SCANNING VOL. 36, 437–443 (2014) © Wiley Periodicals, Inc.
Endodontic Instruments After Torsional Failure: Nanoindentation Test AHMED JAMLEH,1 ALIREZA SADR,2 NAOYUKI NOMURA,3 ARATA EBIHARA,4 YOSHIO YAHATA,5 TAKAO HANAWA,6 JUNJI TAGAMI,2,7 AND HIDEAKI SUDA2,4 1
Department of Endodontics, College of Dentistry, King Saud bin Abdulaziz University for Health Sciences, National Guard Health Affairs, Riyadh, Saudi Arabia 2 Global Center of Excellence (GCOE), International Research Center for Molecular Science in Tooth and Bone Diseases, Tokyo Medical and Dental University, Tokyo, Japan 3 Department of Materials Processing, Graduate School of Engineering, Tohoku University, Tohoku, Japan 4 Department of Pulp Biology and Endodontics, Division of Oral Health Sciences, Medical and Dental Sciences Track, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University (TMDU), Tokyo, Japan 5 Faculty of Dentistry, Department of Endodontics, Showa University, Tokyo, Japan 6 Department of Metallic Biomaterials, Division of Biomedical Materials, Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University (TMDU), Tokyo, Japan 7 Department Cariology and Operative Dentistry, Division of Oral Health Sciences, Medical and Dental Sciences Track, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University (TMDU), Tokyo, Japan
Summary: This study aimed to evaluate effects of torsional loading on the mechanical properties of endodontic instruments using the nanoindentation technique. ProFile (PF; size 30, taper 04; Dentsply Maillefer, Switzerland) and stainless steel (SS; size 30, taper 02; Mani, Japan) instruments were subjected to torsional test. Nanoindentation was then performed adjacent to the edge of fracture (edge) and at the cutting part beside the shank (shank). Hardness and elastic modulus were measured under 100-mN force on 100 locations at each region, and compared to those obtained from the same regions on new instruments. It showed that PF and SS instruments failed at 559 67 and 596 73 rotation degrees and mean maximum torque of 0.90 0.07 and 0.99 0.05 N-cm, respectively. Hardness and elastic modulus ranged 4.8–6.7 and 118–339 GPa in SS, and 2.7–3.2 and 52–81 GPa in PF. Significant differences between torsion-fractured and new instruments in hardness and elastic modulus were detected in the SS system used. While in PF
Conflicts of interest: None. Address for reprints: Arata Ebihara, Department of Pulp Biology and Endodontics, Division of Oral Health Sciences, Medical and Dental Sciences Track, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University (TMDU), 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan E-mail: [email protected] Received 5 December 2013; Accepted with revision 24 January 2014 DOI: 10.1002/sca.21139 Published online 9 March 2014 in Wiley Online Library (wileyonlinelibrary.com).
system, the edge region after torsional fracture had significantly lower hardness and elastic modulus compared to new instruments. The local hardness and modulus of elasticity of endodontic instruments adjacent to the fracture edge are significantly reduced by torsional loading. SCANNING 36:437–443, 2014. © 2014 Wiley Periodicals, Inc. Key words: hardness, elastic modulus, nanoindentation, nickel–titanium, stainless steel
Introduction Root canal therapy is performed to remove inflamed or necrotic pulp tissue from the coronal and radicular parts of the root canal, by cleaning and shaping with endodontic instruments. Traditionally, stainless steel (SS) instruments have been used to accomplish this task. However, these instruments lack the flexibility required to negotiate curved canals. Thus, instruments made of nickel titanium (NiTi) alloy were introduced that added flexibility to the instrument. NiTi instruments have become an integral part of the endodontic treatment due to their exceptional properties of super elasticity and shape memory, which make a root canal treatment more foreseeable. With their wide acceptance, several systems were introduced with various manufacturing processes, metallurgic characteristics, geometries and clinical uses (Parashos and Messer, 2006). NiTi features originate from the change between two main crystallographic phases, namely austenite and
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martensite, by a so-called martensitic transformation (Thompson, 2000). This reversible transformation from austenite to martensite is the main reason for improved flexibility of NiTi instruments, which in turn facilitates preparation of curved root canals with less canal transportation in comparison with SS instruments (Kazemi et al., 2000; Perez et al., 2005). In addition, they are not permanently deformed as easily as SS (Schafer, ’97). In spite of these advantages, the fracture risk of NiTi instruments should not be overlooked (Glossen et al., ’95; Fife et al., 2004). During root canal preparation, even with gentle use, clinicians might encounter unwanted accidents such as defects leadings to intracanal instrument fracture (Di fiore et al., 2006; Arantes et al., 2014). A number of factors could affect the susceptibility of the instrument to fracture, including clinician’s handling (Fife et al., 2004), root canal anatomy (Pruett et al., ’97; Park et al., 2013), geometry (Turpin et al., 2000; Park et al., 2013) and manufacturing process of endodontic instrument (Parashos and Messer, 2006), and its usage history (Yared, 2008). The prospective difficulty in removing fractured instrument from the root canal (Ward et al., 2003) and its negative impact on prognosis of the treatment (Thoden van Velzen et al., ’81) make it a necessity to identify the mechanisms responsible for fracture. Such investigation can provide additional understanding of the problem and potentially result in more effective application of the endodontic instruments. The fracture of NiTi and SS instruments can be due to cyclic fatigue, torsional failure or both (Glossen et al., ’95; Luebke et al., ’95; Kazemi et al., 2000). Fatigue life could be related to the extent to which the instrument is bent when placed in a curved root canal and allowed to rotate, subjecting it to repeated events of tension and compression at the point of maximum flexure (Plotino et al., 2009). Torsional failure occurs when the friction between the instrument and root canal dentin, which leads to an increased rotational torque load, exceeds the torsional strength of the instrument (Sattapan et al., 2000). Nanoindentation technology provides accurate measurements of mechanical properties at different regions within specimens of small sizes such as a single instrument, and this could provide valuable insights on the performance of endodontic instruments (Sadr et al., 2009). A recent nanoindentation study showed that cyclic fatigue process led to a substantial decrease in the hardness and elastic modulus of NiTi instruments, and suggested to investigate changes in mechanical properties after torsional failure using similar technique (Jamleh et al., 2012). Another recent study also reported work softening of NiTi alloy after failure by cyclic fatigue (Gloanec et al., 2013). Several studies have evaluated the torsional resistance of endodontic instruments; it was reported that
repeated torsional loadings affected the large-scale mechanical behavior of endodontic instruments (Luebke et al., ’95; Kazemi et al., 2000; Sattapan et al., 2000; Bahia et al., 2006; Kramkowski and Bahcall, 2009; Yum et al., 2011). However, there were no reports on the local mechanism of torsional failure in terms of nano-scale mechanical properties of the instruments. Thus, the present work was conducted to evaluate the hardness and elastic modulus of the internal structure of presently available instruments, NiTi and SS instruments, after torsional failure using the nanoindentation testing. The null hypothesis of the study was that the local hardness and elastic modulus of endodontic instruments were not affected by torsional loading leading to failure.
