Purpose
With the advance of the precision machining technology and dental materials, the aesthetic limitations of dental prostheses have gradually overcome. The zirconia prosthesis has gained popularity and the development of stronger ceramic materials has led to changes in the existing fabrication methods of dental prostheses. However, the opaque shade of zirconia is an aesthetic drawback. The veneering porcelain has been applied on the zirconia in order to solve the aesthetic problem, but the fracture of veneering porcelain has frequently occurred. The purpose of this study is to assess the impact of the core materials, thickness and fabrication methods of veneered porcelain on prosthesis fracture in the porcelain fused to metal and the porcelain veneered zirconia.

Materials and methods
Forty nickel-chrome alloy cores and 40 zirconia cores were made. Half of each core group was 0.5 mm-in thickness and the other half was 1.0 mm-in thickness. Thus, there were four groups with 20 cores/group. Each group was divided into two subgroups of 10 cores, with two different veneering methods (conventional powder/liquid layering technique and the heat-pressing technique) applied to each subgroup. Tensile strength of the 80 samples in the eight subgroups was tested using the biaxial flexural strength test based on the ISO standard 6872:2008(dental ceramics) and Weibull analysis. Factors influencing fracture strength were analyzed through three-way ANOVA(α≤.05) and the influence of core thickness and veneering method in each core materials was assessed using two-way ANOVA analysis (α≤.05).

Results
The biaxial flexural strength test showed that the fabrication method of veneering porcelain has the largest impact on the fracture strength followed by the core thickness and the core material. Thicker (i.e., 1.0 mm rather than 0.5 mm) metal cores (vs. zirconia cores) and the heat pressing technique (vs. conventional layering technique) showed higher fracture strength. In the metal groups, both the core thickness and the fabrication method of the veneering porcelain significantly influenced on the fracture strength, while only the fabrication method affected the fracture strength in the zirconia groups. Weibull analysis indicated that higher strain increased the probability of fracture probability and the heat pressing method tended to lower fracture probability under the same strain.

Conclusion
The fabrication method is more influential to the strength of a prosthesis compared to the core character determined by material and thickness of the core. The metal core has higher fracture strength than the zirconia core, and the thicker core has higher fracture strength than the thinner core. The heat-pressed veneering technique shows higher fracture strength than the conventional layered veneering technique does.