Journal of Investigative and Clinical Dentistry (2015), 0, 1–6
ORIGINAL ARTICLE Endodontics
Cyclic fatigue of two different single files with varying kinematics in a simulated double-curved canal Prasanna Neelakantan1, Pallavi Reddy2 & James L. Gutmann3 1 Biofilm Research Cluster, Department of Conservative Dentistry and Endodontics, Saveetha Dental College and Hospitals, Saveetha University, Chennai, India 2 Army College of Dental Sciences, Secunderabad, India 3 Baylor College of Dentistry, Texas A&M Health Science Center, Dallas, TX, USA
Keywords canal anatomy, cyclic fatigue, nickel-titanium, reciprocation, rotary, stress. Correspondence P. Neelakantan, Plot 1500, 16th Main Road, Anna Nagar West, Chennai, Tamil Nadu, India. Tel: +91-44-98847-54914 Fax: +91-044-2616-3639 Email: [email protected] Received 29 January 2014; accepted 5 April 2015. doi: 10.1111/jicd.12159
Abstract Objective: To evaluate and compare the cyclic fatigue of a rotary (One Shape) and reciprocating (Reciproc) single file system in a simulated S-shaped canal in static and dynamic models. Methods: The instruments (n = 50 per instrument) were tested in a custom-made device under static (n = 25) and dynamic loads (n = 25) to determine the number of cycles to fracture (NCF). The length of the fractured segments was also measured. Cyclic fatigue data were analyzed by one-way analysis of variance and Tukey honestly significantly different tests, while the data recorded for the fractured fragment lengths were compared using Student’s t-test with the level of significance set at 0.05. Results: The fatigue life of Reciproc was significantly higher in the static (coronal curvature: 717.60 45.32 NCF; apical curvature: 621.80 60.15 NCF) and dynamic models (coronal curvature: 972.40 54.30 NCF, apical curvature: 897.70 47.10 NCF) as compared with One Shape (P < 0.05). The fatigue life was significantly prolonged in the dynamic model for Reciproc (P < 0.05) as compared to One Shape (P > 0.05). Conclusion: The results of the present study showed that the single file reciprocating system (Reciproc) had a longer fatigue life than the single file rotary system (One Shape).
Introduction The objective of root canal instrumentation, in addition to removing the microbiologically contaminated root dentin, is to create a shape that will allow adequate disinfection of the root canal system by irrigants and creation of a space for the root filling material. Traditionally, the instruments used to attain this goal have been made from stainless steel. Nickel-titanium (Ni-Ti) alloys were introduced for this purpose because of their advantage, namely superelasticity, of overcoming the high elastic modulus of stainless steel instruments.1,2 The primary concern with Ni-Ti rotary instrument use has been the possibility of intracanal separation due to cyclic fatigue. This is especially true in the case of canals with severe curvatures. Contemporary research is focused on ª 2015 Wiley Publishing Asia Pty Ltd
identifying different variations of the Ni-Ti alloy, instrument design and manufacturing methods, surface treatments and novel instrumentation techniques that could potentially compensate for this fatigue and thereby minimize instrument separation. In terms of the alloy, M-wire is a type of Ni-Ti alloy developed through a special thermomechanical processing procedure. This alloy has been claimed to demonstrate increased flexibility and resistance to cyclic fatigue.3,4 Surface modification approaches like nitrogen ion implantation5 and electropolishing6 have been shown to enhance the resistance to cyclic fatigue. Ni-Ti instruments are manufactured by a grinding or twisting process, with the instruments manufactured by twisting demonstrating higher resistance to cyclic fatigue.7 Approaches that have received considerable attention to limit cyclic fatigue are variations in techniques of applica1
P. Neelakantan et al.
Cyclic fatigue of single file systems
tion. Following the pioneering work of Yared,8 reciprocating motion was advocated instead of rotation for the preparation of curved canals.9–11 Two single file reciprocating systems are presently available: Reciproc (VDW, Munich, Germany) and WaveOne (Dentsply Maillefer, Ballaigues, Switzerland). These systems offer easier root canal preparations with each system using only one instrument, which reflects a more simple learning curve.12 However, the use of these reciprocating file systems requires a dedicated motor. Claiming this as a drawback of these systems, a single file rotary file system has been introduced (One Shape, Micro-Mega, Besancon Cedex, France). Reciproc files are available in different sizes 25/0.08, 40/0.06 and 50/0.05 tapers. The manufacturer claims that these tapers are limited to the apical 3 mm while the remaining length of the file has a regressive taper up the shaft. The One Shape file is available in only one size (25/0.06). The Reciproc instrument has a S-shaped cross-section along the length of the instrument, while the One Shape instrument has been claimed to have three different cross-sections along the blade. The apical zone has a triangular design with three cutting edges. The second zone, termed the transition zone, has a cross-section that changes from three cutting edges to two, while the coronal zone has an S-shaped cross-section with two cutting edges. Although it has been theorized that reciprocating movement reduces cyclic fatigue, there is no published data on the cyclic fatigue of two single file instruments based on different kinematics in S-shaped canals. The aim of this study was to evaluate and compare the cyclic fatigue of these two instruments (Reciproc and One Shape), in a simulated S-shaped curvature, in both static and dynamic models. The null hypothesis was that there was no significant difference in the cyclic fatigue of the instruments tested in both the testing models. Materials and methods Fifty instruments each of R25 Reciproc (Group 1) and One Shape (Group 2) were used in this study. Twentyfive instruments of each group were used for the static model and the other 25 were used for the dynamic mode. For both the groups, files of 25 mm length were used. The cyclic fatigue testing was performed in a device described by Al-Sudani et al.13 Briefly, this device was used in an artificial canal with a double curvature; the first coronal curve with an angle of curvature of 60° and radius of 5 mm, located 8 mm from the tip of the instrument. The second curve was placed apically with a 70° angle and 2 mm radius of curvature, the center of which was placed at 2 mm from the tip. 2
