A pulpectomy is recommended for irreversibly affected inflamed or infected primary teeth to prevent the premature loss of teeth (Boonchoo et al., 2020). The literature describes various techniques for root canal preparation in the primary teeth, including manual instrumentation with stainless steel hand files, rotary instrumentation with nickel-titanium (NiTi) files, and ultrasonic instrumentation (Kaya et al., 2017; Chauhan et al., 2019). Although the use of manual files for root canal preparation of primary teeth is common, the difficulty in the cooperation of the pediatric patient and being time-consuming are its main disadvantages (Chauhan et al., 2019). Devices that reduce clinical time are of great value in pediatric dentistry, and many studies comparing rotary and manual instrumentation have reported canal preparation with rotary files to be noticeably quicker and easier (Chauhan et al., 2019). NiTi files used for root canal shaping can maintain root canal shape better and overcome the problems associated with the stainless-steel file, namely, procedural errors, ledges, zips, and perforations in the root canal system (Ferreira et al., 2017). NiTi files can be basically divided into two groups according to their movement types as rotary or reciprocating files. Rotary files require more than one file to complete the root canal preparation compared to reciprocating files (Musale et al., 2019; Boonchoo et al., 2020). Precisely for this reason, the control of the operative torque represents an improvement of the technique, reducing the number of instruments useful for completing the endodontic treatment, saving money and time, and selecting those that best adapt to the specific endodontic treatment (Gambarini et al., 2020). With the asymmetric reciprocating motion, the file completes a full cycle, alternating counterclockwise and clockwise to varying degrees, and allowing the root canal preparation to complete with only one file (Yared, 2008).

In the light of the latest developments in technology, new NiTi files produced with different technologies have been introduced to the market that can also be used in primary teeth. AF Baby File (ABF; Fanta Dental Materials Co., Shanghai, China) is a rotary NiTi file system that is specially produced for children. The system consists of 4 files, the orifice opener file Open File (17/.08) 11 mm in length, and shaping files of 20/.04, 25/.04, and 30/.04 16 mm in length. It is produced from H-Wire alloy belonging to ABF NiTi and the cross-sections of the files are triangular. The files are recommended to be used in the order of 20/.04, 30/.04 in wide canals, and in the order of 17/.08, 20/.04, 25/.04 in narrow canals (AF Baby File Brochure, 2023).

T-endo MUST (TEM; Dentac, İstanbul, Türkiye) is a reciprocating single-file NiTi file system. The system consists of 4 files: glide path file TG (13/.04), shaping files M25 (25/.06), M40 (40/.04), and M50 (50/.04). TEM is produced with a proprietary heat treatment called TM-Wire. The glide path file (TG) in the system has a square cross-section and the shaping files have an “S” shaped cross-section. These file systems ensure the practitioners to use multi use single technique (MUST) with high flexibility and allow for an immediate usage from a sterile package. The files in the system are available in the market as 21, 25, and 31 mm (T-endo MUST Brochure, 2023).

There is no study that examines the effect of the different experience levels on the cyclic fatigue resistance of NiTi rotary file systems in pediatric dentistry. Therefore, the aim of this in vitro study was to examine the effect of the operator experience on cyclic fatigue resistance of reciprocating and continuous rotating NiTi files. The null hypothesis of this study was that there would be no significance difference in cyclic fatigue resistance between the NiTi files according to the experience level.

The mean and standard deviation of NCF values of the tested files according to the experience groups are shown in Table 1. The mean and standard deviation of total instrumentation times of the tested files according to the experience groups are shown in Table 2. The NCF values of TEM files were found to be significantly higher than ABF files in all groups (p < 0.05) For both file systems, all experimental groups showed lower NCF values than the control groups (p < 0.05). However, there was no significant difference observed in the NCF values among the experimental groups for the TEM and ABF files (p > 0.05).

Regarding the instrumentation time, TEM files needed significantly lower time for the preparation of root canals compared to the ABF files in all the experimental groups. However, the instrumentation time did not differ according to the experience level for both the file systems (p > 0.05). Additionally, considering the experience of operators and different file systems, fracture lengths did not differ among any of the groups (p > 0.05).

In the images taken at various magnifications (x300, x350, x4000) of the SEM analysis from each group, the rough surface, micropores, and fatigue striations specific to cyclic fatigue were determined (Figure 4).

The current study has analyzed the effects of the experience level (inexperienced, less experienced, and experienced) of the pediatric dentists on the cyclic fatigue resistance and instrumentation time of the reciprocating and continuously rotating files. According to the evaluation, similar fatigue resistance and similar instrumentation time were recorded for each file system for each operator with different levels of experience. To the best of our knowledge, no study has made the comparison, thus, it is difficult to assess the findings of the current study considering the previous reports.

