Fracture Strength of Porcelain-Zirconia Discs: Effect of Varying Thickness Ratio

Objective: to determine the effect of varying total thickness, core thickness and veneering porcelain application technique on the fracture strength of bilayered porcelain-zirconia discs. Methods: One hundred and twenty Zirconia square specimens (e.max ZirCad, Ivoclar Vivadent, Schaan, Liechtenstein) were prepared in three thicknesses (0.6, 0.8 and 1.0 mm). The zirconia discs were veneered with pressable (e.max ZirPress, Ivoclar Vivadent) or manually applied (e.max Ceram, Ivoclar Vivadent) veneering porcelain to produce 12 groups (n=10) as shown in Table 1. The discs were cemented to epoxy resin blocks (5mm in thickness) (Viade Products Inc., Camarillo, CA). The specimens were loaded at the center of the veneering porcelain using an 8mm tungsten carbide ball in a hydraulic-driven universal testing machine (Instron 8501; Instron, Canton, MA) at a cross-head speed of 1mm/min until fracture. The fractured samples were examined to determine fracture mode (involving the veneering layer or both the veneer and core layers).  Fracture load data were analyzed with the generalized linear model to evaluate effect of veneering material application method, core thickness and total thickness. Results: The fracture loads of the study groups are reported in (Table 1).  Factorial ANOVA revealed a statistically-significant effect of the veneering material application method, core thickness, total thickness and the interaction between the total thickness and core thickness on the fracture load (P<0.05). A relationship was observed between the veneer thickness and the fracture mode. An increase in the percentage of the specimens with core fracture was observed as the veneer thickness decreased. Conclusion: The veneering porcelain application method, total thickness and core thickness have significant effects on fracture strength of bilayered porcelain-zirconia discs.

Acknowledgements: University of Toronto, Faculty of Dentistry Research Committee Grant, Education and Research Foundation of Prosthodontics, Ivoclar/Vivadent, Kuraray, and Shaw Dental Lab (Toronto).

Table 1