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Aust Endod J 2014
ORIGINAL RESEARCH
Comparison of cyclic fatigue resistance of novel nickel-titanium rotary instruments Ismail Davut Capar, DDS, PhD1; Huseyin Ertas, DDS, PhD1; and Hakan Arslan DDS, PhD2 1 2
Department of Endodontics, Faculty of Dentistry, I˙zmir Katip Çelebi University, I˙zmir, Turkey Department of Endodontics, Faculty of Dentistry, Atatürk University, Erzurum, Turkey
Keywords cyclic fatigue; heat treatment, nickel-titanium files, ProTaper Next, rotary. Correspondence Dr Ismail Davut Capar, Department of Endodontics, Faculty of Dentistry, I˙zmir Katip Çelebi University, Izmir 35620, Turkey. Email: [email protected] doi:10.1111/aej.12067
Abstract New files (ProTaper Next/HyFlex/OneShape) are made from novel nickeltitanium (NiTi) alloys/treatments. The purpose of this study was to compare the cyclic fatigue resistance of these new instruments with that of Revo-S instruments. Four groups of 20 NiTi endodontic instruments were tested in steel canals with a 3 mm radius and a 60° angle of curvature. The cyclic fatigue of the following NiTi instruments with a tip size 25 and 0.06 taper that were manufactured with different alloys was tested: ProTaper Next X2 (M-Wire), OneShape (conventional NiTi), Revo-S Shaping Universal (conventional NiTi) and HyFlex 25/0.6 (controlled memory NiTi wire). A one-way ANOVA and post-hoc Tukey’s test (α = 0.05) revealed that the HyFlex files had the highest fatigue resistance and the Revo-S had the least fatigue resistance among the groups (P < 0.001).
Introduction During root canal treatment, nickel-titanium (NiTi) rotary instruments may fracture inside the root canal without any sign of previous permanent deformation (1,2), thereby it may jeopardise the outcome of the root canal treatment. In clinical practice, the fracture of NiTi rotary instruments occurs via two different mechanisms: torsional fracture and flexural fatigue (3). Torsional fracture occurs when part of the instrument binds to the dentin while the file continues to rotate (4). Flexural fatigue fracture of the file occurs when the instrument rotates freely in a curvature, generating tension/ compression cycles in the region of maximum flexure until fracture occurs (5). Technological advancements in NiTi instruments have led to new concepts of use, different kinematics and improved alloys that purportedly increase the cyclic fatigue resistance of the instruments. The OneShape (MicroMega, Besancon, France) file is a new file system that is used in a traditional, continuous and rotational motion. It uses a single-file system to shape the root canal completely from start to finish. ProTaper Next (Dentsply Maillefer, Ballaigues, Switzerland) is a novel NiTi file system. It has an off-centred rectangular design and
© 2014 Australian Society of Endodontology
progressive and regressive percentage tapers on a single file. Revo-S (MicroMega) file system has an off-centred design similar to the ProTaper Next file, the off-centred design generates travelling waves of motion along the active part of the file. Recently, HyFlex rotary instruments (Coltene-Whaledent, Allstetten, Switzerland) have been introduced. They are made from a controlled memory (CM) NiTi wire, which is manufactured by a unique process that controls the material’s memory. This CM feature makes the files extremely flexible and makes the instruments more resistant to cyclic fatigue than non-CM NiTi instruments (6). In the literature, there are no data about the cyclic fatigue resistance of the ProTaper Next and OneShape file systems. Thus, the purpose of this study was to compare the cyclic fatigue resistance of these new files with that of HyFlex and Revo-S. The null hypothesis of this study was that there would be no difference in the cyclic fatigue resistance among the tested files.
