Analysis of zirconia mechanical properties after application of a protocol to simulate clinical aging

In order to test the effects of hydrothermal and clinically related aging on zirconia, monolithic disc-shaped samples were milled, sintered and polished from two high translucency zirconia, 3 mol % (HT) and 5 mol % (XT). Samples were divided into three groups: non-aged: control (CT); hydrothermal aging (HA - autoclave aging for 12.5 h at 134 °C, 2 bar); in vitro clinically-related aging (CRA - chewing simulation for 1.2 million cycles followed by 50,000 thermocycles and acidic exposure in HCl, pH 1.2, for 15 h). Mechanical properties (flexural strength, fatigue, hardness and elastic modulus) were analyzed and compared using the analysis of variance (at a level of 5% significance). In vitro clinically-related aging significantly decreased fatigue strength of XT zirconia with no effects on HT zirconia. Surface hardness and elastic modulus were not affected (p = 0.591 and 0.392 respectively). Hydrothermal aging increased fatigue strength for both materials and decreased the surface hardness and modulus of HT zirconia (p ≤ 0.001). Hydrothermal aging and in vitro clinically-related aging have different effects on the mechanical properties of zirconia, when used to simulate five years of clinical service.