## 1 Introduction  
Cutting action is a fundamental characteristic of endodontic files, which permits the dentin to be efficiently removed and to obtain a funnel-shaped canal, facilitating the obturation procedure. In addition, an efficient cutting action is essential to minimize the file torsional stress and consequently the risk of breakage [1]. Because of the pseudoelastic characteristics, NiTi instruments have to be machined rather than twisted, which may cause surface defects within the cutting surfaces with a negative impact on their cutting efficiency [2].  
Despite the advanced technologies and improvements in nickel-titanium (NiTi) files, it is still challenging to mechanically prepare and disinfect the entire root canal system due to its anatomical complexities, especially in oval root canals [3]. Forty percent of oval root canal walls remain untouched after chemo-mechanical preparation, rendering the mechanical debridement of this oval anatomy difficult [4]. Anatomical complexities of oval configurations including buccal and lingual extensions are challenging to be instrumented and filled. It is questionable if the NiTi instruments are able to guarantee a complete and effective preparation of these areas. Indeed, it has been reported that the middle and coronal cross-sections of oval root canals exhibit zones not adequately prepared [5]. Thus, NiTi rotary instruments should be used in a \"brushing\" motion especially in oval canals with the aim to attain more canal walls to make the preparation more efficient [5]. Currently, as reported in previous studies, no technique ensures a full preparation of the walls of oval-shaped canals [6; 7].  
Within this context, heat-treated instruments have been introduced to improve shaping procedures with NiTi endodontic instruments [8]. The 2 Shape system (Micro Mega, Besancon, France) includes TS1 and TS2 files with a 25 tip, a 4% and 6% taper, respectively. The instruments are T-Wire heat-treated and possess a modified triple helix cross-section exhibiting an off-centered blade [9]. The HyFlex CM (Coltene, Cuyahoga Falls, OH, USA) files are made from a thermomechanical process of NiTi alloy with the property of \"Controlled Memory\". These instruments are in martensitic condition at body temperature and possess two types of cross-sections: a quadrangular for the 25.04 file and a triangular for the 25.06 [10]. Heat treatments are able to influence the all-mechanical properties of NiTi instruments such as flexibility and fatigue resistance. The above-mentioned treatments act in enhancing the mechanical behaviour of the instruments due to modifications in microstructure of NiTi alloy [1]. Even if the heat treatments and new metallurgy increase the flexural properties, it was suggested that heat treatments may impair the cutting efficiency [11].  
The cutting efficiency of 2 Shape and HyFlex CM rotary systems and the supplementary effect of a brushing motion have not yet been studied.  
Thus, the primary aim of this study was to assess the influence of brushing motion on cutting efficiency of 2 Shape and Hyflex CM in oval canals using micro computed tomography (micro-CT) imaging. The secondary aim was to compare the percentage of untouched canals generated by the two rotary systems. The first null hypothesis is that there would be no influence of brushing motion on the cutting efficiency of 2 Shape and Hyflex CM in oval canals. The second is that the percentage of untouched canals between the two rotary files would not be significantly different.