Materials and methods The cyclic fatigue of the following new rotary NiTi instruments was tested: ProTaper Next X2, OneShape, Revo-S Shaping Universal and HyFlex. All the instruments were 1
Cyclic Fatigue of Novel Instruments
I. D. Capar et al.
size 25 with a 0.06 taper. Twenty instruments from each brand were evaluated in air at a temperature of 23°C. An artificial canal was made out of a testing block of stainless steel with an inner diameter of 1.5 mm, a 60° angle of curvature and a curvature radius of 3 mm. The canals were covered with glass to prevent the instruments from slipping out. The apparatus used in the cyclic fatigue test was described previously (7). The working length was standardised to 19 mm for all the files. To reduce the friction of the file as it contacted the artificial canal walls, a special oil (WD-40 Company, Milton Keynes, England) was used for lubrication. All the instruments were operated with a low-torque motor (VDW Silver; VDW, Munich, Germany) and were rotated according to the manufacturer’s recommendations as follows: ProTaper Next with 300 rpm and 200 gcm torque, OneShape with 400 rpm and 400 gcm torque, Revo-S with 250 rpm and 200 gcm torque and HyFlex with 500 rpm and 250 gcm torque. The instruments were used until fracture occurred, and the time to fracture was recorded in seconds. The number of cycles to fracture (NCF) was calculated using the following formula: NCF = time (s) to failure × rotational speed/60. The broken instruments were ultrasonically cleaned in alcohol for scanning electron microscopy examination. To verify the fracture mode, the surface of the fractured part of the representative samples from each group was photographed under scanning electron microscopy (Evo LS10; Carl Zeiss, Oberkochen, Germany). After completing the cyclic fatigue test, the NCF data were subjected to a Shapiro–Wilk test to analyse the normality of the continuous variables. The data were then statistically analysed using a one-way ANOVA and post-hoc Tukey’s test (α = 0.05).
Results The NCF of the fractured fragment for each brand are presented in Table 1. The HyFlex files had the highest fatigue resistance and the Revo-S had the least fatigue resistance among the groups (P < 0.001). There was no
Table 1 Comparison of mean number of cycles to fracture (NCF) values
Groups
NCF
Standard deviation
Statistical significance†
ProTaper Next OneShape Revo-S HyFlex
504 510 270 1237
79 78 31 206
a a b c
†Different letters indicate a significant difference between groups.
2
significant difference between the mean NCF of the ProTaper Next and the OneShape (P = 0.998). The scanning electron microscopy images of the fracture surface showed the nature of the mechanical damage of the cyclic fatigue failure in all the groups (Fig. 1).
Discussion Removal of vital and necrotic pulp tissue, microorganisms and their toxins from the root canal system is critical for successful root canal therapy (8–10). During instrumentation if an instrument fractured, the root canal system could not be completely cleaned. In the literature, there are conflicting results on impact of fractured files on outcome. Spili et al. (11) revealed that presence of preoperative periapical lesion is more significant rather than fractured instrument for healing rate. However, Grossman (12) reported lower healing rate for fractured instrument cases with a concomitant preoperative periapical lesion. Therefore, the purpose of this study was to compare the cyclic fatigue resistance of new files (ProTaper Next/HyFlex/OneShape) with that of Revo-S instruments. The mechanical behaviour of NiTi alloys is related to the proportions and the characteristics of the microstructural phases. The fatigue resistance of NiTi endodontic files is affected by the transition temperatures of the NiTi alloys that are mainly adjusted with heat treatment (13,14). In recent years, several novel thermomechanical processing and manufacturing technologies, such as M-Wire, and CM wire, have been developed to optimise the microstructure of NiTi alloys (15). With the development of these technologies, instruments with improved mechanical properties have been manufactured from these new alloys and commercialised. Cyclic fatigue testing devices permit the instruments to rotate until fracture using different curvature. The cyclic fatigue life span of rotary instruments decreases with increasing instrument diameters (16). The metal mass of the instrument at the point of maximum stress is another important factor (17). Therefore, in the present study, the fatigue behaviour of NiTi instruments having a similar taper and tip size was examined. Moreover, M-Wire (i.e. ProTaper Next), traditional NiTi (i.e. Revo-S and OneShape) and CM (HyFlex) instruments were tested to check whether different manufacturing methods influenced the fatigue resistance of the endodontic instruments produced with different alloys. The null hypothesis was rejected. The results showed that the HyFlex instrument was the most cyclic fatigue resistant, with a significant improvement in resistance compared with the other tested instruments. The new manufacturing process (CM wire) of the HyFlex instru-
© 2014 Australian Society of Endodontology
Figure 1 Representative images of fractured instruments. (a) ProTaper Next, (b) OneShape, (c) Revo-S and (d) HyFlex. Left column (a1–d1) shows longitudinal view of instruments at low magnification. Second column (a2–d2) shows high magnification view. Third column (a3–e3) shows lateral view of fracture surfaces of the tested instruments after cyclic fatigue. Fourth column (a4–d4) shows low magnification view of fracture surfaces of the test instruments, with arrows indicating the crack initiation origin. Right column (a5–d5) shows high magnification of central areas with the characteristic surface pattern of tensile failure. The fracture surfaces of all brands revealed similar fractographic features. Presence of fatigue striations and absence of circular abrasion marks indicate the flexural fatigue failure.
