## 4 Discussion
[34], two different cements were used with monolithic and layered zirconia restorations and no influence of cement was observed in contrast with other studies that reported an influence of the cement when restoration thickness decreased from 2 mm to 1.5 mm in thickness, but using IPS-Empress glass ceramic [20]. The present study confirmed that in pre-colored zirconia ceramics of 1.5 mm thickness, the cements used influenced the final color of the restorations, as the \(\Delta\)E values varied depending on the color of the cement. Using two different cements with a 5% difference in translucency, according to the manufacturers' information, allowed confirmation that even slight differences in the color of the cement may have an impact on the final color of the restoration. This should be taken into consideration in cases with discolored crown areas to be covered by a restoration. Nevertheless, differences in cement thickness (0.1 or 0.2 mm) may also slightly affect the final color of monolithic ceramic crowns. However, since cement thickness cannot be completely controlled by clinicians, it should not be considered a reliable method for color correction [16]. According to [PERSON] et al., the perceptibility threshold is \(\leq\)1.2 [35]. In this study, all \(\Delta\)E values exceeded this threshold, and the only brand considered acceptable, as it fell within the acceptability range (>1.3 and \(\leq\)2.7), was AG, specifically for zirconia on natural tooth substrates, regardless of the cement used. Although pre-colored zirconia restorations are expected to facilitate the achievement of the desired tooth shade, as the color is standardized, further research is needed on these zirconia materials, given the limited literature available on the subject. According to the literature, systematic errors in color-measuring devices are difficult to control and can affect their accuracy [21]. During the study, after multiple consecutive color readings, the VITA Easyshade spectrophotometer displayed excessively high \(\Delta\)E values, not consistent with previous results. This required some additional measurements, repeated multiple times. To minimize this issue, the number of consecutive measurements was limited to 3 consecutive measurements per crown. Limitations of the present study can be identified since a limited sample size was used. The number of crowns used in each zirconia disk limited our sample size since disks can vary in intrinsic color, and the purpose was to eliminate every factor that could add differences in color measurements. Color evaluation can be very sensitive to extrinsic factors, such as room light and tip inclination, and easily cause dispersion of the results. The authors tried to limit this factor by adding a pink silicone key that limited variations of tip inclination in each reading. Clinical factors such as tooth coloration, different cements, and various brands of zirconia ceramics can affect the final color of a restoration [16]. Therefore, future research on pre-colored zirconia should explore additional aspects, including increasing the number of commercial brands studied, testing different restoration thicknesses, and incorporating additional brand shade guides. Additionally, it would be valuable to compare these results using resin cements with different opacities (try-in cements), as these factors could influence the outcomes observed in this study. Other potential research directions include measuring color across different thirds of the tooth and comparing the values, as well as analyzing the L\({}^{*}\), C\({}^{*}\), and h\({}^{*}\) values independently to better understand the factors that most influenced the results. Given the limited research on pre-colored zirconia ceramics, further studies should explore factors such as higher sample sizes, different thicknesses of crowns, additional commercial brands, and variations in cement opacity.