Abstract: BACKGROUND: In root canal therapy, the fatigue fracture of NiTi alloy instruments is often caused by stress exceeding the elastic limit of metal plastic deformation or repeated bending at the root canal bend. 
OBJECTIVE: To explore the stress distribution on the inner wall of root canals when NRT FILES are used as root canal preparation materials, and to examine the stress concentration area.
METHODS: Solidworks software was used for 3D modeling of NRT FILES to simulate the state of the instruments in real oral environment. Six groups of simplified root canal models were established: (1) Bending angle of 30°, bending radius of 5 mm, root apex bending; (2) Bending angle of 30°, bending radius of 5 mm, middle root bending; (3) Bending angle of 30°, bending radius of 2 mm, root apex bending; (4) Bending angle of 30°, bending radius of 2 mm, middle root bending; (5) Bending angle of 45°, bending radius of 5 mm, root apex bending; (6) Bending angle of 45°, bending radius of 2 mm, root apex bending. The simplified root canal model and the root canal file model were assembled, and the stress points and stress distribution on the inner wall of the root canal were analyzed. 
RESULTS AND CONCLUSION: The maximum stresses of root canal files in groups (1)-(6) were 1 185, 1 354, 1 371, 1 678, 1 335, and 2 557 MPa, and the maximum strains were 0.010 36, 0.013 54, 0.013 79, 0.016 48, 0.013 45, and 0.022 03, respectively. Different bending parts of root canals could affect the stress borne by the root canal file. In root canal therapy, the lower the root canal file went down along the root canal, the greater the contact radius between these two pieces, the worse the rigidity of the root canal file, the greater the stress it bore, and the easier it was to fracture. The smaller the root canal bending radius, the greater the stress bore on the root canal file. The smaller the bending angle of the root canal, the greater the stress on the file. In the curved part of the root canal tip, the root canal file bore greater stress when the bending radius of the root canal was small, and it was more prone to fracture; similarly, in the curved part of the middle of root canal, the root canal file also bore greater stress with small bending radius. Therefore, it is necessary to pay extra attention to the bending degree of the root canal and select an appropriate root canal file for root canal treatment. 

Key words: curved root canal, NRT FILES nickel titanium file, root canal preparation, stress distribution, finite element analysis