I. D. Capar et al.
© 2014 Australian Society of Endodontology
Cyclic Fatigue of Novel Instruments
3
Cyclic Fatigue of Novel Instruments
ment seems to play a major role in superior cyclic fatigue resistance. This is in agreement with previous studies (18,19). There are no available data in the literature comparing the ProTaper Next and other rotary systems. However, endodontic instruments manufactured with M-Wire alloy are expected to have more flexibility and to be more resistant to fatigue than those made from conventional NiTi wire (20). Thus, the findings of this study can only be compared with studies in which the cyclic fatigue resistance of M-Wire alloy has been evaluated. The findings of the present study revealed that ProTaper Next instruments had similar fatigue resistance to that of OneShape instruments and that Revo-S had the least fatigue resistance. Some previous studies did not find that M-Wire instruments had better cyclic fatigue resistance when compared with conventional NiTi rotary files (21,22). However, Arias et al. (23) showed that unused M-Wire instruments showed higher cyclic fatigue resistance than conventional ones but that conventional ones were significantly more resistant to cyclic fatigue after clinical usage than M-Wire instruments. OneShape and Revo-S instruments are both made of a conventional NiTi alloy. However, similar to our findings, some previous studies (24,25) reported that Revo-S had least fatigue resistance than that of other rotary NiTi instruments. The Revo-S instrument has a variable crosssection and three cutting edges in all parts of the file. In contrast, the OneShape instrument has three variable cross-section zones, which change from three cutting edges near the tip region to two cutting edges at the end of the working part. Moreover, the asymmetric portion of the OneShape instrument is only in the 2 mm of the tip. The difference between the cyclic fatigue behaviour of the OneShape and Revo-S instrument might be due to the different cross-sectional design. The material alloy is not the only important factor influencing the NCF of the instruments. Other factors, such as cross-section shape, cross-sectional dimensions, fluting and speed, may influence the NCF. A finite elemental analysis study showed that NiTi instruments having a triangular cross-sectional geometry provided better fatigue resistance than a square cross-section (26). Versluis et al. (27) showed rectangular and triangular models with the same centre-core areas had similar flexural stiffness, although the rectangular model was 30–40% stiffer. The OneShape, HyFlex and Revo-S instruments have a triangular cross-sectional geometry, whereas that of the ProTaper Next is rectangular. All the tested instruments in the present study have non-cutting tips, a variable pitch and 0.06 taper. However, ProTaper Next instrument has a variable taper design, whereas the design of OneShape, HyFlex and 4
