Comparative study of three yttria-stabilized zirconia formulations in colored vs natural shades

Recently the attention in the field of fixed dental restorations has focused on structural ceramics. Yttria-stabilized tetragonal zirconia (Y-TZP) is proposed as an alternative to conventional ceramic-metal prostheses, since it merges excellent aesthetic quality with outstanding toughness. Before Y-TZP integral structures spread in dental practice, much needs to be studied as regards the mechanical response, the influence of color and of the manufacturing process, the occurrence of aging phenomena. This work aims at a comparative study of three Y-TZP commercial formulations in five shades; the variations introduced by polishing and their recovery after annealing are also addressed. Flexural strength and microhardness are investigated, basing on international standards. Significant differences between the groups and parts' reliability are evaluated through statistical data processing and Weibull analysis. Results show low flexural strength (500-800MPa), at least 45% inferior to technical specifications, and low Weibull modulus. Hardness is instead higher than expected (1500-1700 HV1). The main finding of the research is that the effect of color on mechanical properties is significant in many cases, hence esthetical requirements must be merged with mechanical ones. Y-TZP shows an extreme variability with manufacturing conditions, so nominal characteristics should be assumed with caution and higher reliability is still required.