doi:10.1111/iej.12400
Influence of cyclic torsional preloading on cyclic fatigue resistance of nickel – titanium instruments
1, F. Lo Savio2, S. Boninelli3, G. Plotino4, N. M. Grande4, E. Rapisarda1 & E. Pedulla G. La Rosa2 1
Department of Surgery, University of Catania, Catania; 2Department of Industrial Engineering, University of Catania, Catania; MATIS Institute of Microelectronics and Microsystems, National Research Council, Catania; and 4Department of Endodontics, ‘Sapienza’ University of Rome, Rome, Italy 3
Abstract E, Lo Savio F, Boninelli S, Plotino G, Pedulla Grande NM, Rapisarda E, La Rosa G. Influence of cyclic
torsional
preloading
on
cyclic
fatigue
resistance
of nickel – titanium instruments. International Endodontic Journal.
Aim To evaluate the effect of different torsional preloads on cyclic fatigue resistance of endodontic rotary instruments constructed from conventional nickeltitanium (NiTi), M-Wire or CM-Wire. Methodology Eighty new size 25, 0.06 taper Mtwo instruments (Sweden & Martina), size 25, 0.06 taper HyFlex CM (Coltene/Whaledent, Inc) and X2 ProTaper Next (Dentsply Maillefer) were used. The Torque and distortion angles at failure of new instruments (n = 10) were measured, and 0% (n = 10), 25%, 50% and 75% (n = 20) of the mean ultimate torsional strength as preloading condition were applied according to ISO 3630-1 for each brand. The twenty files tested for every extent of preload were subjected to 20 or 40 torsional cycles (n = 10). After torsional preloading, the number of cycles to failure
Introduction Despite the increased flexibility and strength, compared with stainless steel instruments, NiTi rotary instruments are vulnerable to fracture (Walia et al.
Correspondence: Eugenio Pedulla, Department of Surgery, University of Catania, Via Cervignano, 29, 95129 Catania, Sicily, Italy (Tel.: +39 339 2613264; e-mail: eugeniopedulla@ gmail.com).
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
was evaluated in a simulated canal with 60° angle of curvature and 5 mm of radius of curvature. Data were analysed using two-way analysis of variance. The fracture surface of each fragment was examined with a scanning electron microscope (SEM). Data were analysed by two-way analyses of variance. Results Preload repetitions did not influence the cyclic fatigue of the three brands; however, the 25%, 50% and 75% torsional preloading significantly reduced the fatigue resistance of all instruments tested (P < 0.01, P < 0.001 and P < 0.0001, respectively) except for the HyFlex CM preloaded with 25% of the maximum torsional strength (P > 0.05). Conclusions Torsional preloads reduced the cyclic fatigue resistance of conventional and treated (M-wire and CM-wire) NiTi rotary instruments except for size 25, 0.06 taper HyFlex CM instruments with a 25% of torsional preloading. Keywords: CM-wire, cyclic fatigue M-wire, NiTi fracture, torsional preload.
resistance,
Received 12 August 2014; accepted 23 October 2014
1988, Sattapan et al. 2000, Cheung 2009, Pedull a et al. 2013a). Many variables might contribute to this fracture, but the two main causes are cyclic fatigue and torsional fatigue. Each has been defined (Yum et al. 2011, Bhagabati et al. 2012), and clinically, cyclic fatigue seems to be more prevalent in curved root canals, whereas torsional failure might happen even in a straight canal (Plotino et al. 2010). Although both these failure modes probably occur simultaneously in a clinical situation (Yum et al. 2011), studies have found cyclic fatigue to be the
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et al. Influence of cyclic torsional preloading on cyclic fatigue Pedulla
primary cause of instrument fracture. It accounted for 50–90% of the mechanical failures (Parashos et al. 2004). Torsional fracture occurs when an instrument tip or another part of the instrument is locked in a canal whilst the shank continues to rotate, and the elastic limit of the metal is exceeded (Peters & Barbakow 2002, Parashos & Messer 2006). Instruments fractured by fatigue do not bind in the canal but they rotate freely around a curve, generating tension/compression cycles at the point of maximum flexure until fracture occurs (Pedulla et al. 2013b). Manufacturers are constantly attempting to improve instruments. Two recently introduced innovative systems are made from M-wire and shape memory (Controlled Memory) technology. The M-wire NiTi, in the file system ProTaper Next (Dentsply Maillefer, Ballaigues, Switzerland), is subjected to thermomechanical processing resulting in a reported increased flexibility (Alapati et al. 2009, Ninan & Berzins 2013). CM-wire instruments such as HyFlex CM (HF) (Coltene/Whaledent, Inc, Cuyahoga Falls, OH, USA) represent the latest generation of rotary shape memory NiTi files, which the manufacturer has termed as ‘Controlled Memory’. These instruments, as claimed by the manufacturer, are made by a lower per cent in weight of nickel (52 Ni %wt) compared with the great majority of commercially available NiTi rotary instruments (54.5–57 Ni% wt) (Elnaghy 2014). For this reason and because of its specific manufacturing process, HyFlex CM does not rebound to their original shape as do conventional NiTi instruments, which, combined with their greater flexibility, may lead a reduced risk of ledging, transportation and perforation (Ninan & Berzins 2013). Many fracture simulation studies on NiTi instruments have been conducted separately for cyclic fatigue and torsional failure tests (Yum et al. 2011, Pedull a et al. 2012). Only a few studies have tried to correlate these two factors of fracture (Bahia et al. 2008, Kim et al. 2012, Cheung et al. 2013, Campbell et al. 2014). Some reports (Kim et al. 2012, Campbell et al. 2014) have found that the cyclic preloading (pre-use) of NiTi rotary files reduced the torsional resistance significantly. In particular, no studies have investigated the influence of torsional preloading on cyclic fatigue resistance of endodontic rotary instruments made by traditional NiTi, M-Wire or CM-Wire such as Mtwo (Sweden & Martina, Due Carrare, Padova, Italy), ProTaper Next and HyFlex CM, respectively. The aim of this study was to evaluate the effect of different torsional preload on the cyclic fatigue resistance of NiTi rotary instruments made by NiTi,
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M-Wire or CM-Wire. The null hypothesis is that torsional preloads do not influence the cyclic fatigue of NiTi instruments.
