title: The Effects of Torsional Preloading on the Torsional Resistance of Nickel-titanium Instruments.
abstract: BACKGROUND
  This study evaluated the effect of torsional preloading on the torsional resistance of nickel-titanium (NiTi) endodontic instruments.
  
  
  METHODS
  WaveOne Primary (Dentsply Maillefer, Ballaigues, Switzerland) and ProTaper Universal F2 (Dentsply Maillefer) files were used. The ultimate torsional strength until fracture was determined for each instrument. In the phase 1 experiment, the ProTaper and WaveOne files were loaded to have a maximum load from 2.0 up to 2.7 or 2.8 Ncm, respectively. In the phase 2 experiment, the number of repetitions of preloading for each file was increased from 50 to 200, whereas the preloading torque was fixed at 2.4 Ncm. Using torsionally preloaded specimens from phase 1 and 2, the torsional resistances were calculated to determine the ultimate strength, distortion angle, and toughness. The results were analyzed using 1-way analysis of variance and Duncan post hoc comparison. The fracture surfaces and longitudinal aspect of 5 specimens per group were examined under a scanning electron microscope.
  
  
  RESULTS
  All preloaded groups showed significantly higher ultimate strength than the unpreloaded groups (P < .05). There was no significant difference among all groups for distortion angle and toughness. Although WaveOne had no significant difference between the repetition groups for ultimate strength, fracture angle, and toughness, ProTaper had a higher distortion angle and toughness in the 50-repetition group compared with the other repetition groups (P < .05). Scanning electron microscopic examinations of the fractured surface showed typical features of torsional fracture.
  
  
  CONCLUSIONS
  Torsional preloading within the ultimate values could enhance the torsional strength of NiTi instruments. The total energy until fracture was maintained constantly, regardless of the alloy type.
authors.affiliation: Department of Conservative Dentistry, School of Dentistry, Dental Research Institute, Pusan National University, Yangsan, Korea.
authors.forename: Seung-Hei
authors.initials: SH
authors.lastname: Oh
authors.affiliation: Department of Conservative Dentistry, School of Dentistry, Kyungpook National University, Daegu, Korea.
authors.forename: Jung-Hong
authors.initials: JH
authors.lastname: Ha
authors.affiliation: Department of Conservative Dentistry, School of Dentistry, Dental Research Institute, Pusan National University, Yangsan, Korea.
authors.forename: Sang Won
authors.initials: SW
authors.lastname: Kwak
authors.affiliation: Department of Conservative Dentistry, School of Dentistry, Dental Research Institute, Pusan National University, Yangsan, Korea.
authors.forename: Shin Wook
authors.initials: SW
authors.lastname: Ahn
authors.affiliation: Department of Conservative Dentistry, School of Dentistry, Seoul National University, Seoul, Korea.
authors.forename: WooCheol
authors.initials: W
authors.lastname: Lee
authors.affiliation: Department of Conservative Dentistry, School of Dentistry, Dental Research Institute, Pusan National University, Yangsan, Korea. Electronic address: golddent@pusan.ac.kr.
authors.forename: Hyeon-Cheol
authors.initials: HC
authors.lastname: Kim
mesh_terms: D003762:Dental Instruments / Q000592:standards*; D004708:Endodontics / Q000295:instrumentation*; D004868:Equipment Failure; D019544:Equipment Failure Analysis; D006801:Humans; D008855:Microscopy, Electron, Scanning; D009532:Nickel / Q000737:chemistry; D014025:Titanium / Q000737:chemistry; D054159:Torsion, Mechanical
chemical_list: C040654:titanium nickelide; D009532:Nickel; D014025:Titanium
keywords: M-wire; nickel-titanium rotary file; preloading; torsional load; torsional resistance