while curing to overcome the polymerization shrinkage of the luting agent.
The technique used in marginal finishing also has much to do with marginal staining. The probable mode of clinical failure for porcelain veneers is not known. The marginal staining that is seen is amenable to bleaching agents in maintaining a good appearance for some time. While a small incidence of porcelain fractures will occur, discoloration of the resin luting agent will not be a problem if the luting agent used is of optimal thickness and the marginal fit is good (less than 50 microns).7,33,34
Best Practices
´ Mixing resin and porcelain restorations in the anterior is inadvisable due to optical property differences. Either do all composite (direct or indirect) or all porcelain (crown, veneers).
´ It is best to produce porcelain crowns to match veneers at the same time rather than attempting a match separately. Porcelain-fused-to-metal crowns have a layer of opaque that must be accounted for when side by side with veneers. The tooth preparation for the crown should have a more generous facial shoulder to permit the technician to match porcelain opacity and translucency between crown and veneer. If possible, use all-porcelain crowns with porcelain veneers.
´ Only prepare teeth for veneer when healthy gingival tissue is present. Temporizing porcelain veneer preparations are much easier. You must be capable of maintaining a perfectly dry, clean field during porcelain luting or you will experience more patient postoperative problems, bonding failures and discoloration under the porcelain. Prescribe chlorhexidene if necessary to help the patient maintain excellent gingival health during the provisional phase of treatment.
´ Temporization of porcelain veneer preparations is preferred for esthetic reasons and patient comfort. Temporization can be accomplished using either a directly placed composite material or by indirectly forming the provisional using a template developed either with a diagnostic wax-up or directly in the mouth. Both types of provisionals can be adhered to the teeth using a very small (1mm2 or less) area of etched enamel and bonding resin. Indirect provisionals can also be cemented using a non-eugenol temporary cement.
´ Direct marginal finishing of any type of provisional restoration should be accomplished to ensure adequate gingival health.
´ Maintain clean, dry fields while placing a direct resin veneer and you will significantly decrease finishing time, marginal staining, and increase the longevity of the veneer. If you are replacing veneers because of marginal discoloration, your technique should be adjusted to improve bonding outcomes.
´ Direct resin veneers for diastema closures of 1mm or less are preferable to porcelain veneers because no tooth preparation is required.
´ Diastema closures of greater than 3mm can be accomplished with composites. However, the wider the diastema, the more successful porcelain veneers will be. (Remember the maxim that you can have no thickness of unsupported porcelain greater than 2mm.)
´ When determining space distribution in the anterior, use the golden proportion as a guideline for developing mesio-distal widths of the anterior teeth. If unsure, always mock up on either a diagnostic wax-up or on the patient, using composite without bonding.
´ Whenever possible, keep gingival preparation margins just at the crest of the gingival tissue. This will improve gingival health, minimize marginal staining and make finishing easier.
´ When analyzing anterior tooth shape, size and inclination, be sure you relate incisal edge position to the interpupillary line, maxillary incisal edge to the smile line and facial positioning to emphasize the diminishing golden proportion as the teeth are viewed in a straight facial view on the arch form going posterior.
´ The best procedure for luting porcelain and indirect resin veneers will enable the operator to isolate the tooth preparation from any crevicular fluid or blood during the luting process. It also permits excess material to be present at the porcelain/tooth interface when polymerizing the luting agent. The best technique I have found to accomplish both objectives is to use a Vivadent Contour StripTM while luting a single porcelain veneer. Use of retraction cords, while sometimes effective, also traumatizes the gingival tissue at a time when you wish the tissue to look its best. Rubber dams can also be used, but would require the placement of a 212 retainer to retract the gingival tissue.
´ All internal line angles for porcelain preparations must be rounded to prevent concentration of stress leading to fracture.
´ Resin materials should be polished wet to dissipate the heat from polishing, remove the debris without making it airborne and prevent ditching at margins from overheating (white line).
´ When matching shades of teeth, choose one color for the dentin shade in the gingival one-third of the crown; choose a different color for the incisal one-third. The incisal color is the value of the color, while the dentin color is the chroma. Draw a ñcolor mapî to identify where the incisal enamel begins to be appreciably thick and the dentin more in the background. Also identify the degree to which the enamel is translucent or opaque. Identify variant colors on your color map to simulate fluorescence or white spots or variations in chroma or hue. This should be done for both porcelain and resin veneers.
´ Always take the shade of a tooth before the tooth has begun to dehydrate. If you donÍt, the value of the color you have chosen will be too light.
´ Diagnostic wax-ups can save you and the patient a lot of time and should be used as you begin veneering or crowning more teeth. They are particularly useful in fabricating
provisional restorations for either veneers or crowns.