Materials and methods A total of 240 25-mm-long new Mtwo, ProTaper Next X2 and HyFlex CM instruments, eighty of each brand and all size 25 with a 0.06 taper were tested. Ten instruments for every brand were torsionally tested until fracture to estimate their mean ultimate torsional strength. Four torsional preloading conditions were applied to the remaining instruments of each brand: 0% or no preloading (n = 10) and 25%, 50% and 75% of the mean ultimate torsional strength (n = 20 each). The twenty files tested for every extent of torsional preloading were subdivided into two subgroups (n = 10) on the basis of the number of preloading cycles performed (20 or 40 torsional cycles). Every instrument had been inspected for defects or deformities before the experiment under a stereomicroscope (SZR- 10; Optika, Bergamo, Italy), none was discarded. The torsional load until fracture and established torsional preloads were applied using the same custom-made device produced following the ISO 3630-1. Each file was clamped 5 mm from the tip using a chuck connected to a torque-sensing load cell, after which the shaft of the file was fastened into an opposing chuck able to be rotated with a stepper motor. Each file shaft was rotated in the clockwise direction at a speed of 2 revolutions per minute until file fracture or the preset torque value (25%, 50% and 75%). When the preset torque value was achieved, the motor automatically moved in a counterclockwise direction, at the same rotational rate, until the instrument returned to the original position (i.e. strain = 0). This motion was repeated until the preset number of repetitions (20 or 40) was reached. The torque load (Ncm) and angular rotation (°, degrees) were monitored continuously using a torsiometer (Sabri Dental Enterprises, Downers Grove, IL, USA) at room temperature (21 1 °C), and the ultimate torsional strength and angle of rotation at failure were recorded (Fig. 1a–b). After torsional preloading, a static model for cyclic fatigue testing was conducted in a custom-made device that allowed the reproducible simulation of an instrument confined in an artificial curved canal (Plotino et al. 2010, Al-Sudani et al. 2012, Pedull a et al. 2012, 2013b). The artificial canal was
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
et al. Influence of cyclic torsional preloading on cyclic fatigue Pedulla
(a)
(c)
(b)
(d)
(e)
Figure 1 The experimental set-up for torsional (a–b) and cyclic fatigue tests (c–e). (a) The custom-made device used to apply torsional load until fracture and torsional preloads; (b) A Mtwo file was kept straight during the torsional loads by a chuck that clamped it at 5 mm from the tip and an opposing chuck to secure the instrument shaft; (c) A ProTaper Next X2 inserted in the artificial canal; (d) The fracture point (black arrow) in the centre of the simulated curvature of a ProTaper Next X2 after the cyclic fatigue test; (e) The fractured ProTaper Next X2 in the artificial canal after having removed the separated file segment.
manufactured by reproducing the instrument’s size and taper. It had 5 mm radius of curvature (measured at the internal concave surface of the artificial canal), 60° angle of curvature measured according to the Schneider’s method (Schneider 1971), a centre of curvature 5 mm from the end of the canal and a curved segment of 5 mm in length. The canal was covered with glass to prevent the instruments from slipping out (Pedull a et al. 2013b). The files were activated using a 6:1 reduction handpiece (Sirona Dental Systems GmbH, Bensheim, Germany) powered by a torque-controlled motor (Silver Reciproc; VDW, Munich, Germany) at a speed of 300 RPM. To reduce friction between the instrument and the metal canal walls, a special high-flow synthetic oil designed for the lubrication of mechanical parts (Super Oil; Singer Co Ltd, Elizabethport, NJ, USA) was applied. All instruments were rotated until a fracture occurred. The time was recorded and
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
stopped when a fracture was detected visually and/or audibly (Fig. 1c–e). The number of cycles to failure (NCF) for each instrument was calculated by multiplying the time (in seconds) to failure by the number of rotations or cycles per second (five revolutions per 60 s = 300 rpm). The length of the fractured file tip was measured using a digital microcaliper (Mitutoyo Italiana srl, Lainate, Italy) at 509 under a stereomicroscope (SZR- 10). The fracture surfaces of all fragments were examined under a scanning electron microscope (SEM) (ZEISS Supra 35VP, GmBH, Oberkochen, Germany) looking for topographic features of the fractured instruments. NCF data were analysed using two-way analysis of variance and the Bonferroni’s post hoc test (Prism 5.0; GraphPad Software, Inc, La Jolla, CA, USA) at 0.05 as level of significance.
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Results The mean torque load and angle of rotation until fracture are presented in Table 1. The inferential analysis revealed statistically significant differences amongst the four groups of the same brand considering the value of torsional preloads as the independent variable (two-way analysis of variance, P < 0.0001; interaction = 0.33). There were no significant differences within the same brand when considering the number of preload repetitions as the independent variable (two-way analysis of variance, P > 0.05). The 75% and 50% torsional preloading significantly decreased the cyclic fatigue resistance of instruments compared to nonpreloaded (0%) ones either after 40 or after 20 cycles in every brand (P < 0.0001 and P < 0.001, respectively). The 25% torsional preloading reduced cyclic fatigue resistance of the files ProTaper Next X2 and M2 size 25, 0.06 taper after 20 or 40 cycles (P < 0.01 and P < 0.05, respectively) comparing these with the nonpreloaded ones. HyFlex CM size 25, 0.06 taper did not reduce their cyclic fatigue after 20 or 40 cycles of 25% torsional preloading (P > 0.05) (Table 2). The length of the fractured files was not significantly different for all of the instruments tested (P > 0.05) and which ranged from 4.2 to 6.3 mm (mean = 5.1 mm). SEM images of the fracture surfaces showed similar and typical features of torsional failure with fibrous dimple marks in the centre of rotation for instruments tested only at torsion (Fig. 2a–c). Dimples are visible on all fracture surfaces of the instruments tested for cyclic fatigue (Fig. 2d–f). Smoothed borders and dimples on all fracture surfaces were found in the torsional preloaded files and broken in the following cyclic fatigue test (Fig. 2g–i).
Discussion Although NiTi instruments have improved the quality and speed of root canal preparation, nevertheless,
there is a risk of their failure, as a result of torsional fracture, cyclic fatigue, or a combination of both (Cheung 2009). Often, cyclic fatigue and torsional fracture of various NiTi rotary instruments have been studied separately under simulated conditions (Campbell et al. 2014), probably for convenience or for better control of the loading circumstances (Cheung et al. 2013). Few studies have investigated either the effect of cyclic fatigue preloading on the torsional resistance (Ullmann & Peters 2005, Kim et al. 2012) or the effect of torsional preloads on the cyclic fatigue resistance of conventional NiTi instruments (Galv~ ao Barbosa et al. 2007, Bahia et al. 2008). However, no data have been reported on the influence of torsional preloading on cyclic fatigue of instruments made by different NiTi alloy. Therefore, in this study, CM-wire (HyFlex CM), M-wire (ProTaper Next) and conventional NiTi files (Mtwo) were compared for the effect of different torsional preloads on their cyclic fatigue resistance. NCF value (Yao et al. 2006, Lopes et al. 2009, Pedull a et al. 2014a) and the method described in the ISO standard 3630-1 (Bahia et al. 2008, Gao et al. 2012, Santos Lde et al. 2013) are usually used to measure cyclic fatigue and torsional strength of NiTi instruments, respectively. In the present study, cyclic fatigue was measured by NCF testing the file in a custom-made device (Plotino et al. 2010, Pedull a et al. 2012, 2014a), whilst the torsional loads were applied following the ISO Standard 3630-1 modified only for clamping the testing instruments at the D5 than at the D3 level. This modification was used to provide the torsional preloads at the same level of the maximum stress of the subsequent cyclic fatigue test that was performed using an artificial canal with 5 mm of radius of curvature. The broken fragment after the fatigue tests showed an average length of 5 mm that coincided with the site of torsional preloading application (at D5). Although the comparison amongst the different instruments is difficult to make because of their differences in type of alloy, design and cross-section, in this
Table 1 Mean torque (Ncm) and angle of rotation (°) of instruments tested Torque (Ncm) Instrument ProTaper Next Mtwo HyFlex CM
Mean a
1.32 0.98b 0.84b
Standard deviation 0.25 0.15 0.10
Angle of rotation (°) Min 0.96 0.79 0.76
Max 1.76 1.20 1.06
Mean c
264.16 351.83c 593.83d
Standard deviation
Min
Max
22.67 81.55 93.12
230 230 455
295 440 700
Different superscript letters indicate significant differences amongst groups (P < 0.05).
