Location of the canal curvature and its influence on the resistance to fatigue fracture of two rotary nickel-titanium endodontic instruments

Aim: To evaluate the influence of curvature location along two artificial canals on the cyclic fatigue (CF) of two rotary instruments (Mtwo 35 and Revo-S AS35) by assessing the number of cycles to fracture (NCF) and the morphologic characteristics of the fractured instruments. Methodology: The instruments were tested in artificial curved canals made by milling grooves in stainless steel blocks, with the following measurements: 1.5 mm (width), 20 mm (total length), 3.5 mm (depth), and a U-shaped bottom. In order to enable comparisons, the parameters of curvature varied in two different designs: the canal A curvature was located at the apical portion of the canal, and the canal B curvature was located in the middle of the canal length. All instruments were rotated until fracture and the NCF was recorded. Data were analysed by Student's t test. The fractured surfaces and helical shafts of the instruments were analysed by scanning electron microscopy (SEM). Results: Canals A and B had statistically significantly different (P 0.05) NCF values, with canal B resulting in the lowest NCF values. Plastic deformation of the shafts was not detected in any of the instruments tested. The fractured surfaces displayed morphological characteristics of ductile mode fracture. Conclusions: Placement of the curvature in the middle of the canal length reduced the NCF of the instruments tested. Keywords: canal curvature geometry, cyclic fatigue, rotary nickel-titanium instruments