title: An in vitro study of the torsional properties of new and used rotary nickel-titanium files in plastic blocks.
abstract: OBJECTIVE
  The purpose of this study was to compare torque (in grams per centimeter) and angle of rotation (in degrees) at the time of fracture of new and used nickel-titanium .04 ProFile rotary files. The relationship between the size of the instrument and the torque at fracture was also investigated.
  
  
  METHODS
  The torque and angle of rotation at fracture of new and used nickel-titanium .04 ProFile rotary files Nos. 15 to 40 were determined according to American National Standards Institute/American Dental Association Specification No. 28. Analysis of variance was used to compare the torque and angle of rotation at fracture of the different sizes of the new instruments and of new and used instruments of the same size (alpha =.05). The relationship between torque at fracture and the size of the instrument was determined by means of regression analysis.
  
  
  RESULTS
  The torque at fracture of the new instruments increased significantly with the diameter. The used instruments, Nos. 25 to 40, had significantly lower values of torque at fracture than did the new ones. There were no statistically significant differences between the angle of rotation at fracture of the new and the used instruments (P >.05). A stronger relationship was found between the size of the file and the torque at fracture for the new instruments (P <.0001) than for the used ones (P <.0001).
  
  
  CONCLUSIONS
  The results of the present study suggest that the values of the torque at fracture of new instruments increased significantly with the diameter. The results also suggest that repeated use of ProFile .04 instruments mainly affected the torque at fracture.
authors.affiliation: Department of Endodontics, Faculty of Dentistry, University of Toronto, Ontario, Canada. g.yared@utoronto.ca
authors.forename: Ghassan
authors.initials: G
authors.lastname: Yared
authors.forename: Gajanan K
authors.initials: GK
authors.lastname: Kulkarni
mesh_terms: D000704:Analysis of Variance; D003762:Dental Instruments; D003799:Dental Stress Analysis; D004868:Equipment Failure; D017743:Equipment Reuse; D009532:Nickel; D012044:Regression Analysis; D018915:Root Canal Preparation / Q000295:instrumentation*; D012399:Rotation; D014025:Titanium; D019415:Torque
chemical_list: C040654:titanium nickelide; D009532:Nickel; D014025:Titanium