I. D. Capar et al.
Revo-S tapers are constant. ProTaper Next instrument has 0.06 taper at the apical 3 mm, 0.07 taper at the 3–9 mm, 0.06 taper at the 9–13 mm and 0.04 taper at the 13–16 mm. He and Ni (28) concluded that the bending flexibility decreases with increasing taper. The relatively lower NCF scores of the ProTaper Next system might be due to having 0.07 taper at the flexural stress point. Increased rotational speed was previously associated with increased instrument fracture on the extracted teeth (29). Moreover, recently Peters et al. (30) revealed that increased rotational speed was associated with increased cutting efficiency. However, Pruett et al. (1) reported that cycles to failure was not affected by speed on the conditions of artificial metal canals. In the present study, we used different speed settings (varying from 250 to 500, each of them recommended by manufacturers) for each instrument and similar to previous studies, we compared cyclic fatigue behaviour of the instruments using NCF scores (18,22,31). In contradistinction to this assessment method, some researchers used time to fracture data for failure of instruments (17,23). The use of time to fracture assessment method may be more relevant clinically. However, converting the time to NCF allows fair comparison regardless of the rotational speed (7).
Conclusions Within the limitations of this study, the HyFlex (CM wire) instrument had the best performance in cyclic fatigue resistance, and the rankings of the other instruments in decreasing order of fatigue resistance were as follows: ProTaper Next (M-Wire) = OneShape (single-file conventional NiTi) > and Revo-S (conventional NiTi).
Acknowledgements The authors thank Coltene and Micro Mega for providing the HyFlex and OneShape instruments. The authors deny any conflicts of interest related to this study.
References 1. Pruett JP, Clement DJ, Carnes DL Jr. Cyclic fatigue testing of nickel-titanium endodontic instruments. J Endod 1997; 23: 77–85. 2. Arens FC, Hoen MM, Steiman HR, Dietz GC Jr. Evaluation of single-use rotary nickel-titanium instruments. J Endod 2003; 29: 664–6. 3. Berutti E, Chiandussi G, Paolino DS et al. Canal shaping with WaveOne Primary reciprocating files and ProTaper system: a comparative study. J Endod 2012; 38: 505–9. 4. Peters OA, Barbakow F. Dynamic torque and apical forces of ProFile.04 rotary instruments during preparation of curved canals. Int Endod J 2002; 35: 379–89.
© 2014 Australian Society of Endodontology
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5. Sattapan B, Nervo GJ, Palamara JE, Messer HH. Defects in rotary nickel-titanium files after clinical use. J Endod 2000; 26: 161–5. 6. Shen Y, Qian W, Abtin H, Gao Y, Haapasalo M. Fatigue testing of controlled memory wire nickel-titanium rotary instruments. J Endod 2011; 37: 997–1001. 7. 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. 8. Basmadjian-Charles CL, Farge P, Bourgeois DM, Lebrun T. Factors influencing the long-term results of endodontic treatment: a review of the literature. Int Dent J 2002; 52: 81–6. 9. Wong R. Conventional endodontic failure and retreatment. Dent Clin North Am 2004; 48: 265–89. 10. Siqueira JF Jr, Rocas IN. Clinical implications and microbiology of bacterial persistence after treatment procedures. J Endod 2008; 34: 1291–301. 11. Spili P, Parashos P, Messer HH. The impact of instrument fracture on outcome of endodontic treatment. J Endod 2005; 31: 845–50. 12. Grossman LI. Guidelines for the prevention of fracture of root canal instruments. Oral Surg Oral Med Oral Pathol 1969; 28: 746–52. 13. Frick CP, Ortega AM, Tyber J et al. Thermal processing of polycrystalline NiTi shape memory alloys. Mater Sci Eng A Struct Mater 2005; 405: 34–49. 14. Gutmann JL, Gao Y. Alteration in the inherent metallic and surface properties of nickel-titanium root canal instruments to enhance performance, durability and safety: a focused review. Int Endod J 2012; 45: 113–28. 15. Shen Y, Zhou HM, Zheng YF, Peng B, Haapasalo M. Current challenges and concepts of the thermomechanical treatment of nickel-titanium instruments. J Endod 2013; 39: 163–72. 16. Parashos P, Gordon I, Messer HH. Factors influencing defects of rotary nickel-titanium endodontic instruments after clinical use. J Endod 2004; 30: 722–5. 17. Grande NM, Plotino G, Pecci R, Bedini R, Malagnino VA, Somma F. Cyclic fatigue resistance and threedimensional analysis of instruments from two nickeltitanium rotary systems. Int Endod J 2006; 39: 755–63. 18. Plotino G, Testarelli L, Al-Sudani D, Pongione G, Grande NM, Gambarini G. Fatigue resistance of rotary instruments manufactured using different nickel-titanium alloys: a comparative study. Odontology 2014; 102: 31–5. 19. Pongione G, Pompa G, Milana V et al. Flexibility and resistance to cyclic fatigue of endodontic instruments
