The reproducibility of electronic color measurements of the marginal gingiva

As society ages and more people retain their natural teeth, the incidence of anterior restorations continues to increase, particularly cervical fillings; thus, Class V restorations are increasingly necessary for oral rehabilitation [1, 2]. Class V restorations are applied in areas of occlusal dysfunction and abrasive processes, where the fillings must withstand shrinkage of the resin material and bonding and deformation during static and dynamic occlusal dysfunction [3, 4]. Composite restorations created in this context often require esthetic color adjustment [5]. Typically, color matching refers to the color of the affected tooth. However, in certain situations, to avoid surgical treatment [6], it may be beneficial to use gingiva-colored composites or ceramic materials to achieve better esthetic results and the ideal proportions of red and white [7, 8]. Consequently, color matching of the marginal gingiva is necessary and it is a challenge to achieve color harmonization for natural-looking teeth.

Traditionally, to match tooth or gingival color, the dentist referred to color keys to identify the target color [9]. In the last decade, however, color-measuring devices have been used to overcome the disadvantages of visual color matching, which include observer factors like color blindness, fatigue, light conditions, and health impairments. These color-measurement devices have the potential to detect colors more precisely and reproducibly, by determining the color coordinates and dental color shades and calculating the color differences (ΔE) between objects [10‐18]. The ΔE values are calculated using formulas that enable quantitative evaluation of ΔE. The most frequently used formula is derived from the CIE L*a*b* system. Several studies have reported ΔE thresholds that are clinical acceptable and result in perceptible ΔE [19]. Indeed, 50% of examiners are able to perceive a ΔE value < 1 when observing opal monochromatic samples under controlled conditions. Therefore, the minimum ΔE for discriminating a mismatch between the composite and tooth color in the oral cavity varies from study to study [20]. Although electronic shade-measuring systems are used for color determination, few studies have evaluated the reproducibility of electronic gingival color determination, particularly in the context of adhesive restorations of Class V defects [2, 21, 22].

Therefore, this study evaluated the reproducibility of electronic color-determination system evaluations of the marginal gingiva (Figs. 1).

It was shown both in the L*, a*, and b* color coordinates and in the calculated color differences ΔE different results with wide ranges, so L* color coordinates ranged from 49 to 78.7, a* coordinates from 3.6 to 31.2, b* coordinates from 8.6 to 45.6, and the calculated color differences ΔE for each device from 0.1 to 19.1. Table 2 shows the mean, standard deviations, minimum, and maximum values of L*, a*, and b* coordinates and calculated color differences ΔE for each color measurement device.

Only the L* coordinates and the ΔE values between the system A and C systems differed not significantly (p > 0.016)

Table 3 shows the intraclass correlation coefficients (ICC) for interdevice reliability for the L*, a*, and b* color coordinates and color distances ΔE for the devices tested. These ICC ranged from 0.675 to 0.807.

Pink composite or ceramic restorations are a viable option for restoring Class V defects or perform optimal red esthetics with prosthetic restorations [8]. Even with challenging restorations, such as massive recession defects (Miller class II or above) or pigmentation of the marginal gingiva, composite restorations are an esthetic alternative to invasive surgical procedures for treating recession [6, 23]. However, visual evaluation of the shade of the gingiva has disadvantages because it is affected by light conditions (i.e., metamerism) or dyschromatopsia. Shade guides and electronic measurement systems are available for evaluating the shade of natural teeth. Several shade guides have been developed for different requirements, such as color matching in prosthetic restorations or detecting color changes during a bleaching process. However, only a few shade guide systems are available for evaluating the shade of gingival regions, and these are subject to coverage errors [9]. Ghinea et al. declared that there is no optimal gingival shade guide [24].

Therefore, a more objective and reproducible method for evaluating gingival color would be helpful. While electronic shade-taking systems are generally used for reproducible determination of tooth color [10, 13, 15, 16], these systems may also have the potential to evaluate gingival color reproducibly. The color coordinates of the gingiva vary widely and differ between females and males [25]. It is not clear whether tooth color measurement systems can record the strongly scattered color information of the marginal gingiva, especially because one of the systems tested (device ES) generates color information from reflected light. It is also not clear to what extent the thickness and type of gingiva, the region within the oral cavity [26], and the contact mode of the ES device affect color measurements. In fact and a strength of this study is that it was shown that the color measurement systems tested generated different color coordinates of gingival tissue that is shown by the low ICCs. In contrast to this, a limitation of this study is that no repositioning jig was used. Although such a jig was not used, which could have caused an extra variance of the results, this is not surprising because although dental color measuring systems have high reproducibility, they are not CIE-compliant. It was first demonstrated a difference between these systems and a CIE-compliant system in 2010 [15]. The SDs of the L*, a*, and b* color coordinates ranged from 1.6 to 5.7, and the calculated ΔE for each proband and measuring system also differed, as expected but first shown in this study. Several formulas are available for calculating ΔE [27] and the ΔE of each system has a SD visible to humans [19]. Therefore, the small differences in color of the gingival regions detected could also result from the measurement uncertainty of the systems. This is not surprising given that the instruments were designed for determining the shade of natural teeth, which have different color coordinates. Particularly, contact measurement systems, such as the ES system, could affect tissue perfusion during gingival color measurements, resulting in color changes that is a further limitation when interpreting the results. Therefore, soft tissue colors should be measured with non-contact systems. Considering the inherent uncertainties of gingiva measurements, the SP and CE systems showed the highest reliability for gingival color measurements of the devices tested. However, all these devices allow no high reproducible determination of color coordinates of the marginal gingiva.

The electronic tooth color determination systems evaluated herein are limited in terms of the reproducibility of evaluations of marginal gingival color. Furthermore, the results of the different measurement systems differ enormously; this should be considered in future clinical studies, whereas the systems SP and CE showed a higher reproducibility compared with the other two systems tested in this study. However, all systems tested in this study are not suitable for high reproducible determination of color coordinates of the marginal gingiva.