doi:10.1111/iej.12244
Cyclic fatigue resistance of ProTaper Next nickel-titanium rotary files
A. M. Elnaghy Department of Conservative Dentistry and Endodontics, Faculty of Dentistry, Mansoura University, Mansoura, Egypt
Abstract Elnaghy AM. Cyclic fatigue resistance of ProTaper Next nickel-titanium rotary files. International Endodontic Journal.
Aim To compare the cyclic fatigue resistance of ProTaper Next files (PTN; Dentsply Maillefer, Ballaigues, Switzerland) with Twisted Files (TF; SybronEndo, Orange, CA, USA), HyFlex CM (HF; ColteneEndo/ Whaledent, Inc, Cuyahoga Falls, OH, USA) and ProTaper Universal (PT; Dentsply Maillefer). Methodology Size 25, .06 taper for PTN X2, TF, HF and PT F1 size 20, .07 taper were rotated in simulated canals until failure, and the number of cycles to failure (NCF) was recorded to evaluate their cyclic fatigue resistance. A scanning electron microscope was used to characterize the topographic features of the fracture surfaces of broken files. The data of the NCF and fragment length values were analysed statistically using one-way analysis of variance and Tukey post hoc tests. Statistical significance level was set at P < 0.05.
Introduction Nickel-titanium (NiTi) rotary endodontic files offer greater flexibility and more resistance to torsional fracture than stainless steel files, which result in minimal transportation whilst instrumentation curved canals (Glossen et al. 1995). On the other hand, they have a tendency to fracture unpredictably due to cyclic fatigue, which occurs as a result of the alternating tension-compression cycles to which they are
Correspondence: Dr Amr M. Elnaghy, Department of Conservative Dentistry and Endodontics, Faculty of Dentistry, Mansoura University, Mansoura, PC 35516, Egypt (e-mail: [email protected]).
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Results Twisted Files had a significantly higher resistance to cyclic fatigue than the other instruments (P < 0.05). No significant difference was found in NCF between PTN and HF (P > 0.05); however, there was a significant difference (P < 0.05) of both these systems with PT, which exhibited the lowest mean NCF. The ranking in the NCF values was: TF > PTN > HF > PT. The fracture cross-sections of all brands revealed similar fractographic features, including crack origins, fatigue zone and an overload fast fracture zone. Conclusions The new ProTaper Next had greater resistance to cyclic fatigue compared with ProTaper and HyFlex CM but not the Twisted Files. Keywords: cyclic fatigue, M-Wire, nickel-titanium files, ProTaper Next. Received 6 November 2013; accepted 2 January 2014
subjected when flexed in the region of maximum curvature of the canal (Sattapan et al. 2000). Various manufacturing strategies for NiTi rotary endodontic files have been developed to improve flexibility and resistance to fatigue fracture, including different cross-sectional designs, the use of new alloys that provide superior mechanical properties or improvement in the manufacturing process (Gambarini et al. 2008). One of these improvements was to reduce the number of machining defects. The new manufacturing process of Twisted Files (TF) (SybronEndo, Orange, CA, USA) uses R-phase heat treatment, twisting of a file blank and a special surface treatment to supposedly improve superelasticity and increase cyclic fatigue resistance (SybronEndo 2008, Oh et al. 2010). Alternatively, ProTaper Universal (PT)
International Endodontic Journal
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Cyclic fatigue resistance of ProTaper Next Elnaghy
(Dentsply Maillefer, Ballaigues, Switzerland) is a progressive taper rotary file system. This design is claimed to decrease instrument fatigue, torsional loads and potential breakage (Blum et al. 2003, Simon et al. 2008). In recent years, heat treatment of NiTi alloy has been used to enhance its mechanical properties (Gambarini et al. 2008, 2011, Shen et al. 2011a). HyFlex CM (HF) rotary files (ColteneEndo/Whaledent, Inc, Cuyahoga Falls, OH, USA) are manufactured from a new type of NiTi wire, namely CM wire (controlled memory), using a special thermomechanical process, making the files extremely flexible but without the shape memory of conventional NiTi files (Zhao et al. 2013). The HF instruments display a lower percent in weight of nickel (52 Ni%wt) compared with the great majority of commercially available NiTi rotary instruments (54.5 57 Ni%wt) (Zinelis et al. 2010). The manufacturer claims that these features of HF may give rise to a reduced risk of ledging, transportation or perforation (ColteneEndo 2012). Recently, ProTaper Next files (PTN) (Dentsply Maillefer) have been introduced. PTN files feature an off-centred rectangular cross-section design for greater strength and a unique asymmetric rotary motion that further enhances canal shaping efficiency as claimed by the manufacturer. PTN files are available in five sizes: X1 (tip size 17 with a taper of .04), X2 (tip size 25 with a taper of .06), X3 (tip size 30 with a taper of .07), X4 (tip size 40 with a taper of .06) and X5 (tip size 50 with a taper of .06). They are manufactured using M-Wire NiTi to increase flexibility and cyclic fatigue resistance of the files (Dentsply Maillefer 2013). To date, no study has determined the cyclic fatigue resistance of these recently introduced PTN files. The aim of this study was to compare the cyclic fatigue resistance of the new PTN files with other files including TF, HF and PT.
