title: Influence of cyclic torsional preloading on cyclic fatigue resistance of nickel - titanium instruments.
abstract: OBJECTIVE
  To evaluate the effect of different torsional preloads on cyclic fatigue resistance of endodontic rotary instruments constructed from conventional nickel-titanium (NiTi), M-Wire or CM-Wire.
  
  
  METHODS
  Eighty new size 25, 0.06 taper Mtwo instruments (Sweden & Martina), size 25, 0.06 taper HyFlex CM (Coltene/Whaledent, Inc) and X2 ProTaper Next (Dentsply Maillefer) were used. The Torque and distortion angles at failure of new instruments (n = 10) were measured, and 0% (n = 10), 25%, 50% and 75% (n = 20) of the mean ultimate torsional strength as preloading condition were applied according to ISO 3630-1 for each brand. The twenty files tested for every extent of preload were subjected to 20 or 40 torsional cycles (n = 10). After torsional preloading, the number of cycles to failure was evaluated in a simulated canal with 60° angle of curvature and 5 mm of radius of curvature. Data were analysed using two-way analysis of variance. The fracture surface of each fragment was examined with a scanning electron microscope (SEM). Data were analysed by two-way analyses of variance.
  
  
  RESULTS
  Preload repetitions did not influence the cyclic fatigue of the three brands; however, the 25%, 50% and 75% torsional preloading significantly reduced the fatigue resistance of all instruments tested (P < 0.01, P < 0.001 and P < 0.0001, respectively) except for the HyFlex CM preloaded with 25% of the maximum torsional strength (P > 0.05).
  
  
  CONCLUSIONS
  Torsional preloads reduced the cyclic fatigue resistance of conventional and treated (M-wire and CM-wire) NiTi rotary instruments except for size 25, 0.06 taper HyFlex CM instruments with a 25% of torsional preloading.
authors.affiliation: Department of Surgery, University of Catania, Catania, Italy.
authors.forename: E
authors.initials: E
authors.lastname: Pedullà
authors.affiliation: Department of Industrial Engineering, University of Catania, Catania, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Lo Savio
authors.affiliation: MATIS, Institute of Microelectronics and Microsystems, National Research Council, Catania, Italy.
authors.forename: S
authors.initials: S
authors.lastname: Boninelli
authors.affiliation: Department of Endodontics, 'Sapienza' University of Rome, Rome, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Plotino
authors.affiliation: Department of Endodontics, 'Sapienza' University of Rome, Rome, Italy.
authors.forename: N M
authors.initials: NM
authors.lastname: Grande
authors.affiliation: Department of Surgery, University of Catania, Catania, Italy.
authors.forename: E
authors.initials: E
authors.lastname: Rapisarda
authors.affiliation: Department of Industrial Engineering, University of Catania, Catania, Italy.
authors.forename: G
authors.initials: G
authors.lastname: La Rosa
mesh_terms: D004867:Equipment Design; D004868:Equipment Failure; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D009532:Nickel / Q000737:chemistry*; D018915:Root Canal Preparation / Q000295:instrumentation*; D013314:Stress, Mechanical; D013499:Surface Properties; D014025:Titanium / Q000737:chemistry*; D019415:Torque; D054159:Torsion, Mechanical
chemical_list: C040654:titanium nickelide; D009532:Nickel; D014025:Titanium
keywords: CM-wire; M-wire; NiTi fracture; cyclic fatigue resistance; torsional preload