title: Effect of asymmetry on the behavior of prototype rotary triple helix root canal instruments.
abstract: BACKGROUND
  Two factors affect the choice of instruments for root canal preparation: its ability to achieve the canal's shaping and its safety. These factors depend on the profile of the instrument and the design of its active part. In this study, we tried to assess the role of an asymmetric cross-section on the behavior of the instrument by measuring the stresses generated during the preparation of a simulated canal.
  
  
  METHODS
  Three 25-mm-long, 0.06 taper, ISO size #30 helical nickel-titanium instruments were tested. The first HeroShaper (Micro-Mega, Besançon, France) used had a symmetric triple helix section (H0). The others, based on the HeroShaper design, had a constant asymmetry of 4/100 mm (H4) or 6/100 mm (H6). Six canals were prepared using each instrument, and the experimental conditions (ie, speed and movement) were the same for each sample. A dynamometer with a sensitivity of 0.1 N recorded the stresses transmitted by the instruments.
  
  
  RESULTS
  Torque and apical force increased proportionally with instrument penetration. At the end of the preparations, the axial stress averaged 7.39 N for the symmetric instrument and 5.92 and 5.15 N for the asymmetric instruments, which indicated a significant statistical analysis of variance (P < .001). The average torque was low (1.05-1.13 N.cm), which indicated a nonsignificant statistical analysis of variance (P = .2385).
  
  
  CONCLUSIONS
  In this study, axial stresses decreased, but torque did not change with an asymmetric triple helix cross-section.
authors.affiliation: Faculté de Chirurgie Dentaire de Toulouse, CHU de Toulouse, Toulouse, France. franck.diemer@wanadoo.fr
authors.forename: Franck
authors.initials: F
authors.lastname: Diemer
authors.forename: Jérome
authors.initials: J
authors.lastname: Michetti
authors.forename: Jean-Philippe
authors.initials: JP
authors.lastname: Mallet
authors.forename: Robert
authors.initials: R
authors.lastname: Piquet
mesh_terms: D003722:Dental Alloys / Q000737:chemistry; D004867:Equipment Design; D004869:Equipment Safety; D006801:Humans; D008422:Materials Testing; D009532:Nickel / Q000737:chemistry; D018915:Root Canal Preparation / Q000295:instrumentation*; D012399:Rotation; D013314:Stress, Mechanical; D013499:Surface Properties; D014025:Titanium / Q000737:chemistry; D019415:Torque
chemical_list: D003722:Dental Alloys; C040654:titanium nickelide; D009532:Nickel; D014025:Titanium