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. Author manuscript; available in PMC: 2017 Jan 5.
Published in final edited form as: Scanning. 2014 May 5;36(5):507–511. doi: 10.1002/sca.21146

The Effect of Electrical Treatment on Cyclic Fatigue of NiTi Instruments

Mohammad Ali Saghiri 1,a, Armen Asatourian 2,b, Franklin Garcia-Godoy 3,c, James L Gutmann 4,d, Mehrdad Lotfi 5,c, Nader Sheibani 1,c
PMCID: PMC5214667  NIHMSID: NIHMS839842  PMID: 24798116

Summary

Dentists desire to use NiTi rotary instruments, which do not break inside the root canals of teeth, since the pieces from broken files are difficult to remove. The NiTi rotary instrument breakage is because of cyclic and torsional fatigue. Here the low-voltage (12 V) and high voltage (24 V) electrical treatments were used to enhance the cyclic fatigue of NiTi rotary instruments and increase their durability. In excremental groups, following electrical treatment samples of the NiTi instruments were rotated inside artificial root canals until they broke. Our results showed that electrical treatment with 12-V DC was effective in restoring NiTi instrument’s resistance to cyclic fatigue. The scanning electron microscopy images and fractograph of samples exposed to 12-V electrical treatment showed a more regular texture over the surface with less dimpling on fractured site. These patterns can improve the super elasticity of tested devices during rotational movement, and delay the NiTi instruments separation in root canal preparations.

Keywords: cyclic fatigue, electric stimulation, NCF, NiTi, rotary instrument

Introduction

The use of nickel–titanium (NiTi) superelastic (SE) wires for manufacturing rotary instruments created revolutionary changes in endodontic practice (Miura et al. ’86) after using of stainless steel in endodontic procedures for decades (Thompson 2000). The NiTi intermetallic alloys most important properties are shape memory and super elasticity. These features enable the instruments to rotate through the bends of root canal curvatures (Walia et al. ’88). However, the potential separation (Yared 2004) and corrosion (Saghiri et al. 2012; Fayyad and Mahran 2014) in the clinical scenarios have raised concerns as the instruments are subjected to cyclic fatigue and/or torsional stresses. Crack initiation and transgranular crack growth lead to the occurrence of fatigue failure through a slipping band mechanism and by repeated cyclic loadings (Pruett et al. ’97). Cyclic fatigue occurs during the movement of an instrument inside the curvature of root canal. The cycles of alternating tension and/or compression ultimately result in file separation (Lopes et al. 2007; Azimi et al. 2011).

During the small oscillatory movements between two contacting surfaces in the presence of a corrosive medium, a certain type of corrosion occurs that is known as fretting corrosion (Hoeppner and Chandrasekaran ’94; Saghiri et al. 2012). Fretting corrosion can also occur at the same time when the NiTi endodontic instruments are preparing the root canal wall in the presence of a medium, which can result in the breakdown of the TiO2 layer that is present on titanium and its alloys (Velten et al. 2002). The residence of some parts of endodontic files in the canals may have systematic effects on blood, urine and many other tissues (Saghiri et al. 2013, 2014). In addition, the chemicals used during the instrumentation of root canals can cause pitting or crevice corrosion on NiTi rotary instruments, which can increase their risk of fatigue failure (Darabara et al. 2004; Menan and Henaff 2009).

The electrical stimulation of titanium alloys is used by many investigators for different purposes (Cook et al. 2004; Secinti et al. 2008; Wanga et al. 2009). Some investigators used this method to evaluate its antibacterial effects on the electrically activated vertebral implants (Secinti et al. 2008). Others have used direct electrical current on the titanium inter-body devices in order to improve their bony fusion property (Cook et al. 2004). In the metallurgy industry, several studies have addressed the effects of short-time direct current heating on phase transformation and super elasticity of titanium alloys. These results indicate that direct current heating is able to impose changes in microstructure of titanium alloys where it can either improve or deteriorate its super elasticity (Wanga et al. 2009). Furthermore, some publications have discussed the fact that overheating can make significant microstructural changes and alter the transformation behavior and functional properties of these alloys (Tan and Liu 2004).

Previous investigators have used heat treatment to promote the super-elasticity of nickel titanium orthodontic wires (Miura et al. ’88). Others have shown that the direct electric resistance heat treatment (DERHT) is capable of improving the physiochemical NiTi wire properties, such as surface hardness properties (Kiattiwongse et al. 2008), spring-back, shape memory and super-elasticity (Miura et al. ’88). Other studies have questioned the bending properties and transformation temperature of NiTi orthodontic appliances and demonstrated that through this method the super-elasticity of NiTi wires can be excelled (Yoneyama et al. ’93).

