title: An in vitro study of the torsional properties of new and used K3 instruments.
abstract: OBJECTIVE
  To compare torque and angle of rotation at fracture of new and used K3 rotary instruments. The relation between size of instrument and torque at fracture was also investigated.
  
  
  METHODS
  Nickel-titanium (Ni-Ti).06 K3 rotary instruments were used in a crown-down manner at 300 r.p.m. to prepare canals in resin blocks. The torque and angle of rotation at fracture of new and the used Ni-Ti.06 K3 rotary instruments (sizes 15-40) were determined according to ANSI/ADA Specification no. 28. Analysis of variance was used to compare the torque and angle of rotation at fracture amongst the different sizes of the new instruments and between new instruments and instruments of the same size, which had been used in resin blocks (alpha = 0.05). The relationship between torque at fracture and size of instrument was subjected to regression analysis.
  
  
  RESULTS
  Torque at fracture of the new instruments increased significantly with the diameter. The used instruments (sizes 15, 20, 30, 35 and 40) had lower torque at fracture compared to the new ones (P < 0.0001). The means of angle of rotation at fracture between the different sizes of new instruments were significantly different (P < 0.0001) except for sizes 15-20 (P = 0.2561). The used instruments (sizes 20-40) had lower angle of rotation at fracture compared to the new ones (P < 0.05). A linear relationship was found between the size of the file and the torque at fracture for the new instruments (r2 = 0.84; P < 0.0001) and the used ones (r2 = 0.82; P < 0.0001).
  
  
  CONCLUSIONS
  In general, the results suggested that the torque and angle of rotation at fracture were significantly affected by the repeated use of.06 K3 instruments in resin blocks.
authors.affiliation: Department of Endodontics, Faculty of Dentistry, University of Toronto, Toronto, Ontario, Canada. g.yared@utoronto.ca
authors.forename: G
authors.initials: G
authors.lastname: Yared
authors.forename: G K
authors.initials: GK
authors.lastname: Kulkarni
authors.forename: F
authors.initials: F
authors.lastname: Ghossayn
mesh_terms: D000704:Analysis of Variance; D003722:Dental Alloys; D004867:Equipment Design; D004868:Equipment Failure; D017743:Equipment Reuse; D006801:Humans; D008422:Materials Testing; D009532:Nickel; D012044:Regression Analysis; D018915:Root Canal Preparation / Q000295:instrumentation*; D012399:Rotation; D013499:Surface Properties; D014025:Titanium; D019415:Torque
chemical_list: D003722:Dental Alloys; C040654:titanium nickelide; D009532:Nickel; D014025:Titanium