When 3Y-TZP is exposed to moisture at slightly elevated temperatures the tetragonal grains on the surface start to spontaneously transform to the monoclinic phase. Due to the associated volume expansion, the process is accompanied by surface roughening, grain pull-outs and extensive microcracking, which may ultimately lead to strength degradation. Ageing kinetics of 3Y-TZP ceramics were commonly investigated using accelerated hydrothermal protocols in autoclaves. Our research group approached the problem by using complete dentures as vehicles to hold the zirconia ceramic specimens in patients' mouths during normal function. In this manner the ageing processes could be studied at body temperature in the aggressive environment of the oral cavity. 

Methods. 88 discs were pressed and sintered (1450-1500°C) from two commercial 3Y-TZP compositions As-sintered (AS) and airborne-particle abraded (APA; 50 μm Al2O3) surfaces were investigated. In vivo ageing was performed by incorporating specimens in lingual flanges of complete dentures of 12 edentulous volunteers who wore them continuously for up to 24 months.

Results. Finer and coarser-grained specimens exhibited statistically insignificant differences ageing in vivo. The monoclinic fraction (Xm) on AS surfaces abruptly increased to ~8% after 6 months. The ageing process then proceeded with slower linear kinetics. After 24 months, Xmreached ~12%. The calculated maximum transformed layer was 0.385 μm representing one layer of transformed grains. APA surfaces were highly ageing resistant. The initial Xm of ~4.0% linearly increased by 0.043%/month in vivo. In vitro ageing exhibited an initial induction period, followed by linear ageing kinetics. Coarser-grained AS surfaces aged significantly faster than fine-grained APA discs aged at a rate of 0.43%/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 ageing. After 2 years in vivo, the ageing kinetics were almost 3-times faster than the generally accepted in-vitro-in-vivoextrapolation. In this way, for the first time, in vivo ageing results were presented. This is crucial for the 3Y-TZP life prediction and will be of considerable benefit for to both the dental and the material science communit