title: Failure mechanism of ProTaper Ni-Ti rotary instruments during clinical use: fractographic analysis.
abstract: OBJECTIVE
  To evaluate the failure mechanism of ProTaper Ni-Ti rotary instruments fractured under clinical conditions.
  
  
  METHODS
  A total of 46 ProTaper instruments that failed (fractured and/or plastically deformed) during the clinical use were collected from various dental clinics, whereas a new set of ProTaper instruments served as control. After inspection under stereomicroscopy the instruments were classified into three categories: (i) plastically deformed but not fractured, (ii) fractured with plastic deformation and (iii) fractured without plastic deformation. Three instruments from each group were analysed with computerized X-ray microtomography (micro-XCT) to detect surface and internal defects, whilst all the fracture surfaces were investigated under SEM.
  
  
  RESULTS
  Stereomicroscopic inspection showed that 17.4% of the discarded instruments were only plastically deformed, 8.7% were fractured with plastic deformation and 73.9% were fractured without plastic deformation. Micro-XCT revealed instruments without any surface or bulk defects along with a few files with crack development below the fracture surface. No defects were identified in the unused instruments. SEM examination of fractured surfaces demonstrated the presence of dimples and cones, a typical pattern of dimple rupture developed because of ductile failure.
  
  
  CONCLUSIONS
  The results suggest that a single overloading event causing ductile fracture of ProTaper instruments is the most common fracture mechanism encountered under the clinical conditions.
authors.affiliation: Department of Endodontics, University of Athen's School of Dentistry, Athens, Greece.
authors.forename: A P
authors.initials: AP
authors.lastname: Spanaki-Voreadi
authors.forename: N P
authors.initials: NP
authors.lastname: Kerezoudis
authors.forename: S
authors.initials: S
authors.lastname: Zinelis
mesh_terms: D004868:Equipment Failure; D019544:Equipment Failure Analysis; D008855:Microscopy, Electron, Scanning; D009532:Nickel / Q000737:chemistry; D018915:Root Canal Preparation / Q000295:instrumentation*; D012399:Rotation; D014025:Titanium / Q000737:chemistry
chemical_list: C040654:titanium nickelide; D009532:Nickel; D014025:Titanium