Radial lands and alternating cutting edges contribute to reduced screw-in and torque in curved root canals - an in vitro study.

Introduction: This study aimed to evaluate screw-in force and torsional load generated by six fully rotating nickel-titanium (NiTi) file systems with different geometry, cross-section, and rotational speed.

Methods: Ninety mesial roots of human lower molars were divided into six groups according to the file system used for canal preparation: F360, F6, TruNatomy, RACE EVO, JIZAI I, and JIZAI II. Groups were standardized balanced to working length, chord length, and canal curvature. A custom-built test rig with an attached endodontic motor (Schlumbohm EndoPilot 2) and a six-axis measuring device were used to measure the apically directed, screw-in and torque forces generated along the axis of the tooth as the NiTi file was moved with an amplitude of 3 mm at each peck, increasing penetration depth by 2 mm at each peck. The rotary speed was set individually for each file according to the manufacturer's instructions. Two sets of 3 pecking motions were performed with intermitting cleanings of the file and needle irrigation of the root canal. Additionally, torque forces were recorded by the endodontic motor. Maximum force values for each set of pecks were analyzed using the Kruskal-Wallis test. The correlation between z-axis tooth torque and motor-measured torque was assessed using Spearman's correlation.

Results: Significant differences existed between the files regarding apically directed, screw-in and torque forces (P<0.05). Torque values generated on the tooth along the z-axis and torque on the file correlated significantly (P<.001).

Conclusions: Radial lands or alternating cutting edges affect screw-in effect and torsional load of NiTi files.