INTRODUCTION

Aesthetics is the science of beauty and is a branch of philosophy concerned with the perception of beauty and unattractiveness. In 1984, Goldstein and Lancaster1 conducted a survey to study patient attitudes toward current aesthetic dental treatments, reasons for selecting dental procedures and their satisfaction with their smile. General aesthetic dissatisfaction was found among patients, indicating the need for improved aesthetic dentistry. Color is probably the most important determinant of aesthetics in prosthodontics. The foundation of color was discussed by Clark some 75 years ago. He reported that color has three dimensions, but many dentists in 1931 had not been taught their names or the scales of their measurements.2 Recently, the dental profession has made progress in the study, research and application of the science of color. However, confusion continues to prevail regarding materials and techniques used for constructing aesthetic restorations. The development of new dental systems with the promise of durable, more aesthetic restorations is considered to be a fast growing dental field.

Color measurement is a very complicated process, as color cannot be literally measured by any device, as it is essentially a subjective sensation. What one can do is describe a color in idealized and standardized terms using numbers that correlate with what is perceived. In 1990, Van der Burget and others3 reported visual and instrumental methods to evaluate color. Visual color determination by comparing the patient's tooth with a color standard (shade guides) is the most frequently applied method in clinical dentistry for color communication with dental technicians. The 3D-Master system (Vitazahnfabrik, Bad Sackingen Germany) is based on the theory of color space, which places all imaginable colors within the color space. The color space theory is represented by a sphere for simplicity. Within the color space, the tooth shades are situated in the yellow-red area. In 1999, Vollmann4 discussed the distribution of dental shades within the tooth color space and found that the 3D-Master system provided systematic, uniform and complete colorimetric coverage of the tooth color space in comparison with other commercial shade guides. In spite of the advantages encountered in using the 3-D master shade guide, visual shade matching in general still carries some problems, because of its objectivity. Culpepper5 reported the presence of color matching differences among dentists and for the same dentist for the same tooth on different days as well as the effect of the surrounding environment and light source as was described by Jordan.6 Automated methods of color matching include spectrophotometers, photoelectric colorimeters and color scanners; these devices provide a more objective method for color measurement. Vita Easyshade (Vitazahnfabrik) is a recently introduced intraoral dental spectrophotometer used as a dental shade-matching device. It consists of a base unit and a handpiece with a fiberoptic cable connecting the hand-piece and the base unit. The handpiece contains a fiberoptic probe assembly for illuminating and receiving light from a tooth, multiple spectrometers and a microprocessor for communication with the base unit. Communication between the handpiece and base unit is via USB. The base unit contains a lamp, CPU, vacuum fluorescent display with a touch screen and a removable calibration blank assembly. Easyshade utilizes a 20-watt halogen tungsten filament lamp with a color temperature of 3350 K. For clinical use, the device is supplied with custom-made disposable polyurethane barriers to prevent cross-contamination.7 The Easyshade device represents changes in color results in three forms: numeric change in color values (ΔE), rating the change as Good, Fair or Adjust and graphic and numeric presentations of the difference between the shade and the restoration in terms of value, chroma and hue. With the introduction of more bi-layered aesthetic restorations, the difference between the selected shade tab and the produced restoration is of major concern and can be attributed to the effect of underlying structure on the perceived color. This study evaluated the effect of cores produced by a gold electroforming system on the perceived color of a predetermined dental shade and compared the results obtained to those of In-ceram alumina and cast gold alloy cores veneered using the same shade. The comparison was done by measuring the color difference (ΔE) between a selected shade (2M2) from the 3D Master shade guide and that of crowns made using the three systems.

METHODS AND MATERIALS

The study involved the construction of 30 crowns from an acrylic resin die representing a left maxillary central incisor that was prepared to receive a full coverage restoration. The die was duplicated into 30 dies (Viade Co, Camarillo, CA, USA) of high strength resin material. The dies were divided into three groups: 10 gold electroformed (Gramm Technik GmbH, Ditzingen-Heimerdingen, Germany), 10 In-ceram alumina (Vitazahnfabrik) made by CEREC 3 (Sirona Dental Systems LLC, Charlotte, NC, USA) and 10 porcelain fused to metal using a high noble gold alloy (Classic IV, Jensen Dental, North Haven, CT, USA). The restorations were constructed according to the manufacturer's instructions (Figure 1). The metal-ceramic crowns were constructed in the Smile Tech Dental Lab (Toronto, Canada), the CEREC crowns were constructed at the Faculty of Dentistry, University of Toronto (Toronto, Canada) and the veneering porcelain used was shade 2M2 (3D-Master shade guide, Vitazahnfabrik). The veneering porcelain for the In-ceram alumina blocks was VM7 (Vitazahnfabrik) and the veneering porcelain for the metallic restorations was VMK95 (Vitazahnfabrik). The thicknesses of the veneering porcelain layers were measured and adjusted to obtain the same thickness for each of the tested groups. The thickness of the opaque layer for the two metal systems was 0.3 mm and the dentin (body) thickness was 0.8 mm. The thickness of each veneering ceramic layer was measured after firing to obtain the same thickness for each layer. The Vita Easy Shade (Vitazahnfabrik) was adjusted to measure the change in color between the 2M2 as a target shade and the produced color of the restoration by choosing the restoration setting of the device and selecting the 2M2 shade (Figure 2). The middle third of the restoration was measured and a screen showing the restoration to be “Good,” “Fair” or “Adjust” was displayed, with horizontal lines representing “Value,” “Chroma” and “Hue,” indicating the nominal values for the target shade. As stated by the manufacturer, “Good” means that an expert at shade matching may see little or no difference between the restoration and the target shade. “Fair” means that an expert may see a noticeable but acceptable difference between the restoration and the target shade. For an anterior restoration, “fair” is not acceptable. “Adjust” means that an expert may see a noticeable difference between the restoration and the target shade and that the restoration should be adjusted. The change in color values (ΔE) between the selected shade (2M2) and the resulting crown was also displayed by the device for each crown. The changes in color values (ΔE) were statistically analyzed using ANOVA and post hoc Tukey's test