© 2014 Australian Society of Endodontology
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made with different nickel-titanium alloys: a comparative test. Ann Stomatol (Roma) 2012; 3: 119–22. Pereira ES, Peixoto IF, Viana AC et al. Physical and mechanical properties of a thermomechanically treated NiTi wire used in the manufacture of rotary endodontic instruments. Int Endod J 2012; 45: 469–74. Kramkowski TR, Bahcall J. An in vitro comparison of torsional stress and cyclic fatigue resistance of ProFile GT and ProFile GT Series X rotary nickel-titanium files. J Endod 2009; 35: 404–7. 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. Arias A, Perez-Higueras JJ, de la Macorra JC. Influence of clinical usage of GT and GTX files on cyclic fatigue resistance. Int Endod J 2014; 47: 257–63. Lopes HP, Gambarra-Soares T, Elias CN et al. Comparison of the mechanical properties of rotary instruments made of conventional nickel-titanium wire, M-wire, or nickeltitanium alloy in R-phase. J Endod 2013; 39: 516–20. Sundaram KM, Ebenezar RA, Ghani MF, Martina L, Narayanan A, Mony B. Comparative evaluation of the effects of multiple autoclaving on cyclic fatigue resistance of three different rotary Ni-Ti instruments: an in vitro study. J Conserv Dent 2013; 16: 323–6. Cheung GS, Zhang EW, Zheng YF. A numerical method for predicting the bending fatigue life of NiTi and stainless steel root canal instruments. Int Endod J 2011; 44: 357–61. Versluis A, Kim HC, Lee W, Kim BM, Lee CJ. Flexural stiffness and stresses in nickel-titanium rotary files for various pitch and cross-sectional geometries. J Endod 2012; 38: 1399–403. He R, Ni J. Design improvement and failure reduction of endodontic files through finite element analysis: application to V-Taper file designs. J Endod 2010; 36: 1552–7. Khurana P, Khurana KK. Effect of curvature angle and rotational speed on fracture of various Ni-Ti rotary files used in extracted molars. J Res Dent 2013; 1: 49–54. Peters OA, Morgental RD, Schulze KA, Paqué F, Kopper PM, Vier-Pelisser FV. Determining cutting efficiency of nickel-titanium coronal flaring instruments used in lateral action. Int Endod J 2013; doi: 10.1111/iej.12177 Inan U, Aydin C. Comparison of cyclic fatigue resistance of three different rotary nickel-titanium instruments designed for retreatment. J Endod 2012; 38: 108–11.
5
Aust Endod J 2014
ORIGINAL RESEARCH
Comparison of cyclic fatigue resistance of novel nickel-titanium rotary instruments Ismail Davut Capar, DDS, PhD1; Huseyin Ertas, DDS, PhD1; and Hakan Arslan DDS, PhD2 1 2
Department of Endodontics, Faculty of Dentistry, I˙zmir Katip Çelebi University, I˙zmir, Turkey Department of Endodontics, Faculty of Dentistry, Atatürk University, Erzurum, Turkey
Keywords cyclic fatigue; heat treatment, nickel-titanium files, ProTaper Next, rotary. Correspondence Dr Ismail Davut Capar, Department of Endodontics, Faculty of Dentistry, I˙zmir Katip Çelebi University, Izmir 35620, Turkey. Email: [email protected] doi:10.1111/aej.12067
Abstract New files (ProTaper Next/HyFlex/OneShape) are made from novel nickeltitanium (NiTi) alloys/treatments. The purpose of this study was to compare the cyclic fatigue resistance of these new instruments with that of Revo-S instruments. Four groups of 20 NiTi endodontic instruments were tested in steel canals with a 3 mm radius and a 60° angle of curvature. The cyclic fatigue of the following NiTi instruments with a tip size 25 and 0.06 taper that were manufactured with different alloys was tested: ProTaper Next X2 (M-Wire), OneShape (conventional NiTi), Revo-S Shaping Universal (conventional NiTi) and HyFlex 25/0.6 (controlled memory NiTi wire). A one-way ANOVA and post-hoc Tukey’s test (α = 0.05) revealed that the HyFlex files had the highest fatigue resistance and the Revo-S had the least fatigue resistance among the groups (P < 0.001).
