## 4 Discussion
The tested RaceEvo and R-Motion rotary groups in the current study comprised medical-grade Ni-Ti alloy with a triangular symmetrical cross-section with sharp edges. The active parts of both rotary groups underwent exclusive proprietary heat treatment which triggered the phase transition mentioned above (between martensite and austenite), just below body temperature, i.e., between 32 \({}^{\circ}\)C and 35 \({}^{\circ}\)C [22; 23]. Additionally, it was reported that the thermomechanical treatment of Ni-Ti alloy instruments does not allow the straightening of instruments in curved root canals during preparation and provides less root canal transportation with acceptable shaping abilities [32]. On the other hand, a patented heat-treatment mechanism called controlled memory wire (CM wire) is often used in PTG rotary systems instead of heat-treated Ni-Ti wire, distinct from other all tested rotary and hand instruments in the study. It was proven that instruments produced using CM wire exhibit higher flexibility and greater resistance during root canal preparation by reducing the risk of some complications, such as deviation, instrument fracture, and perforation [33; 34]. Despite these improved physical characteristics of PTG rotary systems with CM wire manufacturing when interpreting the tested instruments from the viewpoint of influence of shape and alloy features on canal transportation and volumetric dentin removal parameters, RaceEvo and R-Motion rotary groups exhibited superior abilities in terms of canal transportation and volumetric dentin removal parameters than the PTG group and manual instrumentation. These unexpected differences could be explained by the progressive taper (9% taper size--F3) and enhanced cutting efficiency with a convex triangular cross-sectional design [29]. Another important issue that should be discussed and compared is _\"the importance and effect of reciprocating motion\"_. The reciprocating motion was first introduced by [PERSON] et al. [35] and theorized as a balanced force during instrumentation on the basis of laws of action and reaction. It is important to emphasize that reciprocating motion minimizes the flexural stresses and provides an opportunity to increase the canal centering ability by reducing the taper lock effect of the instrument during canal preparation. From the viewpoint of pediatric endodontics, the preference of reciprocating motion instead of continuous rotation could reduce the stress and required time for preparation in curved root canals [36]. With these in mind, the obtained superior results of R-Motion regarding the patterns of canal transportation and volumetric dentin removal compared to PTG and manual instrumentation groups may be supported and clarified from a differential aspect. Additionally, the superiority of reciprocating motion in the present study was partially consistent with the results of [PERSON] et al. [37]. A major limitation of the current study was the Hounsfield units (HUs), which were used for thresholding, and are not as reliable as the CT units. Various issues related to limited-field CBCT geometry, basic principles of radiation physics, and the limitations of currently used reconstruction algorithms are associated with the use of HUs in CBCT. However, it is also known that CT units do not have isotropic voxels and small slice thicknesses such as in CBCT units (!1 mm), which are useful for examining small, narrow anatomical structures such as root canals. HUs are considered reliable at these scales. Occasionally, grayscale values (GVs) are used in CBCT instead of HUs, and some inaccuracies occur, such as [38; 39; 40; 41]:  
* The presence of beam hardening, cupping, and doming artifacts;
* Divergence of the X-ray beam;
* Axial slice variability due to different masses of each slice;
* High image noise (which has a minor effect on small-region examinations such as in the current study);
* Absence of GV calibrations in CBCT units by some manufacturers;
* Differences in GVs for the same matter in different CBCT models.