Materials and method Cyclic fatigue resistance test Three NiTi rotary file systems, TF size 25, .06 taper, HF size 25, .06 taper and PT F1, were compared for their cyclic fatigue resistance with PTN X2 size 25, .06 taper. The PT F1 size 20, .07 taper was chosen for having the same diameter (0.52–0.55 mm) similar to the other brands at D5 (i.e. 5 mm from file tip) (Kim et al. 2010). Each file was examined for defects before the test with a dental operating microscope
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(Global Surgical, St. Louis, MO, USA). Cyclic fatigue resistance was evaluated using a custom-made device similar to that described by Gambarini (2001) and Lee et al. (2011). Twenty files from each system were tested in artificial canals made of tempered steel with 5 mm radius and 45° angle of curvature measured according to the method of Schneider (1971). The curved segment of the canal was 5 mm in length, and the centre of the curvature was 5 mm from the tip of the instrument. The files were rotated in an electric motor (X-Smart; Dentsply Maillefer) with a 16 : 1 reduction handpiece at a manufacturer’s recommended speed for each system, TF and HF at 500 rpm and PTN and PT at 300 rpm. The artificial canal wall was flooded with synthetic oil (Super Oil; Singer Co Ltd, Elizabethport, NJ, USA) before each file insertion to reduce friction of file and artificial canal walls (Pedull a et al. 2013). The canals were covered with glass to prevent the files slipping out (Plotino et al. 2006). All files were rotated until fracture occurred. The time to failure was recorded in seconds. Recording of time was stopped as fracture was detected visually and/or audibly. The number of cycles to failure (NCF) for each file was calculated by multiplying the time (seconds) to failure by the rotational speed (Pedull a et al. 2013). The length of the fractured tip was measured using a digital caliper (Mitutoyo, Tokyo, Japan).
SEM analysis A scanning electron microscope (SEM) (JEOL, JXA840A, JEOL Ltd., Tokyo, Japan) was used to characterize the topographic features of the fracture surfaces of broken files.
Statistical analysis Statistical analyses (SPSS 13.0; Chicago, IL, USA) of the NCF and fragment length values were analysed using one-way analysis of variance (ANOVA) and Tukey post hoc tests. Statistical significance level was set at P < 0.05.
Results The mean and standard deviations of the NCF and the length of the fractured fragment for each brand are presented in Table 1. TF had a significantly higher resistance to cyclic fatigue than other instruments (P < 0.05). No significant difference was
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Elnaghy Cyclic fatigue resistance of ProTaper Next
Table 1 Mean SD of the number of cycles to failure (NCF) and fragment length of brand systems Brand systems ProTaper Next Twisted File HyFlex ProTaper
NCF* Mean SD 495.25 834.58 466.08 405.75
46.43b 112.95a 42.99b 59.89c
Fragment length (mm)** Mean SD 5.13 5.19 5.32 5.23
0.21 0.16 0.27 0.29
*Means with the same superscript lowercase letter are not significantly different (P > 0.05). **There was no significant difference in the length of fracture fragment between files (P > 0.05).
found in NCF between PTN and HF (P > 0.05); however, there was a significant difference (P < 0.05) of both these systems with PT, which exhibited the lowest mean NCF. The ranking in the NCF values was as follows: TF > PTN > HF > PT. There was no significant difference in the mean length of the fractured fragments of the four brands tested (P > 0.05).
The fracture cross-sections of all brands revealed similar fractographic features, including crack origins, fatigue zone and an overload fast fracture zone (Fig. 1). The specimens revealed crack initiation at the cutting edges of the fracture cross-section with an area of microscopic dimples on fracture surfaces.
