In-vivo Ageing of Zirconia Ceramics: Results after 12 Months

Objective: An in-vivo study was designed to monitor the propagation of the t-m transformation of two bio-medical grade 3Y-TZP ceramics differing in their mean grain size and transformability, which were directly exposed to the aggressive environment of the oral cavity. Materials and methods: Two bio-medical-grade zirconia powders (TZ-3YB-E and TZ-3YSB-E, Tosoh, Japan) were used to produce disc-shaped specimens (7.8 mm in diameter and 0.8 mm thick) from each material with mean grain sizes of 0.51 ƒÝm and 0.59 ƒÝm, respectively. The specimens of each material were randomly divided into 4 groups of 8 and subjected to the following experimental protocol. Group 1: Left in the air. Group 2: implanted in the lingual flange of the lower denture that was worn 24h/day. After 6 and 12 months the discs were explanted and subjected to the XRD and SEM surface analyses. Groups 3 and 4: subjected to autoclaving at 134 0C in artificial saliva for 3h and 6 h, respectively. The relative amount of transformed monoclinic zirconia on the treated surfaces was determined from the integral intensities of the monoclinic M(11 ) and M(111), and the tetragonal T(111) peaks. Results: The calculated relative amounts of the transformed monoclinic zirconia (XM + SD) on the surface of the Y-TZP ceramics are presented in Table 1. Conclusions: thermal cycling in the chemically aggressive wet environment of the oral cavity fosters the t-m transformation on the surface of the zirconia ceramics implying that the directly exposed surfaces are vulnerable and represent a potential threat of premature failure for all-ceramic dental restorations.