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et al. Influence of cyclic torsional preloading on cyclic fatigue Pedulla
Table 2 Number of cycles to failure (NCF) (mean standard deviation) after different percentage and cycles of torsional pre-
loading Torsional preloading Without torsional preloading (0%)
RNTP
ProTaper Next X2
File
402 34a
Mtwo
616 71b
20 40 20 40 20 40
HyFlex CM
1307 181c
25% 248 283 495 480 1157 1053
30d 62d 59e 91e 357c, 460c,
50%
f f
243 233 501 417 856 939
41d 110d 147e 96e 272f 397f
75% 273 209 447 441 711 554
48d 47d 131e 118e 210f, g 232 g
Different superscript letters indicate significant differences amongst groups (P < 0.05).
study, new HyFlex CM instruments files without torsional preloads had significantly higher cyclic fatigue resistance than Mtwo and both instruments had significantly higher cyclic fatigue than ProTaper Next (P < 0.001 for each comparison). On the other hand, new ProTaper Next X2 had significantly higher values for mean ultimate torsional strength than Mtwo size 25, 0.06 taper and HyFlex CM size 25, 0.06 taper (P < 0.001 for each comparison), whilst no difference was found between the last two files (P > 0.05). Cross-sectional configuration seems to have a decisive influence on torsional behaviour and stress distribution of NiTi rotary instruments. The instruments with a long cross-sectional area should have high torsional and a low cyclic fatigue resistance (Sch€ afer et al. 2003, Melo et al. 2008). In a supplementary examination, the cross-sectional configuration of each instrument was captured at 5 mm from the tip (D5) under SEM and the area measured using software (AutoCAD; Autodesk Inc, San Rafael, CA, USA). Amongst the instruments tested, ProTaper Next X2 had the largest area (approximatively 116.35997 lm2), HyFlex CM size 25, 0.06 taper to have an intermediate value (approximatively 96.45815 lm2) and Mtwo size 25, 0.06 taper the smallest area (approximatively 89.9374 lm2). Thus, the highest value of the mean ultimate torsional strength and the lowest value of NCF associated with ProTaper Next X2 in this study are probably due to the large cross-sectional area of this instrument. However, HyFlex CM size 25, 0.06 taper had slightly larger cross-sectional area compared with Mtwo size 25, 0.06 taper, but succumbed at a lower ultimate torsional strength and a higher NCF. This might be related to the mechanical characteristics of the NiTi alloy. In fact, in agreement with the results of this study, it has been shown that the shape memory HyFlex CM
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
generally possesses lower torque values, greater angle of rotation and greater cyclic fatigue resistance in comparisons with conventional (Mtwo) or M-wire (ProTaper Next) instruments probably due to its reduced torsional stiffness (Table 1) (Ninan & Berzins 2013, Capar et al. 2014). The number of torsional preload repetitions used (20 or 40 cycles) did not affect the cyclic fatigue resistance of the NiTi instruments tested (Cheung et al. 2013). On the other hand, the extent of torsional preloads reduced the cyclic fatigue resistance of the instruments especially when the 75% or the 50% of the mean ultimate torsional strength was applied. These results are in agreement with other studies that have reported a reduced cyclic fatigue resistance of conventional NiTi instruments (e.g. K3, SybronEndo, Orange County, CA, USA) after their torsional preload (Galv~ ao Barbosa et al. 2007, Bahia et al. 2008). However, another recent study has shown that torsional preloads within the superelastic limit of the material may improve cyclic fatigue resistance of conventional nickel-titanium rotary instruments such as ProFile and ProTaper Universal F1 (Cheung et al. 2013). These contrasting findings are probably due to the different instruments and methodology used. The 25% torsional preloads significantly reduced the cyclic fatigue of ProTaper Next X2 and Mtwo size 25, 0.06 taper (P < 0.001), whilst it did not affect the fatigue resistance of HyFlex CM size 25, 0.06 taper (P > 0.05) after 20 as well as 40 cycles. These differences are probably due to the different cross-section and alloy of these instruments. In particular, it was reported that for a given strain, a more flexible file (as HyFlex CM in this report) would experience less stress (Ninan & Berzins 2013). Moreover, because the torsional preloads were normalized as a percentage of the maximum torsional strength depending on the instrument tested, the 25% of torsional preloads
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et al. Influence of cyclic torsional preloading on cyclic fatigue Pedulla
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
Figure 2 SEM appearances of the instruments after fatigue testing (a, d, g = ProTaper Next; b, e, h = Mtwo; c, f, i = HyFlex CM). (a–c) Images of new instruments fracture surface after torsional test, second row (d–f) and third row (g–i) show fracture surface after cyclic fatigue test of new instruments and preloaded files at 75% of the ultimate torsional strength for 40 cycles, respectively. Circular abrasion marks and skewed dimples (white arrows) near the centre of rotation surrounded by a smooth surface are the evidence of torsional failure; smoothed borders (black arrows) due to the torsional load or preload; dimpling involve all fracture surface of new (surrounded area) or torsionally preloaded instruments fractured by cyclic fatigue test. Crack initiation origin at smoothed borders (black arrows) and surface pattern with dimples and cones are observed in the same fracture plane.
of HyFlex CM (0.21 Ncm) was the lowest used in this study considering that these instruments have the lowest maximum torsional strength. The SEM analysis revealed typical fractographic appearances of torsional and/or cyclic fatigue fractures
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that were similar amongst the three brands tested. Cyclic fatigue fracture is characterized by dimples on the entire fracture surface, whilst torsional failure is characterized by circular abrasion marks and dimples near the centre of rotation on the fracture surface (Parashos
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
et al. Influence of cyclic torsional preloading on cyclic fatigue Pedulla
& Messer 2006, Campbell et al. 2014, Pedull a et al. 2014b). As recently reported, NiTi instruments, in which the torsional test was applied after the incomplete flexural fatigue test, showed a fractography that corresponded to the torsional fracture pattern (Campbell et al. 2014). In the same manner, in this study, when the cyclic fatigue test was applied after the incomplete torsional test, instruments of all brands had typical cyclic fatigue fracture patterns in the SEM evaluation. ProTaper Next is able to resist high torsional stresses which may occur in narrow root canals, HyFlex endures high flexural stress which may occur in curved root canals, whilst Mtwo files have medium values of torsional and cyclic fatigue resistance. However, ProTaper Next X2 and Mtwo size 25, 0.06 taper had reduced fatigue resistance after small torsional preloads (25% for 20 cycles). Therefore, clinicians should consider these findings, especially when they prepare curved root canals with the same instrument that has already been used to shape narrow root canals.
