Effect of stress level during mechanical cycling of dental ceramic

Objective: The objective was to determine whether the stress level during mechanical cycling would affect the strength degradation of a yttrium stabilized zirconia, Y-TZP (VITA).

Method: Disc-shaped specimens (12mm x 2mm and 12mm x1mm) were prepared according to manufacturer's recommendations. The flexural strength was determined by biaxial flexure test for the control groups. The ageing protocol involved frequency of 2 Hz and 1 million cycles. Stress levels were 48 MPa for 2mm-thick specimens and 193 MPa for 1mm-thick specimens. After aging, specimens were tested in biaxial flexure. Statistical analysis was performed with Student’s t test (alpha=0.05).

Result: The results are presented in the table. Aging specimens in a chewing simulator reduced the strength of the Y-TZP for both stress levels tested (1 and 2 mm-thick specimens), however these differences in strength were not statistically significant. An increase in the coefficient of variation was observed after mechanical cycling for both specimen’s thicknesses.

Thickness	Condition	σmean (MPa)	σmin (MPa)	σmax (MPa)	p-value
2 mm	Control	856.1 ± 76.0 (9%)	773.5	971.2	0.37
	Cycled	816.8 ± 111.9 (14%)	604.4	1003.9
1 mm	Control	786.1 ± 96.5 (12%)	613.0	966.9	0.66
	Cycled	759.5 ± 174.6 (23%)	531.4	1121.6

Conclusion: A variation in the stress level from 48 to 193 MPa does not affect the strength degradation of Y-TZP discs after chewing simulation at 2 Hz and 1 million cycles.