SCANNING VOL. 37, 82–87 (2015) © Wiley Periodicals, Inc.
Effect From Surface Treatment of Nickel-Titanium Rotary Files on the Fracture Resistance BO HOON KIM,1 JUNG-HONG HA,2 WOO CHEOL LEE,3 SANG-WON KWAK,1 AND HYEON-CHEOL KIM1 1
Department of Conservative Dentistry, School of Dentistry, Dental Research Institute, Pusan National University, Yangsan, Korea 2 Department of Conservative Dentistry, School of Dentistry, Kyungpook National University, Daegu, Korea 3 Department of Conservative Dentistry, School of Dentistry, Dental Research Institute, Seoul National University, Seoul, Korea
Summary: This study was aimed to compare the cyclic fatigue resistance and torsional resistance of rotary instruments with and without surface treatment. G6 A2 (Group A2) with and G6 A2 without surface treatment after machining (Group AN) were compared in this study. ProTaper F2 (Group F2) which has similar dimension and shape was also used for comparison. To evaluate the torsional resistance, ultimate torsional strength and distortion angle until fracture were recorded, and the toughness was calculated. The cyclic fatigue resistance was compared by evaluating the number of cycles to failure in a simulated canal. Statistical analysis was performed by one-way analysis of variance and Tukey post hoc test (p ¼ 0.05). After torsional and cyclic fatigue tests, all fracture fragments were observed under a scanning electron microscope. Group A2 showed higher cyclic fatigue resistance than the groups AN and F2 (p < 0.05). Although group A2 demonstrated lower ultimate torsional strength than the others, there were no significant differences in toughness among the groups. While obvious machining grooves were seen in groups AN and F2, group A2 showed smooth surface resulting from the surface treatment. The specimens of fracture fragments showed typical features of cyclic failure such as micro-cracks, overloaded fast fracture zone, and torsional fracture such as unwinding helix, circular abrasion marks and dimples. Under the conditions of this study, the surface
Conflicts of interest: None. Address for reprints: Hyeon-Cheol Kim, Department of Conservative Dentistry, School of Dentistry, Pusan National University, 20 Geumo-ro, Mulgeum, Yangsan, Gyeongnam, 626-787, Korea E-mail: [email protected] Received 17 September 2014; Accepted with revision 31 October 2014 DOI: 10.1002/sca.21182 Published online 8 December 2014 in Wiley Online Library (wileyonlinelibrary.com).
treated instruments may improve cyclic fatigue resistance while maintaining the torsional resistances and mechanical properties. SCANNING 37:82–87, 2015. © 2014 Wiley Periodicals, Inc. Key words: electro-polishing, fatigue resistance, nickel-titanium rotary file, surface treatment, torsional fracture
Introduction Nickel-Titanium (NiTi) rotary instruments have become an important adjunct in endodontic therapy and shaping with these instruments resulted in more predictable outcome (Peters, 2004). NiTi rotary instruments are frequently used in endodontic procedure because shaping with NiTi rotary files has shown to be faster, easier, and more efficient than hand instrumentation especially in the curved canals (Glossen et al.,’95; Chen and Messer, 2002; Schafer et al., 2004). Due to their efficient shaping ability and flexibility, root canal preparation with NiTi files is related to increase in success rate (Peters, 2004). However, NiTi rotary files still have a higher risk of fracture in clinical situation (Sattapan et al., 2000; Arens et al., 2003). Cyclic flexural fatigue of the instrument occurs when an instrument rotates in a curved canal with repeated compressive and tensile stresses (Cho et al., 2013). Torsional failure results when the tip of the instrument binds in the canal, while the motor continues to rotate (Cheung, 2009). To avert fracture risks, manufacturers developed new manufacturing techniques in an effort to improve the physical and mechanical properties of their instruments for better clinical performance. Some manufacturers have enhanced fracture resistance by removing machining marks and by applying various thermal treatments to NiTi alloy (Bui et al., 2008; Kim et al., 2010; Lopes
B. H. Kim et al.: Effect of surface treatment of rotary NiTi files
et al., 2010). The NiTi files made of/from M-wire, R-phase heat treatment, and controlled memory wire have shown the improved cyclic fatigue resistance in comparison with conventional NiTi files (Kell et al., 2009; Kim et al., 2012; Ye and Gao, 2012; Ha et al., 2013). Meanwhile, in a machined rotary instrument residual tensile stresses may exist on the surface which can accelerate crack initiation and propagation processes, but electro-polishing can produce a smooth, amorphous oxide layer that is free of most defects (Kuhn et al., 2001; Alapati et al., 2005). Rotary NiTi instrument G6 system (Global Top, Goyang, Korea) has been recently introduced with the assertions that it has reduced machining marks or grooves on the surface as a result of special surface treatment (nano-polishing treatment) that could result in slower initiation of fatigue crack or propagation. G6 system is composed of six files (0, 1, 2, A1, A2, and A3) and they have convex triangular cross-sections and same variable taper and tip sizes (external dimensions) with the six files (SX, S1, S2, F1, F2, and F3) of ProTaper system (Dentsply Maillefer, Ballaigues, Switzerland). To date, there were not many studies that compared before and after surface treatments concerning mechanical properties of NiTi rotary files. Therefore, the purpose of this study was to compare the cyclic fatigue resistance and torsional resistance between G6 A2 with and without surface treatment and examine their surface aspects. ProTaper F2 NiTi rotary files having similar dimension and size with G6 A2 but without surface treatment were also used for comparison. The null
83
hypothesis was that there is no significant difference in the mechanical resistances between files with surface treatment and without treatment.
