Basic Research—Technology
Phase Transformation Behavior and Resistance to Bending and Cyclic Fatigue of ProTaper Gold and ProTaper Universal Instruments Ahmed Hieawy, DMD, PhD,* Markus Haapasalo, DDS, PhD,* Huimin Zhou, PhD,† Zhe-jun Wang, DDS, PhD,* and Ya Shen, DDS, PhD* Abstract Introduction: The purpose of this study was to compare the flexibility and cyclic fatigue resistance of ProTaper Universal (PTU; Dentsply Tulsa Dental Specialities, Tulsa, OK) and ProTaper Gold (PTG; Dentsply Tulsa Dental Specialities, Tulsa, OK) instruments in relation to their phase transformation behavior. Methods: Sizes S1, S2, F1, F2, and F3 of PTU and PTG instruments were subjected to rotational bending at a curvature of 40 and a radius of 6 mm. The number of cycles to fracture (NCF) was recorded. The fracture surface of all fragments was examined with a scanning electron microscope. Flexibility was determined by 45 bending tests according to the ISO 3630-1 specification. Unused and fractured instruments were examined by differential scanning calorimetry. Results: PTG had a cyclic fatigue resistance superior to PTU in all sizes (P < .001). The NCF of the nickel-titanium files of sizes S1 and S2 was significantly higher than those of sizes F1 to F3 (P < .001). No significant difference in the NCF of PTU instruments was detected between F1 and F2. The fractured files of both PTU and PTG showed the typical fracture pattern of fatigue failure. The bending load values were significantly lower for PTG than for PTU (P < .05). The differential scanning calorimetry analyses showed that each segment of the PTG instruments had a higher austenite finish temperature (50.1 C 1.7 C) than the PTU instruments (21.2 C 1.9 C) (P < .001). PTG instruments had a 2-stage transformation behavior. There was no significant difference in the austenite finish between unused files and instruments subjected to the fatigue process. Conclusions: PTG files were significantly more flexible and resistant to fatigue than PTU files. PTG exhibited different phase transformation behavior than PTU, which may be attributed to the special heat treatment history of PTG instruments. PTG may be more suited for preparing canals with a more abrupt curvature. (J Endod 2015;-:1–5)
Key Words Differential scanning calorimetry, fatigue resistance, metallurgical properties, nickeltitanium instrument, ProTaper Gold, ProTaper Universal
R
oot canal preparation with nickel-titanium (NiTi) rotary instruments is not only easier and faster but also more likely to lead to an improved success rate than preparation with hand instruments (1). Nevertheless, there is a risk of instrument failure with NiTi rotary instruments. The fracture modes of rotary NiTi files can be broadly classified into 2 types: flexural (cyclic) fatigue and torsional failure (2). Improvements in metallurgy, surface treatment, design, and quality control and the introduction of hands-on training have reduced the extent of file fracture (3–6). Recently, thermal treatment of NiTi alloys (eg, controlled memory wire [CM Wire; DS Dental, Johnson City, TN], M-Wire [Dentsply Tulsa Dental Specialities, Tulsa, OK], and R-phase wire [SybronEndo, Orange, CA]) has been used to modify the mechanical properties of these materials. Thermomechanical processing is frequently used to optimize the microstructure and transformation behavior of NiTi alloys, which in turn has greater influence on the mechanical properties of NiTi files (3, 7–9). Some studies (10–12) have shown that NiTi instruments made of M-Wire have significantly improved fatigue resistance compared with those made of conventional superelastic NiTi alloys. Both new-generation NiTi files, ProFile Vortex (Dentsply Tulsa Dental Specialities) and Vortex Blue (Dentsply Tulsa Dental Specialities) instruments, are made out of M-Wire (10, 11). Vortex Blue instruments have a unique blue color not seen in traditional superelastic NiTi instruments. One study (13) found that Vortex Blue files have metallurgical behavior different from ProFile Vortex instruments. It was suggested that the different manufacturing processes used to produce these instruments could result in different phase transformation behaviors, which gives rise to different mechanical behaviors. ProTaper Universal (PTU, Dentsply Tulsa Dental Specialities) is a much studied NiTi rotary system manufactured with a variable taper over the length of the cutting blades, convex triangular cross sections, and noncutting tips. Recently, ProTaper Gold (PTG, Dentsply Tulsa Dental Specialities) instruments were introduced. The PTG files have a design that features identical geometries as PTU but are more flexible and have been developed with proprietary advanced metallurgy. The manufacturer claims that these instruments have fatigue resistance superior to PTU. These new methods and materials for manufacturing NiTi instruments may advance the science of endodontic rotary instrumentation. However, the properties of the PTG files have not been examined by independent research. The phase transformation behavior
