Materials and Methods: Three different nickel-titanium rotary file systems were used: ProTaper (Dentsply-Maillefer, Ballaiguez, Switzerland), Profile (Dentsply-Maillefer, Ballaiguez, Switzerland) and K3 (Sybron-Endo, U.S.A.). To have the same condition of the taper and tip size of the instrument, ProTaper S2, Profile 0.06/#20 and K3 0.06/#20 files was selected. Instrumentations were done in thirty simulated single-curved root canals in clear resin blocks (REF A0177, Dentsply-Maillefer, Ballaiguez, Switzerland) 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 computer with designed software. 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.
Results: Screw-in forces are given in Newton (mean±SD). The Protaper group (4.32±1.08 N, n=10) produced the most screw-in effect among three file systems (p<0.01), followed by K3 (1.37±0.19 N, n=10) and Profile group (0.38±0.18 N) n=10. There was statistical significance between K3 and ProFile (p<0.01).
Conclusion: There was some significant difference of screw-in effect among the tested nickel-titanium rotary instruments in the present study.