Influence of Pre-Veneering-Treatment on the Bi-Axial Flexure Strength of Zirconia

Objectives: Chipping of veneering-porcelain is on of the major drawbacks of zirconia-based restorations. The manufacturers of veneering-porcelain suggest different treatments for the zirconia-substructure prior to veneering. The purpose of the current study was to examine the influence of different surface treatments, to which zirconia restoration frameworks will routinely be subjected before veneering in the dental laboratory, on the mechanical performance of an yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) dental ceramic.

Methods: The specimens (150 Y-TZP ceramic discs, Everest ZS, KaVo) were prepared according to ISO 6872 and equally divided into 5 groups according to the surface treatment (control, ground by fine diamond, ground by coarse diamond, sandblasted once, sandblasted three times). This treatment was performed by standardized protocols in special apparatus with water spray. Fifteen specimens from each group underwent artificial aging (10000 thermal and 1.2*1000000 mechanical loadings). After treatment, the bi-axial flexure test was used to determine the flexural strength. The reliability of strength was assessed through the Weibull distribution. Statistical analysis was conducted with a two-factor analysis of variance (ANOVA) and post-hoc-test (Student-Newman-Keuls). Treated and fractured surfaces were observed with scanning electron microscopy (SEM).

Results: The following values for strength before and after aging were measured for each group: control 1551/1508 MPa, ground by fine diamond 933/960 MPa, ground by coarse diamond 797/820 MPa, sandblasted once 948/1024 MPa and sandblasted three times 993/1021 MPa. With exception of specimens sandblasted three times the experimental groups showed an increase of Weibull-modulus after aging to the range of 9.8 to 13.6.

Conclusion: The present study suggests that any treatment of the zirconia surface decreases the flexural strength of dental Y-TZP. However, coarse diamonds should not be used as they lower the strength significantly compared to the other techniques tested. Further research evaluating the influence on the shear-bond-strength should be carried out.