|Year : 2017 | Volume
| Issue : 3 | Page : 151-155
Influence of different movement kinematics on cyclic fatigue resistance of nickel-titanium instruments designed for retreatment
Taha Ozyurek, Gülşah Uslu, Koray Yılmaz
Department of Endodontics, Faculty of Dentistry, Ondokuz Mayis University, Samsun, Turkey
|Date of Web Publication||21-Aug-2017|
Department of Endodontics, Faculty of Dentistry, Ondokuz Mayis University, Samsun
Source of Support: None, Conflict of Interest: None
Aim: The objective of this study was to investigate the effect of different types of movement kinematics on the resistance of the ProTaper D3 retreatment file to cyclic fatigue.
Materials and Methods: Eighty ProTaper Universal D3 (20.07) files were utilized in this study. The cyclic fatigue tests were performed using a specially manufactured dynamic cyclic fatigue testing device, which has an artificial stainless steel canal having 60° of curvature angle and 5 mm of curvature radius. The files were randomly divided into four groups (Group 1: Rotary motion [RM]; Group 2: Adaptive motion [AM]; Group 3: Reciprocating motion [RCM]; Group 4: WaveOne motion [WM]). The movements of files until the files broke were recorded in “MOV” format using a device employing a slow-motion camera. The number of cycles to failure was calculated for each group. The data were analyzed statistically using one-way ANOVA and Tukey's honestly significant difference test (P < 0.05).
Results: The cyclic fatigue resistance of the AM group (3357.9 ± 88.5) was significantly higher than that of the other groups (P < 0.001). The lowest cyclic fatigue resistance was observed in the RM (1850.4 ± 93.0) group (P < 0.001).
Conclusion: Within the limitations of the present study, resistance of ProTaper Universal D3 file to the cyclic fatigue significantly increased in the AM group when compared to the RCM and WM groups.
Keywords: Cyclic fatigue, dynamic model, endodontics, nickel-titanium, ProTaper Universal retreatment
|How to cite this article:|
Ozyurek T, Uslu G, Yılmaz K. Influence of different movement kinematics on cyclic fatigue resistance of nickel-titanium instruments designed for retreatment. Saudi Endod J 2017;7:151-5
|How to cite this URL:|
Ozyurek T, Uslu G, Yılmaz K. Influence of different movement kinematics on cyclic fatigue resistance of nickel-titanium instruments designed for retreatment. Saudi Endod J [serial online] 2017 [cited 2018 Jun 20];7:151-5. Available from: http://www.saudiendodj.com/text.asp?2017/7/3/151/213479
| Introduction|| |
As well as, it is for the root canal treatment, the objective in retreatment is to remove the microbial load in the root canals or substantially reduce it. To effectively clean, shape, and obturate the root canals during the retreatment process, the filling material should be completely removed from the root canal system.
Hand files, nickel-titanium (NiTi) rotary systems, ultrasonic, lasers, and heat transfer systems  can be used while removing root canal filling material from the canals. NiTi rotary systems can maintain the original canal shape better than can other systems in root canal preparation and retreatment. Moreover, the NiTi retreatment systems are less time-consuming than other systems.,
Besides the advantages of NiTi rotary files, they have also an important disadvantage; breaking inside the root canal without any prior signal., It is difficult to prevent this undesirable situation. Torsional fatigue occurs when the tip or part of the instrument becomes stuck inside the canal and the shaft of the file continues to rotate. When the torque exceeds the elastic limit of the metal, the tip of the file fractures. When the file is used in curved canals, tensile, or compressive stress occurs along its shaft. In cyclic fatigue, the continuous repetition of these forces leads the file to break unexpectedly.