Both the instruments were used in a VDW Gold Reciproc Motor (VDW, Munich, Germany). The Reciproc instruments were used in the “RECIPROC ALL” setting, whereas the One Shape files were used at 400 rpm and a torque of 4 N/cm as per the manufacturer’s recommendations. In the dynamic model, a mechanical device allowed back-and-forth axial movements, along with the reciprocating movement or continuous rotation inside the canal. The amplitude of the axial movements was 3 mm. In the static model, the back-and forth movements were not employed. All instruments were rotated until fracture occurred; the time to fracture (TtF) was recorded using a 1/100-sec chronometer and registered to the nearest whole number. Continuous movement of the instrument in the canal after fracture occurred at the apical curvature gave the number of cycles to fracture at the coronal curvature. To avoid any bias, the instruments were used by an endodontist who was well trained with both file systems but was neither aware of the objectives of the study nor the sponsorship of the files by the manufacturer. Data presentation and analysis The number of cycles to failure (NCF) for each instrument was calculated by multiplying the time (in seconds) to failure by the number of rotations or cycles per second immaterial of the rotational direction. Continuous motion of the instrument in the canal after fracture occurred at the apical curvature provided the data for calculation of the NCF for the coronal curvature. The NCF was calculated to the nearest whole number. The length of the fractured file tip was measured for each fractured instrument. Cyclic fatigue data were analyzed by one-way analysis of variance and Tukey honestly significantly different tests in order to determine any statistical differences between groups. Student’s t-test was performed between the data recorded for the fractured fragment lengths. The significance level was set at P = 0.05 for both the analyses. Results The NCF data and lengths of the fractured segments were determined as means and standard deviation (Table 1). Reciproc files showed statistically significantly longer fatigue life compared with One Shape instrument in both tests (P < 0.05). Instruments always fractured first in the second apical curvature and then in the coronal curvature. The cyclic fatigue for Reciproc in the coronal and apical curvatures showed no significant difference (P > 0.05). When comparing the results from static and dynamic tests, the fatigue life was markedly prolonged in the dynamic model for Reciproc (P < 0.05) as compared ª 2015 Wiley Publishing Asia Pty Ltd
P. Neelakantan et al.
Cyclic fatigue of single file systems
Table 1. Number of cycles to failure (NCF; mean SD) and fragment length (FL in mm) of the two instruments (n = 50) during static and dynamic cyclic fatigue testing NCF – static test
NCF – dynamic test
Instrument
Coronal curvature
Apical curvature
Reciproc (25/0.08) One Shape (25/0.06)
717.60 45.32 454.30 32.16b,1
621.80 60.15 210.20 42.75c,1
a,1
a,1
FL
Coronal curvature
Apical curvature
972.40 54.30 513.20 45.15b,1
897.70 47.10 320.50 52.35c,1
a,2
a,2
Coronal curvature
Apical curvature
5.3 1.0 5.9 1.1a
1.5 0.18a 2 0.5a
a
Values that share the same superscript letter were not significantly different (P > 0.05) within the same testing method; values that share the same superscript number were not significantly different between the different testing methods (P > 0.05).
with One Shape (P > 0.05). There was no significant difference in the NCF of the One Shape instruments in the static and dynamic models (P > 0.05). There was no significant difference in the length of the fractured segments for both the instruments (P > 0.05). Discussion The present study evaluated the cyclic fatigue of two single file systems that reflect differing kinematics. The results of the present study demonstrated that a single file reciprocating system (Reciproc) demonstrated higher resistance to cyclic fatigue than the single file rotary system (One Shape) in both the static and dynamic testing models. Hence the null hypothesis is rejected. As mentioned earlier, the factors influencing the cyclic fatigue of an instrument could be: nature of the root canal, cross-sectional design of the instrument, metallurgy of the alloy used, and kinematics of use. This study used a double curvature model for testing the cyclic fatigue of the instruments. To the knowledge of the authors, this is the first in vitro study to use the S-shaped simulated canal design to determine the cyclic fatigue of single file systems used in reciprocation and rotation. Another study, which used the same methodology for rotary instruments only, showed that the number of cycles before failure significantly decreases as the complexity of curvature increases.13 This was based on the rationale that in the case of single curvatures, instruments resisted longer before fracture.14,15 The results of the present study showed that instruments always fractured first at the apical curve rather than the coronal. This is in accordance with Al-Sudani et al.,13 and has been attributed to the rather abrupt radius of curvature in the apical curve. The contention that both the angle and radius of curvature are important determinants of cyclic fatigue must be recognized, with the radius being the primary issue. There are presently no guidelines for comparing the cyclic fatigue testing of rotary/reciprocating Ni-Ti instruments. Although the ideal method would involve preparation of root canals in natural teeth, this design cannot be standardized. Manufacturing an artificial canal that was ª 2015 Wiley Publishing Asia Pty Ltd
similar to the instrument size and taper would provide the instrument with a suitable trajectory.15,16 However, in this study standardized simulated curved canals were used because the files tested were single file systems. While the customized approach may hold true for multiple file rotary systems that enlarge the canal, single file systems are meant to work independently in any canal dimension. Furthermore, the mean lengths of the fractured segments in both the groups did not show any significant difference. This showed that the tested instruments were correctly positioned within the canal curvature and also demonstrated that similar stresses were being induced.12 The present study also used a static and dynamic approach to cyclic fatigue testing. Such a method has thus far not been used in an S-shaped curvature. This is of clinical relevance because of the axial oscillation movement, which allows stresses to be distributed along the shaft of the instrument, thereby preventing stress concentration in the same area (as would be the case in the static tests) and reducing the cyclic fatigue.17 This also reinforces the need for performing continuous pecking motions during instrumentation of curved root canals.18 The results for Reciproc were in accordance with the aforementioned work,17 however, for One Shape this did not hold true and hence it must be considered that cyclic fatigue is determined by an interplay of several factors. Although the direct role of the cross-sectional design on cyclic fatigue is yet to be elucidated, several studies have shown that the design does have a bearing on the fatigue life. As mentioned earlier, the Reciproc files have a regressive taper. The D16 diameter of the R25 instrument would therefore be 1.05 mm. The R25 instrument used in this study as the comparative group, that is, One Shape, is available in only one size (tip diameter 0.25 mm, taper 6%). The choice of these sizes was also in accordance with the recommendations of the manufacturers, as these sizes are designated for narrow and curved canals when hand instruments do not passively reach the full working length, which was simulated in the S-shaped curved canal in the present work. The core of One Shape (in lieu of its triangular cross-section) as compared to Reciproc’s S-shaped cross-section could be one of the reasons for 3
