title: Dynamic torsional resistance of nickel-titanium rotary instruments.
abstract: BACKGROUND
  The cyclic fatigue of nickel-titanium (NiTi) rotary instruments has been studied extensively, but there is little information available on torsional fracture. Moreover, a clinical repeated locking effect was not considered in previous studies that evaluated torsional resistance of NiTi instruments. Thus, this study was aimed to compare the repetitive torsional resistance of various NiTi instruments with clinical relevance.
  
  
  METHODS
  Five brands of NiTi rotary instruments were selected: Twisted File (TF; SybronEndo, Orange, CA) and RaCe systems (FKG Dentaire, La Chaux-de-Fonds, Switzerland), both with an equilateral triangular cross-section, and the ProTaper (Dentsply Maillefer, Ballaignes, Switzerland), Helix (DiaDent, Chongju, Korea), and FlexMaster (VDW, Munchen, Germany), which had a convex triangular cross-section. Five millimeters of the tip of each file was embedded in composite resin block, and uniform torsional stresses (300 rpm, 1.0 N.cm) were applied repetitively by an endodontic motor with auto-stop mode until the file succumbed to torsional failure. The number of load applications leading to fracture was recorded. All fracture surfaces were examined under the SEM. Results were analyzed nonparametrically with alpha = 0.05.
  
  
  RESULTS
  Under the mode of load applications in this study, TF had the lowest and FlexMaster the highest torsional resistance among the groups (p < 0.05). Scanning electron microscopy examination revealed a typical pattern of torsional fracture for TF, RaCe, and ProTaper that was characterized by circular abrasion marks and skewed dimples near the center of rotation. In addition to these marks, Helix and FlexMaster presented a rough, torn-off appearance.
  
  
  CONCLUSIONS
  It was concluded that files of same cross-sectional design may exhibit different resistance to fracture probably as a result of the manufacturing process.
authors.affiliation: Department of Conservative Dentistry, School of Dentistry, Pusan National University, Yangsan, Korea.
authors.forename: Su-Young
authors.initials: SY
authors.lastname: Park
authors.forename: Gary S P
authors.initials: GS
authors.lastname: Cheung
authors.forename: Jiwan
authors.initials: J
authors.lastname: Yum
authors.forename: Bock
authors.initials: B
authors.lastname: Hur
authors.forename: Jeong-Kil
authors.initials: JK
authors.lastname: Park
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*; D054159:Torsion, Mechanical
chemical_list: D003722:Dental Alloys; C040654:titanium nickelide; D009532:Nickel; D014025:Titanium