Aust Endod J 2012

ORIGINAL RESEARCH

Effect of reciprocation usage of nickel-titanium rotary files on the cyclic fatigue resistance aej_352

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WooCheol Lee, DDS, PhD1,*; You-Jeong Hwang, DDS, MS2,*; Sung-Yeop You, DDS, MS1; and Hyeon-Cheol Kim, DDS, PhD3 1 Department of Conservative Dentistry, School of Dentistry, Dental Research Institute, Seoul National University, Seoul, Korea 2 Department of Dentistry, CHA Bundang Medical Center, CHA University, Seongnam, Korea 3 Department of Conservative Dentistry, School of Dentistry, Medical Research Institute, Pusan National University, Busan, Korea

Keywords continuous rotation, cyclic fatigue, fracture resistance, nickel-titanium rotary file, reciprocation. Correspondence Professor Hyeon-Cheol Kim, Department of Conservative Dentistry, School of Dentistry, Pusan National University, 3-3 Beomeo-Ri, Mulgeum, Yangsan, Gyeongnam 626-810, Korea. Email: [email protected] doi:10.1111/j.1747-4477.2012.00352.x *These authors contributed equally to this work.

Abstract The use of reciprocating motion for a nickel-titanium file has been claimed to increase its resistance to fatigue in comparison to continuous rotation. The purpose of this study was to investigate the effect using a reciprocating motion instead of continuous rotation for nickel-titanium files on their cyclic fatigue resistance. Cyclic fatigue tests simulating clinical use with ProTaper F2 and ProFile #25/.06 instruments were carried out in an artificial stainless-steel root canal with a 1.5 mm inner diameter, 5 mm radius and 60° angle of curvature. The instruments were driven using either continuous rotation or reciprocation until fracture. For the reciprocation, the rotation angles were set to 140° clockwise and 45° counterclockwise. The number of cycles to fracture was determined by measuring the time to fracture. Statistical analysis was performed using two-way ANOVA. There were no significant differences in the fracture cycles due to the use of reciprocation or continuous rotation for either instrument. The results found using reciprocation were comparable to those found using continuous rotation with regard to the cyclic fatigue resistance of nickel-titanium rotary files.

Introduction Nickel-titanium (NiTi) rotary instruments have been used for more than a decade in endodontic practice. The superelasticity of a NiTi rotary file enables clinicians to prepare root canals with less transportation and a more rounded and tapered canal (1–3). Despite these advantages, NiTi instruments have a high risk of separation during the root canal shaping procedure (4). Fracture of rotary NiTi instruments can be caused by torsional failure and/or flexural cyclic fatigue (1–3). Torsional failure occurs when the tip of the file binds in the root canal while the handpiece that holds the shank of the file continues to rotate. This type of instrument fracture can be avoided by employing a torque control motor (5). On the other hand, flexural cyclic bending fracture of the file occurs as a result of repeated compressive and tensile stresses accumulated at the point of maximum flexure in a curved canal (1–3). To reduce this type of

© 2012 The Authors Australian Endodontic Journal © 2012 Australian Society of Endodontology

failure, operators should avoid a static position of the rotating file but use an in-and-out pecking motion to the point where operator feels resistance exerted on the file during preparation (1–3). Both modes of fracture can happen simultaneously in a clinical situation; however, cyclic fatigue fractures are more prevalent, making up more than 90% of file separation incidents (6). Many studies have investigated the cyclic fatigue of NiTi rotary instruments (1,7,8). It is well known that various aspects of the instrument geometry, such as the size, taper, flute design and cross-sectional shape, are associated with fatigue fracture (5,7,9,10). Root canal shaping strategies, such as crown-down preparation, single-file instrumentation and the application of a pecking motion or brushing out motion, have been suggested to reduce the chance of NiTi file separation (11–13). Recently, the use of reciprocating motion (RM) instead of continuous rotation (CR) has been introduced and shown to extend the lifespan of NiTi rotary 1

Cyclic Fatigue of File by Reciprocating

instruments and increase their resistance to fatigue (14,15). It seems that using a single NiTi instrument under RM is more cost-effective and can relieve the practitioner of a lot of stress involved in learning a new technique (13,15). It is presumed that counterclockwise rotation in RM diminishes the torsional stress exerted on the NiTi file during the active canal shaping procedure, and that this movement ultimately increases the lifespan of the instrument (16). However, there is limited information on the actual mechanism involved in the cyclic fatigue fracture resistance of the files, especially under reciprocating preparation motion. We hypothesised that there is no difference in the cyclic fatigue resistance between CR and RM, regardless of file type. Therefore, the purpose of the present study was to evaluate the cyclic fatigue of two rotary instruments with different cross-sections used under CR and RM with a pecking motion.

