Effect of margin design on the fracture resistance of Procera all ceram cores: an in vitro study.

Aim: The aim of the present in vitro study is to compare the resistance to fracture under a cyclic load applied to chamfer-edged vs. shoulder-edged Procera® All Ceram cores. Materials and Methods: An extracted first maxillary premolar was prepared with a 50° chamfer margin using conventional diamond burs, and an impression was taken using a polyvinylsiloxane. The impression was poured ten times using resin to fabricate dies. The same tooth was retrieved and the 50° chamfer was converted into a 90° shoulder by means of an appropriate diamond bur. An impression was taken and ten more resin dies were fabricated. The resin dies were cast using the lost wax technique in order to obtain brass copies. Impressions of each brass die were taken using a polyvinylsiloxane impression material and poured with die stone. The stone dies were sent to a dental laboratory where densely sintered alumina cores 0.4 mm thick were fabricated using a CAD/CAM process. The alumina cores were then cemented on the brass dies and underwent a fracture test with a cyclic load for 24 hours. Fragments were retrieved for fracture characterization using scanning electron microscopy (SEM). Results: The mean values of fracture resistance for the chamfer samples were 406.10 ± 67.271 N and 643.90 ± 32.912 N for the shoulder samples. The Student's t-test revealed a statistically significant difference between groups. Conclusions: The results of this in vitro study indicate a relationship between the cervical thickness of the alumina cores and their fracture resistance. A shoulder margin could improve the biomechanical performance of posterior single crown alumina restorations.