Introduction During root canal treatment, nickel-titanium (NiTi) rotary instruments may fracture inside the root canal without any sign of previous permanent deformation (1,2), thereby it may jeopardise the outcome of the root canal treatment. In clinical practice, the fracture of NiTi rotary instruments occurs via two different mechanisms: torsional fracture and flexural fatigue (3). Torsional fracture occurs when part of the instrument binds to the dentin while the file continues to rotate (4). Flexural fatigue fracture of the file occurs when the instrument rotates freely in a curvature, generating tension/ compression cycles in the region of maximum flexure until fracture occurs (5). Technological advancements in NiTi instruments have led to new concepts of use, different kinematics and improved alloys that purportedly increase the cyclic fatigue resistance of the instruments. The OneShape (MicroMega, Besancon, France) file is a new file system that is used in a traditional, continuous and rotational motion. It uses a single-file system to shape the root canal completely from start to finish. ProTaper Next (Dentsply Maillefer, Ballaigues, Switzerland) is a novel NiTi file system. It has an off-centred rectangular design and
© 2014 Australian Society of Endodontology
progressive and regressive percentage tapers on a single file. Revo-S (MicroMega) file system has an off-centred design similar to the ProTaper Next file, the off-centred design generates travelling waves of motion along the active part of the file. Recently, HyFlex rotary instruments (Coltene-Whaledent, Allstetten, Switzerland) have been introduced. They are made from a controlled memory (CM) NiTi wire, which is manufactured by a unique process that controls the material’s memory. This CM feature makes the files extremely flexible and makes the instruments more resistant to cyclic fatigue than non-CM NiTi instruments (6). In the literature, there are no data about the cyclic fatigue resistance of the ProTaper Next and OneShape file systems. Thus, the purpose of this study was to compare the cyclic fatigue resistance of these new files with that of HyFlex and Revo-S. The null hypothesis of this study was that there would be no difference in the cyclic fatigue resistance among the tested files.
Materials and methods The cyclic fatigue of the following new rotary NiTi instruments was tested: ProTaper Next X2, OneShape, Revo-S Shaping Universal and HyFlex. All the instruments were 1
Cyclic Fatigue of Novel Instruments
I. D. Capar et al.
size 25 with a 0.06 taper. Twenty instruments from each brand were evaluated in air at a temperature of 23°C. An artificial canal was made out of a testing block of stainless steel with an inner diameter of 1.5 mm, a 60° angle of curvature and a curvature radius of 3 mm. The canals were covered with glass to prevent the instruments from slipping out. The apparatus used in the cyclic fatigue test was described previously (7). The working length was standardised to 19 mm for all the files. To reduce the friction of the file as it contacted the artificial canal walls, a special oil (WD-40 Company, Milton Keynes, England) was used for lubrication. All the instruments were operated with a low-torque motor (VDW Silver; VDW, Munich, Germany) and were rotated according to the manufacturer’s recommendations as follows: ProTaper Next with 300 rpm and 200 gcm torque, OneShape with 400 rpm and 400 gcm torque, Revo-S with 250 rpm and 200 gcm torque and HyFlex with 500 rpm and 250 gcm torque. The instruments were used until fracture occurred, and the time to fracture was recorded in seconds. The number of cycles to fracture (NCF) was calculated using the following formula: NCF = time (s) to failure × rotational speed/60. The broken instruments were ultrasonically cleaned in alcohol for scanning electron microscopy examination. To verify the fracture mode, the surface of the fractured part of the representative samples from each group was photographed under scanning electron microscopy (Evo LS10; Carl Zeiss, Oberkochen, Germany). After completing the cyclic fatigue test, the NCF data were subjected to a Shapiro–Wilk test to analyse the normality of the continuous variables. The data were then statistically analysed using a one-way ANOVA and post-hoc Tukey’s test (α = 0.05).