From the *Division of Endodontics, Department of Oral Biological and Medical Sciences, Faculty of Dentistry, The University of British Columbia, Vancouver, Canada; and †Center for Biomedical Materials and Engineering, Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin, China. Address requests for reprints to Dr Ya Shen, Division of Endodontics, Department of Oral Biological and Medical Sciences, UBC Faculty of Dentistry, 2199 Wesbrook Mall, Vancouver, BC, Canada V6T 1Z3. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2015 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2015.02.030
JOE — Volume -, Number -, - 2015
ProTaper Gold and Universal Instruments
1
Basic Research—Technology determines the mechanical properties of NiTi alloys. The relationship between thermal behavior and fatigue properties of new PTG endodontic instruments has not been investigated. Therefore, the purpose of this study was to examine the flexibility and fatigue behavior of the PTG system and compare it with its predecessor (ie, PTU) and to evaluate the phase transformation behavior of PTG and PTU files using differential scanning calorimetry (DSC) analysis.
Materials and Methods NiTi rotary instruments of PTG and PTU sizes S1 to F3 were subjected to 3-point bending at a curvature of 40 with a 6-mm radius under deionized water at room temperature (23 C 2 C) in the laboratory (14). The instrument was then allowed to rotate at 300 rpm and torque of 150–520 gcm (as recommended by the manufacturer) until it fractured. Each group included 15 instruments. This fatigue testing protocol has been described previously (14). The fatigue life, or the number of cycles to fracture (NCF), was recorded. Detached fragments were measured for length. The fractured instrument was further cleansed in an ultrasonic bath in absolute alcohol, and the fractured surface was faced upward for a fractographic examination using a scanning electron microscope (Helios NanoLab 650; FEI, Eindhoven, Netherlands) operating at 3 kV (15). Flexibility was measured via a bending test, which was performed using a torsiometer (Sabri Dental Enterprises, Downers Grove, IL) at room temperature according to the ISO specification 3630-1 (16). The instruments were secured at a distance of 3 mm from the tip and then bent 45 about their long axis, and the moment of bending at an angular deflection of 45 was recorded. Twelve files were tested for each group. DSC analyses were performed on unused and fractured PTG and PTU instruments of sizes S1, F1, and F2. Five specimens from each group were subjected to these tests. Each specimen consisted of 2 segments 3–4 mm in length and cut from the instruments using a slowspeed water-cooled diamond saw. The DSC analyses of full cycles were conducted (PYRIS Diamond Series DSC; PerkinElmer, Shelton, CT) over a temperature range from 80 C to 80 C using liquid nitrogen cooling to achieve subambient temperatures (17, 18). The transformation temperatures were obtained from the intersection between the extrapolation of the baseline and the maximum gradient line of the lambda-type DSC curve. The austenite finish (Af) was determined. The data for bending moment, NCF, and Af were analyzed statistically using 2-way analysis of variance (SPSS for Windows 11.0; SPSS, Chicago, IL). Post hoc multiple comparison (Tukey test) was used to isolate and compare the means of the results at a significance level of P < .05.
Results The PTG file had a significantly higher NCF than the PTU file (P < .001) (Table 1). S1 and S2 files were more resistant to fatigue
failure compared with F1 to F3 files in both PTG and PTU systems (P < .001). PTG S1 had the highest NCF among all files (P < .001), whereas PTU F3 had the lowest NCF. The fragment length ranged from 3.7–4.8 mm. The scanning electron microscopic topographic appearance of the fracture surfaces of PTG and PTU showed typical features of cyclic fatigue, including 1 or more crack initiation areas, the presence of fatigue striations, and a fast fracture zone with dimples (Fig. 1A–H). The bending moments of the files tested are shown in Table 1. The bending load values were significantly lower for PTG than for PTU (P < .05). There was a significant difference among files (P < .0001) within each file system. DSC plots for both the heating and cooling cycles of different sizes of unused instruments and instruments subjected to the fatigue process are shown in Figure 2A–C. In all DSC plots, the heating curve is shown at the top of the figure, and the cooling curve is shown at the bottom of the figure. The typical DSC curve for PTU instruments exhibited a single and defined peak on cooling and heating, respectively. The Af temperatures for unused PTU files were 21.2 C 1.9 C. Two endothermic peaks (1 weak and 1 intensive peak) were observed on the heating curve of PTG files. The Af temperatures for unused PTG files (50.1 C 1.7 C) were significantly higher than those for PTU files (P < .001). There was no difference in Af temperatures between unused and fractured instruments (P > .05).