Materials and Methods G6 and ProTaper systems were selected for this study because they had similar dimensions in lateral aspect and tip sizes, and were made from same conventional NiTi alloy (1:1 atomic ratio of the major components). G6 A2 (Group A2) and G6 A2 without surface treatment after machining process (Group AN) were selected as one having surface treatment or not, and ProTaper F2 (Group F2) made from a conventional machining process was used for comparison. Both instruments have same ISO 025 tip size, 0.08 taper (at apical 3 mm), and convex triangular cross-section. Sixty new files were used for the cyclic fatigue and torsional resistance tests (n ¼ 10 for each group and each test). Before the experiments, the instruments were visually examined under a stereomicroscope and instruments with any type of defect were discarded. Torsional resistance was evaluated by using an AEndoS (DMJ System, Busan, Korea) (Fig. 1(A)) as described by Yum et al.(2011). The tip of the instrument (5 mm) was secured between polycarbonate blocks while keeping the file straight. The file was rotated clockwise at a constant rotational speed of 2 rpm until fracture occurred. The torsional load (Ncm) and distortion angle (˚) were recorded during the rotation
Fig 1. Customized devices used in this study: (A) AEndoS (DMJ system, Busan, Korea) for torsional strength test and (B) EndoC (DMJ system) for cyclic fatigue resistance test.
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of the files at the rate of 20 Hz. The toughness was computed from the area under the plot by using a software (Origin 6.0; Microcal Software Inc., Northampton, MA). Cyclic fatigue resistance was evaluated by using a custom-made device (EndoC; DMJ System) (Fig. 1(B)). This device was designed to allow reproducible simulation of an instrument confined in a curved canal. An artificial canal block was made of tempered steel with a 6 mm radius and 35˚ angle of curvature (Schneider, ’71; Pruett et al., ’97). The cyclic fatigue test was conducted in a dynamic mode, as described by Li et al. (2002) with continuous up-and-down pecking movements (4 mm in each direction per 0.5 sec and 50 ms of dwell time) to simulate a clinical setting. Synthetic oil (WD-40; WD-40 Company, San Diego, CA) was sprayed into the metal canal walls (Grande et al., 2006). The instruments were rotated at a constant speed of 300 rpm by using a torque-controlled motor (Reciproc Silver: VDW, Munich, Germany) (Gambarini, 2001). The torque was set at maximum (4.0 Ncm) to exclude the influence of torque on the rotational speed. Time was recorded when instrument fracture and the time was converted into number of cycles to failure (NCF). The length of the fractured fragment was measured using a digital microcaliper (Mitutoyo, Kawasaki, Japan). The data were analyzed statistically by using oneway analysis of variance (ANOVA) and Tukey post-hoc test. All statistical analyses were conducted at a significance level of 95% by using SPSS software (SPSS for Windows version 15.0; SPSS, Chicago, IL). After torsional and cyclic fatigue tests, the topographic features of all fractured fragments were evaluated under a scanning electron microscope (SEM) (S-4800 II; Hitachi High Technologies, Pleasanton, CA).
Results The results of the torsional resistance and cyclic fatigue tests are presented in Table I. Group A2 showed higher resistance to cyclic fatigue compared with groups AN and F2 (p < 0.05). Groups AN and F2 showed higher resistance to ultimate torsional strength compared with
group A2 (p < 0.05). However, when they were compared for toughness and fractured angles, no significant differences were found among the groups (p > 0.05). The surface features after cyclic fracture tests under the SEM are shown in Figure 2. While obvious machining grooves were shown in groups AN and F2, group A2 showed the smooth surface resulting from surface treatment (Fig. 2(D)). Group A2 showed irregular flow of micro-cracks on the file surface near the fracture margin (Fig. 2(G)). However, in the groups AN and F2, micro-cracks were found to run along the remained machining grooves (Fig. 2(H) and (I)). SEM examination of the fractured longitudinal surfaces after torsional resistance test showed the typical features of reverse helix of unwinding area (Fig. 3(A)(C)). The lengths of reverse winding area of group A2 were longer than groups AN and F2 (Fig. 3(A)). All cross-sectional aspects showed the typical features of fatigue fracture including crack initiation and propagation areas and also the typical features of torsional fracture including circular abrasion marks and fibrous dimples around the center of rotation.
