Objective This study aimed to establish finite element analysis(FEA) models of the K file, H file, and Protaper file, and analyze the factors leading to the breakdown of these instruments when used in curved root canal. Methods Using the software Pro/Engineer 5.0, we generated three-dimensional FEA models of the cutting edges of the stainless steels K file, H file, and Ni-Ti Protaper file. Curved root canals were classified into four groups according to the angle and radius of the curvature:30°/5 mm, 30°/2 mm, 45°/5 mm, and 45°/2 mm. Pro/Mechanica software performed the FEA on the mechanism of the working instrument in a curved root canal. Stress distribution in the cutting edges under clockwise and counterclockwise torsional loads was analyzed. Results Similar stress distribution patterns were observed in K and H file groups regardless of the direction of torsional load, either clockwise or counter clockwise. In the 45°/2 mm group, stress on the K file is concentrated at the tip of the cutting blade. However, in 30°/5 mm and 45°/5 mm groups, stress on the H file is concentrated at the groove between two cutting edges, whereas in 30°/2 mm and 45°/2 mm, maximum stress is located at the tip of the cutting edge, stress positions and no good or bad times to reverse the direction of the association. In F1 Group, maximum stress is located at the cutting edge tip, in addition to groups(i.e., 45°/5 mm and 30°/2 mm) where stress is acting on the groove between two cutting edges. Conclusion The symmetric cross-section design of the K file and Protaper file is beneficial in the distribution of stress, while the non-symme triccross-section design of the H file is prone to create uneven stress distribution. Thus, the latter is easier to break when used in a curved root canal and under torsional load.
