This study aims to investigate the fracture resistance of endodontically treated maxillary second premolars using fracture resistance test. Materials and Methods: Fifty-five maxillary second premolars were selected. The teeth were divided into four groups (three experimental groups and one control group) each group consists of fifteen teeth. The control group consists of ten teeth. The experimental groups were divided according to the mechanical system used: Endo Star, Revo S, and Protaper. Each group was subdivided into two subgroups according to the presence of the crown or not. Endo star E5 group: (1.a) and (2.a), Revos group :(1.b) and (2.b), Protaper :(1.c) and (2.c).All groups were instrumented and obturated. The subgroups with crown preserved (1.a), (1.b), (1.c) were (MOD) prepared, only the first four subgroups were restored by corono-radicular stabilization method. All the samples were subjected to fracture resistance test. The significant level was set at P≤0.05. Results: In the first four subgroups: the control group (1.d) showed the highest values, there were no differences between the (1.a), the (1.b) and (1.c) groups. All the teeth fractures were favorable.
In the second four subgroups, the highest mean fracture resistance value was obtained by the control group (2.d) followed by EE5 group (2.a). There was no significant difference between Revos (2.b) and Protaper (2.c) groups; both showed the lowest mean fracture resistance values. Conclusion: Although the various endodontically mechanical preparations didn't affect fracture resistance of the coronal portion of the teeth, the fracture resistance of the radicular part was affected