An experimental study of flexural strength and hardness of zirconia and their relation to crown failure loads.

Statement Of Problem: Dental zirconia is often marketed and selected for restorative use based upon the biaxial flexural strength of prefabricated specimens (disks) without considering other mechanical and physical properties.

Purpose: The purpose of this in vitro study was to test whether 4-point flexural strength, biaxial flexural strength, and/or hardness may correlate with failure loads for crowns made of different zirconia materials.

Material And Methods: Three brands of zirconia (BruxZir, Cercon, and Katana) were used to fabricate anatomically contoured crowns, rectangular bars, and circular disks. The sample size was n=15 specimens per zirconia brand and specimen shape. The bars were tested for 4-point flexural strength by using the 4-point bending (4PB) test and Vickers hardness (VH), while the disks were tested for biaxial flexural strength by using a piston-on-3 ball (POB) test. Crowns were attached to resin abutments and compressed with a steel spherical indenter through a polyethylene sheet to assess the failure loads by using the "crunch the crown" (CTC) test. One-way ANOVA (α=.05) was used to test for statistically significant differences between groups, and Weibull analysis was used to assess the variability of the measured flexural strengths, failure load, and hardness.

Results: Statistical differences (P<.001) were found in comparing Cercon, BruxZir, and Katana ([4260 N=4186 N]>3195 N, respectively) with the CTC test and (396 MPa>[281 MPa=275 MPa], respectively) the 4PB test. No statistical differences (P=.084) were found among the zirconia brands (Cercon [384 MPa]=Bruxzir [359 MPa]=Katana [416 MPa]) for the POB test. No significant correlations (P>.05) were found between the 4PB, POB, or VH tests and the corresponding CTC test. The Weibull modulus varied for the different specimen geometries.

Conclusions: The piston-on-3 ball, 4-point bending, and Vickers hardness test results were not correlated with the corresponding crunch-the-crown test.