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Year : 2018  |  Volume : 8  |  Issue : 2  |  Page : 117-122

Effect of environment on cyclic fatigue of ProTaper Next rotary files

Department of Restorative Dentistry, Faculty of Dentistry, King Abdulaziz University, Jeddah, Saudi Arabia

Date of Web Publication5-Apr-2018

Correspondence Address:
Dr. Tariq S Abuhaimed
Department of Restorative Dentistry, Faculty of Dentistry, King Abdulaziz University, Jeddah
Saudi Arabia
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/sej.sej_65_17

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Aim: The aim of the study was to evaluate the effect of different irrigation solutions on cyclic fatigue of ProTaper Next (PTN) rotary files.
Materials and Methods: A total of 50 files were divided into 5 groups according to the environment in which cyclic fatigue test was carried out as follows: chlorhexidine (CHX) 2%, ethylenediaminetetraacetic acid 17%, saline, sodium hypochlorite 6%, and air (control group). A specially designed testing device was assembled utilizing an artificial canal made of a heat-resistant glass tube. The tube had a 1.2 mm diameter, an angle of curvature of 60°, and a radius of curvature 6 mm. Before each fatigue test, the tube was filled with fresh irrigation solution except air group, which was left dry. The mean number of cycles to failure was counted then fractography and length of broken segment were analyzed after failure. Statistical analyses were performed using one-way ANOVA test.
Results: The CHX group was the most resistant to cyclic fatigue compared to all groups (P < 0.05). No significant difference was found between all other solutions or dry group (P > 0.05). Fractographic analysis of all groups showed signs of a fatigue failure. A crack site was seen followed by a striation area leading to a dimples overload area. The length of broken segments was not significantly different among all groups (P = 0.37).
Conclusion: Although liquid media may function as lubrication, all irrigants except CHX failed to increase the fatigue life of PTN files when compared to dry conditions.

Keywords: Cyclic fatigue, irrigation solutions, ProTaper Next, scanning electron microscopy

How to cite this article:
Abuhaimed TS. Effect of environment on cyclic fatigue of ProTaper Next rotary files. Saudi Endod J 2018;8:117-22

How to cite this URL:
Abuhaimed TS. Effect of environment on cyclic fatigue of ProTaper Next rotary files. Saudi Endod J [serial online] 2018 [cited 2020 Jul 14];8:117-22. Available from: http://www.saudiendodj.com/text.asp?2018/8/2/117/229355

  Introduction Top

Undoubtedly, the introduction of nickel-titanium (NiTi) rotary instruments was the key to significant advancement in dentistry. In comparison to hand instruments, NiTi instruments are found to perform considerably faster with less procedural errors and operational fatigue.[1],[2] However, unexpected file separation is one disadvantage that remains a concern among clinicians.[2] Separation of instruments inside curved root canals is not easily retrieved and may jeopardize treatment outcome.[3] Instrument fracture during root canal preparation occurs due to either cyclic or torsional failure. Cyclic fatigue occurs as a result of recurrent tension and compression inside the curved canals until failure. Many factors may play a role in instrument separation including the type metal alloy used, the process of manufacturing, instrument design, canal curvature, and type of irrigation solutions used.[4] Manufactures are consistently focusing on improving the performance of instruments and increasing their resistance to fracture. Such improvements include changes in the file design and cross section,[5] use of different kinematics,[6],[7] and thermal treatment.[8] Thermal treatment optimizes the microstructure and maximizes the flexibility and resistance to cyclic fatigue. ProTaper Next (PTN) is an example of heat-treated rotary NiTi instruments (Dentsply Maillefer, Ballaigues, Switzerland). PTN is made from M-Wire NiTi alloy which was proven to increase flexibility and resistance to fracture.[9] The instrument has a special design with variable taper and off-centered rectangular cross-section that exhibited superior resistance to cyclic fatigue compared to other NiTi files.[10],[11]