In 2008, Yared proposed a new preparation technique employing ProTaper F2 (Dentsply Maillefer, Ballaigues, Switzerland) files in a reciprocating movement. Reciproc (VDW, Munich, Germany) and WaveOne (Dentsply Maillefer, Ballaigues, Switzerland) single-file systems, both of which feature reciprocating movement, have been introduced to the market. Manufacturers claim that the RCM will reduce the stress on the file., The previous studies reported that, when compared to continuous rotation, reciprocating movement increased the cyclic fatigue resistance of NiTi files.,
Recently, a new endodontic motor (Elements Motor; SybronEndo, Orange, CA, USA) aiming to combine the advantages of continuous rotation and RCM has been introduced. The motor utilizes continuous rotation as long as the file is not subjected to any stress inside the canal (or is subjected to minimal stress). The movement stops momentarily on reaching a full clockwise (CW) rotation of 600°, and then it continues its CW rotation. If the instrument becomes stuck in dentine or root canal filling material, then the motor switches its movement to the reciprocating rotation due to the increased stress. The angles of the instrument are not fixed during the reciprocating movement, and the motor modifies the CW and counter CW (CCW) angles from 600/0° up to 370/50°, depending on the stress inside the canal. To the best of our knowledge, in literature, there is no study on the effects of different types of motion on the cyclic fatigue resistance of retreatment files.
The aim of the present study was to investigate the effect of different types of movement kinematics on the cyclic fatigue resistance of the ProTaper D3 retreatment file. The null hypothesis of the present study was that different types of movement would not have any effect on the cyclic fatigue resistance of the ProTaper D3 retreatment file.
| Materials and Methods|| |
Eighty ProTaper Universal D3 (20.07) files (lot no: 1106241) were employed in this study. Before the files were subjected to cyclic fatigue test under a dynamic model, all of the files were examined under a stereomicroscope (Olympus BX43, Olympus Co., Tokyo, Japan) with ×20 magnification to determine any potential deformation on the surfaces of the files.
The cyclic fatigue tests were performed using a specially manufactured dynamic cyclic fatigue testing device. The device has an artificially prepared stainless steel canal having 60° angle of curvature and 5 mm radius of curvature. The center of curvature of the canal is located at 5 mm coronal from the end of the canal, and the internal diameter of the canal is 1.5 mm. The files were randomly divided into four groups (n = 20) and subjected to the following procedures.
Group 1: Rotary motion
The files (n = 20) were used with a VDW Silver Reciproc Motor (VDW, Munich, Germany), which was connected to the dynamic cyclic fatigue-testing device, operating in accordance with the manufacturer's instructions at 500 rpm, and 300 g/cm torque until they broke.
Group 2: Adaptive motion
The files (n = 20) were used with an Elements Motor (SybronEndo), connected to the dynamic cyclic fatigue testing device, using the “TF Adaptive” program until they broke.
Group 3: Reciprocating motion
The files (n = 20) were used with a VDW Silver Reciproc Motor (VDW) connected to the dynamic cyclic fatigue testing device, using the “Reciproc ALL” program until they broke.
Group 4: WaveOne motion
The files (n = 20) were used with a VDW Silver Reciproc Motor (VDW) connected to the dynamic cyclic fatigue testing device, using the “WaveOne ALL” program until they broke.
While the cyclic fatigue test was performed in the adaptive motion (AM), RCM, and WaveOne motion (WM) groups, the movements of files until they broke were recorded in “MOV” format using a device employing a slow-motion camera (iPhone 6 Plus; Apple Inc., Cupertino, CA, USA) adapted for use with the cyclic fatigue testing device. Then, the records were transferred to a computer. The number of cycles to failure (NCF) was calculated in 60 s of slow-motion videos in the AM, RCM, and WM groups. Three different observers counted the NFC. In the rotary motion (RM) group, a value of 500 rpm was used for NCF calculation.
When using the cyclic fatigue testing device, the files were moved back and forth inside the canal using an axially motion at the speed of 3 mm/s to simulate clinical use. Synthetic oil (WD-40 Company; Milton Keynes, U.K.) was used to reduce the friction between files and walls of artificial canal and to enable free rotation. When the file in the cyclic fatigue device broke, then the device automatically stopped, and the time on the screen of device was recorded in seconds. The NCF was calculated for each file according to the following NCF formula: (NCF = rotation speed (rpm) × time (s)/60).