The fracture incidence of reciprocating NiTi files (WaveOne, Dentsply Sirona) clinically by endodontic residents was evaluated in a study by Shen et al. (2016). They reported that file fracture occurrence is quite low (about 1.17%) after a single use. Studies on clinical use show that the incidence of file fracture among undergraduate and graduate students is similar between endodontists. Coelho et al. (2018), reported similar results may be due to case selection (usually the root canal treatments of uncomplicated cases) and intensive preclinical training. However, investigators drew attention to the decrease in the incidence of file fractures. New heat treatment technologies and new types of motion may also have contributed to these results. In the present study, new generation heat-treated file systems were preferred, and files more resistant to cyclic fatigue were used. These factors may have minimized the effect of experience differences between the groups.

Cyclic fatigue resistance of endodontic files has been investigated in various static and dynamic models in the literature. In static test models, the canal file is bent at a fixed working length, and in tests with dynamic models, the canal file is rotated with forward-backward (axial) movements until it fractures (Dederich and Zakariasen, 1986). Additionally, preparing the artificial canals in accordance with the files to be tested is important for the correct and reliable performance of the cyclic fatigue test. Although the use of root canals of extracted human teeth can better reflect clinical conditions, they do not constitute an ideal model because it is not possible to standardize human tooth root canals (Plotino et al., 2009). Since the present study evaluated the effect of the experience levels of pediatric dentists, it was carried out in non-clinical conditions, because the target group is children, in which standardization will be most difficult due to problems such as compliance with treatment and tooth morphology due to physiological resorption. So, the instrumentations with files were performed in the standardized artificial resin teeth, and for the fatigue test artificial stainless-steel canals, which are 1 mm larger than the files, were used to minimize the effects of the parameters that disrupt standardization. Also, a dynamic model including axial movement was used to provide better results in terms of mimicking a more satisfactory similarity of clinical conditions.

DeVasconcelos et al. (2016), compared the cyclic fatigue resistance of different file systems at room temperature (20°C) and body temperature (37°C), and reported that all experimental groups showed a significant decrease in fatigue resistance at 37°C. Thus, in the present study, the test setup was created at 37°C in order to better imitate the intracanal environment, to imitate the liquid environment existing in root canals, and to minimize the friction between the metal artificial canal and the tested NiTi files. It was placed in an oven with water and the tests were carried out in this way. Also, the heat treatment of the NiTi files can affect the cyclic fatigue resistance of the files (Hou et al., 2020; Seracchiani et al., 2021). According to the studies, the austenitic files have less resistance to cyclic fatigue compared to martensitic files (Kwak et al., 2021; Kwak et al., 2022). In the present study different heat-treated NiTi files have been compared. One of the reasons that the significance different found between the groups in the present study.

According to the results of this study, TEM reciprocal NiTi files showed significantly higher cyclic fatigue resistance than ABF rotary NiTi files in all different experienced groups and control groups (p < 0.05). Thus, the null hypothesis of our study was rejected. These results show similarity to the results of studies in the literature reporting that reciprocating NiTi files have higher cyclic fatigue than rotary NiTi files (Özyürek et al., 2017; Özyürek et al., 2018b; Miccoli et al., 2020). Compared to the rotary movement, the reciprocating motion prevents the stress that occurs while the file is rotating in the canal from accumulating continuously in the same region, and distributes it along the shaft of the file, and in this case, the file becomes more resistant to fracture (Bonaccorso et al., 2009; Özyürek et al., 2017). In the present study, the possible reason for the higher resistance of TEM files to cyclic fatigue is the feature of the reciprocating motion, and thus, the dispersion of the stress on the file. Additionally, in the present study, the total instrumentation time for the preparation of artificial resin canals with the TEM reciprocal NiTi files showed significantly lower values than ABF rotary NiTi files. There is no study comparing the reciprocal and rotary NiTi files according to the instrumentation time in the literature. However, in our opinion, the difference in the instrumentation time is probably because the ABF system needs two files and TEM needs one file to prepare the canals.

Within the limitations of this study, the movement characteristics of the files in the root canals affect their resistance to cyclic fatigue. Since the user experience does not affect the fatigue resistance of NiTi files, inexperienced clinicians can use these files in primary and permanent teeth without the risk of fracture. On the other hand, it is recommended to be careful when interpreting the results obtained for clinical use, since there are some limitations such as different hardness values between resin and dentin, and because of the elimination of the difficulties that may arise from anatomical and clinical variations to ensure standardization in in vitro conditions.