رقم املقالة الترقيم عدد الصفحات ص األلوان صور 1 لون صور 4 لون مالحظات 4 137 Metallurgical and Mechanical Properties of Nickel Titanium Instruments Made of Fire-Wire, Controlled Memory Wire, M-Wire, and Conventional Superelastic Wires Hend M. Abou El Nasr * and Shehab El Din M. Saber ** ABSTRACT Aim: To compare the cyclic fatigue resistance of nickel titanium instruments manufactured with different thermomechanical procedures and to underline the possible factors that determine their mechanical behaviour. Methods: Revo-S, Profile Vortex, Typhoon Infinite Flex and EdgeFile files tested for their cyclic fatigue resistance in an artificially constructed stainless steel canal with a 5-mm radius and a 45 angle of curvature. Fractographic analysis was done using Environmental scanning electron microscope (ESEM) to determine the morphological characteristics of the fractured instruments. Chemical analysis using energy-dispersive spectrophotometry (EDS) and X-ray diffraction (XRD) were also done to determine the chemical composition and crystallographic structures of the tested instruments. Results: The EdgeFile had the highest NCF (P < 0.0001) followed by the Vortex file, then the Typhoon with no significant differences between them (P > 0.05). While Revo-S had the lowest NCF which was statistically non-significant from Typhoon (P > 0.05). The fractographic analysis confirmed a predominantly ductile mode of fracture for all instruments. EDS analysis showed that only Vortex files were composed exclusively of nickel (55.7 wt%) and titanium (44.3 wt%), while the other instruments were composed of about 47-48% Ni and 37-38% Ti. XRD examination revealed that the peaks detected in Revo-S were exclusively austenitic; while the other instruments displayed hybrid austenite and martensite phases with different proportions. Conclusion: The microstructure of NiTi raw material used in manufacturing endodontic instruments influences their mechanical behaviour more than does their elemental composition. * Lecturer, Department of Endodontics, Faculty of Oral and Dental Medicine, Cairo University ** Associate Professor, Department of Endodontics, Faculty of Dentistry, Ain Shams University
(2) E.D.J. Vol. 60, No. 2 Hend M. Abou El Nasr and Shehab El-Din M. Saber INTRODUCTION The use of rotary nickel titanium (NiTi) instruments with super-elastic properties has considerably simplified endodontic treatment. However, during root canal shaping, the continuous tensile and compressive stress cycles in the region of maximum canal curvature could lead to cyclic fatigue, which is one of the major causes of sudden file fracture (1, 2) that is not accompanied by visible signs of plastic deformation (3, 4). Moreover, when the NiTi alloy undergoes stress, the parent austenite phase (cubic crystal structure) transforms to martensite (monoclinic structure), which is unstable at the working temperature; so it reverts back to austenite when the stress is discontinued. This crystalline change enables a large recoverable elastic strain and gives rise to the superelastic behavior of the material (5). However, each of these crystallographic phase changes weakens the instrument and further reduces its cyclic fatigue resistance (4). Compared to austenite, the martensite phase is more ductile (5, 6) hence favors reducing the risk of file fracture under high stress because it can be plastically deformed rather than broken (7,8). Therefore, much effort has been placed by manufacturers to produce superelastic NiTi wire blanks that contain stable martensite under clinical conditions. This is possible by varying the Ni content in NiTi alloys (9), or more commonly by modifying the thermomechanical history of the alloy (7). In 2007, Sportswire (Langley, OK) developed the M-Wire, which is a variant of NiTi alloy composed of superelastic 508 nitinol that had undergone a proprietary method of treatment resulting in a material that includes both the martensitic and the premartensitic R-phase (6,10). In 2010, DS Dental (Johnson city, TN) produced the controlled memory (CM) wire using a special technology to control the material s memory (6, 9), and the files manufactured with these wires are used in the martensitic state (7). Very recently EdgeEndo (USA) launched EdgeFile endodontic instruments made of Fire-Wire NiTi alloy that promises performance enhancing durability (11). To date, there is no report of the fatigue behavior of this novel NiTi file. Therefore, the aim of the present study was to compare the cyclic fatigue resistance of EdgeFiles with those manufactured with different thermomechanical procedures and to underline the possible factors that determine their mechanical behaviour. Materials and Methods Cyclic fatigue testing Four types of rotary endodontic instruments were tested: Revo-S (Micro-Mega, Besancon Cedex, France) manufactured from conventional NiTi alloy, Profile Vortex (Dentsply Tulsa Dental Specialties, Tulsa, OK, USA) manufactured