Evaluation of monolithic zirconia with different yttrium ratios in terms of metamerism and translucency

This study aims to examine the metamerism and translucency of monolithic zirconia materials with different yttrium content in different lighting environments before and after thermal aging. For this purpose, 36 square-shaped, 0.5 mm thick, 10x10 mm, colorized, pre-sintered monolithic zirconia material specimens were prepared which have three different yttrium content [4-6% (VITA YZ HT), 6-8% (VITA YZ ST), 8-10% (VITA YZ XT)]. A square shaped 0.2 mm thick polymerized dual-cure self-adhesive resin cement (Bifix QM, VOCO, Germany) was formed using a silicone mold to be used with the prepared zirconia specimens. Neutral color measurement cabinet (Prowite Lightbox 4.0, Turkey) providing four different lighting environments [D65 (Artificial standard daylight), D50 (Mid-day daylight), TL84 (Shop/Office light) and INCA-A (Incandescent light)] for the assessment of metamerism color measurements of monolithic zirconia samples were made with a spectrophotometer (VITA Easyshade Advance 4.0, VITA Zahnfabrik BAD Sackingen, Germany) device in the direction of CIE L*a*b color coordinates on a gray background. Translucency parameters (TP) were also measured and recorded using a black and white background. All samples were aged by applying 10,000 thermal cycles (5-55°C) with a thermal cycle tester (SD Mechatronic Thermocycler, Germany). After thermal aging, translucency parameters and color measurements were repeated and the data obtained from the CIE L*a*b* color system were recorded. Recorded data were analyzed by Instat Statistical Package Program (Instat Graphad Software 6.0, San Diago, CA, USA). Shapiro-Wilk test was used to test whether the data showed a normal distribution. Data of metamerism and translucency measurements were performed using the post-hoc Bonferroni multiple comparison test, followed by two-way ANOVA, statistical analysis of the values between and within groups (before and after aging). Values were given as mean and standard deviation. P<0.05 value was considered statistically significant in all analyses. The results showed that there was no statistical difference between the color measurements under various lighting conditions for the monolithic zirconia ceramics, which is used to evaluate the metamerism (P>0.05). The results for before and after thermal aging were similar as well. It was determined that yttrium content and thermal aging of monolithic zirconia samples have an impact on translucency (P<0.05). It was observed that VITA YZ HT material had a lower TP value than other monolithic zirconia materials and it was observed that thermal aging increased the TP value of VITA YZ HT material (p<0.05). There was no statistical difference between the TP values of VITA YZ ST and VITA YZ XT materials before and after thermal aging (p>0.05). It was observed that TP values decreased after thermal aging of VITA YZ ST and VITA YZ XT materials. While this decrease was statistically significant for VITA YZ ST (p<0.05), it was not statistically significant for VITA YZ XT (p>0.05). Keywords: Monolithic zirconia, Metamerism, Translucency