Clinical Behavior and Intraoral Low-Temperature Degradation of Zirconia Dental Prostheses

The use of zirconia in the field of dental prostheses has grown significantly since its introduction in the 2000s, following the advent of computer-aided design and manufacturing technologies. First-generation zirconia-based restorations (ZBR) are bilayered structures composed of a framework and a glass-ceramic veneer, which imparts an essential esthetic appearance. However, the first clinical reports regarding veneered ZBR indicated a high rate of short-term failures due to cohesive fractures (chipping) of the veneering ceramic. To address this, a retrospective study on veneered ZBR was conducted to investigate the influence of clinical parameters, such as patient-related risk factors, on chipping failures.
Zirconia is now used to fabricate monolithic dental prostheses without the veneering ceramic layer and the only presence of a thin cosmetic glaze. These prostheses were notably developed to remedy chipping and, thus, obtain prostheses with an increased durability. Therefore, zirconia materials exhibiting greater translucency have been developed for monolithic restorations. However, second-generation zirconia materials are prone to show greater metastable behavior, which could promote low-temperature degradation (LTD).
The main objective of this study was to evaluate the intraoral LTD of zirconia monolithic restorations and the influence of occlusal stresses and glaze protection on this process. Secondary objectives included the investigation of the general clinical behavior and material wear of the restorations. This work introduces an original protocol, including ex vivo analyses, to evaluate the LTD process of monolithic zirconia prostheses in the oral environment and to study their general clinical behavior, primarily in terms of material wear.
LTD was shown to develop in 3 mol% yttria-doped tetragonal zirconia polycrystal monolithic restorations six months after intraoral placement and then progresses over time. After two years, the tetragonal-monoclinic transformation became 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. However, in occlusal areas, tribological stress generated surface crushing and grain pull-out from the clusters, causing an underestimation of the aging degree when the evaluation was limited to monoclinic phase quantification. Glazing cannot be considered a protection against LTD because it is worn away in occlusal areas after one year.
Two years after their implantation, the Kaplan-Meier survival rate of restorations was 93.3% (100% for fixed partial dentures) and the success rate was 81.8%. It was found that eighty percent of major failures occurred in patients exhibiting clinical signs of bruxism. Complications such as root fracture, periodontal disease or composite chipping were also observed in antagonistic teeth. Wear in zirconia was observed to be less than 15 μm. The success rate of single-unit restorations was not as high as expected, the weak link being the prosthesis support or the antagonist tooth.
One hypothesis is that zirconia’s stiffness and lack of ability to deform do not promote occlusal stress accommodation, which can be critical in patients affected by bruxism. Furthermore, several unexpected clinical failures were observed, including chipping and major fracture of the material, which may have been induced through LTD. If LTD occurs through the same mechanisms in dental prostheses as in orthopedic prostheses, its clinical impact remains unknown and needs to be evaluated through a thorough analysis of fractured prostheses in the framework of long-term studies.