During chemomechanical preparation of root canals, different irrigants and chelating agents are used for various reasons, including antimicrobial effect,[12] the ability to remove smear layer [13],[14] and reduction of hard tissue structure.[15] Moreover, application of irrigants during root canal preparation helps wash out the content of the root canal, but the contact of these irrigating materials with dentin and rotary instruments may also adversely affect their fracture resistance due to surface corrosion or roughness.[16],[17],[18] Surface corrosion of rotary instruments may act as initiation sites of cracks which further propagate and cause fatigue failure.[17] Sodium hypochlorite (NaOCl), chlorhexidine (CHX), and ethylenediaminetetraacetic acid (EDTA) are commonly used irrigation materials. Their tendency to create on rotary instruments was evaluated in the previous studies which showed conflicting results.[19],[20],[21]

No previous study compared the cyclic fatigue of PTN files in the presence of different irrigants. Thus, the aim of this study was to evaluate the resistance to cyclic fatigue of PTN files in the presence of different environment including CHX, EDTA, NaOCl, saline, and air (dry).

The null hypnosis was that all of the conditions tested would have the same effect on cyclic fatigue of PTN instruments.

  Materials and Methods Top

A total of 50 PTN files size X2 (Dentsply Maillefer, Ballaigues, Switzerland) were used in this experiment. The files were divided into 5 groups (n = 10) according to the environment in which the cyclic fatigue test was carried out [Table 1] as follows: CHX 2%, EDTA 17%, Saline, NaOCl 6%, and AIR (dry), no irrigation medium was used. A special setup was assembled for the current experiment using a universal testing machine (MultiTest 2.5i, Mecmesin, Slinfold, UK) [Figure 1]. The handpiece of an endodontic motor (eNDODUAL, MERIGNAC cedex, France) was attached to the moving arm of the testing machine. At the lower end of the machine, a simulated root canal was attached to a chuck vice, which was fixed to a two-dimensional horizontal micropositional stage to precisely align the simulated canal with the descending file. The simulated root canal was made from a heat resistance borosilicate glass tube (www.mcmaster.com), which had a 1.2 mm internal diameter. The glass tube was formed by using heat to bend the tube into a piece that had a 60° curvature and 6 mm radius according to Pruett et al.[22] At the beginning of each test, the glass tube was filled with fresh irrigant and aligned with the file precisely. After loading the file, the vertical arm holding the handpiece was lowered until the center of curvature was at 4 mm from the tip of the file [Figure 2]. The exact position of the files inside the canal was recorded and saved by a preset computer program that guaranteed repositioning of each file at the same selected point (Emperor, Mecmesin, Sinfold, UK). Only 16 mm of the files were allowed to be immersed in the tube at room temperature of 22°C ± 2°C and relative humidity of 70%. The operating speed was 300 rpm, and torque was set at 2 N. cm according to manufacturer recommendations. The time required for each file to fracture was recorded manually with a stopwatch and converted to number of cycles to failure (NCF) by multiplying the time by the speed. After failure, the file was removed along with the broken piece and saved for length measurements and microscopic analysis. The fractured segments were measured in each group using microcalipers (Filetta, Schut Geometrical Metrology, Ebnatstrasse, Schaffhausen, Switzerland). Four randomly selected samples of broken segments from each group were subjected to fracture analysis under scanning electron microscopy (Quanta 250 FEG, FEI, Eindhoven, Netherlands).
Table 1: Materials used in the study

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Figure 1: Cyclic fatigue setup. (1) Vertical moving stage. (2) Endodontic motor handpiece. (3) Vise holder. (4) Rotary file. (5) Artificial canal. (6) Horizontal micropositioner

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Figure 2: Rotary file in position for fatigue testing

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The tube was cleaned and setup again to repeat the process with another file in the same manner described. The same operator performed all the steps of the laboratory experiments.

Statistical analysis

The statistical package used to analyze the data was IBM's SPSS version 22 (SPSS, Inc., Chicago, IL, USA). Simple descriptive statistics was used to study the variables through the means and standard deviations, which were statistically compared using one-way ANOVA. The tests were performed after examining the normality of the data distribution using the Shapiro-Wilk test. The post hoc (Scheffé) test was used to check out the difference between groups. All significant results were based on the conventional cutoff P < 0.05 for the rejection of the null hypothesis.