Discussion File fracture may occur suddenly and jeopardize the outcome of treatment and healing of periapical tissues (Alapati et al., 2005; Panitvisai et al., 2010). Therefore, it is important to investigate the cyclic and torsional resistance of brand new NiTi rotary files when introduced to the market. Previous studies have evaluated smooth surface instruments (Anderson et al., 2007; Cheung et al., 2007; Kim et al., 2010) and Anderson et al. (2007) concluded that further electro-polishing procedure is likely to yield a reduction in surface irregularities that serve as points for stress concentration and crack initiation. Crack propagation at grain boundaries and surfaces showed that manufacturing process may serve as nucleating sites for the micro-voids, leading to instrument fractures during clinical use (Alapati et al., 2005). The high density of surface defects facilitates the crack nucleation stage and the fatigue failure is largely a crack propagation process (Kuhn et al., 2001). Thus, it is possible that the multitude of machining marks on the
TABLE I Torsional resistance and cyclic fatigue resistance (mean standard deviation) Torsional resistance
A2 AN F2
Cyclic fatigue resistance
Ultimate strength (Ncm)
Fracture angle (˚)
Toughness (˚Ncm)
NCF
Fragment length (mm)
2.26 0.19a 2.81 0.25b 3.03 0.14b
710 349a 615 72a 568 65a
1252 624a 1313 229a 1309 305a
2219 698a 977 325b 843 59b
2.67 0.41a 2.75 0.22a 2.88 0.26a
A2: G6 A2, AN: G6 A2 without surface treatment, F2: ProTaper F2. Different letters means statistical significance between groups: p < 0.05.
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Fig 2. Scanning electron micrographs of the fractured specimens after cyclic fatigue test. Upper row (A, B, and C) shows the typical cross-sectional aspects of cyclic fatigue fracture such as crack initiation area and fibrous fast fracture zone. Middle row (D, E, and F) shows the lateral aspects near the fracture margin. Figure D marked with a star shows highly polished area of the group A2. Figures D and F shows machined micro-grooves from the groups AN and F2. Lower row (G, H, and I) shows the irregular micro-cracks (arrow) on the smoothened surface of group A2 (G) and micro-cracks (arrow) running along the machined grooves in the groups AN (H) and F2 (I).
surface of a ground instrument would lead to crack initiation at multiple locations in which the resolved shear stress is greater than that required for crystallographic slip to occur (Kim et al., 2010). By chance to get the files before surface treatment which has same geometries of final products, this study was possible to be conducted. The manufacturer (Global top) of G6 rotary file system claims that the G6 has highly smoothened surface due to nano-polishing technique and heat treatment thus the aim of this study was to evaluate the effect of surface treatment on the instruments. Although the manufacturer of G6 did not provided the details of nano-polishing technique, an electro-polishing technique is a method of surface treatment to remove surface defects that may remain after machining procedure (Pohl et al., 2004). It might be a controlled chemomechanical process that involves submerging the machined files into an acidic solution which contains nano-particles for surface polishing. Explanations of improved performance of the electro-polished instru-
ments are attributed to reduction in micro-cracks on the surface of the material which eliminates residual stresses (Anderson et al., 2007; Kim et al., 2010). Kuhn et al. (2001) suggested that electro-polishing procedure could be used by the manufacturer to reduce the machining damage effectively on the file surface therefore improving the lifetime of endodontic files. The G6 A2 used in this study possesses the similar geometry of tip size, taper, and cross-section design with the ProTaper F2. As the files of group AN without surface treatment, ProTaper F2 also has machined surface without polishing procedure. In the present results, the group A2 appears to be significantly more resistant to cyclic fatigue than groups AN and F2, and thus it could be concluded as the files with surface treatment is more resistant to fatigue compared to the conventional NiTi instruments without surface treatment. For comparing the torsional resistance of NiTi files, the previous researches usually have used the parameters of the ultimate strength and distortion angle (Bahia
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Fig 3. Scanning electron micrographs of the fractured specimens after torsional fracture test. Upper row (A, B, and C) shows longitudinal lateral aspects near the fracture margin. The indicated areas are the unwound distortion areas with reverse helix. Middle row (D, E, and F) shows the cross-sectional aspects having typical features of torsional fractures such as fibrous dimples (circle) and circular abrasion marks (round arrow). Lower row (G, H, and I) shows the skewed fibrous dimples in the rotation center of torsional fracture area at highly magnified view (1000 ).
