# Color Stability of Monolithic Zirconia in Various Staining Liquids: An In Vitro Study
## 4 Discussion  
This study's findings facilitated the acceptance of the hypothesis as our results revealed that the \(\Delta\)E of MLZ samples was affected after immersion in different liquids. Traditionally, a visual color guide has been used to assess color changes in dental materials, but the accuracy of this method is questionable and dependent on the observer's perception of different colors [22]. A spectrophotometer is a device utilized in dentistry to analyze a material's color and changes in the color. This device is beneficial for the color assessment of various dental restorations and their shade verification [23]. To utilize the benefits of a spectrophotometer for detecting color changes, the technique was applied in this study to analyze the \(\Delta\)E of MLZ samples post-immersion in different staining liquids. To analyze the color differences, we used the CIELAB formula. Although CIEDE 2000 has been introduced, we preferred to stick with the CIELAB formula as numerous previous studies in the literature also used it [16,24]. Previously established standards in this field have indicated that an \(\Delta\)E of between 1 and 3.3 units signifies an important but clinically acceptable difference; however, \(\Delta\)E > 3.3 is considered unacceptable and perceivable even by an inexperienced observer [24,25]. In this study, the greatest \(\Delta\)E was observed when the samples were immersed in the coffee (\(\Delta\)E > 3.3). Former studies have shown that coffee can induce color changes in natural teeth, adhesive resin cement, and dental ceramics [26, 27, 28]. In an earlier study, the immer \begin{table}
\begin{tabular}{c c c} \hline \hline
**Study Groups** & \(\Delta\)**E Mean Difference** & \(p\)**-Value** \\ \hline SAL vs. COF & 2.70 & 0.000 * \\ \hline SAL vs. PSH & 1.44 & 0.000 * \\ \hline SAL vs. HX & 1.80 & 0.000 * \\ \hline SAL vs. CC & 2.23 & 0.000 * \\ \hline COF vs. PSH & 1.33 & 0.000 * \\ \hline COF vs. CHX & 0.97 & 0.006 * \\ \hline COF vs. CC & 0.54 & 0.274 \\ \hline PSH vs. CHX & 0.36 & 0.664 \\ \hline PSH vs. CC & 0.79 & 0.038 \\ \hline CHX vs. CC & 0.43 & 0.500 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Statistical comparison of \(\Delta\)E values among study groups ([PERSON]’s post hoc test). Figure 2: Mean and SD of \(\Delta\)L, \(\Delta\)a, and \(\Delta\)b values among study groups. sion of zirconia ceramics in coffee resulted in significant changes in their \(\Delta\)E values [24]. Another study revealed zirconia samples immersed in coffee expressed increased color differences as opposed to those immersed in other beverages [17]. The current study's findings corroborate with the results of these previous studies as increased \(\Delta\)E values were observed post-immersion in the coffee for MLZ samples. Coffee is a popular drink globally, even though it is known to cause teeth staining and discoloration of materials [29]. Coffee comprises tannin and chlorogenic acids, which are thought to be responsible for its discoloration potential [17]. The marked color changes seen in MLZ samples after immersion in the coffee could be attributed to these two ingredients. It should also be noted that coffee has an acidic pH, which can augment its discoloration potential [17]. However, in this study, the pH of liquids used for immersion experiments was not analyzed. The immersion of MLZ samples in the soft drink also brought marked color changes post-immersion (\(\Delta\)E > 3.3). The discoloring potential of soft drinks is well-known as they contain artificial colorants and citric acid that facilitate discoloration of teeth and restorations [30]. Moreover, the acidic pH of these soft drinks causes demineralization of teeth and discolors dental restorations [30]. The increased \(\Delta\)E after the samples were immersed in the soft drink could be attributed to these two reasons. In the present study, the immersion of MLZ samples in protein shake also discolored the specimens (\(\Delta\)E > 3.3).