Discussion Despite the identical architecture and operation of the PTG and PTU systems, the different manufacturing processes of the instruments clearly affect their stress-strain distribution patterns and fatigue resistance behaviors. Cyclic failure is more common than failure by torque for rotary ProTaper files (19), whereas torsional failure is more common for hand ProTaper files (20). It is important for clinicians to understand the differences between the PTG and PTU files to take advantage of the latest technology and facilitate good choices to meet anatomic challenges. Thermomechanical treatment of NiTi alloys has a strong impact on their transformation behavior. In near-equiatomic NiTi alloys, the martensitic transformation can occur as either a single-stage transformation (austenite [A] martensite [M]) or a 2-stage transformation (A-R-M) depending on the thermomechanical treatments (21). Usually a 1-stage transformation A-to-M occurs in Ni-rich NiTi alloys, and a 2stage transformation A-R-M occurs after additional heat treatment, which creates finely dispersed Ti3Ni4 precipitates in the austenitic matrix (21, 22). The change from 1-stage transformation to 2-stage transformation can be understood by considering that R-phase is another potential martensite phase and by considering the relative preference of the R-phase over martensite in the presence of fine particles. This is because Ti3Ni4 particles strongly resist the formation of martensite, which is associated with a large lattice deformation, but they have much less resistance to the formation of R-phase, which is associated
TABLE 1. Bending Moment (gcm) and Number of Cycles to Fracture (NCF) of ProTaper Gold (PTG) and ProTaper Universal (PTU) at a Curvature 40 with a 6-mm Radius Bending moment (gcm) Files S1 S2 F1 F2 F3
Fatigue test (NCF)
PTGa
PTUb
PTGi
PTUj
4.8 0.8c 8.8 1.3d 14.8 2.9e 34.0 5.0g 39.5 4.3go
9.0 1.9d 21.7 2.3fu 24.4 2.6f 47.1 4.0h 57.2 5.3p
1750.4 129.1k 1388.8 166.5s 1168.2 126.1l 985.2 135.5m 835.5 119.3q
1074.2 168.7lm 813.3 112.8 744.0 151.9 677.6 172.5 564.8 90.7r
Different superscript letters indicate statistically significant difference between groups (P < .05).
2
Hieawy et al.
JOE — Volume -, Number -, - 2015
Basic Research—Technology
Figure 1. Scanning electron micrographs of the fracture surfaces showing the pattern of fatigue failure. (A) PTG F1 with 2 crack origins at the cutting edge (arrows). (B) A higher magnification view of 1 crack origin (arrow). (C) PTU F1 with 1 crack origin (arrow). (D) A higher magnification view of C. (E) PTG F2 with 1 crack origin (arrow). (F) A higher magnification view of F. (G) PTU F2 with 1 crack origin (arrow). (H) A higher magnification view of G.
JOE — Volume -, Number -, - 2015
ProTaper Gold and Universal Instruments
3
Basic Research—Technology
Figure 2. DSC of unused and fractured (A) S1, (B) F1, and (C) F2 PTG and PTU NiTi instruments.