Conclusions Within the limitations of this study, it could be concluded that torsional preloads reduced the cyclic fatigue resistance of conventional and treated (M-wire and CM-wire) NiTi rotary instruments except for HyFlex CM size 25, 0.06 taper with a 25% of torsional preloading.
Acknowledgement The authors thank engineers Salvatore Caltabiano and Salvatore Franco from the University of Catania for the support in the torsional tests. The authors deny any conflict of interests.
References Alapati SB, Brantley WA, Iijima M et al. (2009) Metallurgical characterization of a new nickel-titanium wire for rotary endodontic instruments. Journal of Endodontics 35, 1589–93. Al-Sudani D, Grande NM, Plotino G et al. (2012) Cyclic fatigue of nickel-titanium rotary instruments in a double (S-shaped) simulated curvature. Journal of Endodontics 38, 987–9. Bahia MG, Melo MC, Buono VT (2008) Influence of cyclic torsional loading on the fatigue resistance of K3 instruments. International Endodontic Journal 41, 883–91.
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Bhagabati N, Yadav S, Talwar S (2012) An in vitro cyclic fatigue analysis of different endodontic nickel-titanium rotary instruments. Journal of Endodontics 38, 515–8. Campbell L, Shen Y, Zhou HM, Haapasalo M (2014) Effect of fatigue on torsional failure of nickel-titanium controlled memory instruments. Journal of Endodontics 40, 562–5. Capar ID, Ertas H, Arslan H (2014) Comparison of cyclic fatigue resistance of novel nickel-titanium rotary instruments. Australian Endodontic Journal. doi:10.1111/aej. 12067. [Epub ahead of print]. Cheung GS (2009) Instrument fracture: mechanisms, removal of fragments, and clinical outcomes. Endodontic Topics 16, 1–26. Cheung GS, Oh SH, Ha JH, Kim SK, Park SH, Kim HC (2013) Effect of torsional loading of nickel-titanium instruments on cyclic fatigue resistance. Journal of Endodontics 39, 1593–7. Elnaghy AM (2014) Cyclic fatigue resistance of ProTaper Next nickel-titanium rotary files. International Endodontic Journal 47, 1034–9. Galv~ ao Barbosa FO, Ponciano Gomes JA, Pimenta de Ara ujo MC (2007) Influence of previous angular deformation on flexural fatigue resistance of K3 nickel–titanium rotary instruments. Journal of Endodontics 33, 1477–80. Gao Y, Gutmann JL, Wilkinson K, Maxwell R, Ammon D (2012) Evaluation of the impact of raw materials on the fatigue and mechanical properties of ProFile Vortex rotary instruments. Journal of Endodontics 38, 398–401. Kim JY, Cheung GS, Park SH, Ko DC, Kim JW, Kim HC (2012) Effect from cyclic fatigue of nickel-titanium rotary files on torsional resistance. Journal of Endodontics 38, 527–30. Lopes HP, Ferreira AA, Elias CN, Moreira EJ, de Oliveira JC, Siqueira JF Jr (2009) Influence of rotational speed on the cyclic fatigue of rotary nickel-titanium endodontic instruments. Journal of Endodontics 35, 1013–6. Melo MCC, Pereira ESJ, Viana ACD, Fonseca AMA, Buono VTL, Bahia MGA (2008) Dimensional characterization and mechanical behaviour of K3 rotary instruments. International Endodontic Journal 41, 329–38. Ninan E, Berzins DW (2013) Torsion and bending properties of shape memory and superelastic nickel-titanium rotary instruments. Journal of Endodontics 39, 101–4. Parashos P, Messer HH (2006) Rotary NiTi instrument fracture and its consequences. Journal of Endodontics 32, 1031–43. Parashos P, Gordon I, Messer HH (2004) Factors influencing defects of rotary nickel titanium files after clinical use. Journal of Endodontics 30, 722–5. Pedull a E, Plotino G, Grande NM, Pappalardo A, Rapisarda E (2012) Cyclic fatigue resistance of four nickel-titanium rotary instruments: a comparative study. Annali di Stomatologia 3, 59–63.
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Pedull a E, Grande NM, Plotino G, Palermo F, Gambarini G, Rapisarda E (2013a) Cyclic fatigue resistance of two reciprocating nickel-titanium instruments after immersion in sodium hypochlorite. International Endodontic Journal 46, 155–9. Pedull a E, Grande NM, Plotino G, Gambarini G, Rapisarda E (2013b) Influence of continuous or reciprocating motion on cyclic fatigue resistance of 4 different nickeltitanium rotary instruments. Journal of Endodontics 39, 258–61. Pedull a E, Plotino G, Grande NM et al. (2014a) Influence of rotational speed on the cyclic fatigue of Mtwo instruments. International Endodontic Journal 47, 514–9. Pedull a E, Franciosi G, Ounsi HF, Tricarico M, Rapisarda E, Grandini S (2014b) Cyclic fatigue resistance of nickeltitanium instruments after immersion in irrigant solutions with or without surfactants. Journal of Endodontics 40, 1245–9. Peters OA, Barbakow F (2002) Dynamic torque and apical forces of ProFile. 04 rotary instruments during preparation of curved canals. International Endodontic Journal 35, 379– 89. Plotino G, Grande NM, Melo MC, Bahia MG, Testarelli L, Gambarini G (2010) Cyclic fatigue of NiTi rotary instruments in a simulated apical abrupt curvature. International Endodontic Journal 43, 226–30.
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Santos Lde A, Bahia MG, de Las Casas EB, Buon VT (2013) Comparison of the mechanical behavior between controlled memory and superelastic nickel-titanium files via finite element analysis. Journal of Endodontics 39, 1444–7. Sattapan B, Nervo GJ, Palamara JE, Messer HH (2000) Defects in rotary nickel-titanium files after clinical use. Journal of Endodontics 26, 161–5. Sch€ afer E, Dzepina A, Danesh G (2003) Bending properties of rotary nickel-titanium instruments. Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology and Endodontics 96, 757–63. Schneider SW (1971) A comparison of canal preparations in straight and curved root canals. Oral Surgery, Oral Medicine, and Oral Pathology 32, 271–5. Ullmann CJ, Peters OA (2005) Effect of cyclic fatigue on static fracture loads in ProTaper nickel-titanium rotary instruments. Journal of Endodontics 31, 183–6. Walia HM, Brantley WA, Gerstein H (1988) An initial investigation of the bending and torsional properties of Nitinol root canal files. Journal of Endodontics 14, 346–51. Yao JH, Schwartz SA, Beeson TJ (2006) Cyclic fatigue of three types of rotary nickel-titanium files in a dynamic model. Journal of Endodontics 32, 55–7. Yum J, Cheung GSP, Park JK, Hur B, Kim HC (2011) Torsional strength and toughness of nickel-titanium rotary files. Journal of Endodontics 37, 382–6.