  Results Top

The mean NCF values and each rotary system are represented in [Table 2]. Higher NCF values represent greater resistance to cyclic fatigue. CHX was found to exhibit a significantly greater resistance to cyclic fatigue than all other groups (P< 0.05). There was no significant difference between EDTA, NaOCl, saline, and air (P > 0.05).
Table 2: Mean±standard deviation of number of cycles to failure and segment length

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The mean length of broken segments of all groups ranged from 3.7 to 3.9 mm. No significance differences were found between groups (P > 0.05).

The topographic analysis of the fractured files in all groups exhibited classical signs of fatigue fracture without plastic deformation characterized by the presence of a crack initiation area, fatigue striations, and dimple area [Figure 3]a,[Figure 3]b,[Figure 3]c,[Figure 3]d,[Figure 3]e. However, in NaOCl group, the fractured surface showed two opposing crack sites and striation areas surrounding a central dimple area [Figure 3]d.
Figure 3: Scanning electron microscopy images of cross sections of files failed in different irrigation media, (a) Dry, (b) CHX, (c) EDTA, (d) saline, and (e) sodium hypochlorite. Letter (S) represents striation area. Letter (D) represents Dimple area. Dotted line represents the transition between the two areas. Arrow indicates the crack origin. All samples show one crack origin except the file failed in sodium hypochlorite which shows two crack origins

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  Discussion Top

During clinical shaping of curved root canals, surface interactions between the file and canal walls in a chemically-active irrigation solution simultaneously occur which may cause corrosion or surface roughness which in turn may lead to cracks and eventually cyclic fatigue of the file.[23] The aim of this study was to evaluate the effect of different intracanal conditions including (CHX, EDTA, Saline, NaOCl, and Air) on the cyclic fatigue resistance of PTN files.

In studying the fatigue failure process, some researchers adopted the protocol of preimmersion of NiTi files in irrigation solution followed by testing the file for cyclic fatigue in synthetic oil.[16],[24],[25] This design does not consider the interaction of the file and the irrigation solutions while the file is actually rotating in the irrigation solution. Other studies tested the files in a bath of irrigation solution.[17],[18],[19] However, the interaction of the file with the canal walls was not considered.

In the current study, a specially designed setup that simulates the clinical conditions was used by testing the files while rotating in a curved glass tube filled with the irrigation solution. A heat-resistant curved glass tube was used to avoid the galvanic corrosion that might take place during testing in metal tubes or in contact with metal pins as was described in the previous studies.[18],[26] However, there may be some limitations of using glass tubes that result from chemical reaction with different irrigation solutions that are not present in natural teeth and should not be extrapolated clinically.

The results of the present study indicate that files tested in the CHX group showed the highest resistance to cyclic fatigue compared to all other groups. Due to these findings, the null hypothesis was rejected.

The NiTi alloy is sensitive to temperature. The friction between the file and the canal walls leads to an increase in operating temperature inducing the transformation from martensite to austenite, which is stiffer and less resilient (metal hardening). The stress-induced austenite phase is more liable to fracture due inferior resistance to crack growth.[27]

In theory, using aqueous media will dissipate the frictional heat away and eventually increase the fatigue lifetime of NiTi files.[28] However, according to the results of the present study, the NCF of instrument tested in dry conditions without lubricant or coolant was not significantly different than those tested in EDTA, saline, and NaOCl groups in agreement with several reports [18],[23],[29] and disagreement with others.[16],[19],[24] These conflicting results may be due to one of two factors. The first is that these aqueous solutions have some qualities which may have a negative impact on cyclic fatigue such as the ability to cause corrosion and surface roughness of NiTi alloy. Therefore, fatigue life was not improved. The second factor is the fact that fatigue testing fell in the low-cyclic fatigue region.

Regarding the first factor, earlier reports showed that NaOCl and EDTA could cause corrosion of the files after immersion.[16],[24] However, in these reports, long time and full immersion of the file and the shank caused galvanic corrosion of the files which is clinically not applicable. In contrary, recent reports showed that preimmersion did not cause corrosion [18],[29] or show any negative impact on cyclic fatigue even with increased surface roughness of the files,[20] which is in agreement with the current results.