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Cyclic fatigue of single file systems
the results achieved. Such a correlation has been demonstrated for Wave One and ProTaper files (Dentsply Maillefer, Baillagues, Switzerland), which have shown lesser resistance to cyclic fatigue than Reciproc and Mtwo (VDW GmbH, Munich, Germany).12,19 The rationale that smaller volume per mm results in better resistance to cyclic fatigue has been shown for Mtwo instruments.16 The advantage of measuring the cross-sectional area helps correlate the volume per mm and hence, compare instruments with varying tapers and diameters.16 The crosssectional area at the tip, 5 and 10 mm for the One Shape instrument was 0.07, 0.23 and 0.54 mm2 respectively while that of the Reciproc instrument was 0.05, 0.20 and 0.75 mm2 respectively (Figure 1a–f). Hence, the volume per millimeter of Reciproc has to be higher than One Shape. Future studies should focus on the analysis of volume per millimeter of Reciproc and One Shape instruments. Theoretically, therefore, in the absence of any other differentiating factor, the cycles to fatigue for Reciproc should be lower than that of One Shape. The results of the present work were in contrast to this assumption. This could be attributed to two factors: the exact volume per millimeter of the tested instruments at every millimeter is not known and the type of Ni-Ti alloy used in both these instruments is different. M-wire Ni-Ti is a new alloy produced in an innovative thermal treatment process.20 The M-Wire contains all three crystalline phases, including deformed and micro-
twinned martensite, R-phase, and austenite.21 The Reciproc instrument is manufactured from a M-wire alloy while One Shape is manufactured from a superelastic NiTi alloy. The results of the present study are in accordance with others, which have shown that M-wire-based alloys demonstrate superior resistance to cyclic fatigue than superelastic Ni-Ti alloys.4,22–25 Kinematics of instruments used is one of the most important factors influencing cyclic fatigue of Ni-Ti instruments. Reciprocating movement relieves stress on the instrument and reduces the risk of cyclic fatigue caused by tension and compression.26,27 This movement, which is essentially a modification of the balanced force approach of canal preparation, enables preparation of root canals with one single instrument.8 In general, the reciprocation working motion consists of a counter-clockwise (CCW; cutting direction) and a clockwise motion (CW; release of the instrument). The exact degrees of reciprocation of the Reciproc instrument are not known. It has been reported that the angles may range between 150° CCW and 30° CW.28 The counterclockwise angle is greater than the clockwise one and hence the instrument continuously progresses towards the apex of the root canal. The angles of reciprocation are specific to the design of the particular instruments and are programmed in an electronic motor, which is why the VDW Gold Reciproc motor was used in this study. The angles of reciprocating are smaller than
(a)
(b)
(c)
(d)
(e)
(f)
Figure 1. Microscopic images of fractured surfaces of One Shape (a–c) and Reciproc (d–f) files at the tip (a,d), 5 (b,e) and 10 mm (c,f). The cross-sectional area at the tip, 5 mm and 10 mm for the One Shape instrument was 0.07, 0.23 and 0.54 mm2 respectively while that of the Reciproc instrument was 0.05, 0.20 and 0.75 mm2 respectively.
4
ª 2015 Wiley Publishing Asia Pty Ltd
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Cyclic fatigue of single file systems
the elastic limit of the files in a single reciprocating movement. However, this does not hold true when multiple anticlockwise/clockwise movements are made and the tip of the file binds in the canal.27 This study showed that the reciprocating movement showed superior resistance to cyclic fatigue which was in accordance with previous reports.12,17,29,30 Conclusion The instrument used in reciprocating movement (Reciproc) demonstrated superior resistance to cyclic fatigue than the instrument used in rotary movement (One
References 1 Sch€afer E, Schulz-Bongert U, Tulus G. Comparison of hand stainlesss steel and nickel titanium rotary instrumentation: a clinical study. J Endod 2004; 30: 432–5. 2 Chen JL, Messer HH. A comparison of stainless steel hand and rotary nickel-titanium instrumentation using a silicone impression technique. Aust Dent J 2002; 47: 12–20. 3 Gambarini G, Grande NM, Plotino G et al. Fatigue resistance of engine-driven rotary nickel-titanium instruments produced by new manufacturing methods. J Endod 2008; 34: 1003–5. 4 Johnson E, Lloyd A, Kuttler S, Namerow K. Comparison between a novel nickel-titanium alloy and 508 nitinol on the cyclic fatigue life of ProFile 25/ .04 rotary instruments. J Endod 2008; 34: 1406–9. 5 Gavini G, Pessoa OF, Barletta FB, Vasconcellos MAZ, Caldeira CL. Cyclic fatigue resistance of rotary nickel-titanium instruments submitted to nitrogen ion implantation. J Endod 2010; 36: 1183–6. 6 Praisarnti C, Chang JWW, Cheung GSP. Electropolishing enhances the resistance of nickel-titanium rotary files to corrosion–fatigue failure in hypochlorite. J Endod 2010; 36: 1354–7. 7 Rodrigues RCV, Lopes HP, Elias CN, Amarlal G, Vieira VTL, De Martin AS. Influence of different manufacturing methods on the cyclic fatigue of
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8
9
10
11
12
13
14
15
Shape) in both static and dynamic testing methods in a simulated S-shaped curvature. Reciprocating movement may be considered an important means of prolonging the fatigue life of Ni-Ti instruments, especially in curved canals. Acknowledgements The authors thank VDW GmBH, Germany for providing the instruments for the study. The first author is an opinion leader for VDW GmBH. The authors sincerely thank Dr Josette Camilleri (Malta) and Dr Nicola Grande (Italy) for the technical support.
