|Year : 2019 | Volume
| Issue : 1 | Page : 27-31
Cyclic fatigue life assessment of a M-Wire nickel-titanium reciprocating file in a 90° canal curvature: An in vitro study
Sahil Chanian1, Nandini Suresh1, N Velmurugan1, L Karthik2
1 Department of Conservative Dentistry and Endodontics, Meenakshi Ammal Dental College and Hospital, Chennai, Tamil Nadu, India
2 Central Research Department, Meenakshi Academy of Higher Education and Research, Chennai, Tamil Nadu, India
|Date of Web Publication||11-Oct-2019|
Dr. L Karthik
Meenakshi Academy of Higher Education and Research, Chennai - 600 078, Tamil Nadu
Source of Support: None, Conflict of Interest: None
Objective: The aim of this study is to assess the cyclic fatigue life of WaveOne Gold and WaveOne files in a reciprocating motion using a dynamic testing model. In addition, the present study was also employed to assess the mode of fracture under scanning electron microscope.
Study Design: A total of 24 new reciprocating files were divided into two groups where Group I consisted of 12 samples of size 25, 0.07 taper WaveOne Gold and Group II consisted of 12 samples of size 25, 0.08 taper WaveOne files. After the initial inspection, all the files were subjected to cyclic fatigue testing fabricated with a 90° angle of curvature and a 5 mm radius.
Results: The mean time to fracture for WaveOne Gold and WaveOne was found to be 268.33 and 210.92 s, respectively. There was a statistically significant difference between the WaveOne Gold and WaveOne groups in reciprocating motion. The mean length of the fractured segment of WaveOne Gold and WaveOne were also calculated where the difference was found to be statistically significant.
Conclusion: WaveOne Gold reciprocating files were found to be more resistant and performed better to cyclic fatigue than WaveOne file in a 90° canal curvature.
Keywords: Cyclic fatigue; dynamic test; fatigue resistance; reciprocating movement; WaveOne gold; WaveOne
|How to cite this article:|
Chanian S, Suresh N, Velmurugan N, Karthik L. Cyclic fatigue life assessment of a M-Wire nickel-titanium reciprocating file in a 90° canal curvature: An in vitro study. Indian J Multidiscip Dent 2019;9:27-31
|How to cite this URL:|
Chanian S, Suresh N, Velmurugan N, Karthik L. Cyclic fatigue life assessment of a M-Wire nickel-titanium reciprocating file in a 90° canal curvature: An in vitro study. Indian J Multidiscip Dent [serial online] 2019 [cited 2019 Dec 10];9:27-31. Available from: http://www.ijmdent.com/text.asp?2019/9/1/27/268989
| Introduction|| |
The use of a nickel-titanium (NiTi) alloy called nitinol was initially proposed by Walia et al. for the manufacturing of endodontic instruments characterized by excellent mechanical properties such as superelasticity and shape memory. Despite the good mechanical properties of the NiTi alloy, the possibility of instrument separation remains still a major concern during the clinical use of NiTi files. Fracture may take place in rotary NiTi instruments because of torsion and/or flexion where cyclic fatigue failure are reported to occur unexpectedly without any sign of a previous permanent plastic deformation. Cyclic fatigue occurs when an instrument continues to rotate freely in a curvature where compression and tension cycles are generated until a fracture occurs at the point of maximum flexure. The ultimate cyclic fatigue of a file is an interplay of various factors including flexibility, diameter, mass, design, and the quality of manufacturing of the NiTi files. The cyclic fatigue of the file is directly proportional to the flexibility of the instrument and inversely proportional to the diameter of the file. The principle of NiTi file motion was considered one more factor that influences the cyclic fatigue of the file. Yared proposed a new approach using a single ProTaper F2 instrument (Dentsply Maillefer, Ballaigues, Switzerland) in a reciprocating movement for cleaning and shaping. This has revolutionized the cyclic fatigue resistance of NiTi files. Based on this concept, two files systems, namely WaveOne and Reciproc, were introduced.
