Nickel titanium (NiTi) rotary file systems offer easier and faster root canal shaping procedures than manual hand files.1 NiTi files are designed for shaping the curved canals without damaging the anatomical structure; for this purpose, their elasticity was improved.234 But, even these features cannot prevent the undesired fractures of files.56 Among many factors playing roles in NiTi files' fracture, two of the most important factors are the cyclic and torsional fatigues.7 The fractures originating from cyclic fatigue are observed in files' continuous rotation within the curved root canals; the files are exposed to elastic deformation due to the changing compression and stress forces in these media.8 Because the files may fail due to the cyclic fatigue without any prior sign, the cyclic fatigue resistance is measured by the time to failure under a specific load.9

The variables such as NiTi files' speed, movement kinematic, metallurgic properties, and finishing procedures play roles in cyclic fatigue resistance.7 When compared to the systems performing continuous rotation, the reciprocation systems, which have been developed by purposefully changing the working principles of traditional NiTi rotary files, have been designed in order to decrease the stress on the file and to prevent the failures originating from the cyclic fatigue.10 The reciprocation movement is called oscillation movement, and the files performing reciprocation movement decrease the stress on file by rotating in counterclockwise (CCW) direction before completing the cycle in clockwise direction; they have been reported to improve the files' fatigue life.71112

The commercially available Reciproc (RPC, VDW, Munich, Germany) and WaveOne GOLD (WOG, Dentsply Maillefer, Ballaigues, Switzerland) single file systems work based on the reciprocation movement at different speeds and angles. Reciproc works at 300 rpm speed in 150° CCW and 30° CW direction, while WaveOne GOLD operates at 350 rpm speed in 170° CCW and 50° CW direction and completes 360° in 3 cycles.13 Moreover, Reciproc file is made of M-Wire alloy and employs S-shaped cross-section having 2 cutting edges, while WaveOne GOLD has parallelogram cross-section with 2 cutting edges at apical end and is made of GOLD alloy.

Newly-introduced Tango-Endo (TE, EDS Dental, S. Hackensack, NJ, USA) rotary file system consisting of 2 files (sizes 30/0.02 and 30/0.04) made of conventional NiTi alloy has its unique reciprocation movement using a handpiece, which is specific to this system. Different from the other file systems employing the reciprocation movement, it does not finalize a complete cycle. The manufacturer claims that the cutting efficiency of this file has been improved under favor of 2-patented vertical blades and it is allowed to move until the apex with minimum resistance. The company also states that the file's cyclic fatigue resistance was also improved in this design.14

During our comprehensive literature research, no study on the TE files' cyclic fatigue life was found. The aim of this study was to compare the fatigue life of RPC, WOG, and TE files, which are among the file systems working based on the reciprocation movement, under a static model via artificial canals with different angles of curvature (45°, 60°, and 90°). The null hypothesis of this study was that there would be no difference between the fatigue life of RPC, WOG, and TE files.

RPC R25 (size 25/0.08), WOG Primary (size 25/0.07), and TE (size 30/0.04) instruments were included in this study (n = 20). All the instruments were inspected for any defect or irregularity using stereomicroscope (Olympus BX43, Olympus Co., Tokyo, Japan) under ×20 magnification. Since there were no defective instruments, none of the instruments were discarded.

Artificial canals, which were made of stainless steel with an inner diameter of 1.5 mm, 45°, 60°, and 90° angles of curvature and a radius of curvature of 5 mm, were used for static cyclic fatigue testing. The curvature of the artificial canals was located at the 5 mm coronal of the canal end. To reduce the friction of the files as it contacted the artificial canal walls, synthetic oil (WD-40 Company, Milton Keynes, UK) was used for lubrication.

The files were divided into 3 experimental groups (n = 20) and underwent the following procedures:

Group 1: Reciproc R25

The files in this group were used with VDW Silver Reciproc motor (VDW) connected to cyclic fatigue testing device and used with ‘RECIPROC ALL’ program until they broke.

Group 2: WaveOne GOLD

The files in this group were used with VDW Silver Reciproc motor connected to cyclic fatigue testing device and used with ‘WAVEONE ALL’ program until they broke.

Group 3: Tango-Endo

The files in this group were used with own handpiece connected to cyclic fatigue testing device and used with its 2,500 rpm until they broke.

All the instruments were rotated until fracture occurred, and the time to fracture was recorded in seconds using a digital chronometer.

A total of 6 fractured files, 2 from each group, were examined with SEM device (JSM-7001F, JEOL, Tokyo, Japan) to determine fracture types of the files and photomicrographs were taken from the fractured surfaces.

The ‘time to failure’ values of RPC R25, WOG Primary, and TE files under static model and at 3 different angles of curvature are presented in Table 1. TE files were found to have significantly higher values than WOG and RPC files in terms of fatigue life (p < 0.05). However there was no statistically significant difference between fatigue life of RPC and WOG files. It was determined that increasing the angle of curvature of the stainless steel canals caused significant decreases in fatigue life of all of three files (p < 0.05).

The scanning electron microscopic images of the fracture surface revealed the feature of the mechanical characteristic of the cyclic fatigue failure in all the groups. The fracture surfaces demonstrated one or more crack initiation area(s), fatigue striation(s), and a fast fracture zone (Figure 1).