Discussion File fracture is the main concern throughout canal shaping procedures (Kim et al. 2010). Several factors are responsible for file fracture in use; however, cyclic fatigue has been reported to be a significant cause as the rotary file might be used in curved root canals (Sattapan et al. 2000). Cyclic fatigue occurs as the file is subjected to continual cycles of tension and compression that result in structural breakdown of the metal due to concentration of stress at the propagating crack front and eventually fracture (Cheung 2007). The geometric designs, structural characteristics and surface texture have a significant influence
(a)
(b)
(c)
(d)
Figure 1 Scanning electron micrographs of the fracture surface of separated fragment after cyclic fatigue test. Notice the crack origin (arrows), fatigue zone (F) and overload fast fracture zone (O). (a) ProTaper Next (PTN); (b) Twisted Files (TF); (c) HyFlex CM (HF); and (d) ProTaper Universal (PT).
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
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Cyclic fatigue resistance of ProTaper Next Elnaghy
on the susceptibility of NiTi instruments to fracture mechanically (Kuhn et al. 2001, Kim et al. 2008, 2009). This study compared the cyclic fatigue resistance of the recently introduced PTN file that is manufactured using M-Wire NiTi with a rectangular cross-section design and asymmetric rotary motion (Dentsply Maillefer 2013) with TF, HF and PT. TF was selected as it had the highest results in cyclic fatigue resistance in previous studies (Kim et al. 2010, Bhagabati et al. 2012). In addition, HF (CM wire) and PT (traditional NiTi) were chosen to evaluate different manufacturing process and design on cyclic fatigue resistance. In the present study, PTN files had a significantly greater cyclic fatigue resistance than PT files, similar cyclic fatigue resistance to HF files, and significantly less cyclic fatigue resistance to TF files. Twisted Files was associated with significantly higher number of rotations to failure than the other brands tested. TF is manufactured by twisting a ground blank in combination with a heat-treated wire (R-phase) and special surface conditioning, which conserves the natural grain structure. Consequently, this manufacturing process results in slower crack initiation and propagation with higher fatigue resistance (Gambarini et al. 2008, Kim et al. 2010). PTN had the second rank in the number of rotations to failure. This could be attributed to the manufacturing process including the M-Wire technology and the rectangular cross-section design. M-Wire (Dentsply Tulsa Dental Specialities) is formed by utilizing a series of heat treatments to NiTi wire blanks. M-Wire instruments include Dentsply’s ProFile GT Series X, ProFile Vortex, Vortex Blue and recently PTN (Shen et al. 2013). The M-Wire technology offers greater flexibility and resistance to cyclic fatigue than those instruments made of regular superelastic wire (Gao et al. 2010). In addition, it has been reported that cross-sectional design influenced the cyclic fatigue resistance of different files (Grande et al. 2006, Yao et al. 2006). The off-centred rectangular cross-section design of PTN might enhance the strength (Dentsply Maillefer 2013) and improve the cyclic fatigue resistance beside the MWire technology. For HF files, higher NCF was recorded compared with PT. The manufacturer claims that HF (CM Wire) files are up to 300% more resistant to cyclic fatigue compared to other brands of rotary NiTi instruments made from superelastic wire (ColteneEndo 2012). The conventional superelastic NiTi has an austenite structure at room and oral temperature, whilst files
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manufactured using the CM Wire proprietary technique were reported to have a mixture of martensite and austenite at room temperature (Shen et al. 2011b). Martensite is less stiff compared with austenite. Files manufactured using CM Wire have more martensite at the expense of austenite, and consequently, this influences their fatigue properties (Ninan & Berzins 2013). It has been reported that files manufactured using CM Wire proprietary processing showed increased fatigue resistance (Shen et al. 2011a). At a given strain, a more flexible file would experience less stress, providing a longer fatigue lifetime (Peters et al. 2012, Ninan & Berzins 2013). Consequently, it can be hypothesized that the increased flexibility of HF files is due to the proprietary thermal processing when compared with PT, which have a traditional manufacturing process with no property thermal treatments and, accordingly, different thermomechanical history (Testarelli et al. 2011). ProTaper Universal exhibited the lowest mean NCF amongst the tested brands. This can be attributed to the traditional grinding process that forms microcracks and defects within the internal structure and along the surface of the files, which results in points of stress concentration that weaken the instruments. Consequently, cracks can propagate to failure at a stress level lower than the stress typically experienced during canal instrumentation and result in sudden, unexpected file fracture (Testarelli et al. 2009, Bhagabati et al. 2012). The fracture of the instruments occurred at the point of maximum flexure of the shaft, corresponding to the midpoint of the curvature. The files fractured approximately 5 mm from the tip of the instrument (Table 1), indicating a narrow region of maximum flexure across file groups. These results are in agreement with previous studies (Pruett et al. 1997, Lopes et al. 2007). Scanning electron microscope observations of the fracture cross-sections of all brands showed comparable surface features with noticeable crack origins, fatigue zones and overload fast fracture zones. The formation of dimpled rupture inside the overload zones is the characteristic of ductile fracture. The crack origins typically started at the cutting edge in all files (Shen et al. 2011a). It has been postulated that the cross-sectional design has little effect on the fatigue life of NiTi root canal files made from conventional NiTi (Cheung & Darvell 2007, Shen et al. 2011a). In the present study, HF files with a triangular border and slight convexity had significantly
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Elnaghy Cyclic fatigue resistance of ProTaper Next
higher NCF compared with PT (convex-triangular border), probably related to the CM Wire properties. On the other hand, PTN files have an off-centred rectangular cross-section and asymmetric rotation, which is supposed to increase its strength as claimed by the manufacturer (Dentsply Maillefer 2013). It could also be speculated that the PTN files with nonuniform and reduced contact points between the instrument and the root canal wall could have enhanced the fracture resistance.