WaveOne (Dentsply Maillefer, Ballaigues, Switzerland) is a NiTi instrument that advocates the reciprocation concept, in which the reciprocal motion would reduce the torsional stress by periodically reversing the rotation (170° counterclockwise followed by 50° clockwise rotation for WaveOne) of the file. This reciprocating movement has been believed to ultimately increase the lifespan of the instrument. Recently, WaveOne Gold (Dentsply Maillefer) has been introduced that retains the reciprocating motion of the WaveOne file but possess modified dimensions and geometry. The file is a parallelogram with two cutting edges that comes with the off-center design of ProTaper Next (Dentsply Maillefer) files. The files are manufactured with a gold heat treatment procedure executed manually by heating the file and then cooling slowly, in contrast to the premanufacturing heat treatment of M-Wire technology. It has been proved that this new heat treatment improves the elasticity. Özyürek tested the cyclic fatigue of WaveOne, Reciproc, and WaveOne Gold in a 60° canal curvature and concluded that the cyclic fatigue resistance of the WaveOne Gold Primary single-file system was found to be higher than the WaveOne Primary and Reciproc R25 single-file instruments.
Pruett et al. have found that in cyclic fatigue testing, cycles to failure was inversely proportional to instrument stress. The smaller radius of curvature and increased instrument diameter resulted in fewer cycles to failure caused by higher instrument stress. The cyclic fatigue occurs when the instrument rotates freely at the curvature generating a compression/tension cycles at the point of maximum flexure until the fracture occurs. Sattapan et al. have reported that torsional fracture occurred in 55.7% of all fractured files, whereas flexural fatigue occurred in 44.3%. Fracture of instruments used in rotary motion occurs in two different ways: fracture caused by torsion and fracture caused by flexural fatigue. Excessive force was the major cause of instrument fracture during clinical use. Several reports of evaluation of the cyclic fatigue resistance using ProTaper, WaveOne, Twisted files, Engine-driven NiTi, and Reciproc are available.,,,, Yet, there are no earlier studies reported comparing WaveOne and WaveOne Gold reciprocating files in a 90° curvature. The aim of this study was to assess the cyclic fatigue life of WaveOne Gold and WaveOne files in a reciprocating motion using a dynamic testing model and to assess the mode of fracture under scanning electron microscope (SEM).
| Materials and Methods|| |
A total of 12 new reciprocating WaveOne Gold (Dentsply, ISO tip size = 25, taper = 0.07, length = 25 mm and 12 new WaveOne (Dentsply, ISO tip size = 25, taper 0.08, length = 25 mm) files were selected. All the instruments were previously inspected under an optical stereo microscope (Zoom Stereo Binocular Microscope, Hicksville, NY, USA); with ×20 for any visible signs of deformation. Instruments with visible signs of deformation were excluded from the study. All the files were then subjected to cyclic fatigue testing.
Cyclic fatigue testing device
A custom fabricated dynamic setup was used to test the fatigue resistance of the WaveOne Gold and WaveOne files. Cyclic fatigue testing was performed with a custom-made apparatus specifically designed to allow dynamic testing by simulating the pecking motion. This apparatus consisted of the main metal frame made of iron to which an artificial canal system and support for the handpiece were attached. The canal system, which simulated a root canal, consists of two adjustable metal frames made of brass that can accommodate any instrument to its exact size and taper. It was constructed with a 90° angle of curvature. The curvature started at 5 mm from the tip of the canal. The X-Smart Plus handpiece with WaveOne and WaveOne Gold settings were mounted over the support which also ensured the correct positioning and placement of files to the same appropriate depth for all the samples. In the dynamic setup, the testing was done by stimulating the pecking motion. The whole set of the platform along with the handpiece powered by the electric motor system were reproducing the pecking motion with 2.5 mm/min in each (forward or backward) direction. During the pecking motion, the instruments were well positioned in the artificial groove to avoid displacement of the file. This pecking movement took place at a speed of 1 cycle/second.
