title: Screw-in forces during instrumentation by various file systems.
abstract: OBJECTIVE
  The purpose of this study was to compare the maximum screw-in forces generated during the movement of various Nickel-Titanium (NiTi) file systems.
  
  
  METHODS
  Forty simulated canals in resin blocks were randomly divided into 4 groups for the following instruments: Mtwo size 25/0.07 (MTW, VDW GmbH), Reciproc R25 (RPR, VDW GmbH), ProTaper Universal F2 (PTU, Dentsply Maillefer), and ProTaper Next X2 (PTN, Dentsply Maillefer,n= 10). All the artificial canals were prepared to obtain a standardized lumen by using ProTaper Universal F1. Screw-in forces were measured using a custom-made experimental device (AEndoS-k, DMJ system) during instrumentation with each NiTi file system using the designated movement. The rotation speed was set at 350 rpm with an automatic 4 mm pecking motion at a speed of 1 mm/sec. The pecking depth was increased by 1 mm for each pecking motion until the file reach the working length. Forces were recorded during file movement, and the maximum force was extracted from the data. Maximum screw-in forces were analyzed by one-way ANOVA and Tukey'spost hoccomparison at a significance level of 95%.
  
  
  RESULTS
  Reciproc and ProTaper Universal files generated the highest maximum screw-in forces among all the instruments while M-two and ProTaper Next showed the lowest (p< 0.05).
  
  
  CONCLUSIONS
  Geometrical differences rather than shaping motion and alloys may affect the screw-in force during canal instrumentation. To reduce screw-in forces, the use of NiTi files with smaller cross-sectional area for higher flexibility is recommended.
authors.affiliation: Department of Conservative Dentistry, School of Dentistry, Kyungpook National University, Daegu, Korea.
authors.forename: Jung-Hong
authors.identifier: 0000-0002-0469-4324
authors.initials: JH
authors.lastname: Ha
authors.affiliation: Department of Conservative Dentistry, School of Dentistry, Dental Research Institute, Pusan National University, Yangsan, Korea.
authors.forename: Sang Won
authors.identifier: 0000-0001-7965-977X
authors.initials: SW
authors.lastname: Kwak
authors.affiliation: Department of Conservative Dentistry, School of Dentistry, Kyungpook National University, Daegu, Korea.
authors.forename: Sung-Kyo
authors.identifier: 0000-0002-9160-7187
authors.initials: SK
authors.lastname: Kim
authors.affiliation: Department of Conservative Dentistry, School of Dentistry, Dental Research Institute, Pusan National University, Yangsan, Korea.
authors.forename: Hyeon-Cheol
authors.identifier: 0000-0001-8032-1194
authors.initials: HC
authors.lastname: Kim
keywords: Continuous rotation; Geometry; Nickel-titanium file; Reciprocating; Screw-in force