Conclusions The new ProTaper Next enhanced its resistance to cyclic fatigue compared with traditionally ground ProTaper and HyFlex (CM Wire) but not the Twisted Files.
References Bhagabati N, Yadav S, Talwar S (2012) An in vitro cyclic fatigue analysis of different endodontic nickel-titanium rotary instruments. Journal of Endodontics 38, 515–18. Blum JY, Machtou P, Ruddle C, Micallef JP (2003) Analysis of mechanical preparations in extracted teeth using ProTaper rotary instruments: value of the safety quotient. Journal of Endodontics 29, 567–75. Cheung GSP (2007) Instrument fracture: mechanisms, removal of fragments, and clinical outcomes. Endodontic Topics 16, 1–26. Cheung GS, Darvell BW (2007) Fatigue testing of a NiTi rotary instrument. Part 2: fractographic analysis. International Endodontic Journal 40, 619–25. ColteneEndo (2012) The HyFlex CM Brochure. Available at: http://www.hyflexcm.com/DevDownloads/HyflexCM_bro chure.pdf Dentsply Maillefer (2013) The ProTaper Next Brochure. Available at: http://www.dentsplymaillefer.com/#/218x624/ 218x7718/line_218x7727/product_218x9105/ Gambarini G (2001) Cyclic fatigue of nickel-titanium rotary instruments after clinical use with low- and hightorque endodontic motors. Journal of Endodontics 27, 772–4. Gambarini G, Grande NM, Plotino G et al. (2008) Fatigue resistance of engine-driven rotary nickel-titanium instruments produced by new manufacturing methods. Journal of Endodontics 34, 1003–5. Gambarini G, Plotino G, Grande NM, Al-Sudani D, De Luca M, Testarelli L (2011) Mechanical properties of nickel-titanium rotary instruments produced with a new manufacturing technique. International Endodontic Journal 44, 337–41. Gao Y, Shotton V, Wilkinson K, Phillips G, Johnson WB (2010) Effects of raw material and rotational speed on the
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
cyclic fatigue of ProFile Vortex rotary instruments. Journal of Endodontics 36, 1205–9. 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. Journal of Endodontics 21, 146–51. Grande NM, Plotino G, Pecci R, Bedini R, Malagnino VA, Somma F (2006) Cyclic fatigue resistance and threedimensional analysis of instruments from two nickel-titanium rotary systems. International Endodontic Journal 39, 755–63. Kim HC, Cheung GS, Lee CJ, Kim BM, Park JK, Kang SI (2008) Comparison of forces generated during root canal shaping and residual stresses of three nickel-titanium rotary files by using a three-dimensional finite-element analysis. Journal of Endodontics 34, 743–7. Kim HC, Kim HJ, Lee CJ, Kim BM, Park JK, Versluis A (2009) Mechanical response of nickel-titanium instruments with different cross-sectional designs during shaping of simulated curved canals. International Endodontic Journal 42, 593–602. Kim HC, Yum J, Hur B, Cheung GS (2010) Cyclic fatigue and fracture characteristics of ground and twisted nickeltitanium rotary files. Journal of Endodontics 36, 147–52. Kuhn G, Tavernier B, Jordan L (2001) Influence of structure on nickel-titanium endodontic instruments failure. Journal of Endodontics 27, 516–20. Lee MH, Versluis A, Kim BM, Lee CJ, Hur B, Kim HC (2011) Correlation between experimental cyclic fatigue resistance and numerical stress analysis for nickel-titanium rotary files. Journal of Endodontics 37, 1152–7. Lopes HP, Moreira EJ, Elias CN, de Almeida RA, Neves MS (2007) Cyclic fatigue of ProTaper instruments. Journal of Endodontics 33, 55–7. Ninan E, Berzins DW (2013) Torsion and bending properties of shape memory and superelastic nickel-titanium rotary instruments. Journal of Endodontics 39, 101–4. Oh SR, Chang SW, Lee Y et al. (2010) A comparison of nickel-titanium rotary instruments manufactured using different methods and cross-sectional areas: ability to resist cyclic fatigue. Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology and Endodontology 109, 622–8. Pedull a E, Grande NM, Plotino G, Gambarini G, Rapisarda E (2013) Influence of continuous or reciprocating motion on cyclic fatigue resistance of 4 different nickeltitanium rotary instruments. Journal of Endodontics 39, 258–61. Peters OA, Gluskin AK, Weiss RA, Han JT (2012) An in vitro assessment of the physical properties of novel Hyflex nickel-titanium rotary instruments. International Endodontic Journal 45, 1027–34. Plotino G, Grande NM, Sorci E, Malagnino VA, Somma F (2006) A comparison of cyclic fatigue between used and new Mtwo Ni-Ti rotary instruments. International Endodontic Journal 39, 716–23.
