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ORIGINAL ARTICLE
Year : 2020  |  Volume : 23  |  Issue : 1  |  Page : 41-45

Comparison of the cyclic fatigue resistance of hyflex EDM, vortex blue, protaper gold, and onecurve nickel–Titanium instruments


1 Department of Endodontics, Faculty of Dentistry, Afyonkarahisar Sağlık Bilimleri University, Afyonkarahisar, Turkey
2 Department of Pedodontics, Faculty of Dentistry, Afyonkarahisar Sağlık Bilimleri University, Afyonkarahisar, Turkey

Date of Submission29-Jun-2019
Date of Acceptance01-Sep-2019
Date of Web Publication10-Jan-2020

Correspondence Address:
Dr. A D Uygun
Department of Endodontics, Faculty of Dentistry, Afyonkarahisar Sağlık Bilimleri University, Afyonkarahisar, 03030
Turkey
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/njcp.njcp_343_19

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   Abstract 


Objective: This in vitro study aimed to compare the cyclic fatigue resistance of HyFlex EDM (HEDM), Vortex Blue (VB), ProTaper Gold (PG), and OneCurve (OC) nickel–titanium (NiTi) instruments. Materials and Methods: About 12 HEDM (25/.~), 12 VB (25/.06), 12 PG (25/.08), and 12°C (25/.06) instruments were included in this study. All the instruments were tested with a 60° angle of curvature and a 3-mm radius of curvature. All the instruments were utilized until fracture occurred, and then the number of cycles to failure (NCF) was calculated. The data were analyzed statistically using Kruskal–Wallis H and Mann–Whitney U-tests. The statistical significance level was set at P < 0.05. Results: HEDM instruments had the highest cyclic fatigue resistance among all the other instruments (P < 0.05). The OC instruments had a significantly higher fatigue resistance than the PG and VB instruments (P < 0.05); however, there was no significant difference between PG and VB instruments in the NCF (P > 0.05). Conclusion: Within the limitations of this in vitro study, it was found that the cyclic fatigue resistance was higher for the HEDM instruments than for the VB, PG, and OC instruments.

Keywords: Cyclic fatigue resistance, heat-treated NiTi instruments, HyFlex EDM, OneCurve


How to cite this article:
Uygun A D, Unal M, Falakaloglu S, Guven Y. Comparison of the cyclic fatigue resistance of hyflex EDM, vortex blue, protaper gold, and onecurve nickel–Titanium instruments. Niger J Clin Pract 2020;23:41-5

How to cite this URL:
Uygun A D, Unal M, Falakaloglu S, Guven Y. Comparison of the cyclic fatigue resistance of hyflex EDM, vortex blue, protaper gold, and onecurve nickel–Titanium instruments. Niger J Clin Pract [serial online] 2020 [cited 2020 Sep 28];23:41-5. Available from: http://www.njcponline.com/text.asp?2020/23/1/41/275624




   Introduction Top


Using nickel–titanium (NiTi) instruments has been the gold standard for preparation and shaping of root canals due to their superelasticity and shape memory since Walia et al.[1] introduced Nitinol in 1988. The superelasticity property enables NiTi instruments to revert to their original shape upon unloading following deformation in a curved root canal due to the austenitic-martensitic crystalline form of the transformations.[2],[3],[4] Unfortunately, instrument fracture continues to be a problem in root canal treatments, in spite of past and current advances.[5],[6] It has been suggested that an instrument fracture in the root canal is caused by two mechanism: torsional failure and cyclic fatigue. Torsional failure occurs by exceeding the elastic limit of the instrument when the instrument that is screwed into the root canal continues to rotate.[7] Cyclic fatigue occurs when repeated tension/compression cycles accumulate at the point of maximum flexure in a curved root canal.[8],[9]

Recently, a number of alterations have been made in the manufacturing process, design, and alloys of traditional NiTi instruments to improve their cyclic fatigue resistance in a curved root canal. Heat treatment (thermomechanical process), one of the most popular approaches, enhances the mechanical properties of NiTi instruments; depending on the phase transition temperatures of the NiTi alloy, this can improve their ability to reduce cyclic fatigue.[10],[11],[12]

