# Color Stability of Monolithic Zirconia in Various Staining Liquids: An In Vitro Study
## 4 Discussion
There is a scarcity of similar studies in the literature that have tested the discoloration impact of protein shakes on zirconia samples. The findings of the present study, therefore, report unique results. The young population commonly uses protein shakes to fulfill their protein requirements and stay healthy. Regrettably, protein shakes may contain significant quantities of heavy metals, including lead, arsenic, mercury, and cadmium [31]. These ingredients are not always mentioned on the packaging of the protein powder; therefore, the consumer remains unaware of their presence and potentially harmful effects. It is anticipated that the presence of these heavy metals in the protein shake might have played an essential role in the discoloration of MLZ samples in this study, although more conclusive evidence is required. Chlorwechidine immersion of the MLZ samples also discolored these specimens (\(\Delta\)E < 3.3). Chlorwechidine mouthwash is used in dentistry to restrict bacterial growth and decrease the incidence of periodontal diseases and dental caries [32]. Yet, its extended use has been shown to cause discoloration of natural teeth and dental restorations [16]. Chlorwechidine mouthwash is usually recommended to patients for a period of 7-14 days [20,33]. Although it can be recommended for more than 14-28 days [34], a large increase in extrinsic tooth staining in participants using chlorwechidine mouths Fine for 28-42 days (4-6 weeks) has been reported previously [35]. Therefore, it was decided in our study to immerse the samples in CHX mouthwash for 14 days instead of 28 days. According to a former study, zirconia-based samples are prone to discoloration after they are exposed to chlorwechidine gluconate [32]. Another study revealed that chlorwechidine could affect the color stability of nanoceramic CAD/CAM restorative material within clinically acceptable limits [36]. The findings of the present study agree with these earlier studies as immersion of MLZ samples in chlorwechidine brought changes in their color stability, but the \(\Delta\)E was <3.3 (clinically acceptable level). The precise mechanism via which chlorwechidine induces discoloration in teeth remains unknown. Nevertheless, it is speculated that the chlorwechidine molecule breaks inside the oral cavity and forms parachloraniline, which results in the denaturation of proteins and formation of metal sulfides that cause discoloration of teeth and restorations [37,38]. This could be the reason for the discoloration of MLZ samples in the current study. The results of the present study should be cautiously interpreted. The present study utilized a color difference formula for color stability assessment; however, a visual judgment method is more frequently used in clinical dentistry. In other words, a significant difference in \(\Delta\)E values may not cross the clinically perceptible threshold where it comes to be noticed by the observer [39]. Therefore, along with numerical assessment using the \(\Delta\)E values, the clinical perceptibility threshold and acceptability threshold must also be observed for color stability assessment to conduct a holistic investigation in future studies. Another limitation of the current study was its in vitro design. It should be noted that restorations could act differently inside the oral cavity due to the presence of saliva. Plus, under clinical conditions, as the restorations are bonded to the tooth, they are exposed to the staining liquids unequally (more on one side than the other). In our experiments, they were immersed completely in the liquids; hence, our study could have presented inflated discoloration values. Additionally, oral hygiene maintenance habits (toothbrushing, use of mouthwash) could also affect the color stability of restorations in vivo. Furthermore, only a single type of MLZ was used in this study and combined with a color solution from a particular manufacturer. To improve on that, additional combinations should be explored in future studies.