with a significantly smaller lattice deformation. The presence of Ti3Ni4 particles favors the formation of R-phase, but the alloy requires further cooling to form martensite. Therefore, martensitic transformation occurs in 2 steps: A-R-M (21). Recently, 1 study (18) found that only a single and defined peak (A-to-M) on heating and cooling was detected in ProFile Vortex instruments. Vortex Blue had a 2-stage transformation (13) although Vortex Blue files are also made out of M-Wire. Interestingly, the Af temperatures of Vortex Blue (38 C) were lower than in ProFile Vortex (50 C) instruments (13, 18). Superelasticity or pseudo-elasticity is associated with the occurrence of a phase transformation of the NiTi alloy upon application of stress above a critical level, which takes place when the ambient temperature is above the so-called Af temperature of the material. Therefore, the working temperature for conventional superelastic NiTi files must be above the Af to allow for the use of the pseudo-elasticity. Indeed, superelastic NiTi files, namely ProFile and ProTaper, have Af temperatures below body temperature (23–25). The thermomechanically treated CM Wire files do not have the rebound effect after unloading. Their behavior may be explained by the presence of stable martensite; therefore, the working temperature is below the Af. In the present study, the DSC results showed that PTG instruments had a 2-stage specific transformation behavior, indicating that reverse transformation of the alloy passes through the intermediated R-phase, which reflected the complex phase transformation behavior tracking back to the manufacturing process. Interestingly, the metallurgical characteristics of PTG files had not only a 2-stage specific transformation behavior but also had high Af temperatures, similar to CM Wire (25). The presence of these martensite variants, which can be related to the high transformation temperatures found in PTG files, explains the differences in fatigue resistance between the PTG and PTU files. Figueiredo et al (26) showed that the number of cycles to failure in martensitic NiTi wires can be as much as 100 times higher than in stable and superelastic austenitic NiTi. In the present study, PTG was more flexible than PTU. For a given strain, a more flexible file would experience less stress, allowing for a longer fatigue lifetime if all other factors (cross section, design, and so on) are the same. The fatigue life of a component can be expressed as the number of loading cycles required to initiate a fatigue crack and to propagate the crack to a critical size. The crack propagation mechanism in martensite presents a large number of highly branched cracks that propagate very slowly. In superelastic NiTi, only a few fatigue cracks nucleate, and the propagation is faster (27). A previous study (14) found that CM Wire NiTi files had multiple crack origins on the fracture surface. Although, in the present study, thermally treated PTG instruments exhibited superior cyclic fatigue resistance compared with superelastic PTU instruments, and there was no difference in the fractographic features between the 2 types of instruments. It is generally agreed that the radius of curvature and the size of the instrument affect an instrument’s fatigue life (28–30). In fact, 4
Hieawy et al.
these 2 factors determine the maximum strain on the surface of the instrument. A power function relationship between the lowcycle fatigue life and the surface strain amplitude, which agrees with the Coffin-Mason equation, has been shown for various brands of NiTi files (31). Thus, the larger the strain (generally given by the ratio of the radius of instrument at the breakage point to the radius of curvature), the shorter the fatigue life. Previous studies (29, 30) found that the S1 PTU files had statistically significantly better cyclic fatigue resistance than the F1, F2, and F3 files. This was also confirmed by the present study in which both PTG and PTU showed lower fatigue resistance with an increase in instrument diameter. On the basis of the results in this study, the 2 types of NiTi instruments should be recommended for selective applications according to the canal conditions. For instance, PTG may be more suited for preparing canals with more abrupt curvature because of its good fatigue resistance.
Conclusions Under the limitations of this study, PTG files were significantly more flexible and resistant to fatigue than PTU files. The fatigue life of size S1 and S2 was significantly longer than that of sizes F1–F3 files. PTG exhibited different phase transformation behavior than PTU. The Af temperature of PTG instruments (50.1 C 1.7 C) was higher than the PTU instruments (21.2 C 1.9 C). Furthermore, PTG instruments showed a 2-stage transformation behavior.
Acknowledgments The authors thank Dentsply Tulsa for donating the files used in this study. Supported in part by American Association of Endodontists, the Canadian Academy of Endodontics, and start-up funds provided by the Faculty of Dentistry, University of British Columbia, Canada and Canada Foundation for Innovation (CFI fund; project number 32623). The authors deny any conflicts of interest related to this study.
References 1. Cheung GS, Liu CS. A retrospective study of endodontic treatment outcome between nickel-titanium rotary and stainless steel hand filing techniques. J Endod 2009;35: 938–43. 2. Sattapan B, Nervo GJ, Palamara JE, Messer HH. Defects in rotary nickel-titanium files after clinical use. J Endod 2000;26:161–5. 3. Gambarini G, Grande NM, Plotino G, et al. Fatigue resistance of engine-driven rotary nickel-titanium instruments produced by new manufacturing methods. J Endod 2008;34:1003–5. 4. Gutmann JL, Gao Y. Alteration in the inherent metallic and surface properties of nickeltitanium root canal instruments to enhance performance, durability and safety: a focused review. Int Endod J 2012;45:113–28.
