In vivo aging of zirconia dental ceramics - Part I: Biomedical grade 3Y-TZP.

Objective: In vivo aging of biomedical grade 3Y-TZP ceramics in the oral environment was assessed and compared to artificially accelerated in vitro hydrothermal aging extrapolations at 37°C.

Methods: 88 discs were pressed and sintered (1450-1500°C) from two commercial 3Y-TZP compositions containing 0.25% AlO to generate finer- and coarser-grained specimens. As-sintered (AS) and airborne-particle abraded (APA; 50μm AlO) surfaces were investigated. In vivo aging was performed by incorporating specimens in lingual flanges of complete dentures of 12 edentulous volunteers who wore them continuously for up to 24 months. For comparison, in vitro hydrothermal aging at 134°C was also performed and analysed by XRD and (FIB)-SEM. Data was statistically analysed with linear regression models.

Results: Finer and coarser-grained specimens exhibited statistically insignificant differences in aging in vivo. The monoclinic fraction (X) on AS surfaces abruptly increased to ∼8% after 6 months. The aging process then proceeded with slower linear kinetics (∼0.24%/month). After 24 months, X reached ∼12%. The calculated maximum transformed layer was 0.385μm representing one layer of transformed grains. APA surfaces were highly aging resistant. The initial X of ∼4.0% linearly increased by 0.03%/month in vivo. In vitro aging exhibited an initial induction period, followed by linear aging kinetics. Coarser-grained AS surfaces aged significantly faster than fine-grained (2.41%/h compared to 2.16%/h). APA discs aged at a rate of 0.3%/h in vitro. Microcracking within a single grain and pull-out of grain clusters were observed on aged AS surfaces.

Significance: Biomedical grade 3Y-TZP was susceptible to in vivo aging. After 2 years in vivo, the aging kinetics were almost 3-times faster than the generally accepted in vitro-in vivo extrapolation.