rotary nickel-titanium endodontic instruments. J Endod 2011; 37: 1553– 7. Yared G. Canal preparation using only one Ni-Ti rotary instrument: preliminary observations. Int Endod J 2008; 1: 339–44. De-Deus G, Moreira EJ, Lopes HP, Elias CN. Extended cyclic fatigue life of F2 ProTaper instruments used in reciprocating movement. Int Endod J 2010; 43: 1063–8. Paque F, Zehnder M, De-Deus G. Microtomography-based comparison of reciprocating single-file F2 ProTaper technique versus rotary full sequence. J Endod 2011; 37: 1394–7. You SY, Bae KS, Baek SH, Kum KY, Shon WJ, Lee W. Lifespan of one nickel–titanium rotary file with reciprocating motion in curved root canals. J Endod 2010; 36: 1991–4. Plotino G, Grande NM, Testarelli L, Gambarini G. Cyclic fatigue of Reciproc and WaveOne reciprocating instruments. Int Endod J 2012; 45: 614–18. Al-Sudani D, Grande NM, Plotino G et al. Cyclic fatigue of nickel-itanium rotary instruments in a double (Sshaped) simulated curvature. J Endod 2012; 38: 897–989. Pruett JP, Clement DJ, Carnes DL. Cyclic fatigue testing of nickel-titanium endodontic systems. J Endod 1997; 23: 77–85. Plotino G, Grande NM, Sorci E, Malagnino VA, Somma F. A comparison of cyclic fatigue between used and
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17
18
19
20
21
22
new Mtwo NiTi rotary instruments. Int Endod J 2006; 39: 716–23. Grande NM, Plotino G, Pecci R et al. Cyclic fatigue resistance and threedimensional analysis of instruments from two nickel-titanium rotary systems. Int Endod J 2006; 39: 755–63. Lopes HP, Elias CN, Vieira MVB et al. Fatigue life of Reciproc and Mtwo instruments ubjected to static and dynamic tests. J Endod 2013; 39: 693–6. Lopes HP, Britto IM, Elias CN et al. Cyclic fatigue resistance of ProTaper universal instruments when subjected to static and dynamic tests. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2010; 110: 401–4. Lee MH, Versluis A, Kim BM, Lee CJ, Hur B, Kim HC. Correlation between experimental cyclic fatigue resistance and numerical stress analysis for nickel-titanium rotary files. J Endod 2011; 37: 1152–7. Gambarini G, Plotino G, Grande NM, Al-Sudani D, De Luca M, Testarelli L. Mechanical properties of nickel– titanium rotary instruments produced with a new manufacturing technique. Int Endod J 2011; 44: 337–41. Alapati SB, Brantley WA, Iijima M et al. Metallurgical characterization of a new nickel-titanium wire for rotary endodontic instruments. J Endod 2009; 35: 1589–93. Larsen CM, Watanabe I, Glickman GN, He J. Cyclic fatigue analysis of a new generation of nickel titanium rotary instruments. J Endod 2009; 35: 401–3.
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23 Al-Hadlaq SMS, AlJarbou FA, AlThumairy RI. Evaluation of cyclic flexural fatigue of M-wire nickel-titanium rotary instruments. J Endod 2010; 36: 305–7. 24 Ye J, Gao Y. Metallurgical characterization of M-wire nickel-titanium shape memory alloy used for endodontic rotary instruments during low-cycle fatigue. J Endod 2012; 38: 105–7. 25 Bouska J, Justman B, Williamson A, DeLong C, Qian F. Resistance to cyclic fatigue failure of a new endodontic rotary file. J Endod 2012; 38: 667–9. 26 De-Deus G, Brand~ao MC, Barino B, Di Giorgi K, Fidel RA, Luna AS.
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Assessment of apically extruded debris produced by the single-file ProTaper F2 technique under reciprocating movement. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2011; 110: 390–4. 27 B€ urklein S, Hinschitza K, Dammaschke T, Schafer E. Shaping ability and cleaning effectiveness of two single-file systems in severely curved root canals of extracted teeth: reciproc and WaveOne versus Mtwo and ProTaper. Int Endod J 2012; 45: 449– 61. 28 Kim HC, Kwak SW, Cheung GS et al. Cyclic fatigue and torsional resistance of two new nice titanium instruments
used in reciprocation motion: reciproc versus WaveOne. J Endod 2010; 38: 541–4. 29 Kiefner P, Ban M, De-Deus G. Is the reciprocating movement per se able to improve the cyclic fatigue resistance of instruments? Int Endod J 2014; 47: 430–6. 30 Gambarini G, Gergi R, Grande NM, Osta N, Plotino G, Testarelli L. Cyclic fatigue resistance of newly manufactured rotary nickel titanium instruments used in different rotational directions. Aust Endod J 2013; 39: 151–4.
