fatigue resistance and failure mode of endodontically restored premolars with indirect composite resin restoration-in vitro

background and aim: the present study aimed at determination of the fatigue resistance and failure mode of cusp-replacing onlays in endodontically treated premolars using indirect composite resin restorations. materials and method: in this experimental study, 84 human premolars were randomly divided into 7 groups of 12. these groups were prepared as follows: intact teeth used in group1 as control, the second group covers the endodontically treated teeth, restored with indirect onlays using z-250 composite resin, the next two groups (i.e. 3,4) were similar to the second group, but subjected to 1 and 2 million fatigue load cycles, respectively. groups 5, 6, 7 were similar to groups 2, 3, 4, respectively, however, in these groups tetric ceram used as the restorative material. all specimens were loaded using a universal testing machine until fracture occurred and the mode of failure observed with naked eye. kruskal-wallis and chi-square tests were used to analyze the data (α=.05). results: all specimens withstood the masticating simulation. the mean fracture strength for groups 1 to 7were: 1276.92, 1269.05, 1217.80, 1188.13, 1228.97, 1127.86, 1105.58, respectively. no statistically significant differences were found between the groups in fracture strength and failure mode. conclusion: within the limitation of this study, no statistically significant differences in fracture strength were found between sound teeth and composite onlays that were subjected to 1 and 2 million fatigue load cycles. j dent sch 2010 28(1): 3