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Influence of cyclic torsional preloading on cyclic fatigue resistance of nickel – titanium instruments
1, F. Lo Savio2, S. Boninelli3, G. Plotino4, N. M. Grande4, E. Rapisarda1 & E. Pedulla G. La Rosa2 1
Department of Surgery, University of Catania, Catania; 2Department of Industrial Engineering, University of Catania, Catania; MATIS Institute of Microelectronics and Microsystems, National Research Council, Catania; and 4Department of Endodontics, ‘Sapienza’ University of Rome, Rome, Italy 3
Abstract E, Lo Savio F, Boninelli S, Plotino G, Pedulla Grande NM, Rapisarda E, La Rosa G. Influence of cyclic
torsional
preloading
on
cyclic
fatigue
resistance
of nickel – titanium instruments. International Endodontic Journal.
Aim To evaluate the effect of different torsional preloads on cyclic fatigue resistance of endodontic rotary instruments constructed from conventional nickeltitanium (NiTi), M-Wire or CM-Wire. Methodology Eighty new size 25, 0.06 taper Mtwo instruments (Sweden & Martina), size 25, 0.06 taper HyFlex CM (Coltene/Whaledent, Inc) and X2 ProTaper Next (Dentsply Maillefer) were used. The Torque and distortion angles at failure of new instruments (n = 10) were measured, and 0% (n = 10), 25%, 50% and 75% (n = 20) of the mean ultimate torsional strength as preloading condition were applied according to ISO 3630-1 for each brand. The twenty files tested for every extent of preload were subjected to 20 or 40 torsional cycles (n = 10). After torsional preloading, the number of cycles to failure
Introduction Despite the increased flexibility and strength, compared with stainless steel instruments, NiTi rotary instruments are vulnerable to fracture (Walia et al.
Correspondence: Eugenio Pedulla, Department of Surgery, University of Catania, Via Cervignano, 29, 95129 Catania, Sicily, Italy (Tel.: +39 339 2613264; e-mail: eugeniopedulla@ gmail.com).
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
was evaluated in a simulated canal with 60° angle of curvature and 5 mm of radius of curvature. Data were analysed using two-way analysis of variance. The fracture surface of each fragment was examined with a scanning electron microscope (SEM). Data were analysed by two-way analyses of variance. Results Preload repetitions did not influence the cyclic fatigue of the three brands; however, the 25%, 50% and 75% torsional preloading significantly reduced the fatigue resistance of all instruments tested (P < 0.01, P < 0.001 and P < 0.0001, respectively) except for the HyFlex CM preloaded with 25% of the maximum torsional strength (P > 0.05). Conclusions Torsional preloads reduced the cyclic fatigue resistance of conventional and treated (M-wire and CM-wire) NiTi rotary instruments except for size 25, 0.06 taper HyFlex CM instruments with a 25% of torsional preloading. Keywords: CM-wire, cyclic fatigue M-wire, NiTi fracture, torsional preload.
resistance,
Received 12 August 2014; accepted 23 October 2014
1988, Sattapan et al. 2000, Cheung 2009, Pedull a et al. 2013a). Many variables might contribute to this fracture, but the two main causes are cyclic fatigue and torsional fatigue. Each has been defined (Yum et al. 2011, Bhagabati et al. 2012), and clinically, cyclic fatigue seems to be more prevalent in curved root canals, whereas torsional failure might happen even in a straight canal (Plotino et al. 2010). Although both these failure modes probably occur simultaneously in a clinical situation (Yum et al. 2011), studies have found cyclic fatigue to be the
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primary cause of instrument fracture. It accounted for 50–90% of the mechanical failures (Parashos et al. 2004). Torsional fracture occurs when an instrument tip or another part of the instrument is locked in a canal whilst the shank continues to rotate, and the elastic limit of the metal is exceeded (Peters & Barbakow 2002, Parashos & Messer 2006). Instruments fractured by fatigue do not bind in the canal but they rotate freely around a curve, generating tension/compression cycles at the point of maximum flexure until fracture occurs (Pedulla et al. 2013b). Manufacturers are constantly attempting to improve instruments. Two recently introduced innovative systems are made from M-wire and shape memory (Controlled Memory) technology. The M-wire NiTi, in the file system ProTaper Next (Dentsply Maillefer, Ballaigues, Switzerland), is subjected to thermomechanical processing resulting in a reported increased flexibility (Alapati et al. 2009, Ninan & Berzins 2013). CM-wire instruments such as HyFlex CM (HF) (Coltene/Whaledent, Inc, Cuyahoga Falls, OH, USA) represent the latest generation of rotary shape memory NiTi files, which the manufacturer has termed as ‘Controlled Memory’. These instruments, as claimed by the manufacturer, are made by a lower per cent in weight of nickel (52 Ni %wt) compared with the great majority of commercially available NiTi rotary instruments (54.5–57 Ni% wt) (Elnaghy 2014). For this reason and because of its specific manufacturing process, HyFlex CM does not rebound to their original shape as do conventional NiTi instruments, which, combined with their greater flexibility, may lead a reduced risk of ledging, transportation and perforation (Ninan & Berzins 2013). Many fracture simulation studies on NiTi instruments have been conducted separately for cyclic fatigue and torsional failure tests (Yum et al. 2011, Pedull a et al. 2012). Only a few studies have tried to correlate these two factors of fracture (Bahia et al. 2008, Kim et al. 2012, Cheung et al. 2013, Campbell et al. 2014). Some reports (Kim et al. 2012, Campbell et al. 2014) have found that the cyclic preloading (pre-use) of NiTi rotary files reduced the torsional resistance significantly. In particular, no studies have investigated the influence of torsional preloading on cyclic fatigue resistance of endodontic rotary instruments made by traditional NiTi, M-Wire or CM-Wire such as Mtwo (Sweden & Martina, Due Carrare, Padova, Italy), ProTaper Next and HyFlex CM, respectively. The aim of this study was to evaluate the effect of different torsional preload on the cyclic fatigue resistance of NiTi rotary instruments made by NiTi,
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M-Wire or CM-Wire. The null hypothesis is that torsional preloads do not influence the cyclic fatigue of NiTi instruments.