The HyFlex EDM (HEDM) (Coltene/Whaledent, Altst€atten, Switzerland) heat-treated NiTi instrument system is a novel version of the HyFlex CM system, which was initially produced in 2011. HEDM contains a control memory (CM) wire to optimize the microstructure of the NiTi alloy; however, unlike HyFlex CM, electrical-discharge machining technology is also used. HEDM OneFile (25/~) has a tip size of 25 and a taper of 0.08 in the apical part, with a variable taper.[13] The HEDM instrument has a quadratic cross section at its tip, a trapezoidal cross section in its middle, and an almost triangular cross section at the top.[14]

Vortex Blue (VB) (Dentsply Tulsa Dental Specialties, Tulsa, OK, USA) is another novel rotary system that contains a 0.04 taper and a 0.06 taper, which are available with apical tip sizes ranging from 15 to 50. VB rotary systems have a distinctive blue color with a visible titanium oxide layer. VB instruments have been reported to have a significant increase in cyclic fatigue resistance in comparison to similar sizes of ProFile Vortex instruments that are made of M-wire.[11] ProTaper Gold (PG) (Dentsply Tulsa Dental Specialties) rotary systems feature the same exact geometries as ProTaper Universal (PU) rotary systems with advanced metallurgy. PG is made of gold wire that increases its resistance to cyclic fatigue.[15]

OneCurve (OC) (Micro Méga, Besançon, France) is a single-file rotary system made from a c.wire using a proprietary heat treatment technique used by the manufacturer of OneShape. According to the manufacturer, it is possible to use a single OC file for shaping a root canal but not for preparing it. The instrument's off-centered cross section has a convex triangular tip and its coronal section is s-shaped.[16]

As a result of our own investigations and the findings reported in the current body of research in the literature, these heat-treated files change their shapes at a specific rate with very small amounts of force; they do not regain their original shape once the force is removed. Unlike stainless-steel files, permanent deformation does not occur with these heat-treated files.



All of these heat-treated NiTi rotary systems have similar physical and mechanical properties; they are more flexible and resistant to cyclic fatigue than traditional NiTi rotary systems. To date, only one comparative study has evaluated the cyclic fatigue resistance of HEDM and PG systems.[17] Similarly, only one study has compared the cyclic fatigue resistance of VB and OC systems.[18] Therefore, this study aimed to evaluate and compare the cyclic fatigue resistance of heat-treated NiTi rotary systems. The null hypothesis of this present study was that there would be no difference between the cyclic fatigue resistances of the tested heat-treated NiTi instruments.


   Materials and Methods Top


This study investigated four different heat-treated NiTi instruments with similar dimensions: HEDM (25/.~), PG (25/0.08), VB (25/0.06), and OC (25/0.06). A total of 48 instruments were tested (n = 12 for each group); each had been previously examined for signs of manufacturing defect using an optical stereomicroscope with 20x magnification.

Customized testing device was built for the cyclic fatigue test in this study [Figure 1]. A 1:16 reduction contra-angle handpiece was mounted to maintain standard contact and to ensure that the instrument would not be moved. An artificial stainless-steel canal was mounted onto a mobile device to allow three-dimensional (3D) positioning in the same way that is possible with other instruments. Using the method presented in Pruett et al.,[19] cyclic fatigue testing was performed on the artificial stainless-steel canal with an inner diameter of 1.5 mm, a 60° angle of curvature, and a radius of curvature of 3 mm. In this study, the curvature's center was 5 mm from the tip of the instrument. All the instruments were rotated with a low-torque motor (VDW GOLD; VDW, Munich, Germany) according to the manufacturers' recommendations for speed and torque settings. A glass block was placed over the artificial stainless-steel canal to allow the breakage to be seen and to enable the broken piece of the instrument to be removed. The cyclic fatigue test was carried out in saline solution at 35°C (±2) heated by a device designed for this experiment and the temperature was controlled by means of a thermostat.
Figure 1: The experimental set-up for the cyclic fatigue resistance test