Materials and Methods Instruments
Two endodontic instrument systems; stainless steel K-file (SS; size 30, taper 02; Mani, Japan) hand instruments and Profile (PF; size 30, taper 04; Dentsply Maillefer, Ballaigues, Switzerland) NiTi rotary instruments were employed for investigation. Torsional Test
The experimental design for assessment of the torsional resistance was described previously (Miyai et al., 2006). Briefly, the handle of the instrument was removed, and the diameter at 3 mm from the tip was measured with a dial gauge (Teclock, Nagano, Japan). Subsequently, the instrument was placed in a torsional testing apparatus (Orientec, Tokyo, Japan); the shaft end was fastened into a chuck controlled by a digital display showing degrees rotation preprogrammed to rotate 19.1˚/s in a clockwise direction. The apical 3 mm of the instrument was clamped between two 3-mm thick soft brass inserts that permitted a connection through a conductive plate connected to a digital torque meter memocouple forming a break detection circuit. The instrument rotated until failure occurred by torsion. The test was performed at room temperature, and five instruments of each system were subjected to this test. Nanoindentation Test
Nanoindentation was performed at two regions on each specimen; within 0.1 mm from the fractured edge in specimens fractured by torsion (Fractured-Edge) and their cutting part beside the shank (Fractured-Shank). This was compared to those obtained from the same regions on new specimens; New-Edge (3 mm from the tip of new specimens) and New-Shank (Fig. 1). Specimen preparation and nanoindentation testing were described previously (Jamleh et al., 2012). In
Jamleh et al.: Nanoindentation after torsional fatigue
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Fig 1. Schematic drawings of endodontic instruments showing the tested regions.
short, the specimens were embedded in acrylic resin in a horizontal orientation and then subjected to metallographic preparation. The surfaces of the mounted specimens were ground, polished and then ultrasonically cleaned with distilled water for 10 min. Five torsionfractured and five new instruments from each system were used as specimens. The test was performed at 28˚C with a nanoindentation testing device (ENT-1100a, Elionix, Tokyo, Japan) using a calibrated Berkovich diamond tip with a curvature radius of 100 nm. The indentation points were selected using an optical microscope and chargecoupled device (CCD) camera connected to the testing device. The maximum indenting load value was 100 mN, which was reached at a constant loading rate of 10 mN/s. For each indentation, a curve showing the relationship between the load and displacement was drawn (Fig. 2a and b). The hardness and modulus of elasticity were calculated from loads and displacements measured during indentations according to Oliver and Pharr (’92). The resulting indents were observed with a CCD camera to exclude indentations with unclear shape. Twenty successful indents spaced 20 mm apart in each region specimen were selected (Fig. 2c), totaling 100 indents for each region. Mean values of hardness and elastic modulus were statistically compared using a two-way analysis of
Fig 2. Characteristic loading and unloading curves for the nanoindentation of all regions in PF (a) and SS (b) obtained from successful indents (c).
variance (ANOVA) and followed by multiple comparisons using t-test with Bonferroni correction at a significance level of a ¼ 0.05 using SPSS 16 software (SPSS Inc., Chicago, IL).
Results The average diameters of PF and SS measured at 3 mm from the tip were 0.40 and 0.37 mm, respectively.
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Under torsion, PF and SS instruments had rotated 559 67˚ and 596 73.2˚, respectively, to fracture. The maximum mean torque at fracture was 0.90 0.07 N-cm for PF whereas for SS, it was 0.99 0.05 N-cm. Figure 3a and b show the hardness and elastic modulus values of fractured and new PF and SS instruments at edge and shank regions that were obtained from nanoindentation test. Significant differ-
ences between torsion-fractured and new instruments in hardness and elastic modulus were detected in the SS system used. While in PF system, the edge region after torsional fracture had significantly lower hardness and elastic modulus compared to new instruments. In terms of hardness (Fig. 3a), Fractured-Edge regions in both PF and SS instruments showed the lowest mean values of 2.74 and 4.84 GPa, respectively.
Fig 3. The hardness (a) and elastic modulus (b) values of PF and SS at edge and shank regions obtained from nanoindentation. Each column represents a mean of 100 indents and the asterisk means there is statistical significance. Significant differences between PF and SS at each region (p ¼ 0.00).
Jamleh et al.: Nanoindentation after torsional fatigue
Whereas New-Edge, Fractured-Shank, and New-Shank regions showed 3.17 and 6.06, 3.16 and 5.92, and 3.17 and 6.68 GPa for PF and SS instruments, respectively. Clearly, SS was significantly harder than PF in each region (P < 0.001). In terms of elastic modulus (Fig. 3b) for PF system, Fractured-Edge had the lowest mean value (52.05 GPa) followed by Fractured-Shank (71.16 GPa). Whereas New-Edge and New-Shank for PF system showed 79.26 and 81.22 GPa, respectively. The results of SS instruments showed a similar manner for Fractured-Edge, Fractured-Shank, New-Edge, and New-Shank showing 117.9, 238.9, 225.8, and 339.2 GPa, respectively. Clearly, PF was significantly more flexible than SS within each region (p < 0.001). Two-way ANOVA of hardness showed that the factors (location “edge and shank” and condition “new and fractured”) and their interactions were significant in both PF and SS (p < 0.001). A similar trend was seen for elastic modulus, with the exception of the interaction term in SS, which was not significant (p ¼ 0.46).
