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ORIGINAL ARTICLE
Year : 2017  |  Volume : 7  |  Issue : 2  |  Page : 82-86

Effect of adaptive motion on cyclic fatigue resistance of R-Endo nickel-titanium file


Department of Endodontics, Faculty of Dentistry, Ondokuz Mayis University, Samsun, Turkey

Date of Web Publication25-Apr-2017

Correspondence Address:
Taha Ozyurek
Department of Endodontics, Faculty of Dentistry, Ondokuz Mayis University, Samsun
Turkey
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/1658-5984.205123

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  Abstract 

Aim: The aim of this study is to evaluate R-Endo R3 file's resistance to cyclic fatigue in comparison with continuous rotation movement and "Adaptive Motion" (AM) movement under dynamic model.
Materials and Methods: Forty pieces of R-Endo R3 (25.04) files (lot no: 062011) were included in the study. Cyclic fatigue tests were performed with specifically manufactured dynamic cyclic testing device. The device has an artificially prepared canal with 60° curvature angle and 5 mm curvature radius. The canal inner diameter is 1.5 mm and the center of curvature is located 5 mm coronally from the apex. The files were randomly divided into two groups (n = 20) and the following procedures were performed – Group 1: Rotary motion (RM), Group 2: AM. Time to fracture was recorded and the number of cycles to failure for each instrument was obtained. Statistical analyses were performed using Student's t-test. The statistical significant level was set at P < 0.05.
Results: AM group had a significantly higher cyclic fatigue resistance compared with RM group (P < 0.001).
Conclusion: Within the limitation of the present study, it was found that "Adaptive Motion" statistically increases the resistance to cyclic fatigue of R-Endo R3 files, manufactured for retreatment.

Keywords: Adaptive motion, dynamic cyclic fatigue test, endodontics, nickel-titanium, R-Endo


How to cite this article:
Ozyurek T, Yilmaz K, Uslu G. Effect of adaptive motion on cyclic fatigue resistance of R-Endo nickel-titanium file. Saudi Endod J 2017;7:82-6

How to cite this URL:
Ozyurek T, Yilmaz K, Uslu G. Effect of adaptive motion on cyclic fatigue resistance of R-Endo nickel-titanium file. Saudi Endod J [serial online] 2017 [cited 2017 Aug 17];7:82-6. Available from: http://www.saudiendodj.com/text.asp?2017/7/2/82/205123


  Introduction Top


Root canal retreatment is a difficult and time-consuming process including steps such as complete removal of gutta-percha from the canal, reshaping of the canal, its disinfection, and performing a new hermetic root canal filling.[1] During the root canal retreatment, stainless steel files, nickel-titanium (NiTi) rotary instruments, ultrasonic tips, and laser systems are used.[2]

The use of NiTi rotary instrument systems in removal of root canal filling has become popular in endodontic practice recently. NiTi rotary systems in different tip diameters and in various helix angles have been developed.[3] NiTi instruments have a more flexible structure than stainless steel files, and although they undergo deformation inside the canal, they can easily return to their original form when they are retracted from the canal.[4]

One of the most common problems encountered during removal of gutta-percha from the root canal and reshaping of the canal is fractures occurring in NiTi instruments.[5] Fractures in NiTi rotary instruments are seen depending on torsional or cyclic fatigue.[6] While fractures due to torsional fatigue are seen as a result of a canal instruments being stuck in the canal as it continues to rotate, fractures due to cyclic fatigue occur as a result of repeated compression and tension forces that a file suffers in curved root canals.[7],[8]

Today, various endodontic motors have been developed for the use of NiTi rotary instruments. Elements Motor (SybronEndo, Orange, CA, USA ABD), which was released lately, has been developed in a way to combine advantages of continuous rotation and reciprocation movements to provide a better fit against accumulated stress on the file during canal preparation.[9] In this system, which was called "Adaptive Motion" (AM) by the manufacturing firm, if there is no stress or only minimum stress on the canal instrument, the file motion is 600° continuous rotation clockwise and the instrument stops after completing this cycle and then continues to rotate again. In the case, when the canal file is stuck in the dentin wall or the root canal filling material, the movement transforms into reciprocation depending on the increased stress. The motor converts its motion to reciprocation movement up to 370° clockwise and 50° counterclockwise according to the amount of stress on the file.