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Pruett JP, Clement DJ, Carnes DL Jr (1997) Cyclic fatigue testing of nickel-titanium endodontic instruments. Journal of Endodontics 23, 77–85. Sattapan B, Nervo GJ, Palamara JE, Messer HH (2000) Defects in rotary nickel-titanium files after clinical use. Journal of Endodontics 26, 161–5. Schneider SW (1971) A comparison of canal preparations in straight and curved root canals. Oral Surgery, Oral Medicine, Oral Pathology 32, 271–5. Shen Y, Qian W, Abtin H, Gao Y, Haapasalo M (2011a) Fatigue testing of controlled memory wire nickel-titanium rotary instruments. Journal of Endodontics 37, 997–1001. Shen Y, Zhou HM, Zheng YF, Campbell L, Peng B, Haapasalo M (2011b) Metallurgical characterization of controlled memory wire nickel-titanium rotary instruments. Journal of Endodontics 37, 1566–71. Shen Y, Zhou HM, Zheng YF, Peng B, Haapasalo M (2013) Current challenges and concepts of the thermomechanical treatment of nickel-titanium instruments. Journal of Endodontics 39, 163–72. Simon S, Lumley P, Tomson P, Pertot WJ, Machtou P (2008) Protaper–hybrid technique. Dental Update 35 (110–2), 5–6.
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SybronEndo (2008) The Twisted File Brochure. Available at: http://www.sybronendo.com/index/cms-filesystem-action? file=sybronendo-pdf/tf-brochure.pdf Testarelli L, Grande NM, Plotino G et al. (2009) Cyclic fatigue of different nickel-titanium rotary instruments: a Comparative study. Open Dentistry Journal 3, 55–8. Testarelli L, Plotino G, Al-Sudani D et al. (2011) Bending properties of a new nickel-titanium alloy with a lower percent by weight of nickel. Journal of Endodontics 37, 1293–5. 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. Zhao D, Shen Y, Peng B, Haapasalo M (2013) Micro-computed tomography evaluation of the preparation of mesiobuccal root canals in maxillary first molars with Hyflex CM, Twisted Files, and K3 instruments. Journal of Endodontics 39, 385–8. 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. International Endodontic Journal 43, 125–34.
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Cyclic fatigue resistance of ProTaper Next nickel-titanium rotary files
A. M. Elnaghy Department of Conservative Dentistry and Endodontics, Faculty of Dentistry, Mansoura University, Mansoura, Egypt
Abstract Elnaghy AM. Cyclic fatigue resistance of ProTaper Next nickel-titanium rotary files. International Endodontic Journal.
Aim To compare the cyclic fatigue resistance of ProTaper Next files (PTN; Dentsply Maillefer, Ballaigues, Switzerland) with Twisted Files (TF; SybronEndo, Orange, CA, USA), HyFlex CM (HF; ColteneEndo/ Whaledent, Inc, Cuyahoga Falls, OH, USA) and ProTaper Universal (PT; Dentsply Maillefer). Methodology Size 25, .06 taper for PTN X2, TF, HF and PT F1 size 20, .07 taper were rotated in simulated canals until failure, and the number of cycles to failure (NCF) was recorded to evaluate their cyclic fatigue resistance. A scanning electron microscope was used to characterize the topographic features of the fracture surfaces of broken files. The data of the NCF and fragment length values were analysed statistically using one-way analysis of variance and Tukey post hoc tests. Statistical significance level was set at P < 0.05.
