Abstract
Objectives
The objective of this study was to develop a shade selection program that predicts the shade choice with the smallest CIEDE2000 color difference for dental composite resin restorations when given a backing and target shade. By utilizing previously generated regression models, a database of spectral reflectance information, and principles of Kubelka-Munk layering, a highly accurate shade selection program was designed.
Methods
Using SAS 9.3 Statistical Analysis Software, a shade selection program was developed that incorporated Kubelka-Munk layering data of Herculite Ultra and Estelite Omega composite resins from a characterized database of absorption and scattering information. Test scenarios represented an inquiry based off of a backing selection shade (Shade B) and a target selection shade (Shade T). For the simulation, the thickness of the layer on the backing was 1.9 mm and the CIE illuminant was D65. The selection program was designed to select the shade that would give the lowest CIEDE2000 color difference when selected for the specific target and backing. When using the 3-D Vita shade guide, analysis between direct reflectance data and RGB data regressed to reflectance data was included in order to verify accuracy of the regression.
Results
Test scenarios indicated a systematic and accurate shade selection system by suggesting a shade that resulted in a CIEDE2000 color difference of zero when using the same target and backing shades. Most scenarios of backing and target combinations gave at least one option that was beneath the acceptability threshold indicating a clinically acceptable shade match. Many test scenarios indicated options that were beneath the perceptibility threshold indicating a highly accurate process of shade selection. There was generally little variability in the CIEDE2000 color differences when using the reflectance data versus the RGB regression data as input into the shade selection program further verifying the accuracy of a previously generated regression model.
Conclusions
The shade selection program that was developed is a viable system that could reduce variability in observer selections while increasing patient satisfaction for potential use in clinical situations that require color matching a restoration to a tooth.
1
Introduction
Translation of visual color to selection of a restorative material that best matches the natural dentition is a complicated process that can produce a high amount of inter-observer variability and lack of objectivity . Digital imaging as a method to aid in optimum and efficient shade matching in dentistry has become increasingly more relevant in the modern world of dentistry . Methodology for standardizing the color data that comes from an image has become progressively more refined and reliable for use in color matching as well . Previous studies have confirmed the accuracy of Kubelka-Munk theory of reflectance for making color predictions of resin composite materials . The effects of interfacial reflection corrections on the overall accuracy of Kubelka-Munk theory of reflectance have also been determined, so that utilization of these correction models on the reflectance may provide the highest level of accuracy . Previous computer color matching systems using Kubelka-Munk theory have focused on matching dental porcelains and the mixing of porcelain shades rather than the utilization of fixed shades and layering to obtain a desired result . In addition, the previous systems involving computer aided color matching relied on a spectrophotometer or colorimeter rather than digital imaging with a regression model as implicated in the study set forth in this article .
The specific objectives of this investigation were to design and develop a shade selection program using a precise combination of digital imaging with regression modeling , a previously characterized database of shade information, and Kubelka-Munk theory of reflectance in order to make the process of shade selection more objective and accurate while decreasing the variability in observer selections . Concepts of acceptability and perceptibility thresholds were integrated into the shade selection program in order to enhance the applicability to a clinical setting . Validation of the system was accomplished through exploration of the following null hypothesis: The shade selection program selects a shade with a CIEDE2000 = 0 when given a query with the same target and backing shade. In addition, the results of the shade selection program utilizing the direct method and the method via a previously developed regression model were compared . The null hypothesis for this objective was as follows: There is no difference in the CIEDE2000 color differences in the shade selection output between the direct method and the method using regression estimates.
2
Materials and methods
Information from a previously characterized database of optical information was utilized and expanded by fabrication of Kerr Herculite Ultra (Kerr Corporation, Orange, CA USA) and Estelite Omega (Tokuyama Dental America, Encinitas, CA USA) composite disc shaped samples using circular PVC templates of 20 mm diameter and a thickness of .25 mm and .6 mm in all shades of Herculite Ultra composite materials and in all shades of Estelite Omega composite materials . The previously characterized database also contained three different lots of Herculite Ultra shades A1, B2, and D3 which were analyzed to study potential differences in lot numbers of the same shade . Specimens were first pressed to flat and parallel surfaces utilizing a compressive force of approximately 800 N and were then light cured for approximately 30 s on each side using overlapping irradiation zones . They were then sanded with 600 and 1000 grit sandpaper to remove the matrix rich surface layer and to achieve the desired thickness with uniformity across specimens. Final thicknesses were then determined at the central point . Measurements of radiant energy were made against white, gray, and black opaque backings, with reflectance measurements of 0.8, 0.4, and 0.0 respectively, to simulate a single layering effect . The radiant energy data was converted to spectral reflectance . Kubelka-Munk theory was then used to predict theoretical absorption and scattering coefficients for these specimens on different backings using SAS ® Proprietary Software 9.3, (SAS Institute Inc., Cary, NC, USA.) to perform a non-linear least squares regression analysis to determine the best fit as performed previously . This absorption and scattering information serves as the database upon which the shade selection program draws .