ª 2015 Wiley Publishing Asia Pty Ltd
ORIGINAL ARTICLE Endodontics
Cyclic fatigue of two different single files with varying kinematics in a simulated double-curved canal Prasanna Neelakantan1, Pallavi Reddy2 & James L. Gutmann3 1 Biofilm Research Cluster, Department of Conservative Dentistry and Endodontics, Saveetha Dental College and Hospitals, Saveetha University, Chennai, India 2 Army College of Dental Sciences, Secunderabad, India 3 Baylor College of Dentistry, Texas A&M Health Science Center, Dallas, TX, USA
Keywords canal anatomy, cyclic fatigue, nickel-titanium, reciprocation, rotary, stress. Correspondence P. Neelakantan, Plot 1500, 16th Main Road, Anna Nagar West, Chennai, Tamil Nadu, India. Tel: +91-44-98847-54914 Fax: +91-044-2616-3639 Email: [email protected] Received 29 January 2014; accepted 5 April 2015. doi: 10.1111/jicd.12159
Abstract Objective: To evaluate and compare the cyclic fatigue of a rotary (One Shape) and reciprocating (Reciproc) single file system in a simulated S-shaped canal in static and dynamic models. Methods: The instruments (n = 50 per instrument) were tested in a custom-made device under static (n = 25) and dynamic loads (n = 25) to determine the number of cycles to fracture (NCF). The length of the fractured segments was also measured. Cyclic fatigue data were analyzed by one-way analysis of variance and Tukey honestly significantly different tests, while the data recorded for the fractured fragment lengths were compared using Student’s t-test with the level of significance set at 0.05. Results: The fatigue life of Reciproc was significantly higher in the static (coronal curvature: 717.60 45.32 NCF; apical curvature: 621.80 60.15 NCF) and dynamic models (coronal curvature: 972.40 54.30 NCF, apical curvature: 897.70 47.10 NCF) as compared with One Shape (P < 0.05). The fatigue life was significantly prolonged in the dynamic model for Reciproc (P < 0.05) as compared to One Shape (P > 0.05). Conclusion: The results of the present study showed that the single file reciprocating system (Reciproc) had a longer fatigue life than the single file rotary system (One Shape).
Introduction The objective of root canal instrumentation, in addition to removing the microbiologically contaminated root dentin, is to create a shape that will allow adequate disinfection of the root canal system by irrigants and creation of a space for the root filling material. Traditionally, the instruments used to attain this goal have been made from stainless steel. Nickel-titanium (Ni-Ti) alloys were introduced for this purpose because of their advantage, namely superelasticity, of overcoming the high elastic modulus of stainless steel instruments.1,2 The primary concern with Ni-Ti rotary instrument use has been the possibility of intracanal separation due to cyclic fatigue. This is especially true in the case of canals with severe curvatures. Contemporary research is focused on ª 2015 Wiley Publishing Asia Pty Ltd
identifying different variations of the Ni-Ti alloy, instrument design and manufacturing methods, surface treatments and novel instrumentation techniques that could potentially compensate for this fatigue and thereby minimize instrument separation. In terms of the alloy, M-wire is a type of Ni-Ti alloy developed through a special thermomechanical processing procedure. This alloy has been claimed to demonstrate increased flexibility and resistance to cyclic fatigue.3,4 Surface modification approaches like nitrogen ion implantation5 and electropolishing6 have been shown to enhance the resistance to cyclic fatigue. Ni-Ti instruments are manufactured by a grinding or twisting process, with the instruments manufactured by twisting demonstrating higher resistance to cyclic fatigue.7 Approaches that have received considerable attention to limit cyclic fatigue are variations in techniques of applica1
P. Neelakantan et al.
Cyclic fatigue of single file systems
tion. Following the pioneering work of Yared,8 reciprocating motion was advocated instead of rotation for the preparation of curved canals.9–11 Two single file reciprocating systems are presently available: Reciproc (VDW, Munich, Germany) and WaveOne (Dentsply Maillefer, Ballaigues, Switzerland). These systems offer easier root canal preparations with each system using only one instrument, which reflects a more simple learning curve.12 However, the use of these reciprocating file systems requires a dedicated motor. Claiming this as a drawback of these systems, a single file rotary file system has been introduced (One Shape, Micro-Mega, Besancon Cedex, France). Reciproc files are available in different sizes 25/0.08, 40/0.06 and 50/0.05 tapers. The manufacturer claims that these tapers are limited to the apical 3 mm while the remaining length of the file has a regressive taper up the shaft. The One Shape file is available in only one size (25/0.06). The Reciproc instrument has a S-shaped cross-section along the length of the instrument, while the One Shape instrument has been claimed to have three different cross-sections along the blade. The apical zone has a triangular design with three cutting edges. The second zone, termed the transition zone, has a cross-section that changes from three cutting edges to two, while the coronal zone has an S-shaped cross-section with two cutting edges. Although it has been theorized that reciprocating movement reduces cyclic fatigue, there is no published data on the cyclic fatigue of two single file instruments based on different kinematics in S-shaped canals. The aim of this study was to evaluate and compare the cyclic fatigue of these two instruments (Reciproc and One Shape), in a simulated S-shaped curvature, in both static and dynamic models. The null hypothesis was that there was no significant difference in the cyclic fatigue of the instruments tested in both the testing models. Materials and methods Fifty instruments each of R25 Reciproc (Group 1) and One Shape (Group 2) were used in this study. Twentyfive instruments of each group were used for the static model and the other 25 were used for the dynamic mode. For both the groups, files of 25 mm length were used. The cyclic fatigue testing was performed in a device described by Al-Sudani et al.13 Briefly, this device was used in an artificial canal with a double curvature; the first coronal curve with an angle of curvature of 60° and radius of 5 mm, located 8 mm from the tip of the instrument. The second curve was placed apically with a 70° angle and 2 mm radius of curvature, the center of which was placed at 2 mm from the tip. 2
