## 4 Discussion
Three-dimensional evaluation of cutting efficiency has been introduced for a more detailed understanding of cutting behavior. Micro-CT was reported to be the most accurate technique for detecting non-instrumented areas and examining cross-sectional images [21; 23; 24; 25]. Recently, sophisticated measurement software has been introduced that enables the evaluation of basic geometric parameters, such as the volume of dentin removed, ensuring reliable and reproducible measurements [25]. The cutting efficiency of 2 Shape and Hyflex CM was compared after applying 5 brushing movements in oval canals. [PERSON] et al. (2018) applied 3 brushing movements [26]. However, [PERSON] et al. (2015) found that the greater the number of strokes, the greater the cutting efficiency [14]. Yet, an increased number of strokes can weaken the root structure. Consequently, 5 strokes were considered as a suitable compromise in consideration of the danger zones [14]. The cutting efficiency is generally evaluated at the middle and apical thirds, with the coronal third prepared by a pre-flaring instrument [27]. In this study, no coronal pre-flaring was applied so the entire root canal area was assessed [14]. The mean thickness in the mesial portion of the distal root is smaller in comparison to all other portions of the roots [28]. Thus, the mesial wall was excluded from brushing motion to avoid further weakening a wall with reduced thickness. We have evaluated the cutting efficiency of two heat-treated files when used in combination or not with a supplementary brushing motion. The cutting efficiency of each file was expressed by the resultant modifications in the maximum BL and MD dimensions pre and post instrumentation. According to our results, the cutting efficiency with brushing motion was significantly greater with the 2 Shape in the middle third, in the mesiodistal direction. Also, considering the total space in the buccolingual direction (corresponding to direction of brushing action), 2 Shape was more efficient than Hyflex CM. Thus, the first null hypothesis can be rejected. In both directions, all groups reported similar cutting efficiency at apical levels, as previously reported [14]. The difference observed at midroot level could be linked with the helix angle of the file [14]. Moreover, it has been reported that 2 Shape is principally in the Austenitic phase at room temperature [15]. According to [PERSON] et al. [29] a more flexible instrument would require less pressure against root canal walls and this might justify the higher cutting efficiency observed for stiffer files as 2 Shape [9; 10; 15] in brushing motion [14]. In addition, 2 Shape has a triplehelix cross-sectional consisting of two main and one secondary cutting edge with the two cutting edges demonstrating an excellent cutting performance [30]. In the mesiodistal direction, 2 Shape exhibited a homogenous preparation in all root canal levels while Hyflex CM had significantly greater cutting efficiency at the coronal third. This result is consistent with the investigations of [PERSON] et al. (2013) and [PERSON] et al. (2014) where Hyflex CM proved to be more effective in the coronal section probably because of the alloy thermo-treatments and the design of its flutes [31; 32]. The influence of rotational speed on mechanical properties of NiTi files has been investigated, but no definitive consensus about its effect on cutting ability has been reached [31]. In the current study, both files were used according to manufacturer recommendations. More specifically, 2 Shape was activated at 300 rpm while HyFlex CM at 500 rpm. [PERSON] et al. [31] determined how the rotational speed and number of uses influenced the cutting ability of four nickel-titanium files for coronal flaring. They obtained that an increase in speed resulted in greater cutting efficiency. Even if that study is not comparable with our investigation because of the different methodological conditions, the operating speeds applied for the two tested files could have influenced the results obtained.