Abstract Introduction : Root canal preparation is one of the most important stages of endodontic treatment. It seems that rotary NiTi files are efficient instruments in root canal preparation. Objective: This in vitro study was designed to assess the degree of canal transportation, resulting from instrumentation by a new rotary system (protaper) and to compare it with rotary profile 0.04 taper series 29 system and hand K- flexofiles in a Step- back manner. Materials and Methods: Thirty six mesiobuccal canals of extracted human mandibular molars were instrumented in three groups by each of these methods respectively. Rotary systems were used in a crown- down manner according to the manufacture’s instruction, and stainless steel K- flexofiles were used in a conventional step- back method. Pre- and postoperative X- rays were taken, projected and degree of canal curvature was determined according to the schneider method. The difference of pre- and postoperative canal curvature was evaluated with ANOVA and student tests. Results: The results of statistical analysis indicated that the degree of canal transportion in protaper group was statistically less than that in K- flexofile group that this difference was statistically significant(P<0.0001). There was no significant difference between two rotary groups(P<0.05). Conclusion: On basis of the results obtained from this study, profile 0.04 taper and protaper systems were significantly less likely to change the canal curvature in comparison to stainless K- flexofiles, lie, the former methods compared to the latter one have better capabilities in maintaining the original shape of root canal

Keywords: ٌRoot Canal Prepration, Root Canal Therapy

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