Effect of low-temperature degradation treatment on hardness, color, and translucency of single layers of multilayered zirconia

Statement of problem: Evaluating the effect of low-temperature degradation (LTD) on multilayered zirconia blocks is important for the long-term life of zirconia restorations. Purpose: The purpose of this in vitro study was to analyze the changes in color, translucency, and hardness of a single layer of multilayered zirconia after an LTD treatment. Material and methods: Ninety multilayered 10x10x1.0-mm zirconia specimens (3M Lava Esthetic) were designed using Auto CAD 2006 (Autodesk) and divided into 6 groups (n=15) based on LTD (no treatment, LTD treatment) and the inherent layer type (incisal, transition, and body). The hardness of each specimen was measured using a microVickers hardness tester. The CIELab color coordinates were measured using a spectrophotometer, and the data were used to calculate the color difference and translucency (TP). Data were analyzed using 2-way ANOVA and the Scheff & eacute; test (alpha=.05). Results: LTD treatment did not alter the hardness of multilayered zirconia (P=.572). A significant difference was found in the hardness between the layers of multilayered zirconia (P<.001). LTD treatment did not alter the b & lowast; or L & lowast; values (P=.773, P=.701) but did change the a & lowast; values (P<.001). On LTD treatment, the a & lowast; value was found to decrease. The L & lowast;, a & lowast;, and b & lowast; values differed based on the respective layer within the material (P<.001). As layers progress from the incisal to the body, the L & lowast; value decreased and the a & lowast; and b & lowast; values increased. In all groups, the color difference values were highest in the following order: the incisal and transition layers < the transition and body layers < the incisal and body layers. On LTD treatment, the color difference between the layers decreased. The LTD treatment did not affect the TP value (P=.208), but the TP value for each layer showed a significant difference (P<.001). Conclusions: LTD treatment did not affect the optical properties or hardness of the multilayered zirconia. The hardness of the multilayered zirconia increased from the incisal to the body layer. In multilayered zirconia, the brightness decreased from the incisal to the body layer, and red and yellow color changes were observed. The TP of multilayered zirconia increased from the incisal to the body layer.