The objective in endodontic treatment is to efficiently shape and clean the root canal system and to obturate it hermetically.3 For this purpose, besides various shaping techniques, numerous files having different designs have been developed.15 The file systems, which were developed in recent years and operated in reciprocation movement, aim to decrease the number of file to be used in canal preparation.10 While shaping with single or limited number of files, the files are exposed to high level of stress thus cyclic and torsional fatigues can occur.16 These cyclic and torsional fatigues are among the most important factors playing roles in file failures. For this reason, in the present study, it was aimed to compare the cyclic fatigue life of three different, commercially available file systems operating in reciprocation movement.

One of the most important disadvantages of laboratory studies on comparing the cyclic/torsional fatigue resistance of files is the use of different factors (material characteristics and designs) that might have effect on the results.17 In in vivo studies on the cyclic fatigue resistance of files, it is difficult to obtain accurate results because the torsional fatigue cannot be eliminated.18 In their study on cyclic fatigue, Yao et al.19 have reported that the disadvantage of using the extracted teeth for imitating the clinical conditions is the difficulty of ensuring the standardization. Standard artificial canals in cyclic fatigue studies are useful from the aspect of minimizing the other factors that may affect the results.13 For this reason, standard artificial canals made of stainless steel were employed in the present study. The SEM images were captured from the fractured surfaces of the tested files to determine fracture types of the files. All the images confirmed that the tested files fractured due to cyclic fatigue. The TE file does not complete a cycle. The file performs an oscillating movement. Thus the number of cycles to failure (NCF) values could not be calculated for TE files and the cyclic fatigue life of the tested files was not compared.

According to the results of the present study, TE file was found to have statistically longer fatigue life in all 3 angles of curvature, when compared to RPC and WOG files. For this reason, the null hypothesis of the present study was rejected. But, on the other hand, no significant difference was found between WOG and RPC files in all of canals having 3 different angles of curvature.

There was no study that compares the cyclic fatigue life of TE in the literature. For this reason, the results of the present study cannot be directly compared to those of other studies. It has been reported that the reciprocation movement performed using Giromatic handpiece (Micro Mega, Besancon, France), which has a movement kinematic that is similar to TE files, in 90° CW and 90° CCW direction and the reciprocation movement using M4 handpiece (SybronEndo, Glendora, CA, USA) in 30° CW and 30° CCW decreased the stress that the files were exposed to in canals and increased their fatigue life.2021 From this aspect, we believe that the similarity of low-angle reciprocation movement via a specific handpiece might be the reason for longer fatigue life of TE files than those of RCP and WOG files. Furthermore, 0.30 mm apical diameter of TE files utilized in the present study differs from other files. In literature, the files having similar apical diameters have been compared before. But the aim of the present study in comparing the files having different apical diameters was to investigate the contribution of newly introduced file systems to the endodontic practice.

WOG has been introduced by the manufacturer company as an update to WaveOne (WO, Dentsply Maillefer) file. The reciprocation movement of the file was maintained, while the cross section, dimensions, and geometry were modified. The cross section of the file was changed to the parallelogram design containing 2 cutting edges. The files are manufactured with the heat-treatment procedure for gold alloy. On the contrary to M-Wire technology employing pre-production heat treatment, the heat-treatment for gold alloy utilizes the method of heating and then slowly cooling the file following the production phase. The manufacturer company claims that this new heat-treatment method improves the elasticity and cyclic fatigue resistance of this file.22

There is limited number of studies investigating the cyclic fatigue resistance of WOG files in the literature.2324 According to the results of the present study, the fatigue life of WOG was found to be shorter than TE, but no statistically significant difference was found when compared to RPC files. Similar to present study results, Özyürek24 has reported the cyclic fatigue resistance of WOG files to be higher than RPC files. However, Topcuoglu et al.23 have carried out a study on comparing the number of cycles to the failure of WOG, WO, and RPC files, and reported that the cyclic fatigue resistance of WOG files was higher than other files. We believe that the differences between these studies originate from comparing the NCF of files.

In the literature, there are many studies comparing the cyclic fatigue resistances of WO files, which have been used before WOG files, and RPC files at different angles of curvature.13252627 In their study, Scelza et al.25 have compared the cyclic fatigue resistance of RPC and WO files, and found that of RPC to be higher. Similarly, in their study on canals having 60° angle of curvature, Arias et al.28 have compared RPC files to WO files in terms of cyclic fatigue resistance, and found the cyclic fatigue resistance of RPC files to be higher. Kim et al.13 have repeated the same study at 45°, and obtained the same results. In the literature, there are also studies where the same results have been obtained.132629 On the contrary with these results, Pedullà et al.27 have determined no difference between RPC and WO files. In the studies, the reason for higher cyclic fatigue resistance of RPC files than WO has been shown to be the differences between the cross section and core structures of files.13252829 On the contrary with WO, we believe that WOG files had similar fatigue life values with RPC because of the new alloy and production method of WOG.

Within the limitations of the present study, the cyclic fatigue life of TE files in canals having different angles of curvature was determined to be statistically higher than Reciproc and WaveOne GOLD files. No statistical difference was found between RPC and WOG files.