Color coverage of a newly developed system for color determination and reproduction in dentistry
Introduction
Color of teeth, required for the reproduction in porcelain, is most frequently assessed visually and defined descriptively by means of a code that belongs to a matching shade standard (tab). The defined shade tab is likewise a prescription for the porcelain powder selection for production of the restoration. However, many different variables can negatively influence the accuracy of tooth color reproduction in porcelain, utilized by this approach. Probably the most important variables are the eye's subjectivity and surrounding illumination type followed by the inadequate distribution and inconsistent arrangement of the shade guides within the color range of human teeth.1, 2
The eye's subjectivity and variability of the surrounding illumination can be eliminated using electronic devices in dentistry. From five devices of different optical engineering the spectrophotometer Easy Shade proved to be the most repeatable3 and from three spectrophotometers, an area spectrophotometer SpectroShade-Micro was the most repeatable in clinical conditions.4 The spectral data of the teeth surface can be bundled and represented as a radiance curve that can be compared with the radiance curve of the shade tab in order to define a match. Therefore, dental spectrophotometers have a database of spectral data of the shade guides incorporated. For practical reasons a three coordinate color system CIE L*a*b* is most frequently used in dental research, representing lightness (L*), redness-greenness (a*) and yellowness-blueness (b*).5 In this color system the equal color distances correspond to equal perceptual difference. Therefore, the Euclidean distance (ΔE) of the two color points corresponds to a perceptual difference between the two colors.6 It has been established that the spatial color difference of 1 ΔE unit can be perceived by approximately 50% of experienced observers.7 Given that this perceptibility of color difference depends on the environmental conditions, different authors have established different levels of perceptibility for dental conditions ranging from ΔE = 2.6 for denture teeth8 to ΔE = 1.6 for all ceramic crowns.9 In the similar trend, researchers suggest that perceptible color difference can still be accepted as a clinical optimum, defining different thresholds for acceptability, such as ΔE = 2.710 and ΔE = 6.8.11 However, Lindsey and Wee showed that in controlled circumstances, using computerized models and not allowing for eye adaptation, the perceptibility and acceptability of small color differences are equivalent and lower than earlier established.12
Although the optical properties of teeth are unique for each individual and it is to expect that there are millions of different teeth colors available in the natural environment, the custom shade guide range needs to be restricted to an amount that can be used in the daily dental practice. The highest number of tabs of all contemporary shade guide systems is 38 from Shofu (Menlo Park, CA, USA), but the most widely used shade guide system in dentistry, Vita Classical (Vita Zahnfabrik, Bad Säckingen, Germany) contains 16 tabs. The concept of this system involves four groups: A, B, C and D, which represent reddish, yellowish, reddish-gray and yellowish-gray teeth respectively. Each group has a small range within, which represents a simultaneous increase in pigment saturation and decrease in lightness per tab of the same group. However, the increments of these color gradients are arbitrarily arranged making it very challenging to translate the shade code of the tab into the accurate color reproduction. In order to improve the accuracy of color selection according to three Munsell's dimensions the same company introduced a new shade guide of 29 tabs, Vita 3D Master, which proved to be more uniformly and more widely distributed within the range of human teeth.13, 14 Also the image modalities of Vita classical shade guide showed possibility for improved coverage of teeth colors.15
Dental patients of today have sky-high demands and are very well informed about the excellent esthetic results that can be achieved using porcelain. Therefore, a high number of porcelain restorations are not being accepted due to color miss-match. In order to improve the overall satisfaction and cost effectiveness in dentistry the translation of tooth color into porcelain needs to be improved. A sustainable challenge is to manufacture a new shade guide using dental porcelain in an appropriate way. This system must simplify the color reproduction process by adopting only the most necessary elements and organizing them in a logical manner with uniformed modalities that will always be traceable after color determination.
The basic color science teaches us that the full visible color range can be created by means of mixing three primary colors (red, green and blue Hues). In subtractive color mixing processes, like the color reproduction in dentistry, mixing of more different color pigments will produce a darker effect (decrease in Value) whereas the additional amount of one particular pigment in the mixture will produce more color intensity of that particular pigment (increase of Chroma). Spectral measurements in dentistry showed that mainly yellow and red Hues are clearly present in teeth.16, 17 Tooth color can also have different darkness/lightness levels and different saturation levels.16, 17 Thus, although it is logical to use only yellow and red Hues to produce color standards in dentistry, their darkness/lightness levels need to be simulated by addition of black and white pigments because of the absence of other primary colors. Therefore, it is reasonable to use red, yellow, black and white pigments for subtractive mixing of porcelain in teeth color reproduction. Furthermore, these elements should be precisely weighted in porcelain mixtures and the shade tabs should mimic the thickness of different layers of natural teeth. In doing so a database of differently composed, yet standardized, shade tabs can be created. Using a spectrophotometer a large amount of accurate data from both human teeth and standardized porcelain tabs is collectable. Finally, the color matching between these tabs and teeth can be utilized and a color reproduction recipe can be obtained. In order to keep this system applicable for the visual as well as for the electronic color determination, the redundant color standards can be subsequently eliminated after the amount of tabs, necessary for sufficient color coverage of human teeth, has been defined.
The aim of this study was to evaluate the color coverage of a newly developed, systematically and logically arranged color determination and color reproduction system and to compare this with the color coverage of four different contemporary systems. It is expected that the new system can cover the human teeth range below the level of perceptibility for porcelain restorations ΔE = 1.6 more precisely than the existing ones.
Section snippets
The concept
The concept of the new color determination and color reproduction system is based upon the presence of two Hues (red and yellow), darkness/lightness level and different color intensities in teeth. It adopts those elements and combines them in a comprehensible manner resulting in layered porcelain tabs configured like natural dentin and enamel.
The elementary, highly chromatic red and yellow components of this system were established by long-term experience of dental technicians. The color
Results
Table 2 shows that from all used tabs the NS scored the best with 26 “necessary tabs” out of 42 and the frequency of 133 color matches (out of 198 teeth) established with these 26 tabs. The frequency of matches increased with NE (155), yet the number of necessary tabs did not increase accordingly, which means that more matches could be established with less tabs.
The minimal ΔE values for the new system standard range (NS) and for the expanded range (NE) were established between 0.25 and 8.18 (
Discussion
The reproduction of tooth color has relied on visual methods for many years now and the creation of the ideal optical properties of teeth has been solely achieved by means of trial and error, because there are no standardized guidelines for color reproduction in accordance with the determined shade standard. This has brought high costs within dental industry and often disappointments for dental professionals and their patients. The color mismatch in dentistry can be a consequence of many
Conclusion
Within the limitation of this study it can be concluded that the newly developed color determination and color reproduction system, which is logically and systematically arranged and simplified could cover the range of human teeth below the level of perceptibility (ΔE ≤ 1.6) better than four other available contemporary systems.
Conflict of interest
None declared.
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