Whiteness difference thresholds in dentistry
Introduction
Non-invasive teeth whitening procedures are among the most popular treatments currently used in dental practice to improve tooth color and appearance [1], [2], [3]. Nowadays, there are many vital tooth bleaching procedures, including in-office bleaching, at-home bleaching and over-the-counter bleaching systems [2], [3]. Different bleaching agents and concentrations, product format, application and times of application have been reported [2].
CIELAB color space and its associated color differences formulas are extensively used for color measurement and color evaluation in dentistry. In this sense, whiteness variations are usually evaluated by means of total color differences or differences among one of the three axes that describe the CIELAB color space (ΔL*: differences in lightness; Δa*: differences in red–green axis; and Δb*: differences in yellow–blue axis) [4], [5], [6], [7], [8].
Whiteness (level of white) of a material is adequately portrayed using a whiteness index. Whiteness indexes or formulas based on the CIE1931 XYZ color notation system [9], such as the CIE whiteness index (WIC) [10], [11], [12] and the whiteness index according to ASTM E-313-73 (WI) [13], have been used in dental research [14], [15]. In this sense, an optimized index for evaluation of tooth whiteness (WIO), which preserves the functional form of the CIE WIC index, was developed [16]. Recently, a new customized and easy to use CIELAB-based whiteness index for dentistry (WID) was introduced [17], [18]. WID showed an improved correlation with the visual perception compared to all others CIELAB and CIE1931 XYZ-based whiteness or yellowness indexes tested under laboratory and clinical conditions [17].
Visual thresholds for color discrimination are well-established quality control tools in research and industry. Thus, according to the latest Guidance on color measurements published by the International Organization for Standardization (ISO/TR 28642:2016) [19], color variation should be assessed based on comparisons with 50:50% thresholds. Perceptibility difference threshold (PT) represents the lower perceptual limit and it can be applicable to study discernible colors by the human visual system [20]. However, in many practical situations, noticeable to greater differences above the PT are named acceptable color differences or color tolerances. The industrial interest on these differences is justified, mainly by the need of maintaining the differences under an admissible limit in clinical practice or industrial production. Several studies [21], [22], [23], showed that the acceptability and perceptibility color thresholds (AT and PT, respectively) values were significantly different using both CIELAB and CIEDE2000 color difference formulas. In addition, it has been shown [23], [24], [25] that patients are less discriminating (PT and AT values) than dentists.
A recent study [26] have conducted a psychophysical experiment based on visual assessments of digital images of teeth on a calibrated display to determine the perceptual thresholds for ΔL* (1.14), Δa* (3.24) and Δb* (1.11) in tooth whiteness. Even though these values may be of interest, it is necessary to obtain threshold values based on specific whiteness indexes with a direct application for dentistry. In this context, studies on whiteness thresholds for dentistry that use the recommended CIELAB-based index (WID) and a suitable fitting procedure are not available.
Therefore, the purpose of this study was to determine 50:50% perceptibility and acceptability whiteness thresholds (WPT and WAT) using WID and the Takagi-Sugeno-Kang (TSK) Fuzzy model, testing the hypotheses that (1) there is a difference between WPT and WAT, and (2) WPT and WAT differ between dentists and laypersons.
Section snippets
Stimuli
Reported CIELAB color coordinates of pre- and post-bleaching maxillary central incisors [27] were used to represent computer-simulated teeth. The CIE L*a*b* color coordinate range values for simulated teeth were as follows: L* = 59.9–84.1, a* = 1.9–8.1, and b* = 7.4–14.6, while the WID value ranged from 0.53 to 19.00 units. Whiteness index values were combined to create images of 60 pairs of simulated upper central incisors (SUCI) with WID differences () ranging from 0.22 to 7.83
Results
Fig. 3 shows the values of percentage for perceptibility by all observers against ΔWID. WPT value corresponding to 50% perceptibility was 0.72WID units (CI: 0.0–2.69) with r2 = 0.52.TSK Fuzzy Approximation with 4 equally distributed rules along the x-axis was used. Values of percentage for acceptability by all observers and all groups against ΔWID are plotted in Fig. 4. WAT value for 50% of acceptance was 2.60WID units (CI: 0.2–7 + ) with r2 = 0.57.TSK Fuzzy Approximation with 3 equally distributed
Discussion
Several studies have investigated the thresholds for perceptibility (PT) and acceptability (AT) of color difference for dentistry and dental applications [18], [21], [22], [23], [32], [33], [34], [35], [36]. It is noteworthy a recent multicenter study [23], which determined the 50:50% PT and the 50:50% AT for CIELAB and CIEDE2000 specifically for dentistry. Results from the study were included as reference values in the ISO/TR 28642:2016 [19] and can be applied as a quality control tool to
Conclusion
The present study determined reliable whiteness thresholds with the 50:50% WPT being 0.72 WID units and the 50:50% WAT being 2.60 WID units, which were significantly different. Both, WPT and WAT values, were different for dentists and laypersons.
Acknowledgements
The authors acknowledge funding support from research projects JA TEP-1136 from “Junta de Andalucía”, Spain, MAT2013-43946R from the Spanish Ministry of Economy and Competitiveness.
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