Materials and methods Two different types of NiTi rotary instruments with different cross-sectional shapes and cutting blades were selected to see if there are any differences due to the file types. ProTaper (Dentsply Maillefer, Ballaigues, Switzerland) F2 files with a convex triangular cross-section and ProFile (Dentsply Maillefer) 0.06 taper/size #25 files with a triple U-shaped cross-section were selected. All files were 21 mm long and had diameters of 0.25 mm at D0, and the ProTaper had a progressively changing taper that was larger than that of the ProFile. Each new instrument was inspected for gross defects or deformities prior to the experiment with a dental microscope (Pico; Zeiss, Oberkochen, Germany).

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Cyclic fatigue tests simulating the clinical preparation procedure were done using the experimental design described by Oh et al. (7). In brief, an artificial stainlesssteel root canal with a 1.5 mm inner diameter, 5 mm radius and 60° angle of curvature was assembled between two thin metal plates (Fig. 1). A continuous up (4 mm) and down (4 mm) pecking movement was applied at 1 cycle per second in order to simulate the pecking motion in a real clinical situation. In each group, 12 files were tested using either CR or RM with an electric torquecontrolled motor (ATR Tecnika; Pistoia, Tuscany, Italy); the speed was set at 300 rpm. The rotation angles for the RM were set to 140° clockwise and 45° counterclockwise. The design of the study resulted in four experimental groups (ProTaper and ProFile for either CR or RM; Table 1). The time to failure for each instrument was automatically recorded by a digital chronometer. The number of cycles to failure (NCF) for each instrument was also calculated by multiplying the time (seconds) to failure by the number of rotations per second. The length of the fractured file tip was measured using a digital calliper (Preco Machine Tool Co., Shandong, China). The fractured surfaces were examined blindly by an operator using a scanning electron microscope (SEM, Hitachi S-4700; Hitachi, Tokyo, Japan) at magnifications of ¥200, ¥2000 and ¥10 000 in order to verify the mode of fracture. The NCFs under CR and RM for both file systems were analysed statistically with SPSS v11.5 software (SPSS, Chicago, IL, USA) using two-way ANOVA. The Student’s independent-samples t-test was used to compare the differences due to the use of CR or RM or due to the file types. The lengths of the fractured fragments were also compared using a t-test to find any differences due to the use of CR or RM for either the ProFile or ProTaper files.

Figure 1 Custom-made cyclic fatigue tester used in this study.

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© 2012 The Authors Australian Endodontic Journal © 2012 Australian Society of Endodontology

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Table 1 Number of cycles to failure (NCF) and length of fractured fragment (mm) of each file group (mean ⫾ SD) ProFile*

ProTaper

Group

CR

RM

CR

RM

NCF Fragment length**

1586 ⫾ 298 3.86 ⫾ 0.63

1365 ⫾ 310 4.01 ⫾ 0.27

666 ⫾ 80 4.61 ⫾ 0.96

625 ⫾ 52 4.60 ⫾ 0.97

*ProFile had higher NCFs than ProTaper regardless of instrument movement (P < 0.05). **There were no significant differences between CR and RM for either file system (P > 0.05). CR, continuous rotation; RM, reciprocating motion; SD, standard deviation.

The probability level for the statistical significance was set at 95%.

Results The NCFs for the four experimental groups are presented in Table 1. There were no significant effects caused by mutual action between the rotation movement and file type (P > 0.05). In addition, in both ProFile and ProTaper groups, there were no statistically significant differences in the NCFs for CR and RM (P > 0.05). However, ProFile showed significantly extended fracture resistance compared with ProTaper regardless of the rotational movements used (P < 0.05). The mean lengths of the fractured fragments of ProFile and ProTaper found using both techniques are also presented in Table 1. There were no significant differences in the fractured lengths under CR or RM for either file system (P > 0.05). The fractographic analysis showed similar fractographic features, with the presence of crack initiation origin(s), a crack propagation region and a fast fracture (overload) zone, for both rotational movements and for both files. Clusters of multiple fatigue striations were observed in the crack initiation areas of both NiTi files under highmagnification fractographic analysis (Fig. 2).

Discussion There have been continuous improvements in preparation techniques and instrument design for root canal treatment, with the goal of reducing the occurrence of separation of NiTi rotary instruments during the treatment. Recently, brand new file systems have been introduced to the market with claims that only one file, used with RM, can shape the root canal. In fact, this new preparation technique using RM with only one instrument was previously introduced for the ProTaper F2 file (13). This concept, using a single NiTi instrument to prepare the entire root canal, is interesting, because it could considerably reduce the learning curve by simplifying the technique (15). Moreover, the use of a single NiTi instrument is more cost-effective than the conventional multifile NiTi rotary techniques (15).