R-Endo (MicroMega, Besancon, France) retreatment system, which is used in the procedure of root canal retreatment, consists of three files such as R1, R2, and R3. This system was designed by the manufacturing firm to operate constantly with rotation movement, and it is said to have been made safer with its noncutting tip design. All three files have been designed to be used for and fit different parts of a canal. The R1 file is used in the coronal 1/3 of the canal, R2 is used in the middle 1/3 of the canal, and R3 is used in the apical 1/3 of the canal.

The aim of the present study was to evaluate R-Endo R3 file's resistance to cyclic fatigue in comparison with continuous rotation movement and "Adaptive Motion" movement under dynamic model. The null hypothesis was that R-Endo R3 file used with these two different movement systems would not have any difference in terms of its resistance to cyclic fatigue.


  Materials and Methods Top


Forty pieces of R-Endo R3 (25.04) files (lot no: 062011) were included in the study. Before putting to cyclic fatigue test under dynamic model, they were checked under stereomicroscope (Olympus BX43, Olympus Co., Tokyo, Japan) with ×20 magnification to determine whether or not any deformation exists on their surfaces.

Cyclic fatigue tests were performed with specifically manufactured dynamic cyclic testing device. The device has an artificially prepared canal with 60° curvature angle and 5 mm curvature radius. The canal inner diameter is 1.5 mm and the center of curvature is located 5 mm coronally from the apex [Figure 1]. The files were randomly divided into two groups (n = 20) and the following procedures were performed.
Figure 1: Schematic drawing of dynamic cyclic fatigue device. A: Processor of the device, B: Body of the device, C: Moving parts of the device, D: Handpiece holder, E: Handpiece, F: Stainless steel artificial canal

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Group 1: Rotary motion

The files in this group were used with VDW Silver Motor (VDW Munich, Germany) connected to dynamic cyclic fatigue testing device according to the manufacturer's recommendations at 350 rpm and 80 g/cm torque values until they fracture.

Group 2: Adaptive motion

The files in this group were used with "TF Adaptive" program using Elements Motor (Axis/SybronEndo, Orange, CA, USA) connected to dynamic cyclic fatigue testing device until they fracture.

Back and forth movement of the file in axial direction inside the canal was set to 3 mm/s to stimulate clinical usage. To reduce the effect of friction of the canal files on the artificial canal walls and facilitate their motion, synthetic oil (WD-40 Company; Milton Keynes, England) was used as lubricant. When the files broke under cyclic fatigue mechanism, the device automatically stopped and the time on the device screen in seconds was recorded. The number of cycles to fracture (NCF) for each file was calculated with formula (NCF = Rotation speed (rpm) × Time [s]/60). In NCF calculation of the AM group, a value of 400 rpm was used as number of cycles in 1 min.[10]

A total of four pieces of fractured files, two pieces from each group, were examined with scanning electron microscopic (SEM) device (JEOL, JSM-7001F, Tokyo, Japan) to determine fracture types of the files, and photomicrographs were taken from the fractured surfaces under different magnifications.

Statistical analysis

Data were first verified with Anderson–Darling test for normality of the data distribution and the Levene test for the homogeneity of variances. Than Student's t-test was performed to statistically analyze the cyclic fatigue data via software (SPSS 21.0; IBM-SPSS Inc., Chicago, IL, USA). The statistical significant level was set at P < 0.05.


  Results Top


The mean and standard deviations of the cyclic fatigue resistance for each group are shown in [Table 1]. AM had a significantly higher cyclic fatigue resistance compared with rotary motion (RM) (P < 0.001).
Table 1: Number of cycles to failure (means and standard deviations) of two groups during dynamic cyclic fatigue test

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SEM analysis of the fractured cross-sectional surfaces revealed typical features of cyclic failure including crack origins, fatigue zone, and an overload fast fracture zone [Figure 2].
Figure 2: Scanning electron microscopic appearances of the R-Endo retreatment R3 files after cyclic fatigue testing. General view of R3 instrument with crack origins (a and c; white arrows) high-magnification view R3 instrument showing fatigue striations typical of cyclic fatigue (b and d; white arrows) (a and b: Rotary motion group; c and d: Adaptive motion group)

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  Discussion Top


Although NiTi files' efficacy in removal of root canal filling that are commonly used in retreatment procedures were evaluated in many studies,[11],[12],[13] studies investigating mechanic properties of retreatment files are limited.[14] The files that are used in the retreatment procedures must be not only rigid to remove root canal filling but also flexible enough to avoid a disruption in the root canal.[15] In the literature review, it was seen that there is a study investigating the effect of different movement kinematics of NiTi files, used for the retreatment purposes, on removal of gutta-percha,[16] but a study investigating their effect on resistance to cyclic fatigue was not found. For this reason, in the present study, it was aimed to examine the effect of "Adaptive Motion" on cyclic fatigue of an R-Endo R3 file.

