Double-layer porcelain veneers: Effect of layering on resulting veneer color

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1 Double-layer porcelain veneers: Effect of layering on resulting veneer color Feimin Zhang, DDS, MS, a Guido Heydecke, DDS, Dr Med Dent, b and Michael E. Razzoog, DDS, MS, MPH Nanjing Medical University, Nanjing, China, and University of Michigan, Ann Arbor, Mich. Statement of problem. Densely sintered aluminum oxide veneered with feldspathic porcelain seems to be a promising technology for the fabrication of porcelain laminate veneers, which provide both strength and esthetics. To effectively use this approach for porcelain veneers, practitioners should know how the resulting color is affected by adding a layer of veneering porcelain onto the aluminum oxide. Purpose. This study compared changes in CIE L*a*b* color coordinates of simulated stained teeth when covered with the aluminum oxide core disks alone and after the disks had been veneered with 3 different shades of porcelain. Material and methods. Fifteen aluminum oxide disks were divided into 3 groups. Each of 5 disks was veneered with porcelain of the Vita shades A1, A2, and B4, respectively. The colors of the substrate covered with the nonveneered disks and the veneered disks were measured separately, and the color differences were calculated. Measurements of the disks on a white background were also performed and used as controls. The extracted data were compared with Vita shade values available from the literature. Results. Statistically significant differences in color coordinates of dark substrates were recorded between the substrate covered by an aluminum oxide disk and the addition of veneering porcelain to the aluminum oxide disks. Veneering the aluminum oxide disks with A1, A2, and B4 porcelain produced significant differences in the resulting color when placed on a dark substrate. The direction of the color modifications correlated with values obtained from the literature. The resulting color was also affected by the color of the underlying structures. Conclusion. Although aluminum oxide alone does have a degree of masking capability, the resulting color of porcelain veneers with the use of this material can successfully be modified with the veneering porcelain. (J Prosthet Dent 2000;84: ) CLINICAL IMPLICATIONS This study illustrates that aluminum oxide shells can be used as the core of double-layer veneers. Because the aluminous core is capable of blocking a certain amount of staining, the resulting color can be modified with feldspathic veneering porcelain, increasing the predictability of the esthetic outcome of porcelain veneers. Porcelain laminate veneers are a popular treatment option, often used to change the shape and color of teeth. 1-4 The challenging aspect of treatment with ceramic laminates is to achieve maximum esthetics, which frequently includes color change with limited preparation of the enamel. 5 Several studies have evaluated the effect of porcelain shade, thickness, amount of opaque porcelain, and color of cementing resins on the final appearance of laminates. 6-8 The amount of translucent porcelain used in the process of layering influences the opacity and seems to have a significant impact on the final shade of a veneer aclinical Assistant Professor. b Visiting Assistant Professor, University of Michigan, Ann Arbor, Mich. cprofessor, University of Michigan, Ann Arbor, Mich. restoration. 7,8 However, combining the tasks of achieving the desired shade with limited opacity can be difficult. The resulting color of a veneer restoration often has neither the color of the tooth nor the color of the porcelain used. 7-9 Therefore, to make the shade matching procedure more predictable, a 2-layer porcelain technique has been described that introduces opacity with the first layer and achieves the impression of depth and translucency with the second layer. 10 Chpindel and Cristun 10 also associated an increase in strength with the double-layer technique, although the success rates of porcelain laminates are generally high An all-ceramic system that used dense sintered aluminum oxide as a high-strength core with a flexural strength of 601 ± 73 MPa 15 has been developed for the creation of single crowns (Procera, Nobel Biocare, OCTOBER 2000 THE JOURNAL OF PROSTHETIC DENTISTRY 425

