The purposes of this study were to compare the apical terminus width of simulated curved root canal prepared with three NiTi file systems used by undergraduates for evaluation the effects of flute angle and pitch or radial land on reducing screw-in effect and to determine more safe NiTi file system for inexperienced operators.

Fifty inexperienced undergraduate students prepared 150 simulated curved root canals in resin blocks with three NiTi file systems ; ProFile®, Hero Shaper®, K3™. The electric motor set at a speed of 300 rpm and torque of 30 in a 16 : 1 reduction handpiece was used. The simulated root canal was prepared to ISO #25 sizes with each file system. The scanned images of pre- and post-instrumented canal of resin block were superimposed. To evaluate the screw-in effect of three NiTi file systems, apical terminus width of root canal was measured from superimposed images and statistical analysis was performed.

There were significant differences in three NiTi file systems. ProFile® had significantly smaller width than Hero Shaper® and K3™ (P < 0.05), but no significant difference was observed between K3™ and Hero Shaper®.

Under the condition of this study, active file system (Hero Shaper®, K3™) with variable pitch and helical angle had more screw-in effect than passive file system (ProFile®) with constant pitch and helical angle. It seems that the radial lands play more important role in reducing screw-in effect.