Discussion Using the nanoindentation technique, this study investigated alterations in mechanical properties of the internal structures of NiTi and SS instruments after torsional failure. Traditionally, the mechanical properties of NiTi and SS alloys have been measured separately using bulk tests (such as bending and tensile tests) and Vickers hardness test (Tian and Darvell, 2010; Iijima et al., 2011). However, measurements of mechanical properties obtained from nanoindentation were notably different (Iijima et al., 2011). The difference can be attributed to the different work hardening levels and different material volume sampled. In the conventional Vickers hardness method, a diamond indenter is pressed into the specimen by using a known load applied for a predetermined time. The produced indentation is measured, and the hardness is defined as the maximum load divided by the indented area (Alapati et al., 2006). Nanoindentation differs in that the load and the displacement of the indenter are recorded during the indentation process (Fig. 2), and then analyzed to obtain the mechanical properties, without having to observe the indents manually. Furthermore, simultaneous quantitative measurements of hardness and elastic modulus are performed over on a much smaller volume of the material (Oliver and Pharr, ’92; Sadr et al., 2009), capturing the incidence of localized phase changes. This localized phase change might indicate how greatly a metal is changed in the process of failure (Jamleh et al., 2012). Thus, direct comparison between different testing methods is not appropriate and the results of this study
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should be interpreted with caution. For example, it is known that the elastic modulus of as-received SS instruments by bulk tests is approximately 200 GPa (Alcock et al., 2009), whereas in this study the measurements reached up to 374 GPa. Consistently, another nanoindentation study found that the elastic modulus of SS reached 371 GPa (Alcock et al., 2009). The tested instruments were prepared and examined in a standardized approach to minimize the impact of specimen preparation or alteration in the testing setup to the extent possible. Calibration of the Berkovich indenter tip was initially performed by indenting a standard of known hardness and the modulus of elasticity values. The measured values in each region were remarkably close to each other, resulting in relatively small standard deviations in each region, highlighting the consistency of values at that region. Thus, 20 successful indents spaced 20 mm apart in each region of the specimens appeared to represent the properties of the region investigated. In addition to hardness, elastic modulus was investigated in this study. The hardness of a metal can be defined as its resistance to local deformation through indentation, while the elastic modulus property reveals metal’s stiffness and/or resistance to bending represented in its inter-atomic forces. Unlike hardness, elastic modulus is considered to be inherent (Craig and Powers, 2002). The hardness values for PF NiTi instruments were significantly lower than those observed for SS instruments (p < 0.001; Fig. 3a), which is consistent with a previous study (Serene et al., ’95). Hardness is an essential aspect to consider when assessing the cutting efficiency of endodontic instruments (Bergmans et al., 2001). Although the low hardness could imply a weaker cutting efficiency (Brockhurst and Hau, ’98), NiTi instruments are thought to be more durable (Kazemi et al., ’96) and have shown favorably less root canal transportation than their SS counterparts (Schafer and Lau, ’99). Another noteworthy finding was that the mechanical properties in new SS instruments were significantly different between the edge and shank regions (p < 0.001; not shown in figure); likewise, the elastic modulus values of PF instruments showed statistical significance between the two regions (p < 0.001; not shown in figure). In this regard, it was suggested that potential differences in the thermomechanical processes and degree of work hardening that took place over the length of the instrument during the manufacturing process might affect local mechanical properties of an instrument (Wildey et al., ’92; Alapati et al., 2006; Zinelis et al., 2010; Jamleh et al., 2012); the SS instruments in this study appeared to be more affected by such phenomenon. The differences of measured values between different regions within the same
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instrument show the complexity of manufacturing the instrument, particularly when considering that the variation of hardness and elastic modulus in these regions may not be representative of those throughout the length of the instrument. In the present study, torsional loading has made considerable changes in the local mechanical properties. The mechanical behavior of endodontic instruments under torsion has a load–distortion characteristic. During torsional loading, the stress–strain behavior initially shows elastic deformation. On continuous loading, the imposed strain exceeds the elastic limit and will continue to deform plastically. Then, it passes the point of the highest ultimate strength and subsequently fails (Wolcott and Himel, ’97). In the current study, PF and SS instruments had reached maximum mean torque of 0.90 0.07 and 0.99 0.05 N-cm, respectively. NiTi alloys have two main crystalline phases, named austenite and martensite, where the former might transform to the latter by the effect of stress (Hou et al., 2011). A recent nanoindentation study showed that cyclic fatigue significantly reduced the mechanical properties of NiTi instruments in both edge and shank regions (Jamleh et al., 2012), whereas this experiment reported a localized effect on the fractured edge region after torsional failure. Furthermore, under similar conditions, cyclic fatigue reduced the hardness and elastic modulus of PF NiTi instruments at the edge region to 30% and 40% (Jamleh et al., 2012), whereas torsional failure has caused 13% and 27% decrease, respectively. These figures provide insights on the more deleterious effect of fatigue compared to torsional failure. Consistently, previous studies showed the cyclic fatigue to be the primary cause of NiTi instrument fracture (Parashos et al., 2004; Shen et al., 2006). The SS instruments used in this experiment were made from austenitic SS alloys with face-cantered cubic crystal structure. It is reported that stress application on such alloys will aid the generation of dislocations and hence the martensite transformation having body cubic tetragonal crystal arrangement (Porter and Easterling ’92). This transformation can be related to deterioration of the material property and microstructural changes, as the results of present work confirm. Xray diffraction, atomic force microscopy, electron backscatter diffraction and transmission electron microscopy studies reported also that plastic deformations of austenitic SS have induced martensitic transformation (Wang et al., 2005; Abreu et al., 2007). In the present study, plastic deformation was applied by torsional process and apparently martensitic transformation took place. The martensitic formation was confirmed by the changes from a high hardness and elastic modulus (New-Edge and New-Shank) to low ones (Fractured-Edge and Fractured-Shank) (Fig. 3a and b).
Conclusions Within the limitations of this study, it should be concluded that the instruments fractured by torsion had significant reductions in the local modulus of elasticity and hardness adjacent to the fractured edge. Therefore, the null hypothesis of this study was rejected. The findings may suggest that changes in mechanical properties of SS and PF are likely to be causative for fractures due to torsional movement.
