Background:The smear layer is an amorphous film, i.e., always formed by the action of endodontic instruments inside the root canal during chemomechanical preparation. Different solutions have been used and tested for its removal. For the removal of smear layer, demineralization, and softening of root dentin, chelating agents have been preferred.

Aim:The aim of this study was to compare canal transportation and centering ability of Twisted files (TF) and ProTaper (PT) files with conventional stainless steel (SS) K files. Materials and methods:Thirty mandibular premolar teeth with root curvature of 25 to 35° were selected. The samples were randomly divided into three groups of 10 each according to the type of files used for instrumentation. Group I was instrumented using TF, group II with PT files according to the manufacturer's recommendations, and group III with samples prepared with precurved SS K files using a step-back technique. After preparation, the samples were assessed using spiral computed tomography. Three sections were recorded at coronal, middle, and apical levels. Amount of transportation was assessed comparing with preinstrumentation images. Centering ability was calculated using Gambill's formula. The three groups were statistically compared using Student's t-test.Results: Twisted files showed the least canal transportation and the best centering ability followed by PT file system. Stainless steel K files showed the highest transportation and least centering ability. Conclusion:Twisted files showed the least amount of transportation and better centering compared with the other file systems.Clinical significance: The study gives information of TF, a recently introduced rotary system (2008) with regard to its efficacy in preparing curved root canals. This study has shown that the centering ratio of rotary nickel titanium was better than that of SS files.