Effect of Aging on Contrast Ratio of Conventional/ Translucent Zirconia’s

Objectives: There has been a recent increase in the clinical use of full contour zirconia which will be in direct contact with saliva and accordingly subject to low temperature degradation which may affect its optical properties. This study aimed to investigate the effect of accelerated aging on the contrast ratio (CR) of four different types of zirconia.
Methods: Four zirconia materials were used in this study, Zpex and Zpex Smile (both full contour) and TZ3YE and TZ3YBE (both conventional) (all from TOSOH, Japan). 6 disc samples of each material were prepared by uniaxial and cold isostatic pressing with a diameter of 14.5± 0.5mm and thicknesses 1.00± 0.1 mm. A spherical spectrophotometer (SP64, X-Rite, USA) was used for measuring CR of the samples with D65 /10 degree illumination. Measurement was conducted before and after hydrothermal aging at 134°C for 5 hours at 2.2 bar. Statistical analysis was carried out using one-way ANOVA, followed by Tukey's HSD test (p≤0.05). X-ray diffraction followed by Rietveld analysis (Phillips X'Pert, Panalytical B.V, The Netherlands) was used for phase characterization of the sintered bodies.
Results: There was no statistically significant difference in mean contrast ratio (p≤0.05) as a result of aging, however the difference in contrast ratio between different groups was significant. Zpex Smile showed the lowest contrast ratio(CR) compared to all other groups, followed by Zpex, TZ3YBE and TZ3YE respectively. XRD of all groups showed (tetragonal+cubic) crystals before aging. After aging no detectable transformation was observed for TZ3YE while for TZ3YBE and Zpex Smile, a 5±1.2 vol% of monoclinic fraction phase was detected. Zpex showed the highest amount of transformation to monoclinic (22±1.5 vol%).
Conclusions: There were clear differences in CR amongst the tested materials. Hydrothermal aging had no significant effect on the CR of any of the tested materials, even though there was clear evidence of transformation in most materials, suggesting that their optical properties are robust.