Results The NCF of the fractured fragment for each brand are presented in Table 1. The HyFlex files had the highest fatigue resistance and the Revo-S had the least fatigue resistance among the groups (P < 0.001). There was no
Table 1 Comparison of mean number of cycles to fracture (NCF) values
Groups
NCF
Standard deviation
Statistical significance†
ProTaper Next OneShape Revo-S HyFlex
504 510 270 1237
79 78 31 206
a a b c
†Different letters indicate a significant difference between groups.
2
significant difference between the mean NCF of the ProTaper Next and the OneShape (P = 0.998). The scanning electron microscopy images of the fracture surface showed the nature of the mechanical damage of the cyclic fatigue failure in all the groups (Fig. 1).
Discussion Removal of vital and necrotic pulp tissue, microorganisms and their toxins from the root canal system is critical for successful root canal therapy (8–10). During instrumentation if an instrument fractured, the root canal system could not be completely cleaned. In the literature, there are conflicting results on impact of fractured files on outcome. Spili et al. (11) revealed that presence of preoperative periapical lesion is more significant rather than fractured instrument for healing rate. However, Grossman (12) reported lower healing rate for fractured instrument cases with a concomitant preoperative periapical lesion. Therefore, the purpose of this study was to compare the cyclic fatigue resistance of new files (ProTaper Next/HyFlex/OneShape) with that of Revo-S instruments. The mechanical behaviour of NiTi alloys is related to the proportions and the characteristics of the microstructural phases. The fatigue resistance of NiTi endodontic files is affected by the transition temperatures of the NiTi alloys that are mainly adjusted with heat treatment (13,14). In recent years, several novel thermomechanical processing and manufacturing technologies, such as M-Wire, and CM wire, have been developed to optimise the microstructure of NiTi alloys (15). With the development of these technologies, instruments with improved mechanical properties have been manufactured from these new alloys and commercialised. Cyclic fatigue testing devices permit the instruments to rotate until fracture using different curvature. The cyclic fatigue life span of rotary instruments decreases with increasing instrument diameters (16). The metal mass of the instrument at the point of maximum stress is another important factor (17). Therefore, in the present study, the fatigue behaviour of NiTi instruments having a similar taper and tip size was examined. Moreover, M-Wire (i.e. ProTaper Next), traditional NiTi (i.e. Revo-S and OneShape) and CM (HyFlex) instruments were tested to check whether different manufacturing methods influenced the fatigue resistance of the endodontic instruments produced with different alloys. The null hypothesis was rejected. The results showed that the HyFlex instrument was the most cyclic fatigue resistant, with a significant improvement in resistance compared with the other tested instruments. The new manufacturing process (CM wire) of the HyFlex instru-
© 2014 Australian Society of Endodontology
Figure 1 Representative images of fractured instruments. (a) ProTaper Next, (b) OneShape, (c) Revo-S and (d) HyFlex. Left column (a1–d1) shows longitudinal view of instruments at low magnification. Second column (a2–d2) shows high magnification view. Third column (a3–e3) shows lateral view of fracture surfaces of the tested instruments after cyclic fatigue. Fourth column (a4–d4) shows low magnification view of fracture surfaces of the test instruments, with arrows indicating the crack initiation origin. Right column (a5–d5) shows high magnification of central areas with the characteristic surface pattern of tensile failure. The fracture surfaces of all brands revealed similar fractographic features. Presence of fatigue striations and absence of circular abrasion marks indicate the flexural fatigue failure.
