title: Comparison of the time required to create secondary fracture of separated file fragments by using ultrasonic vibration under various canal conditions.
abstract: BACKGROUND
  Nickel-titanium files often separate because of mechanical fatigue. The purpose of this study was to determine safe preparation techniques for separated file removal by using ultrasonics.
  
  
  METHODS
  Fifty nickel-titanium file fragments were divided into 5 groups. An ultrasonic tip was activated on a file fragment positioned between dentin blocks simulating several canal conditions: Group 1 consisted of the fragment protruding from a pair of straight dentin blocks. For group 2, the fragment was also positioned between 2 straight dentin blocks except one block was positioned 1 mm more apically than the other block, simulating a troughed area that is often created during file removal attempts. For groups 3-5, the fragment was positioned similarly as group 2 but between blocks with 30°, 45°, and 60° curvatures, respectively. The time it took for secondary fracture to occur was recorded, and the data were statistically analyzed.
  
  
  RESULTS
  Fragments with dentin wall supporting on the opposite side of ultrasonic activation site resisted fracture significantly longer than those without it. Fragments in 30° and 45° curved blocks took significantly longer to fracture than the other groups (Fisher protected least significant difference, P < .05).
  
  
  CONCLUSIONS
  Secondary fracture of separated files appeared to be reduced when the ultrasonic tip was applied to the inner curvature of the canal.
authors.affiliation: Pulp Biology and Endodontics Section, Tokyo Medical and Dental University, Tokyo, Japan. Electronic address: yterauchi@intellident.or.jp.
authors.forename: Yoshitsugu
authors.initials: Y
authors.lastname: Terauchi
authors.forename: Le
authors.initials: L
authors.lastname: O'Leary
authors.forename: Takatomo
authors.initials: T
authors.lastname: Yoshioka
authors.forename: Hideaki
authors.initials: H
authors.lastname: Suda
mesh_terms: D003722:Dental Alloys / Q000737:chemistry; D003786:Dental Pulp Cavity / Q000473:pathology*; D003804:Dentin / Q000473:pathology; D004548:Elasticity; D004867:Equipment Design; D004868:Equipment Failure; D005547:Foreign Bodies / Q000628:therapy*; D006801:Humans; D009532:Nickel / Q000737:chemistry; D018583:Pliability; D018915:Root Canal Preparation / Q000009:adverse effects / Q000295:instrumentation*; D013997:Time Factors; D014025:Titanium / Q000737:chemistry; D019415:Torque; D054159:Torsion, Mechanical; D014465:Ultrasonics / Q000295:instrumentation; D014732:Vibration
chemical_list: D003722:Dental Alloys; C040654:titanium nickelide; D009532:Nickel; D014025:Titanium
keywords: Cyclic fatigue; nickel titanium file; secondary fracture; separated file; ultrasonics