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RESULTS

The mean color differences between the shade 2M2 and the fabricated crowns for the different systems are shown in Table 1 and Figure 3. ANOVA revealed a significant difference among the three systems (p<0.001). Further statistical analysis using the post hoc Tukey's test revealed no significant difference between gold electroformed crowns and In-ceram alumina CEREC 3 (p<0.981), while a significant difference was detected between the porcelain-fused-to-metal group and the two other groups (p<0.021).

Table 1 Mean ΔE (SD) for the Three Systems Compared to the 3D-Master Shade 2M2

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DISCUSSION

This study simulated the clinical situation as much as possible by using crowns with the dentin and enamel layers fired over the core layer. Veneering porcelain for all three groups was from the same manufacturer and the veneering porcelain for the two metal groups was of the same brand. Treatment of the three core materials for receiving the veneering porcelain was done according to the manufacturers' instructions. This study involved the use of yellow high noble gold alloy, because it was found to be the alloy of choice for color replication of metal-ceramic restorations, since it could be easily masked by the opaque layer compared to Ni-Cr alloy.8–9 Yellow high noble gold alloy also provided a solid base for comparison with yellow electroformed gold in regard to both color and composition. The In-ceram Alumina CEREC 3 core/veneering ceramic was selected for the study to compare the two other groups to all-ceramic esthetic restorations. The selected shade in the Vita 3D-Master shade guide was 2M2. The first “2” represented the second lightest value within the shade guide. This high value was chosen to be able to determine the effect of the underlying structure on the perceived color. Assuming that, if this selected high value shade was able to mask the color of the underlying structure, the lower value would even have a better masking ability of the core. “M” is the hue and is located in the middle of the chosen value, the last “2” represented the chroma and middle saturation of the color. The whole combination—2M2—represented the center color in the second lightest value. The development of the CIELAB color system has been a cornerstone for the measurement and evaluation of color differences between different materials. Change in color (ΔE) is the difference between two colors in the color space. The literature10–11 ranked the change in color among dental materials as follows: ΔE > 3.7 = a very poor match, ΔE > 2 = clinically unacceptable, ΔE < 2 = clinically acceptable and ΔE < 1 = not appreciable (undetectable even by the most experienced eyes). In this study, the change in color (ΔE) of the selected shade (2M2) and the produced shade was measured using a newly introduced intraoral dental spectrophotometer (Vita Easyshade). By definition, a spectrophotometer “is an instrument for color measurement that measures the spectral reflectance of a color and converts it into a tristimulus value.”12 Easyshade, being an instrumental color measuring device, eliminated the human variation in comparing different shades, allowed multiple and repeated measurements and provided the advantage of directly measuring the change in color between the selected shade and the produced restoration on a dental crown; hence, it did not require specially designed specimens. This instrument allows clinicians and technicians to directly measure tooth colors and produced restorations, thus improving clinician-technician communication regarding dental shades. The current study was designed according to the clinical application and lab procedures for the systems involved. The color reproducibility of dental restorations could be affected by porcelain brands1113 porcelain batch inconsistency,10 multiple glazing cycles, furnace muffle contamination, use of various internal modifiers, external colorants and differences in the overall and component layer thickness.14–15 In the current study, the previously mentioned variables were eliminated by involving only one technician, lab and batch number for each tested group. The mixing ratios of the veneering ceramics were standardized and the condensation technique used was the “brush and vibration” technique. It was reported that this technique resulted in the best shade reproduction, as it produces less and smaller air bubbles.16–17 For the selected two metal systems, the thickness of the opaque layer was 0.3 mm, as it was reported to have the best masking ability to underlying structure.18–20 The dentin (body) thickness involved was 0.8 mm, as the literature showed that this was the best color reproduction when this thickness was used.21–22 The thickness of each veneering ceramic layer was measured after firing to obtain the same thickness for each layer. For the all-ceramic system, the veneering ceramic was applied first as a base dentin layer. This was applied to mask the color of the alumina core, and it was fired after the application of the dentin layer. Two firing cycles were carried for the dentin and a final thickness of 1.1 mm was obtained for the body porcelain. Correction of the ceramic thickness was achieved by surface grinding; it was reported that this grinding did not affect the surface smoothness after the final glaze.23 To ensure that the size of all crowns was the same, a rubber base key index was used. The thickness of the middle third of each crown was measured and adjusted to obtain a total thickness of 2 mm. It is worth mentioning that the change in color between the selected shade and the produced shade for the tested groups might be greatly attributed to the compositional differences among the core materials, with their subsequent effect on the degree of opacity.24 The core materials involved in the study were yellow gold alloy (88%), electroformed gold (99.9%) and In-ceram alumina.

CONCLUSIONS

Within the limitations of the current study, it can be concluded that the use of veneered gold electroformed crowns and In-Ceram alumina crowns resulted in shade matching that was significantly better than that obtained with porcelain-fused-to-metal crowns.