References Abreu H, Carvalho S, Neto P, et al. 2007. Deformation induced martensite in an AISI 301LN stainless steel: characterization and influence on pitting corrosion resistance. Mater Res 10:359–366. Alapati SB, Brantley WA, Nusstein JM, et al. 2006. Vickers hardness investigation of work-hardening in used NiTi rotary instruments. J Endod 32:1191–1193. Alcock JP, Barbour ME, Sandy JR, Ireland AJ. 2009. Nanoindentation of orthodontic archwires: the effect of decontamination and clinical use on hardness, elastic modulus and surface roughness. Dental Mater 25:1039–1043. Arantes WB, da Silva CM, Lage-Marques JL, et al. 2014. SEM analysis of defects and wear on Ni–Ti rotary instruments. Scanning. 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. Bergmans L, van Cleynenbreugel J, Wevers M, Lambrechts P. 2001. Mechanical root canal preparation with NiTi rotary instruments: rationale, performance and safety. Status report for the American Journal of Dentistry. Am J Dent 14:324– 333. Brockhurst P, Hau E. 1998. Hardness and strength of endodontic instruments made from NiTi alloy. Aust Endod J 24:115–119. Craig RG, Powers JM. 2002. Restorative dental materials. 11th edition. St. Louis, MO: Elsevier Science. p 77. Di Fiore PM, Genov KA, Komaroff E, Li Y, Lin L. 2006. Nickel– titanium rotary instrument fracture: a clinical practice assessment. Int Endod J 39:700–708. Fife D, Gambarini G, Britto Lr Lr. 2004. Cyclic fatigue testing of ProTaper NiTi rotary instruments after clinical use. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 97:251–256. Gloanec A, Bilotta G, Gerland M. 2013. Deformation mechanisms in a TiNi shape memory alloy during cyclic loading. Mater Sci Eng A 564:351–358. 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. Hou XM, Yahata Y, Hayashi Y, et al. 2011. Phase transformation behaviour and bending property of twisted nickel–titanium endodontic instruments. Int Endod J 44:253–258. Iijima M, Muguruma T, Brantley WA, Mizoguchi I. 2011. Comparisons of nanoindentation, 3-point bending, and tension tests for orthodontic wires. Am J Orthod Dentofacial Orthop 140:65–71. Jamleh A, Sadr A, Nomura N, et al. 2012. Nano-indentation testing of new and fractured nickel-titanium endodontic instruments. Int Endod J 45:462–468. Kazemi RB, Stenman E, Spa˚ngberg LSW. 1996. Machining efficiency and wear resistance of nickel-titanium endodontic files. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 81:596–602.
Jamleh et al.: Nanoindentation after torsional fatigue Kazemi RB, Stenman E, Spa˚ngberg LS. 2000. A comparison of stainless steel and nickel–titanium H-type instruments ofidentical design: torsional and bending tests. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 90:500–506. Kramkowski TR, Bahcall J. 2009. 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 35:404–407. Luebke NH, Brantley WA, Sabri ZI, Luebke FL, Lausten LL. 1995. Physical dimensions, torsional performance, bending properties, and metallurgical characteristics of rotary endodontic instruments. VI. Canal Master drills. J Endod 21:259– 263. Miyai K, Ebihara A, Hayashi Y, et al. 2006. Influence of phase transformation on the torsional and bending properties of nickel–titanium rotary endodontic instruments. Int Endod J 39:119–126. Oliver WC, Pharr GM. 1992. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res 7:1564–1583. Parashos P, Messer HH. 2006. The diffusion of innovation in dentistry: a review using rotary nickel–titanium technology as an example. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 101:395–401. Parashos P, Gordon I, Messer HH. 2004. Factors influencing defects of rotary nickel–titanium endodontic instruments after clinical use. J Endod 30:722–725. Park SK, Kim YJ, Shon WJ, et al. 2013. Clinical efficiency and reusability of the reciprocating nickel–titanium instruments according to the root canal anatomy. Scanning. Perez F, Schoumacher M, Peli JF. 2005. Shaping ability of two rotary instruments in simulated canals: stainless steel ENDOflash and nickel–titanium HERO Shaper. Int Endod J 38:637–644. Plotino G, Grande NM, Cordaro M, Testarelli L, Gambarini G. 2009. A review of cyclic fatigue testing of nickel–titanium rotary instruments. J Endod 35:1469–1476. Porter D, Easterling K. 1992. Phase transformation in metals and alloys. 2nd edition. London: Chapman & Hall. p 382–440. Pruett JP, Clement DJ, Carnes DL Jr. 1997. Cyclic fatigue testing of nickel–titanium endodontic instruments. J Endod 23:77–85. Sadr A, Shimada Y, Lu H, Tagami J. 2009. The viscoelastic behavior of dental adhesives: a nanoindentation study. Dental Mater 25:13–19. Sattapan B, Nervo GJ, Palamara JE, Messer HH. 2000. Defects in rotary nickel–titanium files after clinical use. J Endod 26:161–165.
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Schafer E. 1997. Root canal instruments for manual use: a review. Endod Dental Traumatol 13:51–64. Schafer E, Lau R. 1999. Comparison of cutting efficiency and instrumentation of curved canals with nickel–titanium and stainless steel instruments. J Endod 25:427–430. Serene TP, Adams JD, Saxena A. 1995. Nickel–titanium instruments. Applications in endodontics. St. Louis, MO: Ishiyaku Euroamerica, Inc. Shen Y, Cheung GS, Bian Z, et al. 2006. Comparison of defects in ProFile and ProTaper systems after clinical use. J Endod 32:61–65. Thoden van Velzen SK, Duivenvoorden HJ, Schuurs AH. 1981. Probabilities of success and failure in endodontic treatment: a Bayesian approach. Oral Surg Oral Med Oral Pathol 52: 85–90. Thompson SA. 2000. An overview of nickel–titanium alloys used in dentistry. Int Endod J 33:297–310. Tian K, Darvell BW. 2010. Determination of the flexural modulus of elasticity of orthodontic archwires. Dental Mater 26:821– 829. Turpin YL, Chagneau F, Vulcain JM. 2000. Impact of two theoretical cross-sections on torsional and bending stresses of nickel titanium root canal instrument models. J Endod 26:414–417. Wang H, Yang J, Bhadeshia HKDH. 2005. Characterisation of severely deformed austenitic stainless steel wire. Mater Sci Technol 21:1323–1328. Ward JR, Parashos P, Messer HH. 2003. Evaluation of an ultrasonic technique to remove fractured rotary nickel– titanium endodontic instruments from root canals: clinical cases. J Endod 29:764–767. Wildey WL, Senia ES, Montgomery S. 1992. Another look at root canal instrumentation. Oral Surg Oral Med Oral Pathol 74:499–507. Wolcott J, Himel VT. 1997. Torsional properties of nickel– titanium versus stainless steel endodontic files. J Endod 23:217–220. Yared G. 2008. Canal preparation using only one Ni–Ti rotary instrument: preliminary observations. Int Endod J 41:339– 344. 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. Zinelis S, Eliades T, Eliades G. 2010. A metallurgical characterization of ten endodontic Ni–Ti instruments: assessing the clinical relevance of shape memory and superelastic properties of Ni–Ti endodontic instruments. Int Endod J 43:125–134.
