Effect of Die Materials on the Fracture Strength of All-Ceramics

Objectives: The purpose of this in vitro study was to investigate the effect of different die materials on the fracture strength of an all-ceramic core material. Methods: Stainless steel, brass, epoxy resin and tooth (24 per group) were prepared to simulate a mandibular molar all-ceramic crown preparation using a CNC milling machine with the following dimensions: 5 mm vertical height, 90 degree, 1 mm shoulder with a total convergence taper of 16 degrees. 24 zirconia core materials were fabricated using a CAD/CAM system for each die. The fracture strength testing was carried out before and after cementation using a universal testing machine at a crosshead speed of 0.5 mm/min. In addition, 3D mathematical model of core and dies were created for finite element stress analysis. Two-way analysis of variance and Tukey HSD tests were used to evaluate the data. Results: Before and after cementation the mean fracture strength and the standard deviation for core materials on the epoxy resin die were 296.22 ± 14.25 N and 497.86 ± 17.18 N, on the tooth die were 282.24 ± 16.95 N and 479.56 ± 17 N, on the brass die were 336.66 ± 14.4 N and 890.24 ± 25.44 N and on the steel die were 346.34 ± 22.72 N and 955.64 ± 36.78 N. A significant difference was found among the die materials (p<0.05). Cemented group showed a significantly higher fracture strength than non-cemented group. Ceramic cores on stainless steel dies showed the highest fracture strength values among the groups. And finite element method showed different stress distributions among groups that supports fracture strength test. Conclusions: Within the limitation of this study, it can be concluded that the elastic modulus of the supporting structure may affect the results of the fracture strength test of all-ceramic materials.