Both the instruments were used in a VDW Gold Reciproc Motor (VDW, Munich, Germany). The Reciproc instruments were used in the “RECIPROC ALL” setting, whereas the One Shape files were used at 400 rpm and a torque of 4 N/cm as per the manufacturer’s recommendations. In the dynamic model, a mechanical device allowed back-and-forth axial movements, along with the reciprocating movement or continuous rotation inside the canal. The amplitude of the axial movements was 3 mm. In the static model, the back-and forth movements were not employed. All instruments were rotated until fracture occurred; the time to fracture (TtF) was recorded using a 1/100-sec chronometer and registered to the nearest whole number. Continuous movement of the instrument in the canal after fracture occurred at the apical curvature gave the number of cycles to fracture at the coronal curvature. To avoid any bias, the instruments were used by an endodontist who was well trained with both file systems but was neither aware of the objectives of the study nor the sponsorship of the files by the manufacturer. Data presentation and analysis The number of cycles to failure (NCF) for each instrument was calculated by multiplying the time (in seconds) to failure by the number of rotations or cycles per second immaterial of the rotational direction. Continuous motion of the instrument in the canal after fracture occurred at the apical curvature provided the data for calculation of the NCF for the coronal curvature. The NCF was calculated to the nearest whole number. The length of the fractured file tip was measured for each fractured instrument. Cyclic fatigue data were analyzed by one-way analysis of variance and Tukey honestly significantly different tests in order to determine any statistical differences between groups. Student’s t-test was performed between the data recorded for the fractured fragment lengths. The significance level was set at P = 0.05 for both the analyses. Results The NCF data and lengths of the fractured segments were determined as means and standard deviation (Table 1). Reciproc files showed statistically significantly longer fatigue life compared with One Shape instrument in both tests (P < 0.05). Instruments always fractured first in the second apical curvature and then in the coronal curvature. The cyclic fatigue for Reciproc in the coronal and apical curvatures showed no significant difference (P > 0.05). When comparing the results from static and dynamic tests, the fatigue life was markedly prolonged in the dynamic model for Reciproc (P < 0.05) as compared ª 2015 Wiley Publishing Asia Pty Ltd
P. Neelakantan et al.
Cyclic fatigue of single file systems
Table 1. Number of cycles to failure (NCF; mean SD) and fragment length (FL in mm) of the two instruments (n = 50) during static and dynamic cyclic fatigue testing NCF – static test
NCF – dynamic test
Instrument
Coronal curvature
Apical curvature
Reciproc (25/0.08) One Shape (25/0.06)
717.60 45.32 454.30 32.16b,1
621.80 60.15 210.20 42.75c,1
a,1
a,1
FL
Coronal curvature
Apical curvature
972.40 54.30 513.20 45.15b,1
897.70 47.10 320.50 52.35c,1
a,2
a,2
Coronal curvature
Apical curvature
5.3 1.0 5.9 1.1a
1.5 0.18a 2 0.5a
a
Values that share the same superscript letter were not significantly different (P > 0.05) within the same testing method; values that share the same superscript number were not significantly different between the different testing methods (P > 0.05).
with One Shape (P > 0.05). There was no significant difference in the NCF of the One Shape instruments in the static and dynamic models (P > 0.05). There was no significant difference in the length of the fractured segments for both the instruments (P > 0.05). Discussion The present study evaluated the cyclic fatigue of two single file systems that reflect differing kinematics. The results of the present study demonstrated that a single file reciprocating system (Reciproc) demonstrated higher resistance to cyclic fatigue than the single file rotary system (One Shape) in both the static and dynamic testing models. Hence the null hypothesis is rejected. As mentioned earlier, the factors influencing the cyclic fatigue of an instrument could be: nature of the root canal, cross-sectional design of the instrument, metallurgy of the alloy used, and kinematics of use. This study used a double curvature model for testing the cyclic fatigue of the instruments. To the knowledge of the authors, this is the first in vitro study to use the S-shaped simulated canal design to determine the cyclic fatigue of single file systems used in reciprocation and rotation. Another study, which used the same methodology for rotary instruments only, showed that the number of cycles before failure significantly decreases as the complexity of curvature increases.13 This was based on the rationale that in the case of single curvatures, instruments resisted longer before fracture.14,15 The results of the present study showed that instruments always fractured first at the apical curve rather than the coronal. This is in accordance with Al-Sudani et al.,13 and has been attributed to the rather abrupt radius of curvature in the apical curve. The contention that both the angle and radius of curvature are important determinants of cyclic fatigue must be recognized, with the radius being the primary issue. There are presently no guidelines for comparing the cyclic fatigue testing of rotary/reciprocating Ni-Ti instruments. Although the ideal method would involve preparation of root canals in natural teeth, this design cannot be standardized. Manufacturing an artificial canal that was ª 2015 Wiley Publishing Asia Pty Ltd
similar to the instrument size and taper would provide the instrument with a suitable trajectory.15,16 However, in this study standardized simulated curved canals were used because the files tested were single file systems. While the customized approach may hold true for multiple file rotary systems that enlarge the canal, single file systems are meant to work independently in any canal dimension. Furthermore, the mean lengths of the fractured segments in both the groups did not show any significant difference. This showed that the tested instruments were correctly positioned within the canal curvature and also demonstrated that similar stresses were being induced.12 The present study also used a static and dynamic approach to cyclic fatigue testing. Such a method has thus far not been used in an S-shaped curvature. This is of clinical relevance because of the axial oscillation movement, which allows stresses to be distributed along the shaft of the instrument, thereby preventing stress concentration in the same area (as would be the case in the static tests) and reducing the cyclic fatigue.17 This also reinforces the need for performing continuous pecking motions during instrumentation of curved root canals.18 The results for Reciproc were in accordance with the aforementioned work,17 however, for One Shape this did not hold true and hence it must be considered that cyclic fatigue is determined by an interplay of several factors. Although the direct role of the cross-sectional design on cyclic fatigue is yet to be elucidated, several studies have shown that the design does have a bearing on the fatigue life. As mentioned earlier, the Reciproc files have a regressive taper. The D16 diameter of the R25 instrument would therefore be 1.05 mm. The R25 instrument used in this study as the comparative group, that is, One Shape, is available in only one size (tip diameter 0.25 mm, taper 6%). The choice of these sizes was also in accordance with the recommendations of the manufacturers, as these sizes are designated for narrow and curved canals when hand instruments do not passively reach the full working length, which was simulated in the S-shaped curved canal in the present work. The core of One Shape (in lieu of its triangular cross-section) as compared to Reciproc’s S-shaped cross-section could be one of the reasons for 3