´ Always identify the existing shade of the prepared tooth when veneering to change color and tell the technician the background color you are starting with, the amount of opacity you think you may need to block discoloration and the end shade(s) you wish the veneer to appear. Pressable ceramics can often provide more opacity than feldspathic porcelain.
´ Matching color requires you match surface texture and reflectivity of the surface as both will affect the perception of whatever color is in the tooth. Describe both in your
laboratory prescription.
References
1. Faunce FR, Myers DR. Laminate veneer restoration of permanent incisors. JADA 1976;93(4):790-2.
2. Rochette AL. A ceramic restoration bonded by etched enamel and resin for fractured incisors. J Prosthet Dent 1975;33(3): 287-93.
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15. Burke FJ. Fracture resistance of teeth restored with dentin bonded crowns constructed of leucite-reinforced ceramic. Dent Mater 1999;15 (5):359-62.
16. Castelnuovo J, Tjan AH, Phillips K, Nicholls JI, Kois JC. Fracture load and mode of failure of ceramic veneers with different preparations. J Prosthet Dent 2000;83 (2):171-80.
17. Magne P, Douglas WH. Optimization of resilience and stress distribution in porcelain veneers for the treatment of crown-fractured incisors. Int J Periodontics Restorative Dent 1999;19 (6):543-53.
18. Magne P, Douglas WH. Interdental design of porcelain veneers in the presence of composite fillings: finite element analysis of composite shrinkage and thermal stresses. Int J Prosthodont 2000;13 (2):117-24.
19. Dhur VB, Lloyd CH. The fracture toughness of repaired composite. J Oral Rehabil 1986;13 (5):413-21.
20. Shiau JY, Rasmussen ST, Phelps AE, Elwo DH, Wolf GR. Analysis of the ñshearî bond strength of pretreated aged composites used in some indirect bonding techniques. J Dent Res 1993;72 (9):1,291-97.
21. Magne P, Kwon KR, Belser UC, Hodges JS, Douglas WH. Crack propensity of porcelain laminate veneers: a simulated operatory evaluation. J Prosthet Dent 1999;81(3):327-34.
22. Magne P, Versluis A, Douglas WH. Effect of luting composite shrinkage and thermal loads on the stress distribution in porcelain laminate veneers. J Prosthet Dent 1999;81(3):335-44.
23. Shortall AC, Uctasli S, Maruis PM. Fracture resistance of anterior, posterior and universal light activated composite restoratives. Oper Dent 2001;26 (1):87-96.
24. Larson TD. Techniques for achieving realistic color distribution in large composite resin restorations. JADA 1986;112(5):669-72.
25. Kreulen CM, Cruegers NH, Meijering AC. Meta-
analysis of anterior veneer restorations in clinical studies. J Dent 1998;26(4):345-53.
26. Kern M, Strub JR, Ly XY. Wear of composite resin veneering materials in a dual axis chewing simulator. J Oral Rehabil 1999;26(5):372-78.
27. Shortall AC, Uctasli S, Marquis PM. Fracture resistance of anterior, posterior and universal light activated composite restoratives. Oper Dent 2001;26(1):87-96.
28. Giordano, R.: Fiber reinforced composite resin systems. Gen Dent 2000;48(3):244-49.
29. Ferracane JL, Mitchem JC, Condon JR, Todd R. Wear and marginal breakdown of composites with various degrees of cure. J Dent Res 76(8):1,508-16, 1997.
30. Asmussen E, Peutzfeldt A. The effect of secondary curing of resin composite on the adherence of resin cement. J Adhes Dent 2000;2(4):315-18.
31. Frankenberger R, Kramer N, Sindel J. Repair strength of etched vs silca-coated metal-ceramic and all-
ceramic restorations. Oper Dent 2000;25(3):209-15.
32. Leirock A, Degenhart M, Behr M, Rosentritt M, Handel G. In vitro study of the effect of thermo- and load-cycling on the bond strength of porcelain repair systems. J Oral Rehabil 1999;26(2):130-37.
33. Peumans M, Van Meerbeek B, Lambrechts P, Vuylsteke- Wauters M, Vanherle G. Five-year clinical perfor-mance of porcelain veneers. Quintessence Int 1998;29(4):
211-21.
34. Coynem BM, Wilson NH. A clinical evaluation of the marginal adaptation of porcelain laminate veneers.
Eur J Prosthodont Restor Dent 1994;3(2):87-90.
*Dr. Larson is Associate Professor, Department of Restorative Sciences, Division of Operative Dentistry, University of Minnesota School of Dentistry, 515 Delaware Street S.E., Minneapolis, Minnesota 55455
Copyright 2003. Minnesota Dental Association
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