et al., 2006). However, the maximum torsional strength and distortion angle are partial parameters of stressstrain curve and subsequently do not always represent the material durability. Yum et al.(2011) suggested comparing the toughness may provide a better understanding of NiTi instrument’s behavior under torsion. Toughness is defined as the total energy for breaking a material, and it can be computed by integration of the stress and strain curve (load and distortional angle under the rotational failure) up to the point of fracture (Gere, 2001). The present results indicated that groups A2, AN, and F2 had no significantly difference in toughness, although groups AN and F2 showed a higher ultimate strength. Usually the files with lower torsional strength would have bigger rotational distortion angle (unwinding ability before fracture) and then the files with higher torsional strength would give clinicians lower chance to notice the distortion (Ha et al., 2013). Thus clinically neither the torsional strength nor the distortional angle would be the representative torsional property of rotating instruments but the experimentally
computed toughness might be the one to decide the torsional property. Therefore it might be concluded that group A2 have similar torsional properties compared with groups AN and F2 from their similar toughness. Considering together that the group A2 showed higher resistance to cyclic fatigue compared with groups AN and F2, surface polishing treatment procedure was shown to improve the surface smoothness of NiTi instruments and thus improve the instrument’s working properties (Cheung et al., 2007; Yum et al., 2011). From the SEM evaluation, the files of group A2 showed highly smoother surface than the group F2 and the micro-cracks near the fracture area of group A2 flowed more irregularly while the micro-cracks ran along the machining groove in groups AN and F2. Like in the non-polished files and ProTaper files with the machined surface, residual tensile stresses may also accelerate the crack propagation as well as the initiation process (Acevedo and Nussbaumer, 2012). Rather electro-polishing can produce a smooth, amorphous oxide layer that is free of most defects. Based on this
B. H. Kim et al.: Effect of surface treatment of rotary NiTi files
test, potential residual stresses on the surface of the G6 instruments were also removed in the process. Previous studies evaluated the effect of surface treatment on fatigue resistance but showed limited results, because they were conducted under the uncontrolled conditions of file’s geometries (Gambarini et al., 2008; Kim et al., 2010). Under the controlled conditions of this study, the electro-polished instruments may have better cyclic fatigue resistance while maintaining similar torsional properties.
References Acevedo C, Nussbaumer A. 2012. Effect of tensile residual stresses on fatigue crack growth and S-N curves in tubular joint in compression. Int J Fatigue 35:171–180. Alapati SB, Brantley WA, Svec TA, Powers JM, Nusstein JM, Daehn GS. 2005. SEM observations of nickel-titanium rotary endodontic instruments that fractured during clinical use. J Endod 31:40–43. Anderson ME, Price JW, Parashos P. 2007. Fracture resistance of electropolished rotary nickel-titanium endodontic instruments. J Endod 33:1212–1216. Arens FC, Hoen MM, Steiman HR, Dietz GC Jr. 2003. Evaluation of single-use rotary nickeltitanium instruments. J Endod 29:664–666. Bahia MG, Melo MC, Buono VT. 2006. Influence of simulated clinical use on the torsional behavior of nickel-titanium rotary endodontic instruments. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 101:675–680. Bui TB, Mitchell JC, Baumgartner JC. 2008. Effect of electropolishing ProFile nickel-titanium rotary instruments on cyclic fatigue resistance, torsional resistance, and cutting efficiency. J Endod 34:190–193. Chen JL, Messer HH. 2002. A comparison of stainless steel hand and rotary nickel-titanium instrumentation using a silicone impression technique. Aust Dent J 47:12–20. Cheung GS, Shen Y, Darvell BW. 2007. Does electropolishing improve the low-cycle fatigue behavior of a nickel-titanium rotary instrument in hypochlorite. J Endod 33:1217–1221. Cheung GS. 2009. Instrument fracture: mechanisms, removal of fragments, and clinical outcomes. Endod Topics 16:1–26. Cho OI, Versluis A, Cheung GS, Ha JH, Hur B, Kim HC. 2013. Cyclic fatigue resistance tests of Nickel-Titanium rotary files using simulated canal and weight loading conditions. Restor Dent Endod 38:31–35. Gambarini G, Grande NM, Plotino G, Somma F, Garala M, De Luca M, Testarelli L. 2008. Fatigue resistance of enginedriven rotary nickel-titanium instruments produced by new manufacturing methods. J Endod 34:1003–1005. Gambarini G. 2001. Cyclic fatigue of nickel-titanium rotary instruments after clinical use with low- and high-torque endodontic motors. J Endod 27:772–774. Gere JM. 2001. Mechanics of materials. 5 ed. Pacific Grove, CA Brooks/Cole 1–66.
