Formulation-dependent hydrothermal degradation of ultratranslucent 5Y-TZP zirconia.

STATEMENT OF PROBLEM: The impact of increased cubic phase content in ultratranslucent yttria-stabilized zirconia (5Y-TZP) on long-term hydrothermal aging resistance and subsequent mechanical and optical stability remains a concern. PURPOSE: The purpose of this in vitro study was to evaluate the microstructural, mechanical, and optical degradation of ultratranslucent zirconia (UTZ) materials after hydrothermal aging compared with conventional zirconia and lithium disilicate controls. MATERIAL AND METHODS: Seven zirconia materials and 1 lithium disilicate were evaluated. Specimens were autoclaved (134 °C, 0.2 MPa) for 0 to 10 hours. Surface topography (scanning electron microscopy [SEM], atomic force microscopy [AFM]), wettability, flexural strength, light transmittance, and color stability (ΔE00) were quantified. Phase evolution and structural integrity were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy. The data were analyzed using robust ANOVA frameworks (α=.05). RESULTS: Hydrothermal aging induced progressive surface hydration without macroscopic tetragonal-to-monoclinic transformation in 5Y-TZP materials. High-strength zirconia and specific UTZs (A-3D, W-MT) demonstrated superior hydrolytic stability. Conversely, pigment-rich UTZs (L-E, L-P, U-TT) and lithium disilicate exhibited significant surface roughening (P<.05), increased wettability, and strength degradation. Color shifts (ΔE00) in susceptible UTZs exceeded clinical acceptability thresholds after intermediate aging. CONCLUSIONS: The aging resistance of 5Y-TZP ceramics was found to be highly formulation-specific, governed by grain-boundary hydration rather than bulk phase transformation. Certain ultratranslucent formulations rival conventional zirconia in stability, while pigment-rich variants exhibit hydrolytic degradation similar to that of lithium disilicate.