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The aim of this study was to investigate the influence of the shape of 3 different artificial canals on the trajectory followed by different nickel-titanium rotary instruments.Ten ProFile and Mtwo instruments, tip sizes 20 and 25, taper .06, were tested in 3 simulated root canals with an angle of curvature of 60° and radius of curvature of 5 mm but with different shapes. Geometric analysis of the trajectory that each instrument followed inside the 3 different artificial canals was performed on digital images, determining 3 parameters: angle and radius of the curvature and the position of the center of the curvature. Mean values were then calculated for each instrument size in all of the artificial canals. Data were analysed using 1-way analysis of variance, Holm t test, and Student t test to determine any statistical difference (P < .05).In all instrument sizes, a statistically significant difference was noted among the artificial canals for the radius and angle of curvature. No statistically significant difference was noted between instruments of the same size for the radius and angle of curvature and the position of the center of the curve when measured in the canal constructed on the dimension of the instruments.Different instruments follow different trajectories in artificial canals constructed with the same parameters of curvature but different shapes, depending on their different bending properties. All of the instruments respected the established parameters of curvature only when the artificial canal is designed on the dimension of the instruments.