Effect of Accelerated Aging (LTD) on Two Translucent Zirconias

Objectives: A new trend has emerged in fixed prosthodontics in the use of full contour zirconia restorations without the need for a veneering porcelain; such materials are fully exposed to the oral environment. This study aimed to investigate the impact of accelerated aging on the properties of discs made from Zpex and Zpex Smile zirconia powders as examples of such monolithic zirconia materials.
Methods: Zpex and Zpex Smile powder (TOSOH, Japan) were used in this study. 60 discs, 1.2±0.2 mm thick and 12±4 mm diameter, of each of the two materials were uniaxially and isostatically pressed and sintered according to the manufacturer’s instructions. Half were subsequently aged hydrothermally at 134 ^OC for 5 hours at 2.2 bar. The biaxial flexural strength (n=15 discs) of the four groups was measured according to ISO 6872:2015 and Weibull analysis carried out. The Vickers hardness (n=5 discs) was determined using microhardness (load of 3 N for 5 s), following normality testing, One Way ANOVA was used to look for differences within and between each material before and after aging. X-ray diffraction (XRD) with subsequent Rietveld analysis was used for phase characterization. Grain size measurement for the samples was conducted by lineal intercept method on digitally calibrated SEM images.
Results: For both Zpex and Zpex Smile Weibull modulus (β) was reduced after aging and the characteristic strength (a) showed a slight change. However, there were no significant differences (p>0.05) in either mean strength or mean hardness as a consequence of aging. Zpex and Zpex Smile showed only the tetragonal and cubic phase before aging. After aging Zpex showed up to 22% of monoclinic while Zpex Smile showed less than 5% of monoclinic. There was no change in the grain size dimensions.
Conclusions: Artificial aging had no significant effect on the mechanical properties of both Zpex and Zpex Smile, however, it had a limited effect on structural properties of Zpex Smile and more significant effect on Zpex. Even after the reduction in Weibull modulus as a result of aging both materials showed a good structural reliability and reproducible flexural strength in excess of the minimum recommended for clinical use, even after accelerated aging mimicking 15-20 years of clinical service.