Introduction Nickel-titanium (NiTi) rotary endodontic files offer greater flexibility and more resistance to torsional fracture than stainless steel files, which result in minimal transportation whilst instrumentation curved canals (Glossen et al. 1995). On the other hand, they have a tendency to fracture unpredictably due to cyclic fatigue, which occurs as a result of the alternating tension-compression cycles to which they are
Correspondence: Dr Amr M. Elnaghy, Department of Conservative Dentistry and Endodontics, Faculty of Dentistry, Mansoura University, Mansoura, PC 35516, Egypt (e-mail: [email protected]).
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Results Twisted Files had a significantly higher resistance to cyclic fatigue than the other instruments (P < 0.05). No significant difference was found in NCF between PTN and HF (P > 0.05); however, there was a significant difference (P < 0.05) of both these systems with PT, which exhibited the lowest mean NCF. The ranking in the NCF values was: TF > PTN > HF > PT. The fracture cross-sections of all brands revealed similar fractographic features, including crack origins, fatigue zone and an overload fast fracture zone. Conclusions The new ProTaper Next had greater resistance to cyclic fatigue compared with ProTaper and HyFlex CM but not the Twisted Files. Keywords: cyclic fatigue, M-Wire, nickel-titanium files, ProTaper Next. Received 6 November 2013; accepted 2 January 2014
subjected when flexed in the region of maximum curvature of the canal (Sattapan et al. 2000). Various manufacturing strategies for NiTi rotary endodontic files have been developed to improve flexibility and resistance to fatigue fracture, including different cross-sectional designs, the use of new alloys that provide superior mechanical properties or improvement in the manufacturing process (Gambarini et al. 2008). One of these improvements was to reduce the number of machining defects. The new manufacturing process of Twisted Files (TF) (SybronEndo, Orange, CA, USA) uses R-phase heat treatment, twisting of a file blank and a special surface treatment to supposedly improve superelasticity and increase cyclic fatigue resistance (SybronEndo 2008, Oh et al. 2010). Alternatively, ProTaper Universal (PT)
International Endodontic Journal
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Cyclic fatigue resistance of ProTaper Next Elnaghy
(Dentsply Maillefer, Ballaigues, Switzerland) is a progressive taper rotary file system. This design is claimed to decrease instrument fatigue, torsional loads and potential breakage (Blum et al. 2003, Simon et al. 2008). In recent years, heat treatment of NiTi alloy has been used to enhance its mechanical properties (Gambarini et al. 2008, 2011, Shen et al. 2011a). HyFlex CM (HF) rotary files (ColteneEndo/Whaledent, Inc, Cuyahoga Falls, OH, USA) are manufactured from a new type of NiTi wire, namely CM wire (controlled memory), using a special thermomechanical process, making the files extremely flexible but without the shape memory of conventional NiTi files (Zhao et al. 2013). The HF instruments display a lower percent in weight of nickel (52 Ni%wt) compared with the great majority of commercially available NiTi rotary instruments (54.5 57 Ni%wt) (Zinelis et al. 2010). The manufacturer claims that these features of HF may give rise to a reduced risk of ledging, transportation or perforation (ColteneEndo 2012). Recently, ProTaper Next files (PTN) (Dentsply Maillefer) have been introduced. PTN files feature an off-centred rectangular cross-section design for greater strength and a unique asymmetric rotary motion that further enhances canal shaping efficiency as claimed by the manufacturer. PTN files are available in five sizes: X1 (tip size 17 with a taper of .04), X2 (tip size 25 with a taper of .06), X3 (tip size 30 with a taper of .07), X4 (tip size 40 with a taper of .06) and X5 (tip size 50 with a taper of .06). They are manufactured using M-Wire NiTi to increase flexibility and cyclic fatigue resistance of the files (Dentsply Maillefer 2013). To date, no study has determined the cyclic fatigue resistance of these recently introduced PTN files. The aim of this study was to compare the cyclic fatigue resistance of the new PTN files with other files including TF, HF and PT.