with M-wire technology, Typhoon Infinite Flex (Clinician s Choice Dental Products Inc., CT, USA) manufactured from a controlled memory wire, and EdgeFile (EdgeEndo, USA) manufactured with Fire-Wire heat treatment. For each system, 10 instruments of similar size and taper (30/.06) were subjected to cyclic fatigue testing using a mechanical device developed for this purpose and already used before (3). The instruments were rotated in a simulated canal at a constant speed of 350 rpm and a fixed torque of 2.5 Ncm using a 16:1 reduction hand piece, powered by an electric motor (NSK, Endo-Mate DT, Tokyo, Japan) in the presence of glycerin as lubricant. The simulated root canal was 18 mm long with a 45 angle of curvature and a 5-mm radius of curvature according to Pruett s method (12). The center of curvature was 6 mm from the instrument tip and the curved segment of the canal was 6 mm. After accurate file positioning to its full length, it was set to rotate freely and was synchronized with timing by a digital stopwatch (Timex, Middlebury, CT, USA) to the thousandth of a second. Timing
Metallurgical and Mechanical Properties of Nickel Titanium Instruments (3) was stopped as fracture was detected visually and audibly. The time to fracture was recorded and multiplied by the number of rotations per minute to obtain the number of cycles to failure (NCF) for each instrument. The NCF data were expressed as mean ± standard deviation (SD) and were analyzed by WinSTAT for Microsoft Excel (v. 2007.1, Germany). One-way analysis of variance (ANOVA) followed by Tukey Kramer multiple-comparison post hoc test was used for comparison between the different tested groups. The significance level was set at P 0.05. Environmental scanning electron microscope (ESEM) Three fractured instruments were randomly selected from each group. They were ultrasonically cleaned in absolute alcohol for approximately 120 seconds then mounted with the fracture end facing upward for fractographic examination by ESEM (AMETEK, NJ, USA) to determine the morphological characteristics of the fractured instruments. Chemical analysis using energy-dispersive spectrophotometry (EDS) The surfaces of 3 new unused instruments from each type were analyzed using EDS (AMETEK, NJ, USA) to identify their metallurgic composition. Analyses were done at 30 kv at room temperature. X-ray diffraction (XRD) XRD (X Pert PRO; PANalytical BV, The Netherlands) was performed to identify phases in the instruments. Analyses were performed on 3 samples from each instrument type at room temperature (25 ), with CuKa monochromic radiation at 40 Kv and a tube current of 30 ma, and 20-100 2θ range. Segments from new unused instruments were prepared as described by Shen et al (8) and placed side by side on the glass sample holder. The peaks were identified by using the pattern library Powder Diffraction File (PDF release 2004; International Center for Diffraction Data, Newton Square, PA). Peaks were indexed to ICDD powder standards for austenite and martensite. Results Cyclic fatigue testing Statistical analysis of NCF (table 1) showed that the EdgeFile had the highest NCF (P < 0.0001) followed by the Vortex file, then the Typhoon with no significant differences between them (P > 0.05). While Revo-S had the lowest NCF which was not statistically significant from Typhoon (P> 0.05). Table 1. Mean values and standard deviations of NCF for all tested instruments. Mean (SD) Revo-S Typhoon Vortex EdgeFile 3027.5 a (1186.5) 3713.5 ab (1120) 5873 b 14406 c (3188.5) (1039.5) Mean values with different superscripts are significantly different at (P 0.05). Fractographic analysis using ESEM ESEM photos (Fig. 1) confirmed a predominantly ductile mode of fracture, as reflected by the numerous dimples on the fractured surface resulting from the formation of microvoids in the center of the metal shaft (overload zone) as it underwent cyclic fatigue. At high magnification; the dimpled surfaces revealed a characteristic population of microvoids for each instrument type. For Revo-S and EdgeFile instruments; the dimples were relatively larger, more elongated and open. For Vortex instruments; the dimples were relatively fewer and smaller, while for Typhoon instruments; the dimples were numerous and symmetrical.
(4) E.D.J. Vol. 60, No. 2 Hend M. Abou El Nasr and Shehab El-Din M. Saber FIG. (1) Scanning electron micrographs of the fractured surfaces obtained from different instruments after cyclic fatigue testing. (A-D) At low magnification showing centrally located overload zones with evidence of multiple crack initiation sites (black arrows) around the perimeter of the instrument shaft. (E-H) At high magnification the overload zones revealed evidence of dimpled rupture that occurred due to the coalescence of the microvoids in the overload zone resulting in the ultimate ductile fracture of the instruments. A large population of microvoids (white arrows) could be seen within the NiTi material.