JOE — Volume -, Number -, - 2015
Basic Research—Technology 5. Shen Y, Zhou HM, Zheng YF, et al. Current challenges and concepts of the thermomechanical treatment of nickel-titanium instruments. J Endod 2013;39:163–72. 6. Kahan RS. Summary of: a survey of adoption of endodontic nickel-titanium rotary instrumentation part 1: general dental practitioners in Wales. Braz Dent J 2013; 214:114–5. 7. Bardsley S, Peters CI, Peters OA. The effect of three rotational speed settings on torque and apical force with vortex rotary instruments in vitro. J Endod 2011;37: 860–4. 8. Gao Y, Gutmann JL, Wilkinson K, et al. Evaluation of the impact of raw materials on the fatigue and mechanical properties of ProFile Vortex rotary instruments. J Endod 2012;38:398–401. 9. Shen Y, Coil JM, Zhou HM, et al. ProFile Vortex instruments after clinical use: a metallurgical properties study. J Endod 2012;38:1613–7. 10. Al-Hadlaq SM, AIJarbou FA, AIThumairy RI. Evaluation of cyclic flexural fatigue of MWire nickel-titanium rotary instruments. J Endod 2010;36:305–7. 11. Gao Y, Shotton V, Wilkinson K, et al. Effects of raw material and rotational speed on the cyclic fatigue of ProFile Vortex rotary instruments. J Endod 2010;36:1205–9. 12. Pereira ES, Gomes RO, Leroy AM, et al. Mechanical behavior of M-Wire and conventional NiTi wire used to manufacture rotary endodontic instruments. Dent Mater 2013;29:e318–24. 13. Tsujimoto M, Irifune Y, Tsujimoto Y, et al. Comparison of conventional and newgeneration nickel-titanium files in regard to their physical properties. J Endod 2014;40:1824–9. 14. Shen Y, Qian W, Abtin H, et al. Fatigue testing of controlled memory wire nickeltitanium rotary instruments. J Endod 2011;37:997–1001. 15. Cheung GS, Peng B, Bian Z, et al. Defects in ProTaper S1 instruments after clinical use: fractographic examination. Int Endod J 2005;38:802–9. 16. International Organization for Standardization ISO 3630-1. Dentistry-Root Canal Instruments: Part 1—General Requirements and Test Methods. Geneva, Switzerland: International Organization for Standardization; 2008. 17. Hou X, Yahata Y, Hayashi Y, et al. Phase transformation behaviour and bending property of twisted nickel-titanium endodontic instruments. Int Endod J 2011;44: 253–8.
JOE — Volume -, Number -, - 2015
18. Shen Y, Zhou HM, Coil JM, et al. ProFile Vortex and Vortex Blue nickel-titanium rotary instruments after clinical use. J Endod. http://dx.doi.org/10.1016/j.joen. 2015.02.003. 19. Wei X, Ling J, Jiang J, et al. Modes of failure of ProTaper nickel-titanium rotary instruments after clinical use. J Endod 2007;33:276–9. 20. Shen Y, Bian Z, Cheung GS, et al. Analysis of defects in ProTaper hand-operated instruments after clinical use. J Endod 2007;33:287–90. 21. Otsuka K, Ren X. Physical metallurgy of Ti-Ni-based shape memory alloys. Prog Mater Sci 2005;50:511–678. 22. Duerig TW, Melton KN, Stockel D, Wayman CM, eds. Engineering Aspects of Shape Memory Alloys. London: Butterworth-Heinemann; 1990:3–35. 23. Alapati SB, Brantley WA, Iijima M, et al. Metallurgical characterization of a new nickel-titanium wire for rotary endodontic instruments. J Endod 2009;35: 1589–93. 24. Miyai K, Ebihara A, Hayashi Y, et al. Influence of phase transformation on the torsional and bending properties of nickel-titanium rotary endodontic instruments. Int Endod J 2006;39:119–26. 25. Shen Y, Zhou HM, Zheng YF, et al. Metallurgical characterization of controlled memory wire nickel-titanium rotary instruments. J Endod 2011;37:1566–71. 26. Figueiredo AM, Modenesi P, Buono V. Low-cycle fatigue life of superelastic NiTi wires. Int J Fatigue 2009;31:751–8. 27. Mckelvey AL, Ritchie RO. Fatigue-crack growth behavior in the superelastic and shape-memory alloy Nitinol. Metall Mater Trans A 2001;32A:731–43. 28. Pruett JP, Clement DJ, Carnes DL Jr. Cyclic fatigue testing of nickel-titanium endodontic instruments. J Endod 1997;23:77–85. 29. Nguyen HH, Fong H, Paranjpe A, et al. Evaluation of the resistance to cyclic fatigue among ProTaper Next, ProTaper Universal, and Vortex Blue rotary instruments. J Endod 2014;40:1190–3. 30. Perez-Higueras JJ, Arias A, de la Macorra JC, Peters OA. Differences in cyclic fatigue resistance between ProTaper Next and ProTaper Universal instruments at different levels. J Endod 2014;40:1477–81. 31. Cheung GS, Darvell BW. Fatigue testing of a NiTi rotary instrument. Part 1: strain-life relationship. Int Endod J 2007;40:612–8.
