## 4 Discussion
Further investigations are required to explore how rotational speed modifies the cutting behavior of NiTi rotary files. Canal volume after bushing was significantly increased with both rotary systems, confirming that mechanical preparation with heat-treated files is effective and safe [11]. No significant difference emerged between 2 Shape and Hyflex CM files for the percentage of untouched canal walls. Thus, the second null hypothesis cannot be rejected. No instrument was able to prepare the entire oval canal, as previously reported [21; 26]. Thus, to enhance the cleaning and disinfection phase, attention must be paid to optimize the efficient delivery of the irrigating solutions in the root canal space. Indeed, effective irrigant delivery and agitation are required to favor the canal disinfection and debris elimination from the most irregular anatomical configurations with a significant improvement of the endodontic treatment outcomes. To date, many systems for mechanical activation of irrigants are available such as special brushes, sonic and ultrasonic tools as well as strategies to increase the intracanal bacterial decontamination including ozone, direct laser action or light-activated disinfection [33]. Beyond the method used, what is important to emphasize in this context is that different irrigation systems can compensate for the limits associated with mechanical shaping, helping clinicians in everyday clinical practice. Because no previous study compared the cutting efficiency of 2 Shape and HyFlex CM with brushing motion, no direct comparison with our results is possible. Considering the files separately, two previous studies assessed the cutting behavior of Hyflex CM rotary files [34; 35]. Yet, the results obtained are not directly comparable with our findings because there are various methodological differences. In the study of [PERSON] et al. [34], the measuring system and general methodological approach applied to determine cutting efficiency ability are different. The cutting ability in that study was expressed by measuring the necessary load to guarantee a constant feed rate while simulated canals were prepared. In addition, the purpose of that study was to explore the effect of repeated simulated clinical use and sterilization on mechanical properties including cutting ability of Hyflex CM rotary instruments. Finally, cutting behavior of Hyflex CM was not compared with other NiTi files. One other study investigated the cutting properties of HyFlex CM [35]. Also in that case, the purpose and methodological approach were not comparable with our investigation. More specifically, the aim was to assess the effect of deep dry cryogenic treatment on the mechanical properties including cutting efficiency of martensitic SM NiTi files, including HyFlex CM. Moreover, cutting efficiency was quantified by measuring the loss of weight in plexiglass simulators. [PERSON] et al. [30] compared the shaping ability of two NiTi rotary files (i.e., 2 Shape and NeoNiTi) in severely curved canals determining also the volume of dentin removed by micro-CT. Even if the findings cannot be directly compared with ours due to the methodological differences, the authors confirmed that 2 Shape was relatively safe to shape root canals and preserve the original canal anatomy. The results are in agreement with other studies [15; 36; 37]. Although the reduced number of sample size could be considered a limitation, the results obtained are deemed reliable considering the evaluation method (i.e., micro-CT) and the similar sample size to some previous studies [6; 30]. Further studies on larger samples are desirable to confirm these results. Despite the standardized methodology, other limitations have to be considered regarding the NiTi instrument type, anatomical configuration of distal root of mandibular molars included and the protocol of brushing motion applied.