Metallurgical and Mechanical Properties of Nickel Titanium Instruments (5) EDS analysis EDS analysis showed that only Vortex files were composed exclusively of nickel (55.7 wt%) and titanium (44.3 wt%). Revo-S files were composed of 48 wt% nickel, 37 wt% titanium with traces of aluminium (11 wt%) and nitrogen (4 wt%). Typhoon files were composed of 47 wt% Ni, 38 wt% Ti, with traces of oxygen (15 wt%), and similarly EdgeFiles were composed of 48 wt% Ni, 38 wt% Ti, with traces of oxygen (14 wt%). XRD Diffraction patterns at room temperature in Revo-S and Vortex contained 3 major peaks for the (110), (200), and (211) crystallographic planes. For Typhoon CM and EdgeFile instruments, XRD patterns contained numerous peaks that could be indexed to crystallographic planes of martensite (Fig. 2). FIG. (2) XRD patterns of Revo-S, Vortex, Typhoon (TYP CM), and EdgeFile (EDGE). Discussion The fatigue life of files produced with the recent thermomechanical technology Firewire (EdgeFile) was compared to those produced with CM Wire (Typhoon Infinite Flex), M-Wire (Profile Vortex), and conventional superelastic wire (Revo-S). To standardize the experimental design, the files were selected featuring similar cross sectional configurations with 3 cutting blades; whether triangular as the Vortex and Typhoon, asymmetrical triangle (Revo-S), or parabolic as described by the manufacturer of EdgeFile. They had similar size and taper (30/.06), rotated at the same speed
(6) E.D.J. Vol. 60, No. 2 Hend M. Abou El Nasr and Shehab El-Din M. Saber (350 rpm) and torque (2.5 Ncm), and were tested in an artificial canal that was constructed with a curvature having 45º angle and 5mm radius to simulate an abruptly curved canal (13-15). A static fatigue test was preferred because could be confined into a precise trajectory (2). The results showed that EdgeFile had the highest NCF while Revo-S had the lowest. Also, Vortex files had significantly more NCF than Revo-S confirming previous findings (16-18). It is well established that the mechanical behavior of endodontic instruments is influenced by their geometrical design (19-21) ; but since they present with comparable cross sections, it is expected that they experience similar stress distribution (20,21). So, this did not interpret the differences in the mechanical behavior. The low NCF expressed by Typhoon and approaching that of Revo-S was also intriguing. Typhoon has been judged as a flexible instrument (19) ; nonetheless, it produced more transportation than Vortex when preparing S-shaped canals (22). This suggests more stiffness than Vortex which might be attributed to its shorter working blade (12mm) and long rigid shaft (20). At this stage, a better insight was needed concerning the metallurgical properties of the instruments; among which the chemical composition, nature and proportions of their (6, 7) microstructural phases are the internal factors while thermomechanical processing and annealing are the external factors (7, 23). EDS and XRD were used to provide information on the chemical composition and crystallographic structure respectively. The Ni content of a NiTi alloy influences the mechanical properties of endodontic instruments (24) ; with the reduction of the Ni content, there is an increased tendency to obtain stable martensite (which is more flexible than austenite) at the working temperature (5). NiTi alloys used for endodontic instruments are made of approximately 56 wt% Ni and 44 wt% Ti (7). Among the instruments tested, only the Vortex presented this composition, while the other 3 instruments were composed of about 47-48% Ni and 37-38% Ti. The low Ni content might have improved the fracture resistance of EdgeFile; however, it seems that the Ni% was not the deciding factor in the superior behavior of Vortex and, conversely, did not improve the fatigue resistance of Revo-S and Typhoon. Therefore, just as Zhou et al (25), the influence of the composition could be ignored in this study. On the other hand, XRD examination revealed that the peaks detected in Revo-S were exclusively austenitic; while the other 3 instruments displayed hybrid phases containing austenite and martensite with different proportions. This was consistent with previous studies (8, 26) and affirmed the flexible properties of those instruments (27). Thus, the fracture resistance during cyclic loading could be substantially ascribed to the crystallographic profiles of the instruments investigated. Those are strictly determined by their respective thermomechanical histories (9), which were not disclosed by the manufacturers. However, heat treatment before, during, and after