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the results achieved. Such a correlation has been demonstrated for Wave One and ProTaper files (Dentsply Maillefer, Baillagues, Switzerland), which have shown lesser resistance to cyclic fatigue than Reciproc and Mtwo (VDW GmbH, Munich, Germany).12,19 The rationale that smaller volume per mm results in better resistance to cyclic fatigue has been shown for Mtwo instruments.16 The advantage of measuring the cross-sectional area helps correlate the volume per mm and hence, compare instruments with varying tapers and diameters.16 The crosssectional area at the tip, 5 and 10 mm for the One Shape instrument was 0.07, 0.23 and 0.54 mm2 respectively while that of the Reciproc instrument was 0.05, 0.20 and 0.75 mm2 respectively (Figure 1a–f). Hence, the volume per millimeter of Reciproc has to be higher than One Shape. Future studies should focus on the analysis of volume per millimeter of Reciproc and One Shape instruments. Theoretically, therefore, in the absence of any other differentiating factor, the cycles to fatigue for Reciproc should be lower than that of One Shape. The results of the present work were in contrast to this assumption. This could be attributed to two factors: the exact volume per millimeter of the tested instruments at every millimeter is not known and the type of Ni-Ti alloy used in both these instruments is different. M-wire Ni-Ti is a new alloy produced in an innovative thermal treatment process.20 The M-Wire contains all three crystalline phases, including deformed and micro-
twinned martensite, R-phase, and austenite.21 The Reciproc instrument is manufactured from a M-wire alloy while One Shape is manufactured from a superelastic NiTi alloy. The results of the present study are in accordance with others, which have shown that M-wire-based alloys demonstrate superior resistance to cyclic fatigue than superelastic Ni-Ti alloys.4,22–25 Kinematics of instruments used is one of the most important factors influencing cyclic fatigue of Ni-Ti instruments. Reciprocating movement relieves stress on the instrument and reduces the risk of cyclic fatigue caused by tension and compression.26,27 This movement, which is essentially a modification of the balanced force approach of canal preparation, enables preparation of root canals with one single instrument.8 In general, the reciprocation working motion consists of a counter-clockwise (CCW; cutting direction) and a clockwise motion (CW; release of the instrument). The exact degrees of reciprocation of the Reciproc instrument are not known. It has been reported that the angles may range between 150° CCW and 30° CW.28 The counterclockwise angle is greater than the clockwise one and hence the instrument continuously progresses towards the apex of the root canal. The angles of reciprocation are specific to the design of the particular instruments and are programmed in an electronic motor, which is why the VDW Gold Reciproc motor was used in this study. The angles of reciprocating are smaller than
(a)
(b)
(c)
(d)
(e)
(f)
Figure 1. Microscopic images of fractured surfaces of One Shape (a–c) and Reciproc (d–f) files at the tip (a,d), 5 (b,e) and 10 mm (c,f). The cross-sectional area at the tip, 5 mm and 10 mm for the One Shape instrument was 0.07, 0.23 and 0.54 mm2 respectively while that of the Reciproc instrument was 0.05, 0.20 and 0.75 mm2 respectively.
4
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the elastic limit of the files in a single reciprocating movement. However, this does not hold true when multiple anticlockwise/clockwise movements are made and the tip of the file binds in the canal.27 This study showed that the reciprocating movement showed superior resistance to cyclic fatigue which was in accordance with previous reports.12,17,29,30 Conclusion The instrument used in reciprocating movement (Reciproc) demonstrated superior resistance to cyclic fatigue than the instrument used in rotary movement (One
References 1 Sch€afer E, Schulz-Bongert U, Tulus G. Comparison of hand stainlesss steel and nickel titanium rotary instrumentation: a clinical study. J Endod 2004; 30: 432–5. 2 Chen JL, Messer HH. A comparison of stainless steel hand and rotary nickel-titanium instrumentation using a silicone impression technique. Aust Dent J 2002; 47: 12–20. 3 Gambarini G, Grande NM, Plotino G et al. Fatigue resistance of engine-driven rotary nickel-titanium instruments produced by new manufacturing methods. J Endod 2008; 34: 1003–5. 4 Johnson E, Lloyd A, Kuttler S, Namerow K. Comparison between a novel nickel-titanium alloy and 508 nitinol on the cyclic fatigue life of ProFile 25/ .04 rotary instruments. J Endod 2008; 34: 1406–9. 5 Gavini G, Pessoa OF, Barletta FB, Vasconcellos MAZ, Caldeira CL. Cyclic fatigue resistance of rotary nickel-titanium instruments submitted to nitrogen ion implantation. J Endod 2010; 36: 1183–6. 6 Praisarnti C, Chang JWW, Cheung GSP. Electropolishing enhances the resistance of nickel-titanium rotary files to corrosion–fatigue failure in hypochlorite. J Endod 2010; 36: 1354–7. 7 Rodrigues RCV, Lopes HP, Elias CN, Amarlal G, Vieira VTL, De Martin AS. Influence of different manufacturing methods on the cyclic fatigue of
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8
9
10
11
12
13
14
15
Shape) in both static and dynamic testing methods in a simulated S-shaped curvature. Reciprocating movement may be considered an important means of prolonging the fatigue life of Ni-Ti instruments, especially in curved canals. Acknowledgements The authors thank VDW GmBH, Germany for providing the instruments for the study. The first author is an opinion leader for VDW GmBH. The authors sincerely thank Dr Josette Camilleri (Malta) and Dr Nicola Grande (Italy) for the technical support.