Based on these findings and their potential application to enhance fatigue life of NiTi rotary endodontic instrument, the aim of the present study was to evaluate the effect of electrical treatment on the cyclic fatigue of the ProTaper series of NiTi instruments. We tested the hypothesis that electrical treatment can produce favorable changes in the fatigue life of NiTi rotary instruments and improve their properties during various dental applications.

Materials and Methods

The methodology use here was similar to that described by Azimi et al. (2011). Briefly, Forty ProTaper® F1 NiTi instruments (Maillefer-Dentsply, Baillagues, Switzerland) were selected and divided into four groups (n =10). A stainless steel block was lathed by a CNC device (Maho MH 400E, Deckel-Maho-Gildemeister, Bielefeld, Germany) to a 110 mm × 100 mm × 10 mm dimension and 63–65 Rockwell hardened stainless steel block. Test canals with a total length of 16 mm, with 30° Curvature and 5 mm radius of curvature were prepared similar to the apparatus used in a previously described studies (Grande et al. 2005; Azimi et al. 2011), which consisted of a frame that supported a hand-piece and the stainless steel block with artificial canals matched for each instrument.

Initially, the instruments in all four groups were introduced into the canals to the full working length by moving the blocks toward the fixed handpiece and canal was lubricate by phosphate buffer saline during instrumentation. The instruments were rotated at 300 rpm using a 1:16 reduction handpiece (Anthogyr, Sallanches, France) with a torque-controlled electric motor (TCM Endo; Nouvag, Goldach, Switzerland) for 30 s. By rotating inside the artificial canals cyclic fatigue occurred inside the structure of ProTaper files. The applied torque was 2-N cm (204 g cm) for ProTaper, according to manufacturer’s instructions.

Treatment of Group A

In this group, all 10 samples continued rotating until breakage, while the other three groups were stopped after 30 s of rotation and underwent electrical treatment as described later.

Treatment of Groups B – D

A customized holding apparatus was fabricated to enable the transfer of direct electrical current (DC) via the shank of the rotary files. In groups B and C the files were placed on the holding device and potential differences for a 12- and 24-V, respectively (55 amp-American Vermerien, Ridgewood, New Jersey) were applied to the rotary files for 5 s. In group D, the files received no treatment just had a 30-s break after 30 s works up to breakage. On the other hand, the difference between groups A and D was the break relaxation time (30-s rotate 30-s pause continually) that the files had in the group D during the cyclic fatigue testing.

Immediately after electrical treatment (approximately 30 s for the whole process including 3-s pass of electricity) samples in groups B and C were again placed on the handpiece and rotated with previous condition until the instrument broke. The intervals time to fracture was recorded by a chronometer accurate to 1:100 of a second and the number of rotations to breakage was calculated subsequently (Number of cyclic fatigue, NCF =time × speed). The raw data of the effects of electrical power and NCF on the instruments was analyzed statistically using a two-factor analysis of variance (ANOVA) followed by a Tukey pairwise comparison of the groups at a significance level of p <0.05.

SEM Analysis

After NiTi instrument separation, they were analyzed using a Scanning Electron Microscope (SEM) operating at 15 kV (Philips XL30 FEG; FEI Company, Hillsboro, VA).

Results

The means ± standard deviations of the number of cyclic fatigue for groups A, B, C, and D were 12,978 ± 646 × 103, 15,115 ± 389 ×103, 12,332 ± 335 × 103, 13,880 ± 334 × 103, respectively. ANOVA and Tukey tests indicated that there were significant differences between group B and other groups (p =0.009). There was a significant difference between groups B and C as well (p =0.0001). The SEM images showed a rougher surface with more irregularities on the surface and cross section of separated instruments of groups A, C, and D in comparison with the samples from group B, which had a more regular pattern. The body of a sample from group B also showed less striations and mirocracks than other groups.

Discussion

The super elasticity of NiTi rotary instruments makes these devices more advantageous in the mechanical preparation of complex and curved root canals with minimal alterations (displacement) in canal consistency (Schäfer and Oitzinger 2008). With respect to all superior characteristics of NiTi instruments, the separation of instruments in the root canal is a concern for clinicians because the retrieval of broken pieces is often difficult (Gutmann and Gao 2012, Madarati et al. 2013; Jamleh et al. 2014). Many authors have tried to provide a solution for this drawback by introducing new manufacturing methods (Gambarini et al. 2008) or heated rotary instruments in order to increase the instrument lifespan and reduce the separation possibility of these instruments during canal preparations (Zinelis et al. 2007).

