Intraoral low-temperature degradation of monolithic zirconia dental prostheses: Results of a prospective clinical study with ex vivo monitoring

Objective. To investigate the intraoral development and kinetics of low-temperature degradation (LTD) in second-generation 3 mol.% yttria-doped tetragonal zirconia polycrystal(3Y-TZP) monolithic prostheses, as well as the influence of masticatory mechanical stress and glaze layer on it. Methods. A total of 101 posterior tooth elements were included in a prospective clinical study, which included ex vivo LTD monitoring (at baseline, 6 months, 1 year, and 2 years) using Raman spectroscopy (n = 2640 monoclinic phase measurement points per evaluation time) and SEM. Four types of areas (1-2 mm(2) surface, 6 on molars, and 4 on premolars) were analyzed on each element surface: occlusal, axial, glazed, or unglazed. Raman depth mapping and high-resolution SEM were performed on the selected samples. Results. LTD developed in 3Y-TZP monolithic restorations 6 months after intraoral placement and progressed with time. After two years, the tetragonal-to-monoclinic transformation was non-uniform, with the presence of localized clusters of transformed grains. In axial areas, the grain aspect was typical of the classical nucleation-growth process reported for LTD, which progresses from the surface to a depth of several tens of microns. However, in occlusal areas, tribological stress generated surface crushing and grain pull-out from the clusters, which induced an underestimation of the aging process when the evaluation was limited to monoclinic phase quantification. Glazing cannot be considered a protection against LTD. Significance. If LTD occurs in dental prostheses in the same way as in orthopedic prostheses, its clinical impact is unknown and needs to be further studied. (C) 2021 Published by Elsevier Inc. on behalf of The Academy of Dental Materials.

Automated summary

The study investigated the intraoral development and kinetics of low-temperature degradation in second-generation 3Y-TZP monolithic prostheses, revealing non-uniform tetragonal-to-monoclinic transformation with localized clusters after two years. Tribological stress in occlusal areas induced an underestimation of the aging process, while glazing did not protect against LTD. Further research is needed to understand the clinical impact of LTD in dental prostheses.