Materials and methods A total of 240 25-mm-long new Mtwo, ProTaper Next X2 and HyFlex CM instruments, eighty of each brand and all size 25 with a 0.06 taper were tested. Ten instruments for every brand were torsionally tested until fracture to estimate their mean ultimate torsional strength. Four torsional preloading conditions were applied to the remaining instruments of each brand: 0% or no preloading (n = 10) and 25%, 50% and 75% of the mean ultimate torsional strength (n = 20 each). The twenty files tested for every extent of torsional preloading were subdivided into two subgroups (n = 10) on the basis of the number of preloading cycles performed (20 or 40 torsional cycles). Every instrument had been inspected for defects or deformities before the experiment under a stereomicroscope (SZR- 10; Optika, Bergamo, Italy), none was discarded. The torsional load until fracture and established torsional preloads were applied using the same custom-made device produced following the ISO 3630-1. Each file was clamped 5 mm from the tip using a chuck connected to a torque-sensing load cell, after which the shaft of the file was fastened into an opposing chuck able to be rotated with a stepper motor. Each file shaft was rotated in the clockwise direction at a speed of 2 revolutions per minute until file fracture or the preset torque value (25%, 50% and 75%). When the preset torque value was achieved, the motor automatically moved in a counterclockwise direction, at the same rotational rate, until the instrument returned to the original position (i.e. strain = 0). This motion was repeated until the preset number of repetitions (20 or 40) was reached. The torque load (Ncm) and angular rotation (°, degrees) were monitored continuously using a torsiometer (Sabri Dental Enterprises, Downers Grove, IL, USA) at room temperature (21 1 °C), and the ultimate torsional strength and angle of rotation at failure were recorded (Fig. 1a–b). After torsional preloading, a static model for cyclic fatigue testing was conducted in a custom-made device that allowed the reproducible simulation of an instrument confined in an artificial curved canal (Plotino et al. 2010, Al-Sudani et al. 2012, Pedull a et al. 2012, 2013b). The artificial canal was
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
et al. Influence of cyclic torsional preloading on cyclic fatigue Pedulla
(a)
(c)
(b)
(d)
(e)
Figure 1 The experimental set-up for torsional (a–b) and cyclic fatigue tests (c–e). (a) The custom-made device used to apply torsional load until fracture and torsional preloads; (b) A Mtwo file was kept straight during the torsional loads by a chuck that clamped it at 5 mm from the tip and an opposing chuck to secure the instrument shaft; (c) A ProTaper Next X2 inserted in the artificial canal; (d) The fracture point (black arrow) in the centre of the simulated curvature of a ProTaper Next X2 after the cyclic fatigue test; (e) The fractured ProTaper Next X2 in the artificial canal after having removed the separated file segment.
manufactured by reproducing the instrument’s size and taper. It had 5 mm radius of curvature (measured at the internal concave surface of the artificial canal), 60° angle of curvature measured according to the Schneider’s method (Schneider 1971), a centre of curvature 5 mm from the end of the canal and a curved segment of 5 mm in length. The canal was covered with glass to prevent the instruments from slipping out (Pedull a et al. 2013b). The files were activated using a 6:1 reduction handpiece (Sirona Dental Systems GmbH, Bensheim, Germany) powered by a torque-controlled motor (Silver Reciproc; VDW, Munich, Germany) at a speed of 300 RPM. To reduce friction between the instrument and the metal canal walls, a special high-flow synthetic oil designed for the lubrication of mechanical parts (Super Oil; Singer Co Ltd, Elizabethport, NJ, USA) was applied. All instruments were rotated until a fracture occurred. The time was recorded and
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
stopped when a fracture was detected visually and/or audibly (Fig. 1c–e). The number of cycles to failure (NCF) for each instrument was calculated by multiplying the time (in seconds) to failure by the number of rotations or cycles per second (five revolutions per 60 s = 300 rpm). The length of the fractured file tip was measured using a digital microcaliper (Mitutoyo Italiana srl, Lainate, Italy) at 509 under a stereomicroscope (SZR- 10). The fracture surfaces of all fragments were examined under a scanning electron microscope (SEM) (ZEISS Supra 35VP, GmBH, Oberkochen, Germany) looking for topographic features of the fractured instruments. NCF data were analysed using two-way analysis of variance and the Bonferroni’s post hoc test (Prism 5.0; GraphPad Software, Inc, La Jolla, CA, USA) at 0.05 as level of significance.
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Results The mean torque load and angle of rotation until fracture are presented in Table 1. The inferential analysis revealed statistically significant differences amongst the four groups of the same brand considering the value of torsional preloads as the independent variable (two-way analysis of variance, P < 0.0001; interaction = 0.33). There were no significant differences within the same brand when considering the number of preload repetitions as the independent variable (two-way analysis of variance, P > 0.05). The 75% and 50% torsional preloading significantly decreased the cyclic fatigue resistance of instruments compared to nonpreloaded (0%) ones either after 40 or after 20 cycles in every brand (P < 0.0001 and P < 0.001, respectively). The 25% torsional preloading reduced cyclic fatigue resistance of the files ProTaper Next X2 and M2 size 25, 0.06 taper after 20 or 40 cycles (P < 0.01 and P < 0.05, respectively) comparing these with the nonpreloaded ones. HyFlex CM size 25, 0.06 taper did not reduce their cyclic fatigue after 20 or 40 cycles of 25% torsional preloading (P > 0.05) (Table 2). The length of the fractured files was not significantly different for all of the instruments tested (P > 0.05) and which ranged from 4.2 to 6.3 mm (mean = 5.1 mm). SEM images of the fracture surfaces showed similar and typical features of torsional failure with fibrous dimple marks in the centre of rotation for instruments tested only at torsion (Fig. 2a–c). Dimples are visible on all fracture surfaces of the instruments tested for cyclic fatigue (Fig. 2d–f). Smoothed borders and dimples on all fracture surfaces were found in the torsional preloaded files and broken in the following cyclic fatigue test (Fig. 2g–i).
Discussion Although NiTi instruments have improved the quality and speed of root canal preparation, nevertheless,
there is a risk of their failure, as a result of torsional fracture, cyclic fatigue, or a combination of both (Cheung 2009). Often, cyclic fatigue and torsional fracture of various NiTi rotary instruments have been studied separately under simulated conditions (Campbell et al. 2014), probably for convenience or for better control of the loading circumstances (Cheung et al. 2013). Few studies have investigated either the effect of cyclic fatigue preloading on the torsional resistance (Ullmann & Peters 2005, Kim et al. 2012) or the effect of torsional preloads on the cyclic fatigue resistance of conventional NiTi instruments (Galv~ ao Barbosa et al. 2007, Bahia et al. 2008). However, no data have been reported on the influence of torsional preloading on cyclic fatigue of instruments made by different NiTi alloy. Therefore, in this study, CM-wire (HyFlex CM), M-wire (ProTaper Next) and conventional NiTi files (Mtwo) were compared for the effect of different torsional preloads on their cyclic fatigue resistance. NCF value (Yao et al. 2006, Lopes et al. 2009, Pedull a et al. 2014a) and the method described in the ISO standard 3630-1 (Bahia et al. 2008, Gao et al. 2012, Santos Lde et al. 2013) are usually used to measure cyclic fatigue and torsional strength of NiTi instruments, respectively. In the present study, cyclic fatigue was measured by NCF testing the file in a custom-made device (Plotino et al. 2010, Pedull a et al. 2012, 2014a), whilst the torsional loads were applied following the ISO Standard 3630-1 modified only for clamping the testing instruments at the D5 than at the D3 level. This modification was used to provide the torsional preloads at the same level of the maximum stress of the subsequent cyclic fatigue test that was performed using an artificial canal with 5 mm of radius of curvature. The broken fragment after the fatigue tests showed an average length of 5 mm that coincided with the site of torsional preloading application (at D5). Although the comparison amongst the different instruments is difficult to make because of their differences in type of alloy, design and cross-section, in this
Table 1 Mean torque (Ncm) and angle of rotation (°) of instruments tested Torque (Ncm) Instrument ProTaper Next Mtwo HyFlex CM
Mean a
1.32 0.98b 0.84b
Standard deviation 0.25 0.15 0.10
Angle of rotation (°) Min 0.96 0.79 0.76
Max 1.76 1.20 1.06
Mean c
264.16 351.83c 593.83d
Standard deviation
Min
Max
22.67 81.55 93.12
230 230 455
295 440 700
Different superscript letters indicate significant differences amongst groups (P < 0.05).