© 2012 The Authors Australian Endodontic Journal © 2012 Australian Society of Endodontology

The use of a RM may reduce the incidence of torsional fracture caused by taper lock and may extend the lifespan of the instrument (3,14,16). Regarding this claim, Shen et al. (17) have demonstrated that the lifespan of NiTi rotary instruments depends on the instrumentation technique rather than the file geometry or number of uses. Hence, the present study was designed to investigate solely the effect of cyclic flexural stress on the files caused by different rotational movements (CR vs. RM) by using two file systems with different cross-sectional shapes. Recently, De-Deus et al. (15) demonstrated that RM promoted an extended cyclic fatigue life for the F2 ProTaper file compared with conventional CR. However, the present study demonstrated that the instruments’ rotational motion, CR or RM, did not affect the ability of NiTi rotary instruments to resist cyclic fatigue, which is thought to be the predominant cause of file separation (18,19). The differences between these results might be attributed to a variation in the manner of file rotation or the method of calculating the NCF. The NCFs for the ProFile used in the present study (1586 and 1365 cycles under CR and RM, respectively) were consistent with that of Oh et al. (7), who applied dynamic cyclic pecking motion during the fatigue test. The dynamic pecking motion may reproduce the clinical situation more closely, as well as effectively reduce the stress concentration points and allow a continuous distribution of the stress on the instrument (20). The pecking motion might be a more critical factor affecting the resistance to fatigue than the type of rotation of the instrument. In the present study, the same condition of pecking motion was applied for all systems and methods. The NCF was calculated from the time to fracture for both CR and RM, because the same flexural stresses were generated during both clockwise and counterclockwise rotations. Two different types of NiTi files were introduced in order to see whether any different results were caused by the different rotational motions, CR or RM. The ProTaper was chosen because its convex triangular cross-sectional shape with neutral cutting edges allows it to cut in both directions under RM and provides a uniform distribution of torsional stress (9). The ProTaper instruments were reported to have a lower cyclic fatigue resistance than 3

Cyclic Fatigue of File by Reciprocating

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Figure 2 Scanning electron micrographs of fracture surfaces of separated fragments after cyclic fatigue fracture tests. Note that they have similar features typical of cyclic fatigue failure (row A, ProFile using continuous rotation (CR) technique; row B, ProFile using reciprocating technique; row C, ProTaper using CR technique; row D, ProTaper using reciprocating technique). Panels A, B, C and D (¥200) show crack initiation origins and rollover fast fracture zones. Central fibrous regions with dimples were similarly observed at the higher magnifications (¥2000; panels a, b, c and d). Clusters of multiple fatigue striations were observed in the crack initiation areas of both NiTi files under high-magnification fractographic analysis (¥10 000; a′, b′, c′ and d′).

other instruments, such as the ProFile (7,21,22). The ProFile has a concave cross-sectional shape with passive cutting edges, and it was shown to have a high fatigue resistance due to its small centre core. As expected, the ProFile showed significantly higher cyclic fatigue resistance than the ProTaper regardless of the instrument 4

motion (CR or RM) in the present study. The superiority of the ProFile instrument has been reported and described in several studies, where the major factors causing its improved flexibility and fatigue resistance were described as not only the small centre core but also the reduced taper of its shaft (7,23–25). The present study also confirmed

© 2012 The Authors Australian Endodontic Journal © 2012 Australian Society of Endodontology

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that the cross-sectional area is a significant factor influencing the fatigue resistance of NiTi rotary instruments. The SEM analysis in this study found that the fractographic appearance of the cyclic fatigue failures was typical, with the presence of crack initiation areas and rollover zones, not only for CR but also for RM. The dimples in the central fibrous regions of the instruments reflected ductile or flexural failures as well. It has been postulated that the extended lifespan of the instrument under RM was due to the superior resistance to torsional fatigue produced by rotation in the reverse direction before the stresses accumulated (15). The present study did not evaluate the influence of the instrument movement on the torsional stress, which might be reduced under RM. A file with higher resistance to flexural fatigue might not survive under (due to) even relatively small torsional stresses. This is because flexural fatigue resistance and torsional resistance are inversely proportional (2,10,26). Therefore, the relationship between the lifespan and the torsional fatigue resistance under RM should be investigated. In conclusion, within the limitations of the present study design, the cross-section of the instrument influenced the flexural fatigue resistance regardless of the rotational motion used. The null hypothesis was accepted, and thus, the method used to rotate the instrument did not affect the cyclic fatigue resistance of either ProFile or ProTaper instruments.

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© 2012 The Authors Australian Endodontic Journal © 2012 Australian Society of Endodontology

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© 2012 The Authors Australian Endodontic Journal © 2012 Australian Society of Endodontology

Effect of reciprocation usage of nickel-titanium rotary files on the cyclic fatigue resistance.

The use of reciprocating motion for a nickel-titanium file has been claimed to increase its resistance to fatigue in comparison to continuous rotation...
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