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Glossen CR, Haller RH, Dove SB, del Rio CE. 1995. A comparison of root canal preparations using Ni-Ti hand, Ni-Ti engine-driven, and K-Flex endodontic instruments. J Endod 21:146–151. Grande NM, Plotino G, Pecci R, Bedini R, Malagnino VA, Somma F. 2006. Cyclic fatigue resistance and three-dimensional analysis of instruments from two nickel-titanium rotary systems. Int Endod J 39:755–763. Ha JH, Kim SK, Cohenca N, Kim HC. 2013. Effect of R-phase heat treatment on torsional resistance and cyclic fatigue fracture. J Endod 39:389–393. Kell T, Azarpazhooh A, Peters OA, El-Mowafy O, Tompson B, Basrani B. 2009. Torsional profiles of new and used 20/.06 GT series X and GT rotary endodontic instruments. J Endod 35:1278–1281. Kim HC, Yum J, Hur B, Cheung GS. 2010. Cyclic fatigue and fracture characteristics of ground and twisted nickel-titanium rotary files. J Endod 36:147–152. Kim HC, Kwak SW, Cheung GS, Ko DH, Chung SM, Lee W. 2012. Cyclic fatigue and torsional resistance of two new nickel-titanium instruments used in reciprocation motion: reciproc versus WaveOne. J Endod 38:541–544. Kuhn G, Tavernier B, Jordan L. 2001. Influence of structure on nickel-titanium endodontic instruments failure. J Endod 27:516–520. Li UM, Lee BS, Shih CT, Lan WH, Lin CP. 2002. Cyclic fatigue of endodontic nickel titanium rotary instruments: static and dynamic tests. J Endod 28:448–451. Lopes HP, Elias CN, Vieira VT, Moreira EJ, Marques RV, de Oliveira JC, Debelian G, Siqueira JF Jr. 2010. Effects of electropolishing surface treatment on the cyclic fatigue resistance of BioRace nickel-titanium rotary instruments. J Endod 36:1653–1657. Panitvisai P, Parunnit P, Sathorn C, Messer HH. 2010. Impact of a retained instrument on treatment outcome: a systematic review and meta-analysis. J Endod 36:775–780. Peters OA. 2004. Current challenges and concepts in the preparation of root canal systems: a review. J Endod 30:559–567. Pohl M, Heßing C, Frenzel J. 2004. Electrolytic processing of NiTi shape memory alloys. Mat Sci Eng A 378:191–199. Pruett JP, Clement DJ, Carnes DL Jr. 1997. Cyclic fatigue test of nickel-titanium endodontic instruments. J Endod 23:77–85. Sattapan B, Nervo GJ, Palamara JE, Messer HH. 2000. Defects in rotary nickel-titanium files after clinical use. J Endod 26:161–165. Schafer E, Schulz-Bongert U, Tulus G. 2004. Comparison of hand stainless steel and nickel titanium rotary instrumentation: a clinical study. J Endod 30:432–435. Schneider SW. 1971. A comparison of canal preparations in straight and curved root canals. Oral Surg Oral Med Oral Pathol 32:271–275. Ye J, Gao Y. 2012. Metallurgical characterization of M-Wire nickel-titanium shape memory alloy used for endodontic rotary instruments during low-cycle fatigue. J Endod 38:105–107. Yum J, Cheung GS, Park JK, Hur B, Kim HC. 2011. Torsional strength and toughness of nickel-titanium rotary files. J Endod 37:382–386.
Effect From Surface Treatment of Nickel-Titanium Rotary Files on the Fracture Resistance BO HOON KIM,1 JUNG-HONG HA,2 WOO CHEOL LEE,3 SANG-WON KWAK,1 AND HYEON-CHEOL KIM1 1
Department of Conservative Dentistry, School of Dentistry, Dental Research Institute, Pusan National University, Yangsan, Korea 2 Department of Conservative Dentistry, School of Dentistry, Kyungpook National University, Daegu, Korea 3 Department of Conservative Dentistry, School of Dentistry, Dental Research Institute, Seoul National University, Seoul, Korea
Summary: This study was aimed to compare the cyclic fatigue resistance and torsional resistance of rotary instruments with and without surface treatment. G6 A2 (Group A2) with and G6 A2 without surface treatment after machining (Group AN) were compared in this study. ProTaper F2 (Group F2) which has similar dimension and shape was also used for comparison. To evaluate the torsional resistance, ultimate torsional strength and distortion angle until fracture were recorded, and the toughness was calculated. The cyclic fatigue resistance was compared by evaluating the number of cycles to failure in a simulated canal. Statistical analysis was performed by one-way analysis of variance and Tukey post hoc test (p ¼ 0.05). After torsional and cyclic fatigue tests, all fracture fragments were observed under a scanning electron microscope. Group A2 showed higher cyclic fatigue resistance than the groups AN and F2 (p < 0.05). Although group A2 demonstrated lower ultimate torsional strength than the others, there were no significant differences in toughness among the groups. While obvious machining grooves were seen in groups AN and F2, group A2 showed smooth surface resulting from the surface treatment. The specimens of fracture fragments showed typical features of cyclic failure such as micro-cracks, overloaded fast fracture zone, and torsional fracture such as unwinding helix, circular abrasion marks and dimples. Under the conditions of this study, the surface
Conflicts of interest: None. Address for reprints: Hyeon-Cheol Kim, Department of Conservative Dentistry, School of Dentistry, Pusan National University, 20 Geumo-ro, Mulgeum, Yangsan, Gyeongnam, 626-787, Korea E-mail: [email protected] Received 17 September 2014; Accepted with revision 31 October 2014 DOI: 10.1002/sca.21182 Published online 8 December 2014 in Wiley Online Library (wileyonlinelibrary.com).