machining is known to produce better arrangement of the crystal structure, thus leading to improved flexibility, and changes in the percentage of phases of the alloy, leading to improved resistance or plastic behavior (24, 28). It allows the release of crystal lattice defects and diminishes the internal strain energy (29) and residual stresses, thereby reducing the rate of crack initiation and propagation (20). Practically, the fatigue life is determined by 2 factors: the rate of crack initiation and the rate of crack propagation (17). Therefore the fractured surfaces in this study were examined using ESEM. All the instruments exhibited characteristic features of fatigue failure. A dimpled overload zone containing extensive microvoids was centrally located. This is a typical picture of ductile fracture and indicates that the metal has undergone plastic deformation before breakage (30). The overload zone was large and surrounded by a smaller fatigue zone
Metallurgical and Mechanical Properties of Nickel Titanium Instruments (7) where slow crack growth occurred. The relative sizes of these two zones suggest that most fatigue failure was spent in the original crack formation, creating a heavy stress concentration that led to instantaneous failure. Moreover, being centrally located, is a sign that the failure was initiated at multiple locations around the perimeter of the shaft and grew inward (30). Indeed, SEM pictures revealed multiple crack origins which confirm that the instruments were under high stress (31). This picture was also described by previous investigators (18,29). Multiple fatigue-crack initiation sites normally indicate a weak resistance to crack nucleation and early propagation (17) ; however, the applied stresses are split up between all the cracks, thus reducing the strain localization and the speed of propagation of the fatigue (3). Moreover, the better reorientation capability of the martensite variants due to the lower symmetry of the monoclinic crystal structure than the cubic crystal structure of austenite makes it more resistant to crack initiation. This favorable reorientation also provides a better accommodation of deformation during bending rotation fatigue and effectively reduces the time in formation and accumulation of microstructural defects such as surface irregularities or subsurface voids in which fatigue cracks could nucleate (17). In addition to the improved resistance against fatigue-crack initiation, a hybrid microstructure with a certain proportion of martensite is more likely to have favorable crack propagation resistance than a fully austenitic microstructure (17). Therefore, within the conditions of the present study, we were able to conclude that the microstructure of NiTi raw material used in manufacturing endodontic instruments influences their cyclic fatigue resistance more than does their elemental composition. Nonetheless, it is worth noting that the clinical situation involves a complex type of stresses. The manufacturing technology of the EdgeFile proved to be superior to the other methods. Although some aspects of the files investigated have been clarified, further studies are needed to disclose more about their microstructure. References 1. Bahia MG, Buono VT. Decrease in the fatigue resistance of nickel-titanium rotary instruments after clinical use in curved root canals. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2005; 100: 249-55. 2. Plotino G, Grande NM, Cordaro M, Testarelli L, Gambarini G. A review of cyclic fatigue testing of nickel-titanium rotary instruments. J Endod 2009; 35: 1469-76. 3. Saber SM. Factors affecting the fracture of rotary nickel titanium instruments. ENDO (Lond Engl) 2008; 2:273-83. 4. Troian CH, Só MV, Figueiredo JA, Oliveira EP. Deformation and fracture of RaCe and K3 endodontic instruments according to the number of uses. Int Endod J 2006; 39: 616-25. 5. Thompson SA. An overview of nickel-titanium alloys used in dentistry. Int Endod J 2000; 33: 297-310. 6. Haapasalo M, Shen Y. Evolution of nickel titanium instruments: from past to future. Endodontic Topics 2013; 29: 3-17. 7. Zhou H, Peng B, Zheng YF. An overview of the mechanical properties of nickel titanium endodontic instruments. Endodontic Topics 2013; 29: 42-54. 8. Shen Y, Zhou HM, Zheng YF, Campbell L, Peng B, Haapasalo M. Metallurgical characterization of controlled memory wire nickel-titanium rotary instruments. J Endod 2011; 37: 1566-71 9. Santos LA, Bahia MGA, de Las Casas EB, Buono VTL. Comparison of the mechanical behavior between controlled memory and superelastic nickel-titanium files via finite element analysis. J Endod 2013; 39: 1444-7. 10. Johnson E, Lloyd A, Kuttler S, Namerow K. Comparison between a novel nickel-titanium alloy and 508 nitinol on the cyclic fatigue life of ProFile 25/.04 rotary instruments. J Endod 2008; 34: 1406-9 11. http://edgeendo.com/products/edgefile/ 12. Pruett JP, Clement DJ, Carnes DL Jr. Cyclic fatigue testing of nickel-titanium endodontic instruments. J Endod 1997; 23: 77-85. 13. Inan U, Aydin C, Tunca YM. Cyclic fatigue of ProTaper rotary nickel-titanium instruments in artificial canals with
