Effect of Optimum Torque Reverse Motion on Torque and Force Generation during Root Canal Instrumentation with Crown-down and Single-length Techniques

J Endod. 2020 Feb;46(2):232-237. doi: 10.1016/j.joen.2019.11.007. Epub 2019 Dec 27.


Introduction: Optimum torque reverse (OTR) motion is a torque-sensitive reciprocal motion in which the motor rotates in alternating 90° counterclockwise and 180° clockwise rotation when the torque exceeds a predetermined value. This study aimed to examine whether OTR motion contributes to torque and force reduction during nickel-titanium rotary instrumentation with the crown-down or single-length technique.

Methods: Twenty-eight simulated straight canals in resin blocks were divided into 2 groups according to the type of motion (OTR or continuous rotation). The groups were further subdivided according to the preparation technique (crown-down or single-length technique, n = 7 each). Automated root canal instrumentation was performed with a torque/force analyzing device (300 rpm, up-and-down speed of 10 mm/min) and EndoWave instruments (FKG Dentaire, La-Chaux-de-Fonds, Switzerland) to size #25/0.06 taper. Maximum torque and apical force were recorded and analyzed with analysis of variance and the Bonferroni test.

Results: During the crown-down preparation phase (#35/0.08, #30/0.06, #25/0.06, and #20/0.06), OTR motion developed lower maximum torque and upward force (representing the screw-in force) than continuous rotation. During the apical preparation phase (#25/0.06), OTR motion generated significantly lower maximum clockwise and counterclockwise torque (P < .05) when the single-length technique was used and significantly lower maximum upward force regardless of the preparation technique (P < .05) compared with continuous rotation.

Conclusions: Under the present experimental conditions, OTR motion reduced both torque and screw-in force during the crown-down preparation phase of the crown-down technique and during the apical preparation phase of the single-length technique.

Keywords: Crown-down technique; nickel-titanium rotary instrumentation; screw-in force; single-length technique; torque; torque-sensitive reciprocal motion.

MeSH terms

  • Crowns
  • Dental Alloys
  • Dental Pulp Cavity*
  • Equipment Design
  • Root Canal Preparation*
  • Rotation
  • Titanium
  • Torque


  • Dental Alloys
  • Titanium