treated instruments may improve cyclic fatigue resistance while maintaining the torsional resistances and mechanical properties. SCANNING 37:82–87, 2015. © 2014 Wiley Periodicals, Inc. Key words: electro-polishing, fatigue resistance, nickel-titanium rotary file, surface treatment, torsional fracture
Introduction Nickel-Titanium (NiTi) rotary instruments have become an important adjunct in endodontic therapy and shaping with these instruments resulted in more predictable outcome (Peters, 2004). NiTi rotary instruments are frequently used in endodontic procedure because shaping with NiTi rotary files has shown to be faster, easier, and more efficient than hand instrumentation especially in the curved canals (Glossen et al.,’95; Chen and Messer, 2002; Schafer et al., 2004). Due to their efficient shaping ability and flexibility, root canal preparation with NiTi files is related to increase in success rate (Peters, 2004). However, NiTi rotary files still have a higher risk of fracture in clinical situation (Sattapan et al., 2000; Arens et al., 2003). Cyclic flexural fatigue of the instrument occurs when an instrument rotates in a curved canal with repeated compressive and tensile stresses (Cho et al., 2013). Torsional failure results when the tip of the instrument binds in the canal, while the motor continues to rotate (Cheung, 2009). To avert fracture risks, manufacturers developed new manufacturing techniques in an effort to improve the physical and mechanical properties of their instruments for better clinical performance. Some manufacturers have enhanced fracture resistance by removing machining marks and by applying various thermal treatments to NiTi alloy (Bui et al., 2008; Kim et al., 2010; Lopes
B. H. Kim et al.: Effect of surface treatment of rotary NiTi files
et al., 2010). The NiTi files made of/from M-wire, R-phase heat treatment, and controlled memory wire have shown the improved cyclic fatigue resistance in comparison with conventional NiTi files (Kell et al., 2009; Kim et al., 2012; Ye and Gao, 2012; Ha et al., 2013). Meanwhile, in a machined rotary instrument residual tensile stresses may exist on the surface which can accelerate crack initiation and propagation processes, but electro-polishing can produce a smooth, amorphous oxide layer that is free of most defects (Kuhn et al., 2001; Alapati et al., 2005). Rotary NiTi instrument G6 system (Global Top, Goyang, Korea) has been recently introduced with the assertions that it has reduced machining marks or grooves on the surface as a result of special surface treatment (nano-polishing treatment) that could result in slower initiation of fatigue crack or propagation. G6 system is composed of six files (0, 1, 2, A1, A2, and A3) and they have convex triangular cross-sections and same variable taper and tip sizes (external dimensions) with the six files (SX, S1, S2, F1, F2, and F3) of ProTaper system (Dentsply Maillefer, Ballaigues, Switzerland). To date, there were not many studies that compared before and after surface treatments concerning mechanical properties of NiTi rotary files. Therefore, the purpose of this study was to compare the cyclic fatigue resistance and torsional resistance between G6 A2 with and without surface treatment and examine their surface aspects. ProTaper F2 NiTi rotary files having similar dimension and size with G6 A2 but without surface treatment were also used for comparison. The null
83
hypothesis was that there is no significant difference in the mechanical resistances between files with surface treatment and without treatment.
Materials and Methods G6 and ProTaper systems were selected for this study because they had similar dimensions in lateral aspect and tip sizes, and were made from same conventional NiTi alloy (1:1 atomic ratio of the major components). G6 A2 (Group A2) and G6 A2 without surface treatment after machining process (Group AN) were selected as one having surface treatment or not, and ProTaper F2 (Group F2) made from a conventional machining process was used for comparison. Both instruments have same ISO 025 tip size, 0.08 taper (at apical 3 mm), and convex triangular cross-section. Sixty new files were used for the cyclic fatigue and torsional resistance tests (n ¼ 10 for each group and each test). Before the experiments, the instruments were visually examined under a stereomicroscope and instruments with any type of defect were discarded. Torsional resistance was evaluated by using an AEndoS (DMJ System, Busan, Korea) (Fig. 1(A)) as described by Yum et al.(2011). The tip of the instrument (5 mm) was secured between polycarbonate blocks while keeping the file straight. The file was rotated clockwise at a constant rotational speed of 2 rpm until fracture occurred. The torsional load (Ncm) and distortion angle (˚) were recorded during the rotation
Fig 1. Customized devices used in this study: (A) AEndoS (DMJ system, Busan, Korea) for torsional strength test and (B) EndoC (DMJ system) for cyclic fatigue resistance test.
