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Introduction: The goal of the present study was to evaluate the influence of surface grooves (peaks and valleys) resulting from machining during the manufacturing process of polished and unpolished nickel-titanium BR4C endodontic files on the fatigue life of the instruments., Methods: Ten electropolished and 10 unpolished endodontic files were provided by the manufacturer. Specimens were from the same batch, but the unpolished instruments were removed from the production line before surface treatment. The instruments were evaluated with a profilometer to quantify the surface roughness on the working part of the instruments. Then the files were subjected to rotating bending fatigue tests., Results: Analysis with the profilometer showed that surface grooves were deeper on the unpolished instruments compared with their electropolished counterparts. In the rotating bending fatigue test, the mean and standard deviation for the number of cycles until fracture (NCF) were greater for instruments with less pronounced grooves. Student t test revealed significant differences in all tests (P < .05)., Conclusions: The results from the present study showed that the depth of the surface grooves on the working part affected the NCF of the instruments tested; the smaller the groove depth, the greater the NCF., (Copyright © 2016 American Association of Endodontists. Published by Elsevier Inc. All rights reserved.)

Introduction: This study evaluated the influence of different features of canal curvature geometry on the number of cycles to fracture of a rotary nickel-titanium endodontic instrument subjected to a cyclic fatigue test., Methods: BioRaCe BR4C instruments (FKG Dentaire, La Chaux-de Fonds, Switzerland) were tested in 4 grooves simulating curved metallic artificial canals, each one measuring 1.5 mm in width, 20 mm in total length, and 3.5 mm in depth with a U-shaped bottom. The parameters of curvature including the radius and arc lengths and the position of the arc differed in the 4 canal designs. Fractured surfaces and helical shafts of the separated instruments were analyzed by scanning electron microscopy., Results: The Student's t test showed that a significantly lower number of cycles to fracture values were observed for instruments tested in canals with the smallest radius, the longest arc, and the arc located in the middle portion of the canal. Scanning electron microscopic analysis of the fracture surfaces revealed morphologic characteristics of ductile fracture. Plastic deformation was not observed in the helical shaft of the fractured instruments., Conclusions: Curvature geometry including the radius and arc lengths and the position of the arc along the root canal influence the number of cycles to fracture of rotary nickel-titanium instruments subjected to flexural load., (Copyright © 2013 American Association of Endodontists. Published by Elsevier Inc. All rights reserved.)