Evaluation the Effect of Change in Convergence Angle of Tooth Preparation and Copping Thickness on Fracture Resistance of All Ceramic Crowns

Low fracture resistance is one of the most important disadvantages of all-ceramic restorations. The aim of the present study was to assess the effect of convergence angle and change in copping thickness related to it on fracture resistance of all ceramic crowns. This study was an experimental study in which 30 brass dies were fabricated as crowns with a height of 5. 5mm، base diameter of 6. 2mm and marginal thickness of 1. 2mm in the following design: (10 shoulder 90˚ → Convergence 6˚)، (20 shoulder 90˚ → Convergence 12˚). With a special index، 0. 6mm thickness acrylic cores were made on 20 dies (10 with 6˚ convergence، 10 with 12˚ convergences). With the 6˚ index and 12˚ dies، 10 cores were made with the thickness of 0. 6mm+difference in thickness of 6˚ and 12˚ convergence angle. Cores were casted with IPS e. max porcelain of core and veneered with another index with parallel cylindrical wall and then cemented with Panavia F2 on the brass dies. After storage in distilled water of 37˚c for 24 hours، the specimens were placed in compressive test ring within an Instron universal testing machine and loaded in the center of occlusal surface with a 4mm diameter stainless steel ball. An axial preload of 20N was applied followed by a compressive load until fracture occurred. The data were analyzed with one way analysis of variance (ANOVA). Average fracture load in 6˚ convergence angle and 0. 6mm core thickness was 1951. 40N، in 12˚ convergence angle and 0. 6mm core thickness، it was 2798. 46N and in 12˚ convergence angle and 0. 6mm+additional core thickness because of increase in convergence angle، it was 3151. 05N. The results revealed no significant difference in the fracture load between two core thickness but fracture load of 12˚ convergence angle was significantly greater than 6˚ (P<0. 001). Copping thickness didn’t affect the fracture load of all ceramic crowns but increase in convergence angle increased the fracture load significantly.