Endodontic Instruments After Torsional Failure: Nanoindentation Test AHMED JAMLEH,1 ALIREZA SADR,2 NAOYUKI NOMURA,3 ARATA EBIHARA,4 YOSHIO YAHATA,5 TAKAO HANAWA,6 JUNJI TAGAMI,2,7 AND HIDEAKI SUDA2,4 1
Department of Endodontics, College of Dentistry, King Saud bin Abdulaziz University for Health Sciences, National Guard Health Affairs, Riyadh, Saudi Arabia 2 Global Center of Excellence (GCOE), International Research Center for Molecular Science in Tooth and Bone Diseases, Tokyo Medical and Dental University, Tokyo, Japan 3 Department of Materials Processing, Graduate School of Engineering, Tohoku University, Tohoku, Japan 4 Department of Pulp Biology and Endodontics, Division of Oral Health Sciences, Medical and Dental Sciences Track, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University (TMDU), Tokyo, Japan 5 Faculty of Dentistry, Department of Endodontics, Showa University, Tokyo, Japan 6 Department of Metallic Biomaterials, Division of Biomedical Materials, Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University (TMDU), Tokyo, Japan 7 Department Cariology and Operative Dentistry, Division of Oral Health Sciences, Medical and Dental Sciences Track, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University (TMDU), Tokyo, Japan
Summary: This study aimed to evaluate effects of torsional loading on the mechanical properties of endodontic instruments using the nanoindentation technique. ProFile (PF; size 30, taper 04; Dentsply Maillefer, Switzerland) and stainless steel (SS; size 30, taper 02; Mani, Japan) instruments were subjected to torsional test. Nanoindentation was then performed adjacent to the edge of fracture (edge) and at the cutting part beside the shank (shank). Hardness and elastic modulus were measured under 100-mN force on 100 locations at each region, and compared to those obtained from the same regions on new instruments. It showed that PF and SS instruments failed at 559 67 and 596 73 rotation degrees and mean maximum torque of 0.90 0.07 and 0.99 0.05 N-cm, respectively. Hardness and elastic modulus ranged 4.8–6.7 and 118–339 GPa in SS, and 2.7–3.2 and 52–81 GPa in PF. Significant differences between torsion-fractured and new instruments in hardness and elastic modulus were detected in the SS system used. While in PF
Conflicts of interest: None. Address for reprints: Arata Ebihara, Department of Pulp Biology and Endodontics, Division of Oral Health Sciences, Medical and Dental Sciences Track, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University (TMDU), 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan E-mail: [email protected] Received 5 December 2013; Accepted with revision 24 January 2014 DOI: 10.1002/sca.21139 Published online 9 March 2014 in Wiley Online Library (wileyonlinelibrary.com).
system, the edge region after torsional fracture had significantly lower hardness and elastic modulus compared to new instruments. The local hardness and modulus of elasticity of endodontic instruments adjacent to the fracture edge are significantly reduced by torsional loading. SCANNING 36:437–443, 2014. © 2014 Wiley Periodicals, Inc. Key words: hardness, elastic modulus, nanoindentation, nickel–titanium, stainless steel
Introduction Root canal therapy is performed to remove inflamed or necrotic pulp tissue from the coronal and radicular parts of the root canal, by cleaning and shaping with endodontic instruments. Traditionally, stainless steel (SS) instruments have been used to accomplish this task. However, these instruments lack the flexibility required to negotiate curved canals. Thus, instruments made of nickel titanium (NiTi) alloy were introduced that added flexibility to the instrument. NiTi instruments have become an integral part of the endodontic treatment due to their exceptional properties of super elasticity and shape memory, which make a root canal treatment more foreseeable. With their wide acceptance, several systems were introduced with various manufacturing processes, metallurgic characteristics, geometries and clinical uses (Parashos and Messer, 2006). NiTi features originate from the change between two main crystallographic phases, namely austenite and
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martensite, by a so-called martensitic transformation (Thompson, 2000). This reversible transformation from austenite to martensite is the main reason for improved flexibility of NiTi instruments, which in turn facilitates preparation of curved root canals with less canal transportation in comparison with SS instruments (Kazemi et al., 2000; Perez et al., 2005). In addition, they are not permanently deformed as easily as SS (Schafer, ’97). In spite of these advantages, the fracture risk of NiTi instruments should not be overlooked (Glossen et al., ’95; Fife et al., 2004). During root canal preparation, even with gentle use, clinicians might encounter unwanted accidents such as defects leadings to intracanal instrument fracture (Di fiore et al., 2006; Arantes et al., 2014). A number of factors could affect the susceptibility of the instrument to fracture, including clinician’s handling (Fife et al., 2004), root canal anatomy (Pruett et al., ’97; Park et al., 2013), geometry (Turpin et al., 2000; Park et al., 2013) and manufacturing process of endodontic instrument (Parashos and Messer, 2006), and its usage history (Yared, 2008). The prospective difficulty in removing fractured instrument from the root canal (Ward et al., 2003) and its negative impact on prognosis of the treatment (Thoden van Velzen et al., ’81) make it a necessity to identify the mechanisms responsible for fracture. Such investigation can provide additional understanding of the problem and potentially result in more effective application of the endodontic instruments. The fracture of NiTi and SS instruments can be due to cyclic fatigue, torsional failure or both (Glossen et al., ’95; Luebke et al., ’95; Kazemi et al., 2000). Fatigue life could be related to the extent to which the instrument is bent when placed in a curved root canal and allowed to rotate, subjecting it to repeated events of tension and compression at the point of maximum flexure (Plotino et al., 2009). Torsional failure occurs when the friction between the instrument and root canal dentin, which leads to an increased rotational torque load, exceeds the torsional strength of the instrument (Sattapan et al., 2000). Nanoindentation technology provides accurate measurements of mechanical properties at different regions within specimens of small sizes such as a single instrument, and this could provide valuable insights on the performance of endodontic instruments (Sadr et al., 2009). A recent nanoindentation study showed that cyclic fatigue process led to a substantial decrease in the hardness and elastic modulus of NiTi instruments, and suggested to investigate changes in mechanical properties after torsional failure using similar technique (Jamleh et al., 2012). Another recent study also reported work softening of NiTi alloy after failure by cyclic fatigue (Gloanec et al., 2013). Several studies have evaluated the torsional resistance of endodontic instruments; it was reported that
repeated torsional loadings affected the large-scale mechanical behavior of endodontic instruments (Luebke et al., ’95; Kazemi et al., 2000; Sattapan et al., 2000; Bahia et al., 2006; Kramkowski and Bahcall, 2009; Yum et al., 2011). However, there were no reports on the local mechanism of torsional failure in terms of nano-scale mechanical properties of the instruments. Thus, the present work was conducted to evaluate the hardness and elastic modulus of the internal structure of presently available instruments, NiTi and SS instruments, after torsional failure using the nanoindentation testing. The null hypothesis of the study was that the local hardness and elastic modulus of endodontic instruments were not affected by torsional loading leading to failure.
