Comparison of Screw-in Effect for Several Nickel-titanium Rotary Instruments in Simulated Resin Root Canal

Screw-in effect is one of the unintended phenomena that occurs during the root canal preparation with nickel-titanium rotary files. The aim of this study was to compare the screw-in effect among various nickel-titanium rotary file systems. Six different nickel-titanium rotary instruments (ISO 20/.06 taper) were used: K3 (SybronEndo, Glendora, CA, USA), Mtwo (VDW GmbH, München, Germany), NRT with safe-tip and with active tip (Mani Inc., Shioya-gun, Japan), ProFile (Dentsply-Maillefer, Ballaigues, Switzerland) and ProTaper (Dentsply-Maillefer, Ballaigues, Switzerland). For ProTaper, S2 was selected because it has size 20. Root canal instrumentations were done in sixty simulated single-curved resin root canals with a rotational speed of 300 rpm and single pecking motion. A special device was designed to measure the force of screw-in effect. A dynamometer of the device recorded the screw-in force during simulated canal preparation and the recorded data was stored in a computer with designed software (LCV-USE-VS, Lorenz Messtechnik GmbH, Alfdorf, Germany). The data were subjected to one-way ANOVA and Tukey's multiple range test for post-hoc test. P value of less than 0.05 was regarded significant. ProTaper produced significantly more screw-in effects than any other instruments in the study (p < 0.001). K3 produced significantly more screw-in effects than Mtwo, and ProFile (p < 0.001). There was no significant difference among Mtwo, NRT, and ProFile (p > 0.05), and between NRT with active tip and NRT with safe one neither (p > 0.05). From the result of the present study, it was concluded, therefore, that there seems significant differences of screw-in effect among the tested nickel-titanium rotary instruments. The radial lands and rake angle of nickel-titanium rotary instrument might be the cause of the difference. [J Kor Acad Cons Dent 35(4):267272, 2010]