Effects of the Thickness Ratio of Zirconia-Lithium Disilicate Bilayered Ceramics on the Translucency and Flexural Strength.

Abstract

To determine the effects of the total thickness and core/veneer thickness ratio of bilayered ceramics on their three-point flexural strength and translucency. Two groups of specimens were prepared with two different total thicknesses, 1.0 mm and 0.6 mm. These groups were divided into five and three subgroups with ten specimens each, respectively, having different core/veneer thickness ratios. Lithium disilicate was pressed on the zirconia surface using the "lost-wax technique." The translucency and three-point flexural strength were measured, and a one-way analysis of variance test (p < 0.05) was performed to determine whether the translucency and three-point flexural strength were affected by the total thickness and core/veneer thickness ratio of the specimens. For the same total thickness, translucency decreased with a increase in the zirconia core proportion (p < 0.001). For the same core/veneer ratio, the translucency decreased with an increase in the total thickness (p < 0.001). If the zirconia thickness was less than half of the total thickness, the bilayered ceramic specimens containing zirconia cores and monolithic lithium disilicate specimens exhibited a similar translucency. The three-point flexural strength increased with the increasing proportion of zirconia; however, it did not change significantly with the total thickness. However, for the same total thickness, the flexural strengths of the bilayered ceramics were higher than those of the monolithic specimens. At the same total thickness, increasing the zirconia core thickness decreased the translucency and increased the three-point flexural strength. Bilayered ceramic specimens with low zirconia ratios exhibited translucency similar to that of monolithic lithium disilicate ceramic but a higher flexural strength.