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of the files at the rate of 20 Hz. The toughness was computed from the area under the plot by using a software (Origin 6.0; Microcal Software Inc., Northampton, MA). Cyclic fatigue resistance was evaluated by using a custom-made device (EndoC; DMJ System) (Fig. 1(B)). This device was designed to allow reproducible simulation of an instrument confined in a curved canal. An artificial canal block was made of tempered steel with a 6 mm radius and 35˚ angle of curvature (Schneider, ’71; Pruett et al., ’97). The cyclic fatigue test was conducted in a dynamic mode, as described by Li et al. (2002) with continuous up-and-down pecking movements (4 mm in each direction per 0.5 sec and 50 ms of dwell time) to simulate a clinical setting. Synthetic oil (WD-40; WD-40 Company, San Diego, CA) was sprayed into the metal canal walls (Grande et al., 2006). The instruments were rotated at a constant speed of 300 rpm by using a torque-controlled motor (Reciproc Silver: VDW, Munich, Germany) (Gambarini, 2001). The torque was set at maximum (4.0 Ncm) to exclude the influence of torque on the rotational speed. Time was recorded when instrument fracture and the time was converted into number of cycles to failure (NCF). The length of the fractured fragment was measured using a digital microcaliper (Mitutoyo, Kawasaki, Japan). The data were analyzed statistically by using oneway analysis of variance (ANOVA) and Tukey post-hoc test. All statistical analyses were conducted at a significance level of 95% by using SPSS software (SPSS for Windows version 15.0; SPSS, Chicago, IL). After torsional and cyclic fatigue tests, the topographic features of all fractured fragments were evaluated under a scanning electron microscope (SEM) (S-4800 II; Hitachi High Technologies, Pleasanton, CA).
Results The results of the torsional resistance and cyclic fatigue tests are presented in Table I. Group A2 showed higher resistance to cyclic fatigue compared with groups AN and F2 (p < 0.05). Groups AN and F2 showed higher resistance to ultimate torsional strength compared with
group A2 (p < 0.05). However, when they were compared for toughness and fractured angles, no significant differences were found among the groups (p > 0.05). The surface features after cyclic fracture tests under the SEM are shown in Figure 2. While obvious machining grooves were shown in groups AN and F2, group A2 showed the smooth surface resulting from surface treatment (Fig. 2(D)). Group A2 showed irregular flow of micro-cracks on the file surface near the fracture margin (Fig. 2(G)). However, in the groups AN and F2, micro-cracks were found to run along the remained machining grooves (Fig. 2(H) and (I)). SEM examination of the fractured longitudinal surfaces after torsional resistance test showed the typical features of reverse helix of unwinding area (Fig. 3(A)(C)). The lengths of reverse winding area of group A2 were longer than groups AN and F2 (Fig. 3(A)). All cross-sectional aspects showed the typical features of fatigue fracture including crack initiation and propagation areas and also the typical features of torsional fracture including circular abrasion marks and fibrous dimples around the center of rotation.
Discussion File fracture may occur suddenly and jeopardize the outcome of treatment and healing of periapical tissues (Alapati et al., 2005; Panitvisai et al., 2010). Therefore, it is important to investigate the cyclic and torsional resistance of brand new NiTi rotary files when introduced to the market. Previous studies have evaluated smooth surface instruments (Anderson et al., 2007; Cheung et al., 2007; Kim et al., 2010) and Anderson et al. (2007) concluded that further electro-polishing procedure is likely to yield a reduction in surface irregularities that serve as points for stress concentration and crack initiation. Crack propagation at grain boundaries and surfaces showed that manufacturing process may serve as nucleating sites for the micro-voids, leading to instrument fractures during clinical use (Alapati et al., 2005). The high density of surface defects facilitates the crack nucleation stage and the fatigue failure is largely a crack propagation process (Kuhn et al., 2001). Thus, it is possible that the multitude of machining marks on the
TABLE I Torsional resistance and cyclic fatigue resistance (mean standard deviation) Torsional resistance
A2 AN F2
Cyclic fatigue resistance
Ultimate strength (Ncm)
Fracture angle (˚)
Toughness (˚Ncm)
NCF
Fragment length (mm)
2.26 0.19a 2.81 0.25b 3.03 0.14b
710 349a 615 72a 568 65a
1252 624a 1313 229a 1309 305a
2219 698a 977 325b 843 59b
2.67 0.41a 2.75 0.22a 2.88 0.26a
A2: G6 A2, AN: G6 A2 without surface treatment, F2: ProTaper F2. Different letters means statistical significance between groups: p < 0.05.