2 THE JOURNAL OF PROSTHETIC DENTISTRY ZHANG, HEYDECKE, AND RAZZOOG Göteborg, Sweden). 16,17 This system embraces the concept of the double-layer technique. The Procera core is semitranslucent because of its density but possesses a certain amount of translucency, depending on the thickness with its shade-matching Vita A1. 18 The aluminum oxide core is veneered with a specific porcelain (AllCeram, Ducera, Germany), which has fluorescent properties that correspond to those of the natural dentition. These properties minimize problems of metamerism and improve the lifelike appearance of the restoration. 16,17,19 The widely recognized CIE L*a*b* system of evaluating color is being increasingly used in dental research. In the CIE L*a*b* color system, the location of a particular shade in the color space is defined by 3 coordinates: L*, a*, and b*. L* describes a color due to the lightness of the object being assessed. The a* values are the chromaticity coordinates in the redgreen axis and b* in the yellow-blue axis. The measure of the total color difference between 2 objects is described by E. Although indicative of a color difference, the magnitude of E gives no information of the character of the color of the specimen because it does not indicate the quantity and direction of the CIE L*a*b* components A color difference of 2 E units has been described as an acceptable clinical shade match in dentistry, whereas color differences above 3.7 E units are unacceptable and are visually detectable. 23,24 If it is possible to use densely sintered aluminum oxide shells, which are subsequently veneered with feldspathic porcelain to fabricate porcelain veneers, it may have the potential to optimize both strength and esthetics. It may be hypothesized that, because of the opacity of the core, the system could be successfully used to mask intrinsic staining. In addition, a predictable shade match could be achieved by using feldspathic porcelain as the veneering material. The purpose of this study was to evaluate the color difference achieved after thin (0.2 mm) aluminum oxide disks were veneered with a 0.4-mm layer of porcelain in 3 different shades that were placed on gray substrates simulating tetracycline-stained tooth structure. 25 The obtained results were compared with the color measurements of the samples on a standard white background and with standard CIE L*a*b* values for the various shades of porcelain available from the literature. MATERIAL AND METHODS Substrate fabrication An aluminum mold was machined as a master die for the fabrication of multiple substrates. An autocuring acrylic resin filling material (Sevriton, Amalgamated Dental Trade Distributions, Ltd, London, UK) was mixed with amalgam powder to impart a gray shade and poured into an impression of the master die to form the substrates. The ratio of the amalgam was recorded so that consistent mixtures of the substrate resin could be fabricated. Fifteen substrates were prepared and divided into 3 groups, and the initial color of each substrate was measured and recorded. Aluminum oxide disk fabrication Fifteen Procera aluminum oxide disks mm in diameter and 0.2 mm in thickness were used for the experiment. Before the test, each disk was measured in 3 locations with a Digimatic Caliper (Mitutoyo Corp, Tokyo, Japan) to determine the actual thickness. The average thickness of all samples was 0.21 mm (mean 0.21 mm, SD mm). The 15 Procera aluminum oxide disks were divided into 3 groups for the veneering process. Samples in group 1 were veneered with porcelain of shade A1, group 2 with shade A2, and group 3 with shade B4. After the veneering, each aluminum oxide disk was paired with the substrate for the length of the experiment. Veneering of the aluminum oxide disks The veneering porcelain (AllCeram Porcelain, Ducera, Rosbach, Germany) was applied after the aluminum oxide disks were cleaned in an ultrasonic cleaner with distilled water. Porcelain shades of A1, A2, and B4 were used on the disks of the respective groups. The initial thickness of the veneering layer was slightly more than 0.4 mm, and the excess was ground with a diamond bur to achieve a uniform thickness of 0.4 mm. All the surfaces of veneers were polished with 600-grit polishing paper (Ecomet 3, Bruchler Ltd, Lake Bluff, Ill.) to produce a smooth surface, which was microscopically examined to ensure lack of porosity. The final thickness was verified to be 0.4 mm (±20 µm) with a Digimatic caliper. Finally, all 15 veneered disks were self-glazed at 880 C. Measurement