Materials and method Cyclic fatigue resistance test Three NiTi rotary file systems, TF size 25, .06 taper, HF size 25, .06 taper and PT F1, were compared for their cyclic fatigue resistance with PTN X2 size 25, .06 taper. The PT F1 size 20, .07 taper was chosen for having the same diameter (0.52–0.55 mm) similar to the other brands at D5 (i.e. 5 mm from file tip) (Kim et al. 2010). Each file was examined for defects before the test with a dental operating microscope
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(Global Surgical, St. Louis, MO, USA). Cyclic fatigue resistance was evaluated using a custom-made device similar to that described by Gambarini (2001) and Lee et al. (2011). Twenty files from each system were tested in artificial canals made of tempered steel with 5 mm radius and 45° angle of curvature measured according to the method of Schneider (1971). The curved segment of the canal was 5 mm in length, and the centre of the curvature was 5 mm from the tip of the instrument. The files were rotated in an electric motor (X-Smart; Dentsply Maillefer) with a 16 : 1 reduction handpiece at a manufacturer’s recommended speed for each system, TF and HF at 500 rpm and PTN and PT at 300 rpm. The artificial canal wall was flooded with synthetic oil (Super Oil; Singer Co Ltd, Elizabethport, NJ, USA) before each file insertion to reduce friction of file and artificial canal walls (Pedull a et al. 2013). The canals were covered with glass to prevent the files slipping out (Plotino et al. 2006). All files were rotated until fracture occurred. The time to failure was recorded in seconds. Recording of time was stopped as fracture was detected visually and/or audibly. The number of cycles to failure (NCF) for each file was calculated by multiplying the time (seconds) to failure by the rotational speed (Pedull a et al. 2013). The length of the fractured tip was measured using a digital caliper (Mitutoyo, Tokyo, Japan).
SEM analysis A scanning electron microscope (SEM) (JEOL, JXA840A, JEOL Ltd., Tokyo, Japan) was used to characterize the topographic features of the fracture surfaces of broken files.
Statistical analysis Statistical analyses (SPSS 13.0; Chicago, IL, USA) of the NCF and fragment length values were analysed using one-way analysis of variance (ANOVA) and Tukey post hoc tests. Statistical significance level was set at P < 0.05.
Results The mean and standard deviations of the NCF and the length of the fractured fragment for each brand are presented in Table 1. TF had a significantly higher resistance to cyclic fatigue than other instruments (P < 0.05). No significant difference was
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
Elnaghy Cyclic fatigue resistance of ProTaper Next
Table 1 Mean SD of the number of cycles to failure (NCF) and fragment length of brand systems Brand systems ProTaper Next Twisted File HyFlex ProTaper
NCF* Mean SD 495.25 834.58 466.08 405.75
46.43b 112.95a 42.99b 59.89c
Fragment length (mm)** Mean SD 5.13 5.19 5.32 5.23
0.21 0.16 0.27 0.29
*Means with the same superscript lowercase letter are not significantly different (P > 0.05). **There was no significant difference in the length of fracture fragment between files (P > 0.05).
found in NCF between PTN and HF (P > 0.05); however, there was a significant difference (P < 0.05) of both these systems with PT, which exhibited the lowest mean NCF. The ranking in the NCF values was as follows: TF > PTN > HF > PT. There was no significant difference in the mean length of the fractured fragments of the four brands tested (P > 0.05).
The fracture cross-sections of all brands revealed similar fractographic features, including crack origins, fatigue zone and an overload fast fracture zone (Fig. 1). The specimens revealed crack initiation at the cutting edges of the fracture cross-section with an area of microscopic dimples on fracture surfaces.
Discussion File fracture is the main concern throughout canal shaping procedures (Kim et al. 2010). Several factors are responsible for file fracture in use; however, cyclic fatigue has been reported to be a significant cause as the rotary file might be used in curved root canals (Sattapan et al. 2000). Cyclic fatigue occurs as the file is subjected to continual cycles of tension and compression that result in structural breakdown of the metal due to concentration of stress at the propagating crack front and eventually fracture (Cheung 2007). The geometric designs, structural characteristics and surface texture have a significant influence
(a)
(b)
(c)
(d)
Figure 1 Scanning electron micrographs of the fracture surface of separated fragment after cyclic fatigue test. Notice the crack origin (arrows), fatigue zone (F) and overload fast fracture zone (O). (a) ProTaper Next (PTN); (b) Twisted Files (TF); (c) HyFlex CM (HF); and (d) ProTaper Universal (PT).
