Investigation of the optical properties of different ceramic materials containing zirconia

Monolithic zirconia materials, despite their superior mechanical properties, may exhibit aesthetic shortcomings. In recent years, with the rise in aesthetic expectations, high translucency zirconia materials stabilized with 4 mol and 5 mol yttria, which possess both high mechanical strength due to their monolithic structure and superior aesthetic properties, have been developed. Multilayer zirconia ceramics have been introduced as a solution to improve the color of monolithic zirconia restorations. Although the fit of restorations produced with CAD-CAM systems is generally very good, clinical adjustments are often necessary. The aim of this study is to investigate the effects of various surface polishing procedures, performed after clinical adjustments, on the optical properties and phase changes of three different multilayer monolithic zirconia materials. In this study, a total of 90 samples, 30 samples of 1 mm thickness, 10 mm in diameter, and A1 color, were obtained from Katana UTML, Katana STML, and IPS e.max ZirCAD MT Multi blocks. Each material sample group was randomly divided into three separate groups: control, polishing, and glazing. The L*, a*, and b* values of each sample were analyzed using a spectrophotometer after the surface finishing procedures. The ∆E00 and TP00 values of the samples were calculated using the CIEDE200 formula. XRD and SEM analyses were performed on one sample from each subgroup. Data analyses for ∆E00 measurements were conducted using IBM SPSS 25, while TP00 measurements were analyzed using IBM SPSS V23. The normal distribution of the multivariate data was checked using the Shapiro-Wilk test. To examine the effects of different materials and surface finishing procedures on ∆E00, a Two-Way Repeated Measures ANOVA (Greenhouse-Geisser statistic) was applied, while a Two-Way ANOVA was used for comparing TP00 measurements. Post Hoc Bonferroni tests were utilized to identify the group or groups causing the difference. Homogeneity of variances was checked using the Levene test. Multiple group comparisons were examined using the Bonferroni test. The level of statistical significance was set at "p<.05." According to the results obtained, statistically significant differences were found between the groups SP and SG (p<.001), UP and UG (p<.001), and ZG and ZP (p=.002) in terms of ∆E00 values. It was observed that the color change caused by the glazing process was greater than that of polishing. Statistically significant differences in ∆E00 values were found between the ZG group and the SG and UG groups (p=.035 and p=.001, p<.05). The color change measurements of the SG and UG groups were found to be higher than those of the ZG group, with the highest color change observed in the UG group. The effect of the surface treatments and material types on the average TP00 values was found to be statistically insignificant (p=.132, p>.05). In all material groups, the average TP00 values increased with the polishing procedure, but this increase was not statistically significant. With the glazing process, the TP00 values of all material groups decreased, but this decrease was also statistically insignificant. According to the evaluations from SEM analysis, both surface finishing procedures resulted in smoother surfaces compared to the control group. It was observed that the glazing process created a smoother surface compared to the polishing procedure. However, both surface finishing procedures did not completely eliminate deep scratches, pits, and grooves in the samples. The results of the XRD analysis indicated that the graphs and peak values of all samples were consistent with the cubic zirconia graphs and peak values found in the diffractometer's database. No monoclinic phase peaks were observed in any of the examined samples. There were no findings suggesting that a phase transformation occurred within the material.