Torsional properties of stainless steel and nickel-titanium endodontic files

The torsional properties of stainless steel (SS) endodontic files (Flex-O-File; Maillefer/L. D. Caulk Co., Milford, DE) and recently developed nickel-titanium (NiTi) endodontic files (Quality Dental Products, Inc., Johnson City, TN) were compared. File sizes 15, 25, 35, 45, and 55 were subjected to torsional load in clockwise (CW) and counterclockwise (CCW) directions independently. Results showed that SS files had a significantly greater rotation to failure in the CW direction, whereas the NiTi files had a significantly greater rotation to failure in the CCW direction. Despite these differences in rotation to fracture, there was essentially no difference between the SS and NiTi instruments in the torque that it took to cause failure in both the CW and the CCW directions. Therefore, whereas the number of CW and CCW rotations to failure differed for the two instruments, the actual force that it took to cause that failure was the same.