Purpose: The purpose of this study was to compare and evaluate canal transportation and centering ability of 2 single nickel-titanium rotary instruments, one manufactured by twisting (Twisted File) and the other by grinding (OneShape) using Cone- Beam Computed Tomography scanning (CBCTs).Materials and Methods: Forty mesiobuccal
canals of mandibular molars with an angle of curvature ranging from 25° to 35° and radius <15 mm were divided according to the instrument used in canal preparation into 2 groups (20 samples each): Group I: instrumented with Twisted File (TF) and Group II: instrumented with OneShape (OS). Each group was subdivided into subgroups (SG) A and B according to the technique of root canal preparation (single or modified). Canals were scanned using an i-CAT CBCT scanner before and after preparation to evaluate the change in the angle and radius of root canal curvature as well as
the amount of canal transportation and centering ratio at 1, 3, and 8 mm from the apex.
The significance level was set at P≤0.05. Results: At 1mm level, TF instruments used with modified technique produced a statistically significant less canal transportation compared to OS instruments (P ≤ 0.05). While at 8mm level, OS instruments used with single file technique produced a statistically significant less canal transportation and
more centered canal preparation compared to TF instruments (P ≤ 0.05). Conclusions: TF and OS single files systems used in continuous clockwise rotation are capable of preparing root canals with moderate and sever curvature, with minimal apical transportation. Cervical pre-flaring of the canal has an impact on canal shaping where it enhances the performance of Twisted File instruments. Twisted files with 0.08 tapers could be used for apical preparation without creating severe aberrations.