Using SAS ® Proprietary Software 9.3, an all-encompassing model that incorporated the regressions previously generated with the spectral reflectance and K-M layering data , a shade selection program was designed. Test scenarios were developed that represented an inquiry based off of a backing selection shade (Shade B) and a target selection shade (Shade T). For the simulation, the thickness of the layer on the backing was 1.9 mm and the CIE illuminant was D65 . The selection program was designed to select the shade that would give the lowest CIEDE2000 color difference when selected for the specific target and backing as well as to indicate perceptibility and acceptability of the proposed shade match . The null hypothesis was tested by setting up a test scenario where the backing and target shades were the same in order to determine CIEDE2000 of the output shade to the target shade. When using the VITA Linearguide 3D-MASTER ® (VITA North America 22705 Savi Ranch Parkway, Suite #100 Yorba Linda, CA 92887), statistical analysis in the format of a paired t -test (α = 0.05) between direct reflectance data and RGB data regressed to reflectance data was included in order to further verify accuracy of the regression model used .
2
Materials and methods
Information from a previously characterized database of optical information was utilized and expanded by fabrication of Kerr Herculite Ultra (Kerr Corporation, Orange, CA USA) and Estelite Omega (Tokuyama Dental America, Encinitas, CA USA) composite disc shaped samples using circular PVC templates of 20 mm diameter and a thickness of .25 mm and .6 mm in all shades of Herculite Ultra composite materials and in all shades of Estelite Omega composite materials . The previously characterized database also contained three different lots of Herculite Ultra shades A1, B2, and D3 which were analyzed to study potential differences in lot numbers of the same shade . Specimens were first pressed to flat and parallel surfaces utilizing a compressive force of approximately 800 N and were then light cured for approximately 30 s on each side using overlapping irradiation zones . They were then sanded with 600 and 1000 grit sandpaper to remove the matrix rich surface layer and to achieve the desired thickness with uniformity across specimens. Final thicknesses were then determined at the central point . Measurements of radiant energy were made against white, gray, and black opaque backings, with reflectance measurements of 0.8, 0.4, and 0.0 respectively, to simulate a single layering effect . The radiant energy data was converted to spectral reflectance . Kubelka-Munk theory was then used to predict theoretical absorption and scattering coefficients for these specimens on different backings using SAS ® Proprietary Software 9.3, (SAS Institute Inc., Cary, NC, USA.) to perform a non-linear least squares regression analysis to determine the best fit as performed previously . This absorption and scattering information serves as the database upon which the shade selection program draws .
Using SAS ® Proprietary Software 9.3, an all-encompassing model that incorporated the regressions previously generated with the spectral reflectance and K-M layering data , a shade selection program was designed. Test scenarios were developed that represented an inquiry based off of a backing selection shade (Shade B) and a target selection shade (Shade T). For the simulation, the thickness of the layer on the backing was 1.9 mm and the CIE illuminant was D65 . The selection program was designed to select the shade that would give the lowest CIEDE2000 color difference when selected for the specific target and backing as well as to indicate perceptibility and acceptability of the proposed shade match . The null hypothesis was tested by setting up a test scenario where the backing and target shades were the same in order to determine CIEDE2000 of the output shade to the target shade. When using the VITA Linearguide 3D-MASTER ® (VITA North America 22705 Savi Ranch Parkway, Suite #100 Yorba Linda, CA 92887), statistical analysis in the format of a paired t -test (α = 0.05) between direct reflectance data and RGB data regressed to reflectance data was included in order to further verify accuracy of the regression model used .
3
Results
The CIEL*a*b* values for the selected combinations of target and backing used as test scenarios for the shade selection program are shown in Table 1 .
| Target Shade | Backing Shade | Target | Backing | ||||
|---|---|---|---|---|---|---|---|
| L* | a* | b* | L* | a* | b* | ||
| A1 | A1 | 76.3 | 1.4 | 16.2 | 76.3 | 1.4 | 16.2 |
| A1 | B2 | 76.3 | 1.4 | 16.2 | 75.4 | −0.8 | 16.9 |
| A1 | D3 | 76.3 | 1.4 | 16.2 | 65.3 | 1.4 | 13.9 |
| B2 | A1 | 75.4 | −0.8 | 16.9 | 76.3 | 1.4 | 16.2 |
| B2 | B2 | 75.4 | −0.8 | 16.9 | 75.4 | −0.8 | 16.9 |
| B2 | D3 | 75.4 | −0.8 | 16.9 | 65.3 | 1.4 | 13.9 |
| D3 | A1 | 65.3 | 1.4 | 13.9 | 76.3 | 1.4 | 16.2 |
| D3 | B2 | 65.3 | 1.4 | 13.9 | 75.4 | −0.8 | 16.9 |
| D3 | D3 | 65.3 | 1.4 | 13.9 | 65.3 | 1.4 | 13.9 |
| 2M1 | 2M1 | 73.5 | 0.0 | 12.2 | 73.5 | 0.0 | 12.2 |
| 1M1 | 1M1 | 77.8 | −0.4 | 11.0 | 77.8 | −0.4 | 11.0 |
| 2L1.5 | 2L1.5 | 73.5 | −0.4 | 16.8 | 73.5 | −0.4 | 16.8 |
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