Eight broken files (two from each group) were examined through a scanning electron microscope (SEM) (JEOL, JSM-7001F, Tokyo, Japan), and photomicrographs of the fractured surfaces were taken under different magnifications to determine the fracture type.
The normality of the data distribution was first verified with the Anderson–Darling test, and then the Levene test was used for the homogeneity of variances. SPSS version 21.0 (IBM-SPSS Inc., Chicago, IL, USA) was used for the statistical analysis, and the results of the cyclic fatigue analysis were analyzed using a one-way ANOVA and Tukey's honestly significant difference test. The statistical significant level was set at P < 0.05.
| Results|| |
The rpm of each file group, according to the video recordings, are presented in [Table 1]. The mean and standard deviations of the cyclic fatigue resistance of each group are presented in [Table 2]. The cyclic fatigue resistance of AM group (3357.9 ± 88.5) was significantly higher than that of the other groups (P < 0.001). The lowest cyclic fatigue resistance was observed in WM (1850.4 ± 93.0) group (P < 0.001).
|Table 2. Number of cycles to failure (means and standard deviations) of 4 groups during static cyclic fatigue test|
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The SEM analysis of the fractured cross-sectional surfaces revealed typical features of cyclic failure including crack origins, fatigue zones, and an overload fast fracture zone [Figure 1].
|Figure 1: Scanning electron microscope appearances of the ProTaper Universal retreatment D3 files after cyclic fatigue testing. General view of D3 instrument with crack origins (a, c, and e; white arrows) high-magnification view D3 instrument showing fatigue striations typical of cyclic fatigue (b, d, and f; white arrows). (a and b: Rotary motion group; c and d: Adaptive motion group; e and f: Reciprocating motion group; g and h WaveOne motion group)|
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| Discussion|| |
Instrument fractures that occur during the root canal shaping and retreatment procedures are a source of major concern in the clinic.,, To prevent this complication in a clinical setting, manufacturers initially made design changes in features of the files such as the flute, tip configuration, and cross section. Then, they altered the composition of the files (metal alloys). Today, to address the problem, research is underway on the motion kinematics of files.
In 2008, Yared  suggested the RCM at different angles (144° CCW and 72° CW at 400 rpm) for root canal preparation. Then, VDW Company (Munich, Germany) introduced Reciproc, and Dentsply Company (Maillefer, Ballaigues, Switzerland) introduced WaveOne single-files. Then, Axis/Sybron (Orange, CA, USA) introduced the Elements Motor, which does not perform continuous reciprocation but switches the movement of the file to reciprocation when there is stress on the file. Different reports can be seen in the literature on the reciprocation angle and rpm of these systems since they introduced in the market. A previous study on the cyclic fatigue resistance of Reciproc and WaveOne files compared the cyclic fatigue resistance of the files based on the time to fracture because the authors stated that there was no information in the literature on the rotation angle and rpm of these systems. Kim et al. investigated the resistance of Reciproc and WaveOne files to cyclic fatigue, and researchers utilized the files together with their own software. According to the study of Kim et al., the rpm of the Reciproc and WaveOne systems was 300 and 350 rpm, respectively. Then, the authors calculated the NCF of WaveOne and Reciproc files using these values. Since then, the studies investigating the resistance of the Reciproc and WaveOne files to cyclic fatigue have cited the study by Kim et al. as a source of these systems' rpm values. Fidler  used a high-speed camera to determine the actual rpm of Reciproc and WaveOne files in the “Reciproc ALL” and “WaveOne ALL” programs. He found that the rpm of the Reciproc and WaveOne files was 282.92 and 343.36, respectively. Gambarini and Glassman  investigated the resistance of the Twisted File to cyclic fatigue under continuous rotation and AM, and they compared the time to fracture instead of the NCF. Higuera et al. compared the resistance of Reciproc, WaveOne, and TF Adaptive systems to cyclic fatigue and determined that the rpm of AM was 400 rpm. They then calculated the NCF of TF Adaptive files using this value.