(8) E.D.J. Vol. 60, No. 2 Hend M. Abou El Nasr and Shehab El-Din M. Saber 2 different radii of curvature. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2007; 104: 837-40. 14. Lopes HP, Vieira MVB, Elias CN, Siqueira Jr JF, Gonçalves LS, Vieira VTL. Location of the canal curvature and its influence on the resistance to fatigue fracture of two rotary nickel-titanium endodontic instruments. ENDO (Lond Engl) 2013; 7: 53 8 15. Rodrigues RC, Lopes HP, Elias CN, Amaral G, Vieira VT, De Martin AS. Influence of different manufacturing methods on the cyclic fatigue of rotary nickel-titanium endodontic instruments. J Endod 2011; 37: 1553-7 16. Al-Hadlaq SM, Aljarbou FA, AlThumairy RI. Evaluation of cyclic flexural fatigue of M-wire nickel-titanium rotary instruments. J Endod 2010; 36: 305-7 17. Gao Y, Shotton V, Wilkinson K, Phillips G, Johnson WB. Effects of raw material and rotational speed on the cyclic fatigue of ProFile Vortex rotary instruments. J Endod 2010; 36: 1205-9. 18. Lopes HP, Gambarra-Soares T, Elias CN, Siqueira JF Jr, Inojosa IF, Lopes WS, Vieira VT. Comparison of the mechanical properties of rotary instruments made of conventional nickel-titanium wire, M-wire, or nickeltitanium alloy in R-phase. J Endod 2013; 39: 516-20. 19. Shen Y, Qian W, Abtin H, Gao Y, Haapasalo M. Fatigue testing of controlled memory wire nickel-titanium rotary instruments. J Endod 2011; 37: 997-1001 20. Kim TO, Cheung GS, Lee JM, Kim BM, Hur B, Kim HC. Stress distribution of three NiTi rotary files under bending and torsional conditions using a mathematic analysis. Int Endod J 2009; 42: 14-21. 21. Xu X, Eng M, Zheng Y, Eng D. Comparative study of torsional and bending properties for six models of nickeltitanium root canal instruments with different crosssections. J Endod 2006; 32: 372-5 22. Burroughs JR, Bergeron BE, Roberts MD, Hagan JL, Himel VT. Shaping ability of three nickel-titanium endodontic file systems in simulated S-shaped root canals. J Endod 2012; 38: 1618-21 23. Kuhn G, Jordan L. Fatigue and mechanical properties of nickel-titanium endodontic instruments. J Endod 2002; 28: 716-20. 24. Testarelli L, Plotino G, Al-Sudani D, Vincenzi V, Giansiracusa A, Grande NM, Gambarini G. Bending properties of a new nickel-titanium alloy with a lower percent by weight of nickel. J Endod 2011; 37: 1293-5. 25. Zhou HM, Shen Y, Zheng W, Li L, Zheng YF, Haapasalo M. Mechanical properties of controlled memory and superelastic nickel-titanium wires used in the manufacture of rotary endodontic instruments. J Endod 2012; 38:1535-40. 26. Alapati SB, Brantley WA, Iijima M, Clark WA, Kovarik L, Buie C, Liu J, Ben Johnson W. Metallurgical characterization of a new nickel-titanium wire for rotary endodontic instruments. J Endod 2009; 35: 1589-93. 27. Condorelli GG, Bonaccorso A, Smecca E, Schäfer E, Cantatore G, Tripi TR. Improvement of the fatigue resistance of NiTi endodontic files by surface and bulk modifications. Int Endod J 2010; 43: 866-73 28. Plotino G, Testarelli L, Al-Sudani D, Pongione G, Grande NM, Gambarini G. Fatigue resistance of rotary instruments manufactured using different nickel-titanium alloys: a comparative study. Odontology 2014; 102: 31-5 29. Shen Y, Zhou HM, Wang Z, Campbell L, Zheng YF, Haapasalo M. Phase transformation behavior and mechanical properties of thermomechanically treated K3XF nickel-titanium instruments. J Endod 2013; 39: 919-23. 30. Ounsi HF, Salameh Z, Al-Shalan T, Ferrari M, Grandini S, Pashley DH, Tay FR. Effect of clinical use on the cyclic fatigue resistance of ProTaper nickel-titanium rotary instruments. J Endod 2007; 33: 737-41. 31. Sachs NW. Understanding the surface features of fatigue fractures: How they describe the failure cause and the failure history. JFAP 2005; 5: 11-5 32. Pirani C, Ruggeri O, Cirulli PP, Pelliccioni GA, Gandolfi MG, Prati C. Metallurgical analysis and fatigue resistance of WaveOne and ProTaper nickel-titanium instruments. Odontology DOI 10.1007/s10266-013-0113-6 (in press) 33. Alapati SB, Brantley WA, Svec TA, Powers JM, Mitchell JC. Scanning electron microscope observations of new and used nickel-titanium rotary files. J Endod 2003;29: 667-9.