Materials and Methods Instruments
Two endodontic instrument systems; stainless steel K-file (SS; size 30, taper 02; Mani, Japan) hand instruments and Profile (PF; size 30, taper 04; Dentsply Maillefer, Ballaigues, Switzerland) NiTi rotary instruments were employed for investigation. Torsional Test
The experimental design for assessment of the torsional resistance was described previously (Miyai et al., 2006). Briefly, the handle of the instrument was removed, and the diameter at 3 mm from the tip was measured with a dial gauge (Teclock, Nagano, Japan). Subsequently, the instrument was placed in a torsional testing apparatus (Orientec, Tokyo, Japan); the shaft end was fastened into a chuck controlled by a digital display showing degrees rotation preprogrammed to rotate 19.1˚/s in a clockwise direction. The apical 3 mm of the instrument was clamped between two 3-mm thick soft brass inserts that permitted a connection through a conductive plate connected to a digital torque meter memocouple forming a break detection circuit. The instrument rotated until failure occurred by torsion. The test was performed at room temperature, and five instruments of each system were subjected to this test. Nanoindentation Test
Nanoindentation was performed at two regions on each specimen; within 0.1 mm from the fractured edge in specimens fractured by torsion (Fractured-Edge) and their cutting part beside the shank (Fractured-Shank). This was compared to those obtained from the same regions on new specimens; New-Edge (3 mm from the tip of new specimens) and New-Shank (Fig. 1). Specimen preparation and nanoindentation testing were described previously (Jamleh et al., 2012). In
Jamleh et al.: Nanoindentation after torsional fatigue
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Fig 1. Schematic drawings of endodontic instruments showing the tested regions.
short, the specimens were embedded in acrylic resin in a horizontal orientation and then subjected to metallographic preparation. The surfaces of the mounted specimens were ground, polished and then ultrasonically cleaned with distilled water for 10 min. Five torsionfractured and five new instruments from each system were used as specimens. The test was performed at 28˚C with a nanoindentation testing device (ENT-1100a, Elionix, Tokyo, Japan) using a calibrated Berkovich diamond tip with a curvature radius of 100 nm. The indentation points were selected using an optical microscope and chargecoupled device (CCD) camera connected to the testing device. The maximum indenting load value was 100 mN, which was reached at a constant loading rate of 10 mN/s. For each indentation, a curve showing the relationship between the load and displacement was drawn (Fig. 2a and b). The hardness and modulus of elasticity were calculated from loads and displacements measured during indentations according to Oliver and Pharr (’92). The resulting indents were observed with a CCD camera to exclude indentations with unclear shape. Twenty successful indents spaced 20 mm apart in each region specimen were selected (Fig. 2c), totaling 100 indents for each region. Mean values of hardness and elastic modulus were statistically compared using a two-way analysis of
Fig 2. Characteristic loading and unloading curves for the nanoindentation of all regions in PF (a) and SS (b) obtained from successful indents (c).
variance (ANOVA) and followed by multiple comparisons using t-test with Bonferroni correction at a significance level of a ¼ 0.05 using SPSS 16 software (SPSS Inc., Chicago, IL).
Results The average diameters of PF and SS measured at 3 mm from the tip were 0.40 and 0.37 mm, respectively.
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Under torsion, PF and SS instruments had rotated 559 67˚ and 596 73.2˚, respectively, to fracture. The maximum mean torque at fracture was 0.90 0.07 N-cm for PF whereas for SS, it was 0.99 0.05 N-cm. Figure 3a and b show the hardness and elastic modulus values of fractured and new PF and SS instruments at edge and shank regions that were obtained from nanoindentation test. Significant differ-
ences between torsion-fractured and new instruments in hardness and elastic modulus were detected in the SS system used. While in PF system, the edge region after torsional fracture had significantly lower hardness and elastic modulus compared to new instruments. In terms of hardness (Fig. 3a), Fractured-Edge regions in both PF and SS instruments showed the lowest mean values of 2.74 and 4.84 GPa, respectively.
Fig 3. The hardness (a) and elastic modulus (b) values of PF and SS at edge and shank regions obtained from nanoindentation. Each column represents a mean of 100 indents and the asterisk means there is statistical significance. Significant differences between PF and SS at each region (p ¼ 0.00).
Jamleh et al.: Nanoindentation after torsional fatigue
Whereas New-Edge, Fractured-Shank, and New-Shank regions showed 3.17 and 6.06, 3.16 and 5.92, and 3.17 and 6.68 GPa for PF and SS instruments, respectively. Clearly, SS was significantly harder than PF in each region (P < 0.001). In terms of elastic modulus (Fig. 3b) for PF system, Fractured-Edge had the lowest mean value (52.05 GPa) followed by Fractured-Shank (71.16 GPa). Whereas New-Edge and New-Shank for PF system showed 79.26 and 81.22 GPa, respectively. The results of SS instruments showed a similar manner for Fractured-Edge, Fractured-Shank, New-Edge, and New-Shank showing 117.9, 238.9, 225.8, and 339.2 GPa, respectively. Clearly, PF was significantly more flexible than SS within each region (p < 0.001). Two-way ANOVA of hardness showed that the factors (location “edge and shank” and condition “new and fractured”) and their interactions were significant in both PF and SS (p < 0.001). A similar trend was seen for elastic modulus, with the exception of the interaction term in SS, which was not significant (p ¼ 0.46).
