Effect of Zirconia Substructure Design on the In-Vitro Fracture Load of Molar Zirconia Core Crowns

In the available all-ceramic systems the zirconia core is commonly fabricated as one layer with an even thickness, but the veneering porcelain has different thicknesses in different parts of the restoration and therefore, undergoes chipping and fracture more rapidly under masticatory forces. This study aimed at comparing the fracture load of all-ceramic crowns with 2 different zirconia core designs in Cercon system and in-vitro conditions. In the present experimental study, 10 metal dies of mandibular first molar were fabricated with NNB base metal alloy using lost wax technique. Ten standard zirconia cores were then fabricated with an even thickness of 0.5 mm using Cercon CAD/CAM System. Also, 10 more were fabricated in the customized form with a 1 mm labial collar and 2 mm lingual shoulder. Porcelain was applied on all samples by an expert technician using an index. The prepared crowns were placed on their respective dies and cemented with Panavia F resin cement and under the constant pressure of 25 N. Vertical compressive force was applied to the samples by means of a stainless steel ball at a crosshead speed of 0.5 mm/min using Universal Testing Machine until failure. Data were analyzed using student t test. Our study results demonstrated the fracture load of 1852.11±587.9 N for zirconia core crowns with standard core design and 3332.63±916.38 N for custom design crowns. Statistical analyses demonstrated that the fracture load was significantly higher for customized core designs than the standard design cores(P<0.0001). Considering the obtained results we can conclude that crowns with customized core design have a greater fracture resistance than those with standard design.