## 1 Introduction  
Today, endodontic approaches (pullectomy procedure) could be considered as the most appropriate solution to provide maintenance of primary molars diagnosed with irreversible pulpitis or pulpal necrosis in the oral cavity until their required physiological shedding time [1, 2]. In correlation with permanent dentition, the clinical success rate of root canal treatment in primary dentition is mainly related to perfect biomechanical preparation, the removal of microorganisms, and infected pulpal remnants from the root canal system [3]. However, complexities or variations in root canal anatomy and the high root resorption probability of primary teeth jeopardize and complicate root canal therapy (RCT) [4]. The characteristics of primary molars with curved and shorter roots, ribbon-shaped morphology, and ectopic surface resorption, especially, must be taken into consideration during RCT [5]. Along with these challenging characteristics, the selection of a suitable instrument that can preserve the original shape of the root canal without creating any variations and providing regular dentin removal should also be kept in mind during biomechanical preparation. Generally, manual instrumentation is still preferred as the main option for the preparation of the primary root canals. However, various clinical complications during manual instrumentation such as perforations, ledge formation, canal transportation, dentinal cracks, dentin compaction, and broken files were reported [6; 7]. Firstly, the rotary file systems for endodontic treatment approaches were introduced in primary dentition by [PERSON] and colleagues in 2000 to overcome these complex anatomical morphology and clinical errors [8]. The capability of rotary systems that for slight debris extrusion through the apical foramen to the surrounding periapical tissues is considered one of the main advantages. Furthermore, significantly minimized chair time for pediatric dental patients, no excessive dental tissue removal, and no canal transportation in curved canals were reported as the other major advantages of rotary systems [9; 10]. Over the last few decades, numerous new generations of rotary files systems were developed. Specifically, the clinical efficiencies (less canal transportation, successful in preserving root dentinal walls, retention of original root canal shape) of ProTaper Gold, ProTaper Next, Wave One, and Kedo-S files in primary and permanent dentition were reported by many authors [11; 12; 13; 14]. Recently, the ProTaper Gold (PTG) rotary file system, which works with the principle of a rotating mechanism, was produced. PTG has a progressive taper that results in a more effective cutting ability and improved safety on dentinal tissue [15]. On the other hand, reciprocating systems were developed as an alternative to the continuous rotation for the preparation of curved canals by using a single file. In this technique, the safety of root canals is achieved by decreasing the possibility of fractures on root canal walls and removing less dentinal tissue [6]. Most recently, RaceEvo and R-motion rotary systems were developed with greater flexibility, cutting efficiency and improved cross-sectional design that are significant factors in the canal transportation of curved root canals. However, to the best of our knowledge, there are no published data available concerning the three-dimensional (3D, e.g., volumetric) and two-dimensional (2D, e.g., canal transportation) efficiencies of these newly designed rotary systems on primary curved root canals. Thus, from a clinically significant point of view, it is imperative to assess the relevant parameters, shaping quality regarding canal transportation, canal straightening, and unnecessary hard dental tissue removal conditions during the preparation of curved primary root canals to perform successful long-term clinical trials with minimally invasive dentistry.