of the color coordinate variables A colorimeter (Minolta Chroma Meter II, Minolta Inc, Osaka, Japan) was used for all color measurements. The colorimeter has a 3-mm head and a diffuse illumination vertical viewing (0 ) geometry for color measurement. CIE L*a*b* notations were used for all color measurements, and the standard illuminant D65 was selected for all measurements. Before the experimental measurements, the colorimeter was calibrated to a white standard tile supplied by the manufacturer. The L*a*b* color notation of each substrate was measured 3 times consecutively, and an average of the 3 readings was calculated to give the initial color of the substrate. The color after covering the substrate with the nonveneered aluminum oxide disk was recorded making 3 separate measurements (Fig. 1). The measurements were repeated with the veneered aluminum oxide disks. All nonveneered and veneered aluminum 426 VOLUME 84 NUMBER 4

3 ZHANG, HEYDECKE, AND RAZZOOG THE JOURNAL OF PROSTHETIC DENTISTRY Fig. 2. Color measurements of substrates. Fig. 1. Specimen consisting of simulated stained tooth substrate and veneered porcelain disk. Chromameter head in place. oxide disks were measured on a standard white background after the procedures previously described. Calculation of the color differences Color differences were calculated (a) between the substrates and the substrates covered with the nonveneered and veneered disks, (b) between the nonveneered disks and the veneered disks on the substrates, (c) the 4 groups of disks (nonveneered and veneered with shades A1, A2, B4) on the gray substrates and on a standard white background, and (d) among the 4 types of disks on a white background and color measurements of Vita shade tabs available from the literature. 24 The CIE L*a*b* color coordinates of each disk-background sample were compared using the formula E=([L*1 L*2] 2 + [a*1 a*2] 2 + [b*1 b *2] 2 ) 0.5 where L*1, a*1, and b*1 represent pretreatment color coordinates of each sample, and L*2, a*2, and b*2 represent posttreatment color coordinates of each sample. 22 Statistical analysis A 1-way analysis of variance (ANOVA) test was used to compare the 3 groups of substrates with each other to detect color coordinate differences. The mean values for the color coordinates L*, a*, and b* of the disk-substrate pairs for each porcelain shade were compared with the nonveneered disks by using multiple paired t tests. The total color difference was statistically evaluated by comparison of the group mean with a hypothesized mean for E of 3. One-sided t tests were performed to identify signficant color differences larger than this value. For the comparison between the disks measured on a white background and on the sub- Table I. Statistical evaluation of the color differences among the 3 groups of substrates (1-way analysis of variance) Source of variation SSq DF MSq F P L* by group Sample Within cells Total a* by group Sample Within cells Total b* by group Sample Within cells Total strates, no tests were performed because of the magnitude of the differences. RESULTS The analysis of the measurements of the substrates did not reveal significant differences between the 3 groups for the color coordinate variables (Table I). The color coordinate values are presented in Figure 2. The result of a 1-way ANOVA showed that there was no significant difference among the 3 groups before adding the AllCeram veneering porcelain on the aluminum oxide disks in L* (P=.2025), a* (P=.0624), and b* (P=.1167). The color difference of substrates covered with pure aluminum oxide disks and with veneered aluminum oxide disks revealed a nonsignificant color difference of 2.3 E* units in group 1 (A1). In group 2 (A2) and group 3 (B4), color differences of 4.3 and 8.8 E* units were detected, statistically significant in group 2 and highly significant in group 3 (Table II). In the A1 group, a slight increase in lightness and in the b* OCTOBER

