# Color Stability of Monolithic Zirconia in Various Staining Liquids: An In Vitro Study
###### Abstract  
This study aimed to compare the color stability of monolithic zirconia (MLZ) after immersion in various staining liquids. Fifty MLZ samples (\(n\) = 50) were color infiltrated by brushing/painting the samples pre-sintering. All zirconia samples were then brushed inside a toothbrushing simulator machine to create abrasion. Ten samples each were immersed in artificial saliva (gp-1), coffee (gp-2), protein shake (gp-3), chlorhexidine mouthwash (gp-4), and a soft drink (gp-5). Besides chlorhexidine mouthwash (immersion time: 14 days), the samples were immersed in the liquids for 28 days. A spectrophotometer was utilized to observe the color differences (\(\Delta\)E) at the baseline vs. 28 days post-immersion. The means and standard deviations were compared using analysis of variance (ANOVA) and a multiple comparisons test. A _p-_value < 0.05 was considered significant. The greatest \(\Delta\)E (4.57) was observed after immersion in the coffee, followed by a soft drink (\(\Delta\)E: 4.03). Chlorhexidine mouthwash immersion of MLZ samples also significantly changed their color stability (\(\Delta\)E: 3.60). The inter-group comparisons revealed statistically significant differences (_p_ < 0.05) when \(\Delta\)E values of gp-1 (artificial saliva) were compared with all other groups. Significant differences (_p_ < 0.05) were also seen when \(\Delta\)E values of gp-2 (coffee) were compared with gp-3 (protein shake) and gp-4 (chlorhexidine). All other inter-group comparisons revealed non-significant results (_p_ > 0.05). The results of the present study demonstrate that coffee immersion of MLZ samples causes more significant discoloration (increased \(\Delta\)E values) than other liquids. Further in vitro and clinical studies are warranted to corroborate the present study's findings.  
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helps the color to infiltrate the ceramic in this permeable, pre-sintered stage [11]. However, this addition of color can significantly reduce its strength [12], which affects its clinical success. Plus, under in vivo conditions, exposure to various colored consumable drinks can further deteriorate the color stability [13]. In an in vitro environment, the discoloration produced by different drinks in vivo can be reproduced by immersing the material in a consumable drink, which is considered a good test to evaluate its potential to discolor by measuring the staining levels [14].  
Color stability is of utmost importance among various optical properties of dental restoration [15]. In an earlier in vitro study, the color stability of different zirconia types was studied after exposure to chlorheoxidine, coffee, and green tea [16]. The study's results demonstrated that MLZ samples significantly discolored after coffee immersion [16]. Another previous study also revealed that coffee produced more noticeable color changes in zirconia samples after immersion than all the other beverages [17]. These former studies have verified that while zirconia has commendable mechanical properties and aesthetics, its color stability can still be compromised due to the exposure to various beverages. Hence, it is necessary to test this material further with different testing conditions and multiple beverages to provide a more robust challenge to test its color stability.  
In that context, the present study aimed at coloring the zirconia samples and testing their color stability after exposure to different staining liquids including coffee, protein shake, chlorheoxidine mouthwash, and soft drink (artificial saliva was used as a control). It was hypothesized that the immersion of zirconia samples in various liquids would affect its color stability differently.