© 2014 International Endodontic Journal. Published by John Wiley & Sons Ltd
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on the susceptibility of NiTi instruments to fracture mechanically (Kuhn et al. 2001, Kim et al. 2008, 2009). This study compared the cyclic fatigue resistance of the recently introduced PTN file that is manufactured using M-Wire NiTi with a rectangular cross-section design and asymmetric rotary motion (Dentsply Maillefer 2013) with TF, HF and PT. TF was selected as it had the highest results in cyclic fatigue resistance in previous studies (Kim et al. 2010, Bhagabati et al. 2012). In addition, HF (CM wire) and PT (traditional NiTi) were chosen to evaluate different manufacturing process and design on cyclic fatigue resistance. In the present study, PTN files had a significantly greater cyclic fatigue resistance than PT files, similar cyclic fatigue resistance to HF files, and significantly less cyclic fatigue resistance to TF files. Twisted Files was associated with significantly higher number of rotations to failure than the other brands tested. TF is manufactured by twisting a ground blank in combination with a heat-treated wire (R-phase) and special surface conditioning, which conserves the natural grain structure. Consequently, this manufacturing process results in slower crack initiation and propagation with higher fatigue resistance (Gambarini et al. 2008, Kim et al. 2010). PTN had the second rank in the number of rotations to failure. This could be attributed to the manufacturing process including the M-Wire technology and the rectangular cross-section design. M-Wire (Dentsply Tulsa Dental Specialities) is formed by utilizing a series of heat treatments to NiTi wire blanks. M-Wire instruments include Dentsply’s ProFile GT Series X, ProFile Vortex, Vortex Blue and recently PTN (Shen et al. 2013). The M-Wire technology offers greater flexibility and resistance to cyclic fatigue than those instruments made of regular superelastic wire (Gao et al. 2010). In addition, it has been reported that cross-sectional design influenced the cyclic fatigue resistance of different files (Grande et al. 2006, Yao et al. 2006). The off-centred rectangular cross-section design of PTN might enhance the strength (Dentsply Maillefer 2013) and improve the cyclic fatigue resistance beside the MWire technology. For HF files, higher NCF was recorded compared with PT. The manufacturer claims that HF (CM Wire) files are up to 300% more resistant to cyclic fatigue compared to other brands of rotary NiTi instruments made from superelastic wire (ColteneEndo 2012). The conventional superelastic NiTi has an austenite structure at room and oral temperature, whilst files
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manufactured using the CM Wire proprietary technique were reported to have a mixture of martensite and austenite at room temperature (Shen et al. 2011b). Martensite is less stiff compared with austenite. Files manufactured using CM Wire have more martensite at the expense of austenite, and consequently, this influences their fatigue properties (Ninan & Berzins 2013). It has been reported that files manufactured using CM Wire proprietary processing showed increased fatigue resistance (Shen et al. 2011a). At a given strain, a more flexible file would experience less stress, providing a longer fatigue lifetime (Peters et al. 2012, Ninan & Berzins 2013). Consequently, it can be hypothesized that the increased flexibility of HF files is due to the proprietary thermal processing when compared with PT, which have a traditional manufacturing process with no property thermal treatments and, accordingly, different thermomechanical history (Testarelli et al. 2011). ProTaper Universal exhibited the lowest mean NCF amongst the tested brands. This can be attributed to the traditional grinding process that forms microcracks and defects within the internal structure and along the surface of the files, which results in points of stress concentration that weaken the instruments. Consequently, cracks can propagate to failure at a stress level lower than the stress typically experienced during canal instrumentation and result in sudden, unexpected file fracture (Testarelli et al. 2009, Bhagabati et al. 2012). The fracture of the instruments occurred at the point of maximum flexure of the shaft, corresponding to the midpoint of the curvature. The files fractured approximately 5 mm from the tip of the instrument (Table 1), indicating a narrow region of maximum flexure across file groups. These results are in agreement with previous studies (Pruett et al. 1997, Lopes et al. 2007). Scanning electron microscope observations of the fracture cross-sections of all brands showed comparable surface features with noticeable crack origins, fatigue zones and overload fast fracture zones. The formation of dimpled rupture inside the overload zones is the characteristic of ductile fracture. The crack origins typically started at the cutting edge in all files (Shen et al. 2011a). It has been postulated that the cross-sectional design has little effect on the fatigue life of NiTi root canal files made from conventional NiTi (Cheung & Darvell 2007, Shen et al. 2011a). In the present study, HF files with a triangular border and slight convexity had significantly
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Elnaghy Cyclic fatigue resistance of ProTaper Next
higher NCF compared with PT (convex-triangular border), probably related to the CM Wire properties. On the other hand, PTN files have an off-centred rectangular cross-section and asymmetric rotation, which is supposed to increase its strength as claimed by the manufacturer (Dentsply Maillefer 2013). It could also be speculated that the PTN files with nonuniform and reduced contact points between the instrument and the root canal wall could have enhanced the fracture resistance.
Conclusions The new ProTaper Next enhanced its resistance to cyclic fatigue compared with traditionally ground ProTaper and HyFlex (CM Wire) but not the Twisted Files.
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