Discussion Using the nanoindentation technique, this study investigated alterations in mechanical properties of the internal structures of NiTi and SS instruments after torsional failure. Traditionally, the mechanical properties of NiTi and SS alloys have been measured separately using bulk tests (such as bending and tensile tests) and Vickers hardness test (Tian and Darvell, 2010; Iijima et al., 2011). However, measurements of mechanical properties obtained from nanoindentation were notably different (Iijima et al., 2011). The difference can be attributed to the different work hardening levels and different material volume sampled. In the conventional Vickers hardness method, a diamond indenter is pressed into the specimen by using a known load applied for a predetermined time. The produced indentation is measured, and the hardness is defined as the maximum load divided by the indented area (Alapati et al., 2006). Nanoindentation differs in that the load and the displacement of the indenter are recorded during the indentation process (Fig. 2), and then analyzed to obtain the mechanical properties, without having to observe the indents manually. Furthermore, simultaneous quantitative measurements of hardness and elastic modulus are performed over on a much smaller volume of the material (Oliver and Pharr, ’92; Sadr et al., 2009), capturing the incidence of localized phase changes. This localized phase change might indicate how greatly a metal is changed in the process of failure (Jamleh et al., 2012). Thus, direct comparison between different testing methods is not appropriate and the results of this study
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should be interpreted with caution. For example, it is known that the elastic modulus of as-received SS instruments by bulk tests is approximately 200 GPa (Alcock et al., 2009), whereas in this study the measurements reached up to 374 GPa. Consistently, another nanoindentation study found that the elastic modulus of SS reached 371 GPa (Alcock et al., 2009). The tested instruments were prepared and examined in a standardized approach to minimize the impact of specimen preparation or alteration in the testing setup to the extent possible. Calibration of the Berkovich indenter tip was initially performed by indenting a standard of known hardness and the modulus of elasticity values. The measured values in each region were remarkably close to each other, resulting in relatively small standard deviations in each region, highlighting the consistency of values at that region. Thus, 20 successful indents spaced 20 mm apart in each region of the specimens appeared to represent the properties of the region investigated. In addition to hardness, elastic modulus was investigated in this study. The hardness of a metal can be defined as its resistance to local deformation through indentation, while the elastic modulus property reveals metal’s stiffness and/or resistance to bending represented in its inter-atomic forces. Unlike hardness, elastic modulus is considered to be inherent (Craig and Powers, 2002). The hardness values for PF NiTi instruments were significantly lower than those observed for SS instruments (p < 0.001; Fig. 3a), which is consistent with a previous study (Serene et al., ’95). Hardness is an essential aspect to consider when assessing the cutting efficiency of endodontic instruments (Bergmans et al., 2001). Although the low hardness could imply a weaker cutting efficiency (Brockhurst and Hau, ’98), NiTi instruments are thought to be more durable (Kazemi et al., ’96) and have shown favorably less root canal transportation than their SS counterparts (Schafer and Lau, ’99). Another noteworthy finding was that the mechanical properties in new SS instruments were significantly different between the edge and shank regions (p < 0.001; not shown in figure); likewise, the elastic modulus values of PF instruments showed statistical significance between the two regions (p < 0.001; not shown in figure). In this regard, it was suggested that potential differences in the thermomechanical processes and degree of work hardening that took place over the length of the instrument during the manufacturing process might affect local mechanical properties of an instrument (Wildey et al., ’92; Alapati et al., 2006; Zinelis et al., 2010; Jamleh et al., 2012); the SS instruments in this study appeared to be more affected by such phenomenon. The differences of measured values between different regions within the same
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instrument show the complexity of manufacturing the instrument, particularly when considering that the variation of hardness and elastic modulus in these regions may not be representative of those throughout the length of the instrument. In the present study, torsional loading has made considerable changes in the local mechanical properties. The mechanical behavior of endodontic instruments under torsion has a load–distortion characteristic. During torsional loading, the stress–strain behavior initially shows elastic deformation. On continuous loading, the imposed strain exceeds the elastic limit and will continue to deform plastically. Then, it passes the point of the highest ultimate strength and subsequently fails (Wolcott and Himel, ’97). In the current study, PF and SS instruments had reached maximum mean torque of 0.90 0.07 and 0.99 0.05 N-cm, respectively. NiTi alloys have two main crystalline phases, named austenite and martensite, where the former might transform to the latter by the effect of stress (Hou et al., 2011). A recent nanoindentation study showed that cyclic fatigue significantly reduced the mechanical properties of NiTi instruments in both edge and shank regions (Jamleh et al., 2012), whereas this experiment reported a localized effect on the fractured edge region after torsional failure. Furthermore, under similar conditions, cyclic fatigue reduced the hardness and elastic modulus of PF NiTi instruments at the edge region to 30% and 40% (Jamleh et al., 2012), whereas torsional failure has caused 13% and 27% decrease, respectively. These figures provide insights on the more deleterious effect of fatigue compared to torsional failure. Consistently, previous studies showed the cyclic fatigue to be the primary cause of NiTi instrument fracture (Parashos et al., 2004; Shen et al., 2006). The SS instruments used in this experiment were made from austenitic SS alloys with face-cantered cubic crystal structure. It is reported that stress application on such alloys will aid the generation of dislocations and hence the martensite transformation having body cubic tetragonal crystal arrangement (Porter and Easterling ’92). This transformation can be related to deterioration of the material property and microstructural changes, as the results of present work confirm. Xray diffraction, atomic force microscopy, electron backscatter diffraction and transmission electron microscopy studies reported also that plastic deformations of austenitic SS have induced martensitic transformation (Wang et al., 2005; Abreu et al., 2007). In the present study, plastic deformation was applied by torsional process and apparently martensitic transformation took place. The martensitic formation was confirmed by the changes from a high hardness and elastic modulus (New-Edge and New-Shank) to low ones (Fractured-Edge and Fractured-Shank) (Fig. 3a and b).
Conclusions Within the limitations of this study, it should be concluded that the instruments fractured by torsion had significant reductions in the local modulus of elasticity and hardness adjacent to the fractured edge. Therefore, the null hypothesis of this study was rejected. The findings may suggest that changes in mechanical properties of SS and PF are likely to be causative for fractures due to torsional movement.
