Methods: Highly curved mesial roots from extracted, fully formed human molars were collected and were randomly divided in five groups (A-E). The root canals were subjected to chemomechanical treatment up to No 30/.04 file size by EndoSequence, K3, Liberator, ProFile and ProTaper files per group. Five sets of each file were used. The EndoSequence files were used at 500 rpm, the K3 files at 350 rpm, the Liberator files at 1000 rpm, the ProFile files at 300 rpm and the ProTaper files at 280 rpm. All files were subjected to macroscopic inspection after each use and were discarded when signs of plastic deformation or fracture occurred. Each set of files was used to prepare up to eight root canals or until all the initial files of set failed. One unused file of No 30/.04 and more than fifteen failed specimens from each file type were ultrasonically cleaned in the 17% neutral-EDTA solution for 9 min, and examined under a SEM.
Results: All files presented flat regions allocated mostly at the periphery of their cross sections and in some cases extended up to the center of the cross section. These flat areas comprised of elongated dimples along the horizontal plane, a characteristic surface pattern of shear overloading. According to the results of the fractographic assessment, it seems that the fracture was originated from the sides of the instruments probably due to the higher stress concentration developed at these regions due to the file geometry.
Conclusions: Under the conditions of the present study, for all the files tested, fracture seems to be caused by a simple overloading incidence that leads to ductile fracture, rather than any cumulative damage mechanism (i.e. fatigue mechanism).