## 4 Discussion  
In addition to the effective enlargement of a root canal by providing an opportunity for the removal of organic and inorganic debris, the accomplishment and preservation of an original continuous/tapered root canal anatomy from apical to coronal through adequate dentin thickness during the instrumentation of tortuous root canals of primary molars are also noteworthy [17]. However, a number of procedural errors, such as canal transportation and inappropriate dentin removal complications, can lead to subsequently decreased fracture resistance, endangering original canal curvature, increasing the risk of debris extrusion and postoperative pain [18; 19]. Current improvements in rotary Ni-Ti file systems create a paradigm shift in the root canal therapy of primary teeth. Therefore, it is necessary to perform a comprehensive evaluation regarding efficiencies of newly manufactured instruments for root canal shaping in terms of different parameters. Hence, the current study was conducted to compare and evaluate the abilities of relatively novel RaceEvo, R-Motion, ProTaper Gold systems in curved primary root canals with regard to the patterns of canal transportation and volumetric dentin removal by using CBCT. Numerous methodologies involving serial sectioning, microscopic analyses, muffle systems, radiographs, silicone impressions, endodontic tubes, multislice computed tomography, CBCT, etc., were described to assess different instrumentation parameters [20]. CBCT has become more popular and is a favorable alternative tool in the contemporary endodontic field, especially for the evaluation of three-dimensional (3D) root canal anatomy, determination of the amount of dentin for removal, measurements of canal transportation, centering ability, and conducting comparisons between before and after the instrumentation of root canals. Additionally, CBCT can be used to measure and calculate the volumetric units of enamel, dentin, and pulp tissues [20; 21]. In the current study, CBCT was preferred to determine the inclusive criteria and compare the parameters of overall canal transportation and volumetric dentin removal patterns in curved primary root canals prepared by manual and rotary instrumentation systems. The determination of canal transportation patterns was selected as a primary parameter to assess the shaping effectiveness of tested instruments in each experimental group. The canal transportation was evaluated overall for each prepared root canal length. For overall canal transportation analysis among all groups, the results showed that the newly generated RaceEvo and R-motion rotary systems clearly exhibited fewer canal transportation complications compared to the ProTaper Gold (PTG) rotary system, while preserving the original curved root canal shapes following instrumentation. The highest mean transportation values were observed in the PTG group. These important differences could be attributed to the manufactured techniques and characterizations of the tested instruments. The instruments of R-Motion rotary systems were designed with thinner core sizes, increased flexibility, rounded triangular cross-sections with sharp cutting edges and new, optimized file tips [22]. With that in mind, concerning these developed properties, the R-Motion rotary systems provide better respect to the curved primary root canal anatomies without creating significant canal transportation complications while preserving and reducing the stress on dental tissue compared to PTG systems. Moreover, the difference in mean transportation levels between R-Motion and PTG rotary systems could be elucidated by the lower screwing effect design of R-Motion rotary systems that allow the clinician to provide a higher control efficiency during the progression in root canals [22]. A similar efficiency in canal transportation with R-Motion was also observed in the RaceEvo rotary system compared to PTG and manual instrumentation.