Effect of artificial aging on optical properties and crystalline structure of high-translucency zirconia

Purpose: To investigate the effect of different in vitro aging protocols on the optical properties and crystalline structure of high-translucency (HT) zirconia. Materials and methods: Thirty-six specimens of HT and extra-high translucency (XT) zirconia were divided into three groups: control (CO)—no treatment; hydrothermal aging (HA)—autoclave aging for 12.5 h at 134°C, 2 bar; clinically related aging (CRA)—aging in the chewing simulator for 1.2 million cycles, followed by 50,000 thermocycles (5–55°C) and immersion in HCl (pH 1.2) for 15 h. Optical properties, crystalline structure, and surface roughness were analyzed and compared using analysis of variance (5% significance level). Results: There was no statistically significant effect of aging on translucency (p = 0.10), but CRA promoted the development of a high contrast ratio (p = 0.03). Aging did not cause significant color changes for HT (p = 0.65) or XT (p = 0.36). The proportion of monoclinic crystals increased to 40% for HT-zirconia after HA and 5% after CRA. No monoclinic crystals were detected for XT groups. There was no effect of aging on surface roughness (p = 0.77). Conclusions: Although hydrothermal aging has been widely used to verify zirconia crystalline stability, it did not generate an effect similar to clinically related aging on the optical properties and crystalline structure of zirconia. HA affected the crystalline structure of HT-zirconia, and CRA compromised the optical properties of XT zirconia.