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Table 2 Number of cycles to failure (NCF) (mean standard deviation) after different percentage and cycles of torsional pre-
loading Torsional preloading Without torsional preloading (0%)
RNTP
ProTaper Next X2
File
402 34a
Mtwo
616 71b
20 40 20 40 20 40
HyFlex CM
1307 181c
25% 248 283 495 480 1157 1053
30d 62d 59e 91e 357c, 460c,
50%
f f
243 233 501 417 856 939
41d 110d 147e 96e 272f 397f
75% 273 209 447 441 711 554
48d 47d 131e 118e 210f, g 232 g
Different superscript letters indicate significant differences amongst groups (P < 0.05).
study, new HyFlex CM instruments files without torsional preloads had significantly higher cyclic fatigue resistance than Mtwo and both instruments had significantly higher cyclic fatigue than ProTaper Next (P < 0.001 for each comparison). On the other hand, new ProTaper Next X2 had significantly higher values for mean ultimate torsional strength than Mtwo size 25, 0.06 taper and HyFlex CM size 25, 0.06 taper (P < 0.001 for each comparison), whilst no difference was found between the last two files (P > 0.05). Cross-sectional configuration seems to have a decisive influence on torsional behaviour and stress distribution of NiTi rotary instruments. The instruments with a long cross-sectional area should have high torsional and a low cyclic fatigue resistance (Sch€ afer et al. 2003, Melo et al. 2008). In a supplementary examination, the cross-sectional configuration of each instrument was captured at 5 mm from the tip (D5) under SEM and the area measured using software (AutoCAD; Autodesk Inc, San Rafael, CA, USA). Amongst the instruments tested, ProTaper Next X2 had the largest area (approximatively 116.35997 lm2), HyFlex CM size 25, 0.06 taper to have an intermediate value (approximatively 96.45815 lm2) and Mtwo size 25, 0.06 taper the smallest area (approximatively 89.9374 lm2). Thus, the highest value of the mean ultimate torsional strength and the lowest value of NCF associated with ProTaper Next X2 in this study are probably due to the large cross-sectional area of this instrument. However, HyFlex CM size 25, 0.06 taper had slightly larger cross-sectional area compared with Mtwo size 25, 0.06 taper, but succumbed at a lower ultimate torsional strength and a higher NCF. This might be related to the mechanical characteristics of the NiTi alloy. In fact, in agreement with the results of this study, it has been shown that the shape memory HyFlex CM
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
generally possesses lower torque values, greater angle of rotation and greater cyclic fatigue resistance in comparisons with conventional (Mtwo) or M-wire (ProTaper Next) instruments probably due to its reduced torsional stiffness (Table 1) (Ninan & Berzins 2013, Capar et al. 2014). The number of torsional preload repetitions used (20 or 40 cycles) did not affect the cyclic fatigue resistance of the NiTi instruments tested (Cheung et al. 2013). On the other hand, the extent of torsional preloads reduced the cyclic fatigue resistance of the instruments especially when the 75% or the 50% of the mean ultimate torsional strength was applied. These results are in agreement with other studies that have reported a reduced cyclic fatigue resistance of conventional NiTi instruments (e.g. K3, SybronEndo, Orange County, CA, USA) after their torsional preload (Galv~ ao Barbosa et al. 2007, Bahia et al. 2008). However, another recent study has shown that torsional preloads within the superelastic limit of the material may improve cyclic fatigue resistance of conventional nickel-titanium rotary instruments such as ProFile and ProTaper Universal F1 (Cheung et al. 2013). These contrasting findings are probably due to the different instruments and methodology used. The 25% torsional preloads significantly reduced the cyclic fatigue of ProTaper Next X2 and Mtwo size 25, 0.06 taper (P < 0.001), whilst it did not affect the fatigue resistance of HyFlex CM size 25, 0.06 taper (P > 0.05) after 20 as well as 40 cycles. These differences are probably due to the different cross-section and alloy of these instruments. In particular, it was reported that for a given strain, a more flexible file (as HyFlex CM in this report) would experience less stress (Ninan & Berzins 2013). Moreover, because the torsional preloads were normalized as a percentage of the maximum torsional strength depending on the instrument tested, the 25% of torsional preloads
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et al. Influence of cyclic torsional preloading on cyclic fatigue Pedulla
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
Figure 2 SEM appearances of the instruments after fatigue testing (a, d, g = ProTaper Next; b, e, h = Mtwo; c, f, i = HyFlex CM). (a–c) Images of new instruments fracture surface after torsional test, second row (d–f) and third row (g–i) show fracture surface after cyclic fatigue test of new instruments and preloaded files at 75% of the ultimate torsional strength for 40 cycles, respectively. Circular abrasion marks and skewed dimples (white arrows) near the centre of rotation surrounded by a smooth surface are the evidence of torsional failure; smoothed borders (black arrows) due to the torsional load or preload; dimpling involve all fracture surface of new (surrounded area) or torsionally preloaded instruments fractured by cyclic fatigue test. Crack initiation origin at smoothed borders (black arrows) and surface pattern with dimples and cones are observed in the same fracture plane.
of HyFlex CM (0.21 Ncm) was the lowest used in this study considering that these instruments have the lowest maximum torsional strength. The SEM analysis revealed typical fractographic appearances of torsional and/or cyclic fatigue fractures
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that were similar amongst the three brands tested. Cyclic fatigue fracture is characterized by dimples on the entire fracture surface, whilst torsional failure is characterized by circular abrasion marks and dimples near the centre of rotation on the fracture surface (Parashos
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et al. Influence of cyclic torsional preloading on cyclic fatigue Pedulla
& Messer 2006, Campbell et al. 2014, Pedull a et al. 2014b). As recently reported, NiTi instruments, in which the torsional test was applied after the incomplete flexural fatigue test, showed a fractography that corresponded to the torsional fracture pattern (Campbell et al. 2014). In the same manner, in this study, when the cyclic fatigue test was applied after the incomplete torsional test, instruments of all brands had typical cyclic fatigue fracture patterns in the SEM evaluation. ProTaper Next is able to resist high torsional stresses which may occur in narrow root canals, HyFlex endures high flexural stress which may occur in curved root canals, whilst Mtwo files have medium values of torsional and cyclic fatigue resistance. However, ProTaper Next X2 and Mtwo size 25, 0.06 taper had reduced fatigue resistance after small torsional preloads (25% for 20 cycles). Therefore, clinicians should consider these findings, especially when they prepare curved root canals with the same instrument that has already been used to shape narrow root canals.
