Statement of Problem: There are many controversies regarding the usable number of rotary files in the literature.
Purpose:The aim of this study was to compare three Rotary Systems in Terms of Number of Severely Curved Root Canals Preparation, Till Either Defect or Fracture.
Material and Methods: In this in vitro study, 60 mandibular or maxillary molar mesial roots with a mature apex and a curvature of 35-80º were evenly divided into three groups according to their radius and curvature angle. The Pruett method was used to define the curvature degree. In all the three groups, cleaning and shaping were accomplished with a working length of 16mm.Then, the number of the prepared canals until any defect or fracture (using a 8x magnifier) was recorded and canal preparation was done with the same devices in the remaining canals. Chi-square test was applied for inters categorical variables comparisons. Kaplan-Meier curves were used to display the number of prepared canals till any defect or fracture with further comparison by a log-rank test. To compare the curve angle and radius, Kruskal-Wallis test was used. p.value of less than 0.05 was considered as statistically significant.
Results: The average number (±SD) of the prepared canals leading to defect or fracture in flexmaster, Hero and Race systems was 11.87 (±2.07), 13.28 (±2.46), and 9.66 (±1.92), respectively ( p <0.05). Overall, 20 devices were found to be fractured or defective, among which 7 had defects and 13 had fractures. The rates of defects in Hero, Race and Flexmaster were 50%, 33.3% and 28.6% with the fracture rate of 50%, 60% and 71.4%, respectively ( p  > 0.05).
Conclusion: Hero system showed higher numbers of prepared canals and revealed less fracture rate. Meanwhile, Race system revealed the least number of prepared canals with the highest rate of the fracture.  However, there were no statistically significant differences between the systems.