Effect of long term mechanical cycling on Y-TZP strength

Method: Disc-shaped specimens (12mm x 1mm) of Y-TZP (In-Ceram YZ/Vita) were prepared by slicing, reshaping and sintering of pre-sintered CAD-CAM blocks according to manufacturer's recommendations. The flexural strength was determined by biaxial flexure test for the control group using a universal testing machine at constant stress rate of 0.5mm/min in distilled water at 37^oC. The ageing protocol involved chewing simulation at frequency of 2 Hz and stress level of 193 MPa for 1 million or 3 million cycles performed in a chewing simulator (Biocycle, Biopdi, Brazil). After mechanical cycling, specimens were fast fractured like the control group. Statistical analysis was done by with one-way ANOVA (alpha=0.05).

Result: The results are shown in the table. The ageing protocol of 1 million cycles caused a reduction of 3.4% in the mean strength of the Y-TZP, however this difference was not statistically significant. The 3 million cycles protocol resulted in an increase of 1.1% in strength compared to 1 million cycles results, but again this difference was not significant. The lack of strength degradation after chewing simulation may be explained by the occurrence of phase transformation and creation of compressive stresses around superficial defects on the tensile surface of the specimens after ageing.

Conclusion: None of the chewing simulation ageing protocols used in this study (equivalent to up to 3 years in vivo) degraded significantly the flexural strength of the Y-TZP compared to the control.