4 THE JOURNAL OF PROSTHETIC DENTISTRY ZHANG, HEYDECKE, AND RAZZOOG Table II. Statistical evaluation of the color coordinate differences (L*, a*, b*) between the pure (core) and veneered alumina disks for the 3 porcelain shades (paired t tests). For the evaluation of E, a threshold level of E>3 was chosen and a 1-sided t test was performed A1 A2 B4 n Mean SD n Mean SD n Mean SD L* Core Core + veneer Difference tailed P a* Core Core + veneer Difference tailed P b* Core Core + veneer way ANOVA tailed P E Core + veneer core Hypothesized tailed P ANOVA = Analysis of variance. Table III. Absolute color coordinate measurements of the sample disks on a white background and measurements on the dark substrates. Also presented are the color differences L* a* b* E Group White Substr. White Substr. White Substr. E (s-w) Core Mean SD (0.45) (0.55) (0.08) (0.07) (0.69) (0.55) (0.71) A1 Mean SD (0.87) (0.99) (0.06) (0.15) (0.36) (0.33) (0.64) A2 Mean SD (0.52) (1.23) (0.21) (0.22) (0.49) (0.20) (0.61) B4 Mean SD (0.55) (1.34) (0.22) (0.38) (0.74) (0.41) (0.81) Substr. = Substrate; w = white; s = dark substrates; E = color differences. coordinate could be observed but were not significant. In the A2 and B4 groups, a marked decrease in lightness ( L*) was significant in both groups, which is basically the reflection of a darker tooth color. Also, differences in the b* axis were apparent, even more in the B4 group, indicating a shift toward yellow. The total color differences were a strong function of the changes occurring in the b* coordinate (Fig. 3). Although the simulated veneers could produce a color difference of (SD 0.51) E units without any veneering porcelain and (SD 0.52) for A1, (SD 0.64) for A2, and (SD 1.05) E units for the B4 shades compared with the color of the substrates, the resulting color was still affected by the background used. The color shift is clearly shown using a standard white background. The color coordinate measurements are presented in Table III. Total E values (Fig. 4) demonstrate large differences between the resulting veneer color on the gray substrate and on a white background, ranging from to E units. These are far beyond a clinically acceptable difference of 3 units. 428 VOLUME 84 NUMBER 4

5 ZHANG, HEYDECKE, AND RAZZOOG THE JOURNAL OF PROSTHETIC DENTISTRY Fig. 3. Color coordinate ( L*, a*, b*) and total color differences ( E) between nonveneered and veneered alumina core material disks. Fig. 4. Total color differences between porcelain disks (core material and with 3 porcelain shades) on white background and dark substrates. Fig. 5. Color coordinate (L*, a*, b*) and total differences ( E) between standard values 24 and measurements of samples on standard white background. Despite this dependence of the resulting appearance on the background color, the differences between the simulated veneers on a white background and the given literature values for the shades A1, A2, and B4 24 were closer to the standards. The differences were close to acceptability for the A1 and A2 groups but still large in the B4 group (Table IV, Fig. 5). DISCUSSION Because of their lifelike appearance and long-term dependability, porcelain laminate veneers are often selected for the esthetic restorations of the anterior dentition. The traditional technique for the fabrication of a veneer consists of layering feldspathic porcelain onto a refractory die material or a platinum foil material. A new procedure introduced for the fabrication of porcelain laminate veneers uses a thin aluminum oxide base material veneered with feldspathic porcelain. The core layer is made of densely sintered, high-purity aluminum oxide, which improves the strength and is hypothesized to be able to mask tooth discoloration. The densely sintered aluminum oxide is translucent and has an initial color close to A1 in the Vita shade system. This system has been previously documented for single crown restorations. 16,17 It was hypothesized that the aluminous core could mask discolorations introduced by a stained tooth. A model simulating tetracycline staining was therefore created. Although color measurements of stained teeth are not available in the literature, they have been described as appearing OCTOBER

6 THE JOURNAL OF PROSTHETIC DENTISTRY ZHANG, HEYDECKE, AND RAZZOOG Table IV. Color coordinate and total difference between measurements of the veneered disks on a standard white background and values of standard shade tabs 24 Group L* a* b* E A A B yellow, brown, or gray to black. 3,25 Therefore, the model used in this experiment seems realistic for the described scenario. Our study demonstrated that a color difference of 22.4 E units can be achieved by using the core material alone. This is a marked change, but in clinical use, the core is veneered with a specific porcelain to simulate translucency and achieve the desired shade. The thickness of porcelain laminate veneers is limited because the veneer should be attached to at least 50% of the enamel. The mean thickness of enamel of maxillary anterior teeth is 1.0 to 1.3 mm in the incisal third and 0.4 to 0.5 mm in the gingival third. 