Effect of Thickness of Zirconia Ceramic on Its Masking Ability: An In Vitro Study

Color-masking ability of zirconia ceramics as coping materials has not been clearly understood in zirconia-based restorations. This in vitro study aimed to evaluate the effect of zirconia ceramic thickness on its masking ability, and to define a thickness cutoff for the tested ceramic.

Ninety zirconia disc specimens, in nine thickness groups including 0.4, 0.6, 0.8, 1, 1.2, 1.4, 1.6, 1.8, and 2 mm were fabricated. A white substrate and a black substrate were prepared. The disc specimens were placed onto the substrates for spectrophotometric measurements. A spectrophotometer measured L^* , a^* , and b^* color attributes for the specimens. ΔE value was calculated to determine the color difference of specimens on the white and black substrate. Then the ΔE values were compared with an acceptable clinical threshold (ΔE = 5.5) and an ideal threshold (ΔE = 2.6). Welch test, Games-Howell Post Hoc, one-sample t-test, and a regression analysis were performed (p < 0.05).

The means of ΔE values for the thickness groups of 0.4, 0.6, 0.8, 1, 1.2, 1.4, 1.6, 1.8, and 2 mm were 13.0, 11.9, 9.7, 5.0, 4.2, 3.9, 2.2, 1.9, and 1.5, respectively. A significant difference was found in the ΔE value among the groups (p < 0.0001). An exponential relation was detected between the thickness and the ΔE by the regression analysis (∆E = 25.68e^{-1.45 (thickness)} ).

The masking ability increased as the zirconia ceramic thickness increased. The tested zirconia ceramic had an acceptable clinical masking ability with a minimum thickness of 1 mm, and an ideal masking ability with a minimum thickness of 1.6 mm.