Aim: The aim of this study was to determine root surface strain (RSS) developed by two reciprocating NiTi files and the resultant cracks. Materials and Methods: Thirty mesial roots of mandibular molars were used. Ten roots were prepared using AF Blue R3 Reciprocating One-File System (FBR) (Fanta; ShangHai, China) for the mesiobuccal canal and WaveOne Gold primary (WOG) (Dentsply Tulsa Dental; Tulsa, OK, USA) for the mesiolingual canal. Ten roots were prepared using WOG and ten roots using FBR.

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... n electrical strain gauge was fixed on the root surface connected to a strain amplifier. Maximum strain and time were recorded. Roots were cut transversely to evaluate for cracks. Results: WOG has developed RSS of (84.16±52.57) and cracks (2/10); FBR has developed RSS of (128.33±128.55) and cracks (5/10) without statistically significant difference. Preparation time of WOG was (55.28±26.24) which was statistically significantly less than FBR (109±57.3). Conclusion: Both files have developed RSS and cracks during canal shaping.