Influence of hydrothermal aging on the optical properties of a translucent zirconia for anterior teeth

Objectives: To evaluate low temperature degradation effects on biaxial flexural strength (BFS), optical properties and microstructure of a translucent zirconia for anterior teeth (ZMA), a translucent zirconia for posterior teeth (ZMP) and zirconia for frame (ZF)
Methods: Specimens with 12 mm of diameter and 1.2 mm of thickness shaped from a framework and tow monolithic zirconia blocks were subjected to one of the following treatments: aging in water steam at 120° C for 8 hours under 2 kgf/cm of pressure, and no treatment (control)(n=12). Before and after hydrothermal aging, spectral reflectance light data were recorded from 400 to 700 nm spectrum at 10 nm intervals, under the D65 standard illuminant, 2° observer and 37°C of temperature using a computer-controlled spectrophotometer (CM-2600d, Konica Minolta). Optical properties of opacity, translucency and sum of scattering and absorption were determined since color coordinates of L* (lightness), a* (red-green chromaticity index), b* (yellow-blue chromaticity index), C* (chroma) and h (hue angle). The specimens were subsequently loaded to fracture by means of biaxial bending test (ISO 6872:2008). Specimens microstructure was analyzed by scanning electron microscopy examination before and after hydrothermal aging, and the mean grain area was measured by ImageJ software (n=3).
Results: The 2-way-ANOVA for BFS was significant, and Tukey test showed that the ZMA was less resistant then ZF and ZMP. Hydrothermal aging affected ZF and ZMA only. SEM images showed expanded grains and empty spaces after hydrothermal aging, and significant increase on grain volume for the all types of zirconia studied. The optical properties were not changed by hydrothermal aging. ZMA had lower opacity (%) and higher translucency and scattering/absortion, while ZMP and ZF did not differ each other.
Conclusions: Hydrothermal aging apparently interfered on the mechanical resistance and microstructure, but did not significantly affected optical properties.