During root canal preparation, NiTi files face with wide range of stresses inside the root canal because of various canal anatomies (different curves, etc.). To cope with these different stresses, the Elements Motor adjusts the number of rotations of the files, according to the rate of stress. As a result, the rpm values of the files change. In conclusion, it can be stated that the rpm values of the reciprocation NiTi file systems are subjected to numerous discussions. For these reasons, in the present study, the slow motion videos of the AM, RCM, and WM groups were captured using a camera capable of recording 240 frames/min, and they were recorded in “MOV” format. Then, the rpm values of files were determined by counting [Table 1].
In the static cyclic fatigue test model, the compressive and tensile stresses accumulate in one area because the file does not move axially (back and forth). These cumulative stresses induce microstructural changes in the file. Therefore, the cyclic fatigue tests in the present study were performed using a dynamic test model on a stainless steel canal having 60° curvature angle, 5 mm curvature radius, and 1.5 mm internal diameter. The curvature center was situated at 5 mm in the coronal end of the artificial canal.,,,,
Although several studies have investigated the cyclic fatigue resistance of NiTi files used for root canal preparation,, only a few studies have examined the cyclic fatigue resistance of NiTi files designed specifically for retreatment.,, In the present study, the D3 file of the ProTaper Universal Retreatment NiTi rotary system was tested. According to the results, the cyclic fatigue resistance from highest to lowest was as follows; AM group, RCM group, WM group, and RM group. There was a statistically significant difference between the cyclic fatigue resistances of all the groups (P < 0.05). Therefore, the null hypothesis of the present study was rejected.
No previous studies used a high-speed camera and counted the NCF to compare the cyclic fatigue of files. Thus, the results of this study cannot be directly compared to other studies. The previous studies have shown that, regardless of the type of file, the reciprocation movement increased the resistance of the files to cyclic fatigue when compared to continuous rotation movement.,,,, The present study results showed that the RCM significantly increase the fatigue life of the PTR files. Similar to the present study results, Gambarini et al. investigated the cyclic fatigue resistance of Twisted Files in RCM and full RM and the authors reported that the RCM significantly increased the cyclic fatigue life of the files. In addition, Karatas et al. investigated the cyclic fatigue resistance of OneShape and WaveOne files in different motion kinematics, and they reported that the RCM significantly increased the cyclic fatigue life of the files. We believe that the RCM decreases the stress rate that the file faces during preparation. As a result of decreased stress rate the cyclic fatigue life of file increases.
Higuera et al. compared the resistance of TF Adaptive, WaveOne, and Reciproc files to cyclic fatigue and stated that the rpm of the systems were 400, 350, and 300 rpm, respectively. In contrast, in the present study, the rpm of the “Reciproc ALL” and “WaveOne ALL” movements were 180 and 210 rpm, respectively. We believe that the difference in the results may be explained by the use of the high-speed video camera, which provided accurate recordings of the actual rpm values. Higuera et al. also reported that the rpm value of TF Adaptive movement was 400 rpm. In the present study, the rpm of the ProTaper Universal D3 file during AM was 430. We believe that the difference in the results is due to the variations in the tip diameters and tapers of the files used in these two studies. In addition, the discord between the studies may be explained by the difference in the amount of stress that the files were exposed to inside the artificial canal and the fact that the Elements Motor adjusted the ratio of the reciprocating angle and speed.
| Conclusion|| |
Within the limitations of the present study, we conclude that the resistance of the ProTaper Universal D3 file to cyclic fatigue increased significantly in the AM group (AM) compared to the RCM and WM groups.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2]