title: Effect from cyclic fatigue of nickel-titanium rotary files on torsional resistance.
abstract: BACKGROUND
  The aim of this study was to evaluate the effect of cyclic fatigue on the torsional resistance of nickel-titanium (NiTi) rotary instruments.
  
  
  METHODS
  ProFile (#25/0.06; Dentsply Maillefer, Ballaigues, Switzerland) and ProTaper (F1, Dentsply Maillefer), both of which have the same external diameter at D5, were tested using a fatigue testing machine for the mean number of cycles of failure (mNCF). Then, new files were cyclic precycled to 4 conditions (ie, 0%, 25%, 50%, and 75% of the mNCF) before the torsional resistance test was performed on these cyclic preloaded files. A uniform clockwise rotation was applied to the file in a straight state in a torsion tester. The torsional load and distortion angle were recorded during rotation until the file succumbed to the torque. The toughness was computed. The results were analyzed statistically using one-way analysis of variance and the Duncan post hoc comparison to find any differences between groups at a significance level of 95%.
  
  
  RESULTS
  In both ProFile and ProTaper groups, the 75% preloading groups had significantly lower torsional strength than other preloaded files. In the ProFile group, the 50% and 75% preloading groups had a smaller distortion angle until fracture than the 25% and no preloading groups. The 75% preloading group showed a lower toughness value than the 25% and no preloading groups. In the ProTaper group, all preloading groups had less distortion and toughness than the no preloading group. Fractographic examinations revealed the 75% preloaded files showed less amount of reverse-wound flute than other preloading groups.
  
  
  CONCLUSIONS
  Within the limitations of the methodology, it could be concluded that approximate 75% cyclic fatigue may reduce the torsional resistance of NiTi rotary instruments significantly.
authors.affiliation: Department of Conservative Dentistry, School of Dentistry, Pusan National University, Yangsan, Korea.
authors.forename: Ju-Yeong
authors.initials: JY
authors.lastname: Kim
authors.forename: Gary Shun-Pan
authors.initials: GS
authors.lastname: Cheung
authors.forename: Se-Hee
authors.initials: SH
authors.lastname: Park
authors.forename: Dae-Cheol
authors.initials: DC
authors.lastname: Ko
authors.forename: Jin-Wook
authors.initials: JW
authors.lastname: Kim
authors.forename: Hyeon-Cheol
authors.initials: HC
authors.lastname: Kim
mesh_terms: D003722:Dental Alloys / Q000737:chemistry*; D004867:Equipment Design; D004868:Equipment Failure; D006801:Humans; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D009532:Nickel / Q000737:chemistry*; D018915:Root Canal Preparation / Q000295:instrumentation*; D012399:Rotation; D013314:Stress, Mechanical; D013499:Surface Properties; D014025:Titanium / Q000737:chemistry*; D019415:Torque; D054159:Torsion, Mechanical
chemical_list: D003722:Dental Alloys; C040654:titanium nickelide; D009532:Nickel; D014025:Titanium