Cyclic fatigue test
A total of 24 samples were randomly divided into two groups each ( n = 12) according to the type of instrument used. In Group I (WaveOne Gold), files were allowed to rotate in a reciprocating motion set at WaveOne Gold setting, whereas in Group II (WaveOne files), files were allowed to rotate in a reciprocating motion set at the WaveOne setting in the X-Smart Plus motor according to the manufacturer's instructions. Glycerine was used as a lubricant after the use of each file during instrumentation. Instruments were allowed to reciprocate until fracture occurred. The cyclic fatigue testing was performed under a dental operating microscope at a magnification of ×25 to precisely determine the time of fracture. All files were tested by one operator, while the other operator simultaneously operated the stopwatch. The length of the broken fragments was measured using a digital vernier caliper (Aerospace Digimatic Vernier Caliper, 0–150 mm).
Scanning electron microscopic analysis
Five representative samples from each group were selected randomly. The fractured surfaces of the files were examined under a SEM at ×200–×1500 to determine the model characteristics of fracture. The images were checked for the presence of any dimples, craters, valleys, or microbubbles. Following this, a longitudinal photograph of one new file from each group was taken with a dental operating microscope [Figure 1]a, [Figure 1]b and [Figure 2]a, [Figure 2]b. The mean and standard deviation of the time to fracture and the length of the fractured fragment were calculated for all the experimental groups. The Kolmogorov–Smirnov test and Shapiro–Wilk test were used to assess the normality. The intergroup analysis was performed using Independent sample t-test (Software: SPSS Version 16, IBM Corporation, USA) for both the time to fracture and the length of the fractured segment. P < 0.05 was considered statistically significant.
| Results|| |
The mean time to fracture for WaveOne Gold and WaveOne was found to be 268.33 and 210.92, s respectively [Table 1]. There was a statistically significant difference observed between the WaveOne Gold and WaveOne groups in reciprocating motion. The mean length of the fractured segment of WaveOne Gold and WaveOne was found to be 2.7033 mm and 3.6517 mm, respectively [Table 2]. Descriptive statistics for the group are listed in [Table 1] and [Table 2]. Statistically significant difference was observed between the two groups pertaining to the length of the fractured fragment.
|Table 1: Descriptive statistics of time taken to fracture of WaveOne Gold and WaveOne files|
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|Table 2: Descriptive statistics of the mean length of fragment of WaveOne Gold and WaveOne|
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Scanning electron microscopic analysis results
SEM analysis of fractured surfaces of all the samples in reciprocating motion revealed crater-like formation along with numerous valleys, dimples, and microbubbles [Figure 3]a,[Figure 3]b,[Figure 3]c,[Figure 3]d,[Figure 3]e,[Figure 3]f and [Figure 4]a,[Figure 4]b,[Figure 4]c,[Figure 4]d,[Figure 4]e,[Figure 4]f. These fracture pattern features indicate that the instrument has undergone a ductile mode of fracture, which was predominantly observed in cyclic fatigue failure.
|Figure 3: (a-f) Scanning electron microscope images of WaveOne files at various magnifications|
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|Figure 4: (a-f) Scanning electron microscope images of WaveOne Gold files at various magnifications|
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| Discussion|| |
Ever since NiTi was introduced into endodontics, it has greatly transformed the methods of root canal cleaning and shaping. It has drastically minimized the unwanted complications often encountered during instrumentation in narrow and canals with curvatures. Files made from NiTi alloy are biologically compatible and flexible than stainless steel files. Despite its improved strength and flexibility, instrument fracture and separation is still a concern with NiTi instruments, and they have been reported to undergo unexpected fractures. One of the reasons for the fracture are said to be cyclic fatigue which occurs due to repeated compressive-tensile stress. Rotation motion subjects an endodontic instrument to both compressive and tensile stress in the root canal curvature. Instruments placed in curved canals deform and create stress within the instrument itself. Every rotation inside a curved canal causes an instrument to endure one complete tension-compression stress cycle. This is the most damaging form of cyclic burdening where the more severe the curvature, more stresses are concentrated on the rotary file.