The second factor seems to be more plausible. The NCF in the current setup ranged from 602 to 867 corresponding to 1.5–3 min which fell in the region of low-cyclic fatigue (estimated <1000 cycles).[17],[28] This time frame is insufficient to show the effect of the different conditions. Shen et al.[18] evaluated the NCF of different files in dry conditions and in a bath of irrigation solutions including water, EDTA, NaOCl, and found that there was no difference in NCF when the number of cycles fell in the low-cyclic fatigue life. However, liquid media showed improvement of cyclic fatigue in long-fatigue life when compared to dry conditions. In the classic study of Tobushi et al.,[28] they found that there was no difference between water and dry conditions in low-fatigue region. Similarly, Pedullà et al.[25] showed that immersion of NiTi files in NaOCl and EDTA had no negative effects on the NCF. In contrary, Elnaghy and Elsaka [19] found that testing WaveOne Gold in air reported higher resistance to fatigue compared to aqueous media. The conflict in results could be attributed to the methodology, instrument design, and the type of motion used.

Another factor that also may mask the impact of irrigation solutions on cyclic fatigue is the degree of curvature. It is known that a file passing through steep curvature is subjected to a high-strain amplitude which expedites the failure process.[17] In the current setup, a 60° curvature was utilized. Consequently, there was insufficient time to show the impact of different environment (low-fatigue region). Hasegawa et al.[23] showed that 3 different files tested in different environments gave similar NCF when tested in 60° curvature. However, different NFC was reported when the files were tested in 30° curvature where water improved the NCF of the files.

Therefore, it may be expected that longer testing times would show better performance with aqueous solutions.

Interestingly, the CHX group exhibited a significantly higher NCF compared to all groups within the low-cyclic fatigue region which may indicate that it has a superior lubrication quality. Chlorhexidine is a cationic surfactant synthetic biguanide. As a surfactant, it behaved as a lubricant reducing the friction of the file with the glass surface of the tube. Moreover, CHX does not cause surface damage to the NiTi alloy. There is scarce literature on the impact of CHX on cyclic fatigue of NiTi files, which will be one of our future goals.

The lengths of the broken pieces of all groups were almost identical (≈4 mm) [Table 2] without any significant difference between groups. This coincides with the location of the file at the maximum curvature (4 mm from the tip). Similar results were reported previously.[30] This confirms that the fracture is usually expected to occur at the maximum curvature of the canal at the maximum strain amplitude.[18] It also confirms that all files were positioned at the same location.

Fractographic analysis of failed files showed typical signs of fatigue failure. Each file is characterized by a crack initiation site followed by an area of striation and finally dimples overload area. Similar results were reported previously.[19],[25],[31],[32] In addition, in agreement with other studies, this study found that there were no signs of corrosion at or around the fractured surfaces.[18],[19] Interestingly, in the NaOCl group, there were 2 crack sites initiated from opposing cutting edges that propagated toward the center of the file. This could be related to surface roughness, which may act as crack propagation sites. Hence, multiple cracks may explain the lower number of cycles in this group.

The limitation of the present study is the short time of testing. It would be valuable to evaluate the effect of irrigation solutions in high-cyclic fatigue region. In addition, it may be useful to evaluate the frictional forces that result from each solution which may have an impact on the NCF.

  Conclusion Top

Irrigation environment may affect the cyclic fatigue of NiTi files. CHX improved the NCF of PTN files. However, all other environments showed no difference primarily due to the short time of testing (low-cyclic fatigue region).

The results presented in this in vitro study should not be directly extrapolated to clinical conditions since the time needed for cyclic fatigue is much longer than that used for shaping natural teeth. Therefore, clinical conclusions should be drawn with caution.

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Conflicts of interest

There are no conflicts of interest.

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Madarati AA, Watts DC, Qualtrough AJ. Factors contributing to the separation of endodontic files. Br Dent J 2008;204:241-5.  Back to cited text no. 4
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Pedullà E, Grande NM, Plotino G, Gambarini G, Rapisarda E. Influence of continuous or reciprocating motion on cyclic fatigue resistance of 4 different nickel-titanium rotary instruments. J Endod 2013;39:258-61.  Back to cited text no. 6
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  [Figure 1], [Figure 2], [Figure 3]

  [Table 1], [Table 2]


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