I. D. Capar et al.
© 2014 Australian Society of Endodontology
Cyclic Fatigue of Novel Instruments
3
Cyclic Fatigue of Novel Instruments
ment seems to play a major role in superior cyclic fatigue resistance. This is in agreement with previous studies (18,19). There are no available data in the literature comparing the ProTaper Next and other rotary systems. However, endodontic instruments manufactured with M-Wire alloy are expected to have more flexibility and to be more resistant to fatigue than those made from conventional NiTi wire (20). Thus, the findings of this study can only be compared with studies in which the cyclic fatigue resistance of M-Wire alloy has been evaluated. The findings of the present study revealed that ProTaper Next instruments had similar fatigue resistance to that of OneShape instruments and that Revo-S had the least fatigue resistance. Some previous studies did not find that M-Wire instruments had better cyclic fatigue resistance when compared with conventional NiTi rotary files (21,22). However, Arias et al. (23) showed that unused M-Wire instruments showed higher cyclic fatigue resistance than conventional ones but that conventional ones were significantly more resistant to cyclic fatigue after clinical usage than M-Wire instruments. OneShape and Revo-S instruments are both made of a conventional NiTi alloy. However, similar to our findings, some previous studies (24,25) reported that Revo-S had least fatigue resistance than that of other rotary NiTi instruments. The Revo-S instrument has a variable crosssection and three cutting edges in all parts of the file. In contrast, the OneShape instrument has three variable cross-section zones, which change from three cutting edges near the tip region to two cutting edges at the end of the working part. Moreover, the asymmetric portion of the OneShape instrument is only in the 2 mm of the tip. The difference between the cyclic fatigue behaviour of the OneShape and Revo-S instrument might be due to the different cross-sectional design. The material alloy is not the only important factor influencing the NCF of the instruments. Other factors, such as cross-section shape, cross-sectional dimensions, fluting and speed, may influence the NCF. A finite elemental analysis study showed that NiTi instruments having a triangular cross-sectional geometry provided better fatigue resistance than a square cross-section (26). Versluis et al. (27) showed rectangular and triangular models with the same centre-core areas had similar flexural stiffness, although the rectangular model was 30–40% stiffer. The OneShape, HyFlex and Revo-S instruments have a triangular cross-sectional geometry, whereas that of the ProTaper Next is rectangular. All the tested instruments in the present study have non-cutting tips, a variable pitch and 0.06 taper. However, ProTaper Next instrument has a variable taper design, whereas the design of OneShape, HyFlex and 4
I. D. Capar et al.
Revo-S tapers are constant. ProTaper Next instrument has 0.06 taper at the apical 3 mm, 0.07 taper at the 3–9 mm, 0.06 taper at the 9–13 mm and 0.04 taper at the 13–16 mm. He and Ni (28) concluded that the bending flexibility decreases with increasing taper. The relatively lower NCF scores of the ProTaper Next system might be due to having 0.07 taper at the flexural stress point. Increased rotational speed was previously associated with increased instrument fracture on the extracted teeth (29). Moreover, recently Peters et al. (30) revealed that increased rotational speed was associated with increased cutting efficiency. However, Pruett et al. (1) reported that cycles to failure was not affected by speed on the conditions of artificial metal canals. In the present study, we used different speed settings (varying from 250 to 500, each of them recommended by manufacturers) for each instrument and similar to previous studies, we compared cyclic fatigue behaviour of the instruments using NCF scores (18,22,31). In contradistinction to this assessment method, some researchers used time to fracture data for failure of instruments (17,23). The use of time to fracture assessment method may be more relevant clinically. However, converting the time to NCF allows fair comparison regardless of the rotational speed (7).
Conclusions Within the limitations of this study, the HyFlex (CM wire) instrument had the best performance in cyclic fatigue resistance, and the rankings of the other instruments in decreasing order of fatigue resistance were as follows: ProTaper Next (M-Wire) = OneShape (single-file conventional NiTi) > and Revo-S (conventional NiTi).
Acknowledgements The authors thank Coltene and Micro Mega for providing the HyFlex and OneShape instruments. The authors deny any conflicts of interest related to this study.
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