5 A reduction of 0.5 to 0.8 mm of the enamel is usually recommended for laminates so that the total restoration is not thicker than 0.5 to 0.8 mm. 3,14 A total thickness of 0.6 mm is therefore realistic and was chosen as the thickness for the samples used in this experiment. As a consequence, any additional shade modification beyond the amount achieved with the aluminum oxide disks had to be achieved with 0.4 mm of veneering porcelain. It could be demonstrated that an additional color difference of 2.31 to 8.78 E units can be predicted by adding the veneering porcelain, depending on the shade. The question to be answered is, if the resulting color difference is in the designated direction in the color space, namely, if by veneering the core material with B4 porcelain, a B4 shade is achieved. Different authors have, in separate studies, reported on the color differences among Vita shades. 21,24 The decrease of the L* coordinate and the increase of the a* and b* coordinates from A1 to A2 and from A1, A2 to B4 in our study are well coordinated with the findings by those authors. It may be concluded that, after partly masking stains with a layer of aluminum oxide, the tooth can be modified to the desired shade using the veneering porcelain. However, it must be stated that these findings are indicators of only the direction of the color change and that they do not correlate in magnitude. The comparison of the disks on the discolored substrates with disks on a white background shows that the resulting color is largely affected by the background color. In this study, color differences of up to 21 E units were found that illustrate the dependence on the background color. The dependency has also been reported in another study on thin porcelain samples by Davis et al. 7 In that study, samples were measured on tooth-colored substrates and compared with measurements on a gray background. The color differences reported were smaller than the differences found in our experiments, which is probably because of the use of different backgrounds. The white background in our study caused a larger increase in the L* coordinate. However, the custom shade samples reported in the study by Davis et al 7 could not be compared with shade values from the literature because of the use of the gray background. Therefore, it is unclear whether their samples are accurate representations of the shade standards. But even our measurement of the samples on a white background that were used as controls does not necessarily represent the actual shade predicted by the shade system. As illustrated by the comparison of the disks measured on a white background, color differences between the obtained values and those used for comparison 24 do reach significant levels. The findings are similar to those of others, who found the shades of the used porcelain to be too high in the L* coordinate. 8 Consequently, even the measurements of the disks on a white background do not accurately represent shades in the Vita system. Nonetheless, the similarity to the literature values is surprising considering the differences in the method. In other studies, different measurement devices were used; we performed all investigations with a colorimeter as opposed to a spectrophotometer. The characteristics of the samples were also different. We used flat, relatively thin disk samples. Some authors have measured shade tabs directly, which is fraught with limitations because of the influence of the surface shape and texture. Others have used shade tabs custom made from the veneering porcelain that may not represent the actual color from the shade guide. 7,8,24 Therefore, a standard for dental shades does not exist. Another factor influencing the final color of veneers made from semiopaque systems using an aluminum oxide core is the thickness of the core layer 8 and the amount of opaque porcelain used. However, in esthetic veneer restorations, an increase in thickness is not an option, nor is an increase in opacity. Additional shade modifications may always be performed through adjustment of the composite cement color. The key to success seems to be a close communication with the dental technician to achieve the desired amount of masking without jeopardizing the esthetic outcome by introduction of too much opacity. It may be concluded that the masking of discoloration from the underlying tooth structure is possible within the described limits by using a double layer (aluminum oxide core/veneering porcelain) technique for porcelain laminate veneer restorations. With the use of 0.6-mm thick porcelain laminate veneer simulations, the measured color was 430 VOLUME 84 NUMBER 4