rotary nickel-titanium endodontic instruments. J Endod 2011; 37: 1553– 7. Yared G. Canal preparation using only one Ni-Ti rotary instrument: preliminary observations. Int Endod J 2008; 1: 339–44. De-Deus G, Moreira EJ, Lopes HP, Elias CN. Extended cyclic fatigue life of F2 ProTaper instruments used in reciprocating movement. Int Endod J 2010; 43: 1063–8. Paque F, Zehnder M, De-Deus G. Microtomography-based comparison of reciprocating single-file F2 ProTaper technique versus rotary full sequence. J Endod 2011; 37: 1394–7. You SY, Bae KS, Baek SH, Kum KY, Shon WJ, Lee W. Lifespan of one nickel–titanium rotary file with reciprocating motion in curved root canals. J Endod 2010; 36: 1991–4. Plotino G, Grande NM, Testarelli L, Gambarini G. Cyclic fatigue of Reciproc and WaveOne reciprocating instruments. Int Endod J 2012; 45: 614–18. Al-Sudani D, Grande NM, Plotino G et al. Cyclic fatigue of nickel-itanium rotary instruments in a double (Sshaped) simulated curvature. J Endod 2012; 38: 897–989. Pruett JP, Clement DJ, Carnes DL. Cyclic fatigue testing of nickel-titanium endodontic systems. J Endod 1997; 23: 77–85. Plotino G, Grande NM, Sorci E, Malagnino VA, Somma F. A comparison of cyclic fatigue between used and
16
17
18
19
20
21
22
new Mtwo NiTi rotary instruments. Int Endod J 2006; 39: 716–23. Grande NM, Plotino G, Pecci R et al. Cyclic fatigue resistance and threedimensional analysis of instruments from two nickel-titanium rotary systems. Int Endod J 2006; 39: 755–63. Lopes HP, Elias CN, Vieira MVB et al. Fatigue life of Reciproc and Mtwo instruments ubjected to static and dynamic tests. J Endod 2013; 39: 693–6. Lopes HP, Britto IM, Elias CN et al. Cyclic fatigue resistance of ProTaper universal instruments when subjected to static and dynamic tests. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2010; 110: 401–4. Lee MH, Versluis A, Kim BM, Lee CJ, Hur B, Kim HC. Correlation between experimental cyclic fatigue resistance and numerical stress analysis for nickel-titanium rotary files. J Endod 2011; 37: 1152–7. Gambarini G, Plotino G, Grande NM, Al-Sudani D, De Luca M, Testarelli L. Mechanical properties of nickel– titanium rotary instruments produced with a new manufacturing technique. Int Endod J 2011; 44: 337–41. Alapati SB, Brantley WA, Iijima M et al. Metallurgical characterization of a new nickel-titanium wire for rotary endodontic instruments. J Endod 2009; 35: 1589–93. Larsen CM, Watanabe I, Glickman GN, He J. Cyclic fatigue analysis of a new generation of nickel titanium rotary instruments. J Endod 2009; 35: 401–3.
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23 Al-Hadlaq SMS, AlJarbou FA, AlThumairy RI. Evaluation of cyclic flexural fatigue of M-wire nickel-titanium rotary instruments. J Endod 2010; 36: 305–7. 24 Ye J, Gao Y. Metallurgical characterization of M-wire nickel-titanium shape memory alloy used for endodontic rotary instruments during low-cycle fatigue. J Endod 2012; 38: 105–7. 25 Bouska J, Justman B, Williamson A, DeLong C, Qian F. Resistance to cyclic fatigue failure of a new endodontic rotary file. J Endod 2012; 38: 667–9. 26 De-Deus G, Brand~ao MC, Barino B, Di Giorgi K, Fidel RA, Luna AS.
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Assessment of apically extruded debris produced by the single-file ProTaper F2 technique under reciprocating movement. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2011; 110: 390–4. 27 B€ urklein S, Hinschitza K, Dammaschke T, Schafer E. Shaping ability and cleaning effectiveness of two single-file systems in severely curved root canals of extracted teeth: reciproc and WaveOne versus Mtwo and ProTaper. Int Endod J 2012; 45: 449– 61. 28 Kim HC, Kwak SW, Cheung GS et al. Cyclic fatigue and torsional resistance of two new nice titanium instruments
used in reciprocation motion: reciproc versus WaveOne. J Endod 2010; 38: 541–4. 29 Kiefner P, Ban M, De-Deus G. Is the reciprocating movement per se able to improve the cyclic fatigue resistance of instruments? Int Endod J 2014; 47: 430–6. 30 Gambarini G, Gergi R, Grande NM, Osta N, Plotino G, Testarelli L. Cyclic fatigue resistance of newly manufactured rotary nickel titanium instruments used in different rotational directions. Aust Endod J 2013; 39: 151–4.
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