Comparative Mechanical Evaluation Of Three Y-TZP Formulations In Five Shades

Objectives: As integral structures in Y-TZP spread in dental practice, verifying the mechanical characteristics reported in literature and if they are affected by coloration becomes imperative. This study aims at determining flexural strength and hardness of three Y-TZP types in five different colours. Methods: Three commercial formulations of Y-TZP (A, B, C) were considered: as-received natural white and 4 different shades. Specimens were ASTM C1161, five for each typology (75 parts in 15 groups). Blocks of pre-sintered material were CAD/CAM dry machined, then some were coloured by dipping, before the last step of sintering (Tmax=1450°C). Three point flexural tests were performed to obtain flexural strength (S). After polishing Vickers hardness (HV1) was measured (ISO 14705). T-test for independent samples was carried out on the data, to evaluate the presence of significant differences (positive t-test) between the groups. Results: Mean S was between 500 and 800MPa, much lower than declared by the producers (1200 MPa). For B and C S tends to decrease with colour, but not significantly. For A S increases with colour and two cases present positive t-tests compared to natural white. There aren't significant differences between white specimens of the three companies. As to other shades, the t-test is positive between B and C for three colours over four and between A and B for one colour. HV1 results between 15.8 and 17GPa (in technical specifications HV=12.2 GPa). Colours significantly influence hardness, shifting towards higher values, for nearly all the studied comparisons. Conclusion: Material properties resulted greatly different from technical specifications. Flexural strength values suggest a careful design of frameworks connectors, especially in posterior prostheses. The studied materials resulted harder than expected, in particular when coloured. Colour proved to affect the performances in many cases, requiring esthetical and mechanical requirements to be merged