References Abreu H, Carvalho S, Neto P, et al. 2007. Deformation induced martensite in an AISI 301LN stainless steel: characterization and influence on pitting corrosion resistance. Mater Res 10:359–366. Alapati SB, Brantley WA, Nusstein JM, et al. 2006. Vickers hardness investigation of work-hardening in used NiTi rotary instruments. J Endod 32:1191–1193. Alcock JP, Barbour ME, Sandy JR, Ireland AJ. 2009. Nanoindentation of orthodontic archwires: the effect of decontamination and clinical use on hardness, elastic modulus and surface roughness. Dental Mater 25:1039–1043. Arantes WB, da Silva CM, Lage-Marques JL, et al. 2014. SEM analysis of defects and wear on Ni–Ti rotary instruments. Scanning. 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. Bergmans L, van Cleynenbreugel J, Wevers M, Lambrechts P. 2001. Mechanical root canal preparation with NiTi rotary instruments: rationale, performance and safety. Status report for the American Journal of Dentistry. Am J Dent 14:324– 333. Brockhurst P, Hau E. 1998. Hardness and strength of endodontic instruments made from NiTi alloy. Aust Endod J 24:115–119. Craig RG, Powers JM. 2002. Restorative dental materials. 11th edition. St. Louis, MO: Elsevier Science. p 77. Di Fiore PM, Genov KA, Komaroff E, Li Y, Lin L. 2006. Nickel– titanium rotary instrument fracture: a clinical practice assessment. Int Endod J 39:700–708. Fife D, Gambarini G, Britto Lr Lr. 2004. Cyclic fatigue testing of ProTaper NiTi rotary instruments after clinical use. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 97:251–256. Gloanec A, Bilotta G, Gerland M. 2013. Deformation mechanisms in a TiNi shape memory alloy during cyclic loading. Mater Sci Eng A 564:351–358. 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. Hou XM, Yahata Y, Hayashi Y, et al. 2011. Phase transformation behaviour and bending property of twisted nickel–titanium endodontic instruments. Int Endod J 44:253–258. Iijima M, Muguruma T, Brantley WA, Mizoguchi I. 2011. Comparisons of nanoindentation, 3-point bending, and tension tests for orthodontic wires. Am J Orthod Dentofacial Orthop 140:65–71. Jamleh A, Sadr A, Nomura N, et al. 2012. Nano-indentation testing of new and fractured nickel-titanium endodontic instruments. Int Endod J 45:462–468. Kazemi RB, Stenman E, Spa˚ngberg LSW. 1996. Machining efficiency and wear resistance of nickel-titanium endodontic files. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 81:596–602.
Jamleh et al.: Nanoindentation after torsional fatigue Kazemi RB, Stenman E, Spa˚ngberg LS. 2000. A comparison of stainless steel and nickel–titanium H-type instruments ofidentical design: torsional and bending tests. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 90:500–506. Kramkowski TR, Bahcall J. 2009. 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 35:404–407. Luebke NH, Brantley WA, Sabri ZI, Luebke FL, Lausten LL. 1995. Physical dimensions, torsional performance, bending properties, and metallurgical characteristics of rotary endodontic instruments. VI. Canal Master drills. J Endod 21:259– 263. Miyai K, Ebihara A, Hayashi Y, et al. 2006. Influence of phase transformation on the torsional and bending properties of nickel–titanium rotary endodontic instruments. Int Endod J 39:119–126. Oliver WC, Pharr GM. 1992. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res 7:1564–1583. Parashos P, Messer HH. 2006. The diffusion of innovation in dentistry: a review using rotary nickel–titanium technology as an example. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 101:395–401. Parashos P, Gordon I, Messer HH. 2004. Factors influencing defects of rotary nickel–titanium endodontic instruments after clinical use. J Endod 30:722–725. Park SK, Kim YJ, Shon WJ, et al. 2013. Clinical efficiency and reusability of the reciprocating nickel–titanium instruments according to the root canal anatomy. Scanning. Perez F, Schoumacher M, Peli JF. 2005. Shaping ability of two rotary instruments in simulated canals: stainless steel ENDOflash and nickel–titanium HERO Shaper. Int Endod J 38:637–644. Plotino G, Grande NM, Cordaro M, Testarelli L, Gambarini G. 2009. A review of cyclic fatigue testing of nickel–titanium rotary instruments. J Endod 35:1469–1476. Porter D, Easterling K. 1992. Phase transformation in metals and alloys. 2nd edition. London: Chapman & Hall. p 382–440. Pruett JP, Clement DJ, Carnes DL Jr. 1997. Cyclic fatigue testing of nickel–titanium endodontic instruments. J Endod 23:77–85. Sadr A, Shimada Y, Lu H, Tagami J. 2009. The viscoelastic behavior of dental adhesives: a nanoindentation study. Dental Mater 25:13–19. Sattapan B, Nervo GJ, Palamara JE, Messer HH. 2000. Defects in rotary nickel–titanium files after clinical use. J Endod 26:161–165.
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Schafer E. 1997. Root canal instruments for manual use: a review. Endod Dental Traumatol 13:51–64. Schafer E, Lau R. 1999. Comparison of cutting efficiency and instrumentation of curved canals with nickel–titanium and stainless steel instruments. J Endod 25:427–430. Serene TP, Adams JD, Saxena A. 1995. Nickel–titanium instruments. Applications in endodontics. St. Louis, MO: Ishiyaku Euroamerica, Inc. Shen Y, Cheung GS, Bian Z, et al. 2006. Comparison of defects in ProFile and ProTaper systems after clinical use. J Endod 32:61–65. Thoden van Velzen SK, Duivenvoorden HJ, Schuurs AH. 1981. Probabilities of success and failure in endodontic treatment: a Bayesian approach. Oral Surg Oral Med Oral Pathol 52: 85–90. Thompson SA. 2000. An overview of nickel–titanium alloys used in dentistry. Int Endod J 33:297–310. Tian K, Darvell BW. 2010. Determination of the flexural modulus of elasticity of orthodontic archwires. Dental Mater 26:821– 829. Turpin YL, Chagneau F, Vulcain JM. 2000. Impact of two theoretical cross-sections on torsional and bending stresses of nickel titanium root canal instrument models. J Endod 26:414–417. Wang H, Yang J, Bhadeshia HKDH. 2005. Characterisation of severely deformed austenitic stainless steel wire. Mater Sci Technol 21:1323–1328. Ward JR, Parashos P, Messer HH. 2003. Evaluation of an ultrasonic technique to remove fractured rotary nickel– titanium endodontic instruments from root canals: clinical cases. J Endod 29:764–767. Wildey WL, Senia ES, Montgomery S. 1992. Another look at root canal instrumentation. Oral Surg Oral Med Oral Pathol 74:499–507. Wolcott J, Himel VT. 1997. Torsional properties of nickel– titanium versus stainless steel endodontic files. J Endod 23:217–220. Yared G. 2008. Canal preparation using only one Ni–Ti rotary instrument: preliminary observations. Int Endod J 41:339– 344. 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. Zinelis S, Eliades T, Eliades G. 2010. A metallurgical characterization of ten endodontic Ni–Ti instruments: assessing the clinical relevance of shape memory and superelastic properties of Ni–Ti endodontic instruments. Int Endod J 43:125–134.