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The instruments were rotated until they were broken, and the time to fracture was recorded in seconds using a chronometer. Because the four different types of rotary instruments were tested at different revolutions per minute (rpm) values, the number of cycles to failure (NCF) was further calculated using the following formula: (NFC = revolutions per minute (rpm) × time to fracture (s)/60). A digital caliper was used to measure the length of the fractured fragment. One of the fractured instruments in each of the four groups was examined under a scanning electron microscope [Figure 2]. NCF and fragment length (FL) data were analyzed using the Shapiro–Wilk test to verify the assumption of normality and Levene's test was used to determine the homogeneity of variances. The Kruskal–Wallis H test was then performed to statistically analyze the NCF and FL data, and the Mann–Whitney U-test was used to determine any statistical significance between two of the rotary instruments. The statistical significance level was set at P < 0.05 (SPSS v. 23.0; IBM Corp, Armonk, NY).
Figure 2: Scanning electron micrographs of the fracture surface of separated fragments HEDM (a and e), VB (b and f), PG (c and g), and OC (d and h). Left column (250 × magnification) and right column (3,000 × magnification). The white arrows indicate the origins of the crack

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   Results Top


The mean and standard deviation of the NCF values and the FL of the segments are shown in [Table 1]. The null hypothesis tested in this study was rejected because the NCF values were significantly higher in the HEDM instruments than in the other instruments (P < 0.05). The OC instrument had a significantly higher fatigue resistance than the PG and VB instruments (P < 0.05); moreover, no significant difference in the NCF was found for the PG and VB instruments (P > 0.05). The mean of the FL was significantly longer for the HEDM instruments than for the VB, PG, and OC instruments (P < 0.05). The OC instruments were found to have the shortest FL of all the tested instruments; the difference was statistically significant.
Table 1: The Mean and Standard Deviation Values for the Number of Cycles to Fracture (NCF) and the Length (mm) of the Fractured Fragment of the Tested Instruments

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   Discussion Top


Cyclic fatigue is the major reason for the instrument fracture that occurs unexpectedly at the instrument's maximum flexure point while it is rotating freely inside curved root canals.[20] Therefore, it is very important to conduct cyclic fatigue tests on novel NiTi instrument systems; doing so provides clinicians with information about their resistance to fracture.[21] Standardization of natural teeth cannot be achieved due to their different morphological properties; therefore, they are not suitable for a cyclic fatigue test. We used a nontooth model as have many other studies in the literature. An artificial stainless-steel simulated root canal with a 60° angle of curvature and a 3-mm radius of curvature was used to test the instruments in the cyclic fatigue model.

This study compared the fatigue resistance of newly developed HEDM, VB, PG, and OC instruments, manufactured using different heat treatments, in an artificial root canal. All of these instruments had common features, such as flexibility and superelasticity. Superelasticity is an isothermal phenomenon in which the material can recover from a high degree of strain triggered by mechanical stress.[22] The instruments in this study can change their shape against very low mechanical stress and do not tend to return to their previous shape upon removal of the stress, unlike traditional NiTi instruments. Therefore, they can be easily utilized even in the most anatomically complex root canal. Each manufacturer uses a different word to describe the alloys in the instruments it produces, such as CM wire, Gold wire, Blue wire, or C. Wire. In the current body of literature, these instruments or instrument alloys are not classified using a common definition. Using a common term to define these instruments would help clinicians determine which instrument they would prefer to use. In some studies, such NiTi instruments are given in names such as shape memory.[23],[24] However, it is not appropriate to call these instruments' shape memory instruments because this term refers to the ability of a deformed material to recover its predeformed shape after it is heated.[22] According to the manufacturer of HEDM, its instrument has a shape memory effect; thus, after autoclave sterilization, the shape of a used instrument can be recovered. Therefore, the manufacturer claims that it can be used more than once.[14] However, to our knowledge, it was not specified whether the other instruments used in this study had such a property by their manufacturers.