7 ZHANG, HEYDECKE, AND RAZZOOG THE JOURNAL OF PROSTHETIC DENTISTRY associated with the shade of the used veneering porcelain. CONCLUSIONS 1. With the use of a simulation of double-layer veneers on stained teeth, significant color differences can be achieved of up to 27 E units. 2. The color differences among the shades compare favorably in the direction with values extracted from the literature; thus shades can be expected to match the desired color. Despite the masking ability of aluminous porcelain, the resulting color is still affected by the background color. REFERENCES 1. Nixon RL. Masking severely tetracycline-stained teeth with ceramic laminate veneers. Pract Periodontics Aesthet Dent 1996;8:227-35; quiz Belser UC, Magne P, Magne M. Ceramic laminate veneers: continuous evolution of indications. J Esthet Dent 1997;9: Faunce F. Management of discolored teeth. Dent Clin North Am 1983;27: McLaughlin G. Porcelain veneers. Dent Clin North Am 1998;42:653-6, ix. 5. Ferrari M, Patroni S, Balleri P. Measurement of enamel thickness in relation to reduction for etched laminate veneers. Int J Periodontics Restorative Dent 1992;12: Yaman P, Qazi SR, Dennison JB, Razzoog ME. Effect of adding opaque porcelain on the final color of porcelain laminates. J Prosthet Dent 1997;77: Davis BK, Aquilino SA, Lund PS, Diaz-Arnold AM, Denehy GE. Colorimetric evaluation of the effect of porcelain opacity on the resultant color of porcelain veneers. Int J Prosthodont 1992;5: Douglas RD, Przybylska M. Predicting porcelain thickness required for dental shade matches. J Prosthet Dent 1999;82: Davis BK, Aquilino SA, Lund PS, Diaz-Arnold AM, Denehy GE. Subjective evaluation of the effect of porcelain opacity on the resultant color of porcelain veneers. Int J Prosthodont 1990;3: Chpindel P, Cristou M. Tooth preparation and fabrication of porcelain veneers using a double-layer technique. Pract Periodontics Aesthet Dent 1994;6:19-28; quiz Fradeani M. Six-year follow-up with Empress veneers. Int J Periodontics Restorative Dent 1998;18: Kihn PW, Barnes DM. The clinical longevity of porcelain veneers: a 48- month clinical evaluation. J Am Dent Assoc 1998;129: Kreulen CM, Creugers NH, Meijering AC. Meta-analysis of anterior veneer restorations in clinical studies. J Dent 1998;26: Peumans M, Van Meerbeek B, Lambrechts P, Vuylsteke-Wauters M, Vanherle G. Five-year clinical performance of porcelain veneers. Quintessence Int 1998;29: Wagner WC, Chu TM. Biaxial flexural strength and indentation fracture toughness of three new dental core ceramics. J Prosthet Dent 1996;76: Andersson M, Oden A. A new all-ceramic crown. A dense-sintered, high-purity alumina coping with porcelain. Acta Odontol Scand 1993;51: Oden A, Andersson M, Krystek-Ondracek I, Magnusson D. Five-year clinical evaluation of Procera AllCeram crowns. J Prosthet Dent 1998;80: Razzoog ME, Lang LA, McAndrew KS. AllCeram crowns for single replacement implant abutments. J Prosthet Dent 1997;78: Hegenbarth EA. Procera aluminum oxide ceramics: a new way to achieve stability, precision and esthetics in all-ceramic restorations. Quintessence Dent Technol 1996;19: Sproull RC. Color matching in dentistry. Part I. The three-dimensional nature of color. J Prosthet Dent 1973;29: Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics. 2nd ed. St Louis: Mosby; p Hunter RS, Harold RW. The measure of appearance. 2nd ed. New York: John Wiley; p Johnston WM, Kao EC. Assessment of appearance match by visual observation and clinical colorimetry. J Dent Res 1989;68: O Brien WJ, Groh CL, Boenke KM. A new, small-color-difference equation for dental shades. J Dent Res 1990;69: Haywood VB, Leonard RH, Dickinson GL. Efficacy of six months of nightguard vital bleaching of tetracycline-stained teeth. J Esthet Dent 1997;9:13-9. Reprint requests to: DR MICHAEL RAZZOOG UNIVERSITY OF MICHIGAN SCHOOL OF DENTISTRY ANN ARBOR, MI FAX: (734) merim@umich.edu Copyright 2000 by The Editorial Council of The Journal of Prosthetic Dentistry /2000/$ /1/ doi: /mpr OCTOBER