Mechanical Performance of Highly-translucent Zirconia Ceramics for Monolithic Restorations

Objectives: Many variants of zirconia ceramics are nowadays introduced in the dental field. Although zirconia ceramics are known to be strong and tough, their performance can greatly vary depending on the stabilizer type, content, microstructure, etc.. The aim of this work was to investigate the mechanical behavior of three zirconia ceramics stabilized by 3, 4 and 5 mol% yttria to represent zirconia that require a higher translucency for monolithic reconstructions.
Methods: Ceramic blocks were own-synthesized from Zpex, Zpex4 and ZpexSmile powders (Tosoh, Japan). In addition to the generally studied mechanical properties (bending strength with Weibull analysis, fracture toughness, hardness and wear), slow-crack growth (SCG) from fast fracture to threshold were assessed for the three zirconia ceramics by double-torsion methods in comparison to lithium-disilicate glass-ceramic (‘LS₂’: Ivoclar Vivadent, Liechtenstein). Four-unit fixed dental prostheses (FDPs) were also fabricated from the same three zirconia grades with varying yttria content (Pritidenta, Germany). The fracture load of all FDPs (n=24/group) were tested and correlated with their microstructure and phase compositions.
Results: 4 and 5 mol% yttria-stabilized zirconia revealed lower toughness than conventional 3Y-TZP, as expected; whereas the bending strength of 4Y-zirconia was similar to that of 3Y-TZP (about 1000 MPa) but significantly higher than that of 5Y-zirconia (about 500 MPa). The same trend was found for the fracture load of four-unit FDPs: 3Y-FDPs ~ 4Y-FDPs > 5Y-FDPs. Nevertheless, all zirconia ceramics, irrespective of yttria content, exhibited similar wear behavior and relative susceptibility to SCG, while being more wear/SCG resistant than LS₂. However, a lower Weibull modulus was obtained for 4 and 5 mol% yttria-stabilized zirconia than for 3Y-TZP and LS₂.
Conclusions: The new translucent zirconia ceramics fill a part of the material’s window between conventional 3Y-TZP and LS₂ glass ceramic. Due to the lack of transformability, 5Y-zirconia with reduced loading capability and low reliability may approach the bottom limit.