Previously, Kaval et al.[17] evaluated the cyclic fatigue resistance of HEDM, PG, and PU instruments. They reported that HEDM instruments have the highest cyclic fatigue resistance, followed by PG and PU instruments. Gündoǧar et al.[25] reported that HEDM instruments resisted cyclic fatigue significantly more than OneShape, Reciproc Blue, and WaveOne Gold instruments. Reciproc Blue and WaveOne Gold are also new heat treated NiTi instruments, and they are used in reciprocal movement; however, they are not included in this study because we only compared systems that work with a clockwise rotary motion. Most previous studies have proven the superiority of HEDM instrument. Our finding is in agreement with the results reported in those studies. In this study, the NCF was significantly higher in the HEDM instruments than the OC, VB, and PG instruments (P < 0.05). This can be because the processing of the CM wire using electrical-discharge machining technology increases the phase transformation temperatures and hardness.[26] Different factors influence the cyclic fatigue resistance of NiTi instruments including instrument size, taper, cross-sectional design, and manufacturing techniques.[27],[28] The size in the point of maximum stress during a cyclic fatigue test could affect the fatigue life of NiTi rotary instruments. Whereas the larger the metal volume, the lower the fatigue resistance.[29] However, these instruments are called files with variable taper that means taper changes in different levels of the file, so it is not correct to estimate the cyclic fatigue resistance based on the taper angles that manufacturer companies have said. According to the results of our study, although VB was less taper than PG, there was no statistically significant difference between them.

According to the results of this study, the OC instruments showed the higher resistance to cyclic fatigue than PG and VB instruments. The OC instrument is a new product of the company that produces the OneShape instrument. Although there are similarities between the two products, such as cross-sectional designs, cross-sectional area at 5 mm of OC is smaller than OS instruments. Not only for this reason, OC instrument, unlike the OS instrument, has a higher fracture resistance due to the fact that it is produced from an alloy with a martensitic transformation called C. Wire.[16] There is not much research comparing OC instrument with other instruments in literature. Elnaghy et al.[18] reported that VB instruments exhibited greater cyclic fatigue resistance than OC instruments in artificial root canals with single and double curvatures. It can be assumed that these two studies are incompatible because they used different experimental setups. Further research is needed on this issue.

Over the past several years, many studies have suggested that a cyclic fatigue resistance test should be done at body temperature.[30],[31],[32] However, during a root canal treatment, the root canal has an unstable temperature between room and body temperature because the endodontic access cavity is opened under water cooling, the root canals are irrigated with a solution at room temperature, and the dentin does not transmit the heat from periodontal ligaments quickly enough. In a previous study about effect of temperature on collagen dissolving ability of sodium hypochlorite, researchers claimed that the intracanal temperature was 31°C–33.5°C,[33] but later, de Hemptinne et al.[34] reported that the mean intracanal temperature of the completed root canal preparation was 35.1°C (±1). In this study, cyclic fatigue resistance test was performed in saline solution at 35°C (±2).

In this study, the mean FL was significantly different in the HEDM instruments in comparison to the VB, PG, and OC instruments. Similarly, some previous studies found a significant difference between the mean length of the fractured fragments.[17],[35],[36] Differences in the designs and alloys of the instruments may cause the location of the maximum stress points to vary. A longer fractured fragment can be easier and more convenient for a clinician to remove.


   Conclusion Top


Within the limitations of the presentin vitro study, the HEDM instruments were found to more effectively resist cyclic fatigue than the OC, VB, and PG instruments. Moreover, the OC instruments were found to be more resistant to cyclic fatigue than the PG and VB instruments.

Acknowledgements

The authors deny any conflicts of interest related to this study.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
   References Top

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Elnaghy AM, Elsaka SE. Cyclic fatigue resistance of One Curve, 2Shape, ProFile Vortex, Vortex Blue, and RaCe nickel-titanium rotary instruments in single and double curvature canals. J Endod 2018;44:1725-30.  Back to cited text no. 18
    
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33.
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35.
Shen Y, Hieawy A, Huang X, Wang ZJ, Maezono H, Haapasalo M. Fatigue resistance of a 3-dimensional conforming nickel-titanium rotary ınstrument in double curvatures. J Endod 2016;42:961-4.  Back to cited text no. 35
    
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