Conclusions Within the limitations of this study, it could be concluded that torsional preloads reduced the cyclic fatigue resistance of conventional and treated (M-wire and CM-wire) NiTi rotary instruments except for HyFlex CM size 25, 0.06 taper with a 25% of torsional preloading.
Acknowledgement The authors thank engineers Salvatore Caltabiano and Salvatore Franco from the University of Catania for the support in the torsional tests. The authors deny any conflict of interests.
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© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Bhagabati N, Yadav S, Talwar S (2012) An in vitro cyclic fatigue analysis of different endodontic nickel-titanium rotary instruments. Journal of Endodontics 38, 515–8. Campbell L, Shen Y, Zhou HM, Haapasalo M (2014) Effect of fatigue on torsional failure of nickel-titanium controlled memory instruments. Journal of Endodontics 40, 562–5. Capar ID, Ertas H, Arslan H (2014) Comparison of cyclic fatigue resistance of novel nickel-titanium rotary instruments. Australian Endodontic Journal. doi:10.1111/aej. 12067. [Epub ahead of print]. Cheung GS (2009) Instrument fracture: mechanisms, removal of fragments, and clinical outcomes. Endodontic Topics 16, 1–26. Cheung GS, Oh SH, Ha JH, Kim SK, Park SH, Kim HC (2013) Effect of torsional loading of nickel-titanium instruments on cyclic fatigue resistance. Journal of Endodontics 39, 1593–7. Elnaghy AM (2014) Cyclic fatigue resistance of ProTaper Next nickel-titanium rotary files. International Endodontic Journal 47, 1034–9. Galv~ ao Barbosa FO, Ponciano Gomes JA, Pimenta de Ara ujo MC (2007) Influence of previous angular deformation on flexural fatigue resistance of K3 nickel–titanium rotary instruments. Journal of Endodontics 33, 1477–80. Gao Y, Gutmann JL, Wilkinson K, Maxwell R, Ammon D (2012) Evaluation of the impact of raw materials on the fatigue and mechanical properties of ProFile Vortex rotary instruments. Journal of Endodontics 38, 398–401. Kim JY, Cheung GS, Park SH, Ko DC, Kim JW, Kim HC (2012) Effect from cyclic fatigue of nickel-titanium rotary files on torsional resistance. Journal of Endodontics 38, 527–30. Lopes HP, Ferreira AA, Elias CN, Moreira EJ, de Oliveira JC, Siqueira JF Jr (2009) Influence of rotational speed on the cyclic fatigue of rotary nickel-titanium endodontic instruments. Journal of Endodontics 35, 1013–6. Melo MCC, Pereira ESJ, Viana ACD, Fonseca AMA, Buono VTL, Bahia MGA (2008) Dimensional characterization and mechanical behaviour of K3 rotary instruments. International Endodontic Journal 41, 329–38. Ninan E, Berzins DW (2013) Torsion and bending properties of shape memory and superelastic nickel-titanium rotary instruments. Journal of Endodontics 39, 101–4. Parashos P, Messer HH (2006) Rotary NiTi instrument fracture and its consequences. Journal of Endodontics 32, 1031–43. Parashos P, Gordon I, Messer HH (2004) Factors influencing defects of rotary nickel titanium files after clinical use. Journal of Endodontics 30, 722–5. Pedull a E, Plotino G, Grande NM, Pappalardo A, Rapisarda E (2012) Cyclic fatigue resistance of four nickel-titanium rotary instruments: a comparative study. Annali di Stomatologia 3, 59–63.
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Pedull a E, Grande NM, Plotino G, Palermo F, Gambarini G, Rapisarda E (2013a) Cyclic fatigue resistance of two reciprocating nickel-titanium instruments after immersion in sodium hypochlorite. International Endodontic Journal 46, 155–9. Pedull a E, Grande NM, Plotino G, Gambarini G, Rapisarda E (2013b) Influence of continuous or reciprocating motion on cyclic fatigue resistance of 4 different nickeltitanium rotary instruments. Journal of Endodontics 39, 258–61. Pedull a E, Plotino G, Grande NM et al. (2014a) Influence of rotational speed on the cyclic fatigue of Mtwo instruments. International Endodontic Journal 47, 514–9. Pedull a E, Franciosi G, Ounsi HF, Tricarico M, Rapisarda E, Grandini S (2014b) Cyclic fatigue resistance of nickeltitanium instruments after immersion in irrigant solutions with or without surfactants. Journal of Endodontics 40, 1245–9. Peters OA, Barbakow F (2002) Dynamic torque and apical forces of ProFile. 04 rotary instruments during preparation of curved canals. International Endodontic Journal 35, 379– 89. Plotino G, Grande NM, Melo MC, Bahia MG, Testarelli L, Gambarini G (2010) Cyclic fatigue of NiTi rotary instruments in a simulated apical abrupt curvature. International Endodontic Journal 43, 226–30.
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Santos Lde A, Bahia MG, de Las Casas EB, Buon VT (2013) Comparison of the mechanical behavior between controlled memory and superelastic nickel-titanium files via finite element analysis. Journal of Endodontics 39, 1444–7. Sattapan B, Nervo GJ, Palamara JE, Messer HH (2000) Defects in rotary nickel-titanium files after clinical use. Journal of Endodontics 26, 161–5. Sch€ afer E, Dzepina A, Danesh G (2003) Bending properties of rotary nickel-titanium instruments. Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology and Endodontics 96, 757–63. Schneider SW (1971) A comparison of canal preparations in straight and curved root canals. Oral Surgery, Oral Medicine, and Oral Pathology 32, 271–5. Ullmann CJ, Peters OA (2005) Effect of cyclic fatigue on static fracture loads in ProTaper nickel-titanium rotary instruments. Journal of Endodontics 31, 183–6. Walia HM, Brantley WA, Gerstein H (1988) An initial investigation of the bending and torsional properties of Nitinol root canal files. Journal of Endodontics 14, 346–51. Yao JH, Schwartz SA, Beeson TJ (2006) Cyclic fatigue of three types of rotary nickel-titanium files in a dynamic model. Journal of Endodontics 32, 55–7. Yum J, Cheung GSP, Park JK, Hur B, Kim HC (2011) Torsional strength and toughness of nickel-titanium rotary files. Journal of Endodontics 37, 382–6.
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