Effect of hydrothermal aging on dental multilayered zirconia for monolithic restorations: An in vitro study

Abstract

The aim of this in vitro study was to investigate the changes in mechanical, optical, and surface properties of multilayered zirconia during hydrothermal aging.One conventional block (Katana Zirconia HT) and three multilayered blocks (Katana Zirconia ML, STML, and UTML) of monolithic zirconia were examined. Bar-shaped specimens were autoclaved at 134°C and 0.2MPa for 0, 5, and 10 h. The Young's modulus, three-point flexural strength, and nanoindentation hardness were measured to evaluate the mechanical properties. The surface roughness, phase distribution, surface microstructure, and elemental composition were measured to analyze the surface properties. The contrast ratio and total transmittance were measured via spectrophotometry to evaluate the optical properties. Statistical differences were analyzed using appropriate ANOVA, Tukey HSD post hoc tests, and independent and paired sample t-tests (α = .05).The monoclinic phase increased gradually after hydrothermal aging. The yttrium and zirconium concentrations decreased, and the oxygen concentration and the surface roughness increased in all specimens (P<.05) after the aging process. All specimens showed significant grain push-out and microcracks. The total transmittance increased, and the contrast ratio and Young's modulus decreased in all specimens (P<.05) after the aging process. The nanoindentation hardness and three-point flexural strength exhibited a decreasing tendency after the aging process. However, there were no statistical differences (P>.05) between the materials. Significant interactions between material grades and hydrothermal aging were found for all the properties studied (P<.001).Microstructural alterations and significant phase transformations were detected on the surface of the multilayered zirconia after hydrothermal aging. The hydrothermal aging led to increased surface roughness, opaqueness, and elasticity of multilayered zirconia. The optical, mechanical, and surface properties of multilayered zirconia were influenced by the grade of the material after hydrothermal aging. Careful consideration of the grade of materials is necessary for the appropriate selection of multilayered zirconia ceramics for monolithic restorations.