In our study, it was observed that WaveOne Gold had a better cyclic fatigue resistance than WaveOne. The time to fracture of WaveOne Gold and WaveOne were observed to be 268.33 s and 210.92 s, respectively. The WaveOne Gold file performed 1.3 times better than WaveOne file. An earlier study done by Taha Ozyurek reported that WaveOne Gold primary had a better cyclic fatigue resistance when compared to WaveOne primary in a 60° canal curvature. He proved that the NCF of WaveOne Gold was 1.4 times more than that of WaveOne. Topçuoǧlu et al. found that WaveOne Gold had a higher cyclic fatigue as compared to WaveOne and Reciproc in an S-shaped canal curvature. This could be due to the newer heat treatment of the Gold Wire and the high Austentic finishing (Af) of the metal similar to the gold wire used in ProTaper Gold (PTG) system. In a study done by Hieawy et al., the differential scanning calorimeter results showed that gold wire instruments had a two-stage specific transformation behavior, indicating that reverse transformation of the alloy passes through the intermediated R-phase. Interestingly, the metallurgical characteristics of PTG files had not only a two-stage specific transformation behavior but also had high Af temperatures.
On longitudinal analysis of the files, it was observed that WaveOne Gold had seven flutes and WaveOne had nine flutes. A large number of flutes mean that there are more concentration points during flexion which might lead to increased cyclic fatigue and fracture. The pitch of WaveOne Gold and WaveOne files were found variable over the active portion of both the files. The WaveOne Gold exhibited an apical taper of 0.07, whereas the WaveOne possessed an apical taper of 0.08 which may provide more flexibility to the WaveOne Gold as compared to WaveOne. The shape of the file at its circumference also determines a file's resistance to fatigue. WaveOne has a convex triangular cross section with three cutting edges, whereas WaveOne Gold has a parallelogram cross-section with an off-center design of ProTaper Next (Dentsply Maillefer) file with two cutting edges. This might have led to increased stress in WaveOne when compared to WaveOne Gold. In the fractographic analysis of the study, almost all SEM images of the fractured instruments showed specific patterns of striations and final rupture of a ductile nature that is characterized by micro voids, craters, valleys, and dimple appearance. This proves that the fracture is ductile in nature.,
These appearances bear a direct relationship to the micromechanisms of fracture, and therefore, they are of fundamental importance to failure analysis. The limitations of this laboratory study have been that the various contributing factors such as the material properties, design, and dimensions of each file, which are specific to each brand tested were cannot be totally eliminated. This makes it difficult to quantify the effect of a single variable on fatigue behavior. The clinical relevance of the results of such tests become difficult to assess because this condition differs from intracanal instrumentation in which the fracture occurs due to several factors (including torsional stress) that act together at the same time. Differential scanning calorimeter and the Af values were not calculated in the present study. Within the limitations of this study, WaveOne Gold reciprocating files are found to be more resistant to cyclic fatigue than WaveOne file in a reciprocating motion in a 90° canal curvature.
| Conclusion|| |
The purpose of this study was to evaluate the cyclic fatigue resistance of WaveOne Gold and WaveOne rotary files in a reciprocating motion using a dynamic model and to assess the mode of fracture under SEM. A total of 24 new reciprocating WaveOne Gold files and WaveOne files were divided into 2 groups ( n = 12). A cyclic fatigue dynamic testing device was fabricated with a 90° angle of curvature and a 5 mm radius. The files were subjected to dynamic assays device moved by an electric motor which permitted the reproduction of pecking motion. All instruments were reciprocated until fracture occurred in dynamic motion. The time taken for each instrument to fracture and the length of the broken fragments were recorded. All the fractured files were analyzed under a SEM to detect the mode of fracture. The statistical analysis was performed to evaluate the cyclic fatigue resistance of WaveOne Gold and WaveOne files. The time taken for the WaveOne Gold file under reciprocating motion to fail under cyclic loading was significantly longer when compared to that of the WaveOne file ( P < 0.001). There was a statistically significant difference observed between WaveOne Gold and WaveOne groups. SEM observations showed that the instruments of all groups had undergone a ductile mode of fracture. To summarize, within the limitations of this study, it was observed that, in reciprocating motion, WaveOne Gold reciprocating files performed better than WaveOne reciprocating files. The results obtained from SEM images of all representative samples indicated that the instrument had undergone a ductile mode of fracture. Further research in this area has to be carried out in order to facilitate all the parameters of fatigue testing.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2]