In the present study, we used low and high voltage electrical direct current (DC) for 5 s to improve the cyclic fatigue of NiTi rotary files. In a previous study (Wanga et al. 2009), it was shown that short-term direct current heating (e.g., <6 s) can be safe without changing the original properties of the titanium alloy, while prolonged DC heating (e.g., >20 s) deteriorated the super elasticity. Here, in two of the experimental groups this treatment was used with two different voltages, while in other two groups, the files remained untreated. The difference between untreated groups was the relaxation time (pause rotating for 30 s) of cyclic fatigue testing in order to evaluate whether the element of time is effective on the cyclic fatigue of files or it is not a matter of concern.

The results of number of cyclic fatigue (NCF) testing indicated that low voltage direct current electrical stimulation can significantly increase the cyclic fatigue resistance in comparison with untreated rotary files. This result is consistent with the findings of the previous study, where authors indicated that short term DC heating has a positive effect on super-elasticity of titanium alloy and improved the properties of these alloys (Wanga et al. 2009; Miura 1988; Yoneyama et al. ’93). It seems that low voltage direct current can improve the super elasticity property of titanium alloy by phase transformation (Wanga et al. 2009).

The results of untreated samples in groups A and D showed no significant difference (Fig. 1), which indicated that the interruption (approximately pause for 30 s) has no significant effect on the final results of fatigue failure. In other word, the time interval between samples of groups A and D has no effect on cycled fatigue values of tested samples. However, between groups B and C where rotary files were treated with low or high voltage, significantly better results were noticed in favor of low voltage group. This difference can be explained by the intensity of direct current that was transferred through the body of the instrument. In the 12-V direct current treated samples, the heat generation caused micro structural changes in titanium alloy, which could affect the phase transformation leading to the elasticity improvement of devices and preventing cracks from growing or propagating inside the fatigued instruments. This issue was also detected on the SEM images of separated instruments from group B, where the surface of separated files revealed smoother and more regular pattern of fracture (Fig. 2). In contrast, in files treated with high voltage the increased heat generation adversely affected the NCF of the instruments. The effects of overheating of titanium alloys produced drastic changes in the transformation of these alloys, which can deteriorate the super elasticity of the instrument. The changes in structure of the samples in this group showed more irregularities on the surface of separated file and also several pits on their blades (Fig. 2).

Fig 1.

Fig 1

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Box plots of the number of cyclic fatigue (NCF) for the four groups. (A) without electrical treatment, (B) 12-V electrical treatment, (C) 24-V electrical treatment, and (D) without electrical treatment but with relaxation time.

Fig 2.

Fig 2

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SEM images of samples after experiment. (A) The body of file shows deep erosion fatigue striations and microcracks on the edge of group A. (B) The fractograph of group B showing dimple pattern of facture side. (C) The body of a separated sample of group C showing several pits on the blade. (D) The fractograph of a separated sample of group C showing more irregularities and dimplings versus Group B. (E) The fractograph of a separated sample of group B showing much more regular surface with less dimpling versus other groups (F) (Scale bar 50 μm).

The SEM results indicated that in the groups A and D the separated file surfaces were rougher with more irregularities. These images are similar to previous studies reported elongated dimple patterns on separated surfaces (Alapati et al. 2005; Arantes et al. 2014). The SEM images of high voltage treated samples showed more dimpling and irregularities on the surface in comparison with low voltage treated samples (Fig. 2D and E). This difference might be explained by the increased oxide layer on the surface of samples treated by high voltage direct current that can result in more dimpling of the surface. Although the results of current study revealed the promising use of electrical current to enhance the fatigue life of endodontic instruments, future investigations by X-ray diffractometer (XRD) are needed to address any phase changes into the structure of endodontic instrument following electrical treatment.

Conclusions

In accordance with results of the present study, the following conclusions can be made:

  • The application of a 12-V electrical current to NiTi rotary instruments during simulated root canal instrumentation can significantly increase their cyclic fatigue life, which can delay the time of separating in NiTi devices inside dental canal.

  • The usage of a low voltage (12-V electrical current) with NiTi rotary instruments during simulated root canal instrumentation can produce a smoother and regular wear pattern, and enhance more free fatigue life other than high voltage (24-V electrical current).

Acknowledgments

The authors thank Neda Bayati, Amir Pasha Mahmud Zadeh, and Jafar Orangi for their contributions.

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