B. H. Kim et al.: Effect of surface treatment of rotary NiTi files
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Fig 2. Scanning electron micrographs of the fractured specimens after cyclic fatigue test. Upper row (A, B, and C) shows the typical cross-sectional aspects of cyclic fatigue fracture such as crack initiation area and fibrous fast fracture zone. Middle row (D, E, and F) shows the lateral aspects near the fracture margin. Figure D marked with a star shows highly polished area of the group A2. Figures D and F shows machined micro-grooves from the groups AN and F2. Lower row (G, H, and I) shows the irregular micro-cracks (arrow) on the smoothened surface of group A2 (G) and micro-cracks (arrow) running along the machined grooves in the groups AN (H) and F2 (I).
surface of a ground instrument would lead to crack initiation at multiple locations in which the resolved shear stress is greater than that required for crystallographic slip to occur (Kim et al., 2010). By chance to get the files before surface treatment which has same geometries of final products, this study was possible to be conducted. The manufacturer (Global top) of G6 rotary file system claims that the G6 has highly smoothened surface due to nano-polishing technique and heat treatment thus the aim of this study was to evaluate the effect of surface treatment on the instruments. Although the manufacturer of G6 did not provided the details of nano-polishing technique, an electro-polishing technique is a method of surface treatment to remove surface defects that may remain after machining procedure (Pohl et al., 2004). It might be a controlled chemomechanical process that involves submerging the machined files into an acidic solution which contains nano-particles for surface polishing. Explanations of improved performance of the electro-polished instru-
ments are attributed to reduction in micro-cracks on the surface of the material which eliminates residual stresses (Anderson et al., 2007; Kim et al., 2010). Kuhn et al. (2001) suggested that electro-polishing procedure could be used by the manufacturer to reduce the machining damage effectively on the file surface therefore improving the lifetime of endodontic files. The G6 A2 used in this study possesses the similar geometry of tip size, taper, and cross-section design with the ProTaper F2. As the files of group AN without surface treatment, ProTaper F2 also has machined surface without polishing procedure. In the present results, the group A2 appears to be significantly more resistant to cyclic fatigue than groups AN and F2, and thus it could be concluded as the files with surface treatment is more resistant to fatigue compared to the conventional NiTi instruments without surface treatment. For comparing the torsional resistance of NiTi files, the previous researches usually have used the parameters of the ultimate strength and distortion angle (Bahia
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Fig 3. Scanning electron micrographs of the fractured specimens after torsional fracture test. Upper row (A, B, and C) shows longitudinal lateral aspects near the fracture margin. The indicated areas are the unwound distortion areas with reverse helix. Middle row (D, E, and F) shows the cross-sectional aspects having typical features of torsional fractures such as fibrous dimples (circle) and circular abrasion marks (round arrow). Lower row (G, H, and I) shows the skewed fibrous dimples in the rotation center of torsional fracture area at highly magnified view (1000 ).
et al., 2006). However, the maximum torsional strength and distortion angle are partial parameters of stressstrain curve and subsequently do not always represent the material durability. Yum et al.(2011) suggested comparing the toughness may provide a better understanding of NiTi instrument’s behavior under torsion. Toughness is defined as the total energy for breaking a material, and it can be computed by integration of the stress and strain curve (load and distortional angle under the rotational failure) up to the point of fracture (Gere, 2001). The present results indicated that groups A2, AN, and F2 had no significantly difference in toughness, although groups AN and F2 showed a higher ultimate strength. Usually the files with lower torsional strength would have bigger rotational distortion angle (unwinding ability before fracture) and then the files with higher torsional strength would give clinicians lower chance to notice the distortion (Ha et al., 2013). Thus clinically neither the torsional strength nor the distortional angle would be the representative torsional property of rotating instruments but the experimentally
computed toughness might be the one to decide the torsional property. Therefore it might be concluded that group A2 have similar torsional properties compared with groups AN and F2 from their similar toughness. Considering together that the group A2 showed higher resistance to cyclic fatigue compared with groups AN and F2, surface polishing treatment procedure was shown to improve the surface smoothness of NiTi instruments and thus improve the instrument’s working properties (Cheung et al., 2007; Yum et al., 2011). From the SEM evaluation, the files of group A2 showed highly smoother surface than the group F2 and the micro-cracks near the fracture area of group A2 flowed more irregularly while the micro-cracks ran along the machining groove in groups AN and F2. Like in the non-polished files and ProTaper files with the machined surface, residual tensile stresses may also accelerate the crack propagation as well as the initiation process (Acevedo and Nussbaumer, 2012). Rather electro-polishing can produce a smooth, amorphous oxide layer that is free of most defects. Based on this
B. H. Kim et al.: Effect of surface treatment of rotary NiTi files
test, potential residual stresses on the surface of the G6 instruments were also removed in the process. Previous studies evaluated the effect of surface treatment on fatigue resistance but showed limited results, because they were conducted under the uncontrolled conditions of file’s geometries (Gambarini et al., 2008; Kim et al., 2010). Under the controlled conditions of this study, the electro-polished instruments may have better cyclic fatigue resistance while maintaining similar torsional properties.
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