title: 'Screw-in' tendency of rotary nickel-titanium files due to design geometry.
abstract: OBJECTIVE
  To examine the effect of several standard geometric characteristics of rotary instruments on the 'screw-in' forces and stresses generated on root dentine using 3D finite element analysis (FEA).
  
  
  METHODS
  Four cross-sectional designs (triangular, slender-rectangular, rectangular and square) were evaluated. The area of the triangular cross-section and of the slender-rectangular model were the same. Another rectangular model had the same centre-core diameter as the triangular one. Each design was twisted into a file model with 5, 10 or 15 threads over its 16-mm-long working section. Three curved root canals were simulated as rigid surface models: θ = 15 degrees/R = 36 mm radius; θ = 30/R = 18; and θ = 45/R = 12. A commercial FEA package was used to simulate the file rotating in the canal to determine the 'screw-in' force and reaction torque on the instrument.
  
  
  RESULTS
  Instruments of a square cross-section had the highest 'screw-in' force and reaction torsional stresses followed by the rectangle, the triangle design and the slender-rectangle design, respectively. The file with closer pitch generated lower stresses, compared with that with longer pitch. The greater the root canal curvature, the higher the 'screw-in' force and reaction torque generated.
  
  
  CONCLUSIONS
  This study demonstrated that the 'screw-in' tendency depends on both the instrument geometry and canal curvature. Clinicians should be aware that certain instrument designs are prone to develop high 'screw-in' forces, requiring the operator to maintain control of the handpiece or to use a brushing action to prevent instruments being pulled into the canal.
authors.affiliation: Department of Conservative Dentistry, School of Dentistry, Kyungpook National University, Daegu, Korea.
authors.forename: J H
authors.initials: JH
authors.lastname: Ha
authors.affiliation: Area of Endodontics, Faculty of Dentistry, The University of Hong Kong, Hong Kong, China.
authors.forename: G S P
authors.initials: GS
authors.lastname: Cheung
authors.affiliation: Department of Bioscience Research, College of Dentistry, University of Tennessee Health Science Center, Memphis, TN, USA.
authors.forename: A
authors.initials: A
authors.lastname: Versluis
authors.affiliation: Dongnam Regional Division, Korea Institute of Industrial Technology, Jinju, Korea.
authors.forename: C J
authors.initials: CJ
authors.lastname: Lee
authors.affiliation: Department of Conservative Dentistry, School of Dentistry, Pusan National University, Dental Research Institute, Yangsan, Korea.
authors.forename: S W
authors.initials: SW
authors.lastname: Kwak
authors.affiliation: Department of Conservative Dentistry, School of Dentistry, Pusan National University, Dental Research Institute, Yangsan, Korea.
authors.forename: H C
authors.initials: HC
authors.lastname: Kim
mesh_terms: D003722:Dental Alloys; D003762:Dental Instruments; D003799:Dental Stress Analysis / Q000379:methods; D004548:Elasticity; D004867:Equipment Design; D020342:Finite Element Analysis; D009532:Nickel; D018915:Root Canal Preparation / Q000295:instrumentation*; D013314:Stress, Mechanical; D014025:Titanium; D054159:Torsion, Mechanical
chemical_list: D003722:Dental Alloys; C040654:titanium nickelide; D009532:Nickel; D014025:Titanium
keywords: cross-section; finite element analysis; geometry; pitch; rotary nickel-titanium file; screw-in effect