In the studies, it was reported that regardless of brands and production methods of the files, the number of cycles these files make until they fracture is significantly higher in application of dynamic test than static test.[17],[18],[19],[20] In static test model, pressure and tensile stresses accumulate in one place as the file does not move (forward-backward). These cumulative stresses induce microstructural changes. In dynamic test model, on the other hand, stress disperses through the shaft of the file as the file moves back and forth axially. Researches state that resistance of a file to fracture increases by preventing accumulation of stress this way.[19],[20],[21] Therefore, in many studies, cyclic fatigue tests were conducted with dynamic test model.[17],[18],[19],[20],[21],[22]

In addition, in studies, the amount of movement in axial direction was reported as 3 mm/s in dynamic test models.[22],[23] For this reason, in the present study, dynamic cyclic fatigue testing device stimulating clinical use was used and the amount of axial back and forth movement was set to 3 mm/s.

In the previous studies, artificial canals that are made of different shapes and materials such as glass tube,[24] curved metal tube [23] with different canal inner diameter, different canal curvature angle, and radius and grooved blocks [25] were used. In the present study, similar to the previous studies, stainless steel artificial canal that had 60° curvature angle and 5 mm curvature radius, of which curvature center was located at the coronal 5 mm from the apical, was used.[26],[27],[28]

According to the results of the present study, the AM group was found to be statistically more resistant to cyclic fracture than the RM group. For this reason, the null hypothesis of the present study was rejected. In their study, where they compared resistance of R-Endo, ProTaper Universal Retreatment, and Mtwo-R files to cyclic fatigue under static model, İnan and Aydin [28] reported that the R-Endo R3 file was statistically more resistant to cyclic fatigue than other groups. Hussne et al.[29] reported in their study where they examined mechanical properties of R-Endo, ProTaper Universal Retreatment, and Mtwo-R files that the Mtwo-R file was statistically more flexible than other files. In studies where the effect of "Adaptive Motion" on the cyclic fatigue of NiTi files was investigated, it was reported that "Adaptive Motion" increased cyclic fatigue of files as compared to continues RM.[30] Gambarini and Glassman [30] reported in their study that when Twisted File (SybronEndo, Orange, CA, USA) used with "Adaptive Motion" its resistance to cyclic fatigue increases as compared to continues RM. Again, in many studies, where effect of reciprocation movement on resistance to cyclic fatigue was investigated, it was shown that reciprocation movement used with various rotation angles increased the resistance of files to cyclic fatigue.[31],[32],[33] Pedullà et al.[34] compared the cyclic fatigue resistance of four different NiTi files (Reciproc, WaveOne, Twisted File, and Mtwo) in three different movements (Reciproc ALL, WaveOne ALL, and continuous rotation), and they reported that Reciproc ALL and WaveOne ALL groups showed significantly higher cyclic fatigue than the continuous rotation group. Arslan et al.[35] compared the cyclic fatigue resistance of Reciproc file in four different movement, and they reported that reciprocation motion significantly increased the NCF of files. The results of the present study show similarity with the results of these studies. When NiTi files are operated with "Adaptive Motion," according to the degree of strain the files are subjected to inside the canal, their movement is changed from 600° clockwise constant rotation to 370° clockwise and 50° counterclockwise, and stress on the file is diminished. In the present study, it is thought that thereby "Adaptive Motion" increases the fatigue life due to a reduction in the magnitude of cyclic stress.


  Conclusion Top


Within the limitation of the present study, it was found that "Adaptive Motion" statistically increases the resistance to cyclic fatigue of R-Endo R3 files, manufactured for the retreatment. Further studies investigating clinical performance of retreatment files used with "Adaptive Motion" are needed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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34.
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