ProTaper Gold and Universal Instruments
5
Phase Transformation Behavior and Resistance to Bending and Cyclic Fatigue of ProTaper Gold and ProTaper Universal Instruments Ahmed Hieawy, DMD, PhD,* Markus Haapasalo, DDS, PhD,* Huimin Zhou, PhD,† Zhe-jun Wang, DDS, PhD,* and Ya Shen, DDS, PhD* Abstract Introduction: The purpose of this study was to compare the flexibility and cyclic fatigue resistance of ProTaper Universal (PTU; Dentsply Tulsa Dental Specialities, Tulsa, OK) and ProTaper Gold (PTG; Dentsply Tulsa Dental Specialities, Tulsa, OK) instruments in relation to their phase transformation behavior. Methods: Sizes S1, S2, F1, F2, and F3 of PTU and PTG instruments were subjected to rotational bending at a curvature of 40 and a radius of 6 mm. The number of cycles to fracture (NCF) was recorded. The fracture surface of all fragments was examined with a scanning electron microscope. Flexibility was determined by 45 bending tests according to the ISO 3630-1 specification. Unused and fractured instruments were examined by differential scanning calorimetry. Results: PTG had a cyclic fatigue resistance superior to PTU in all sizes (P < .001). The NCF of the nickel-titanium files of sizes S1 and S2 was significantly higher than those of sizes F1 to F3 (P < .001). No significant difference in the NCF of PTU instruments was detected between F1 and F2. The fractured files of both PTU and PTG showed the typical fracture pattern of fatigue failure. The bending load values were significantly lower for PTG than for PTU (P < .05). The differential scanning calorimetry analyses showed that each segment of the PTG instruments had a higher austenite finish temperature (50.1 C 1.7 C) than the PTU instruments (21.2 C 1.9 C) (P < .001). PTG instruments had a 2-stage transformation behavior. There was no significant difference in the austenite finish between unused files and instruments subjected to the fatigue process. Conclusions: PTG files were significantly more flexible and resistant to fatigue than PTU files. PTG exhibited different phase transformation behavior than PTU, which may be attributed to the special heat treatment history of PTG instruments. PTG may be more suited for preparing canals with a more abrupt curvature. (J Endod 2015;-:1–5)
Key Words Differential scanning calorimetry, fatigue resistance, metallurgical properties, nickeltitanium instrument, ProTaper Gold, ProTaper Universal
R
oot canal preparation with nickel-titanium (NiTi) rotary instruments is not only easier and faster but also more likely to lead to an improved success rate than preparation with hand instruments (1). Nevertheless, there is a risk of instrument failure with NiTi rotary instruments. The fracture modes of rotary NiTi files can be broadly classified into 2 types: flexural (cyclic) fatigue and torsional failure (2). Improvements in metallurgy, surface treatment, design, and quality control and the introduction of hands-on training have reduced the extent of file fracture (3–6). Recently, thermal treatment of NiTi alloys (eg, controlled memory wire [CM Wire; DS Dental, Johnson City, TN], M-Wire [Dentsply Tulsa Dental Specialities, Tulsa, OK], and R-phase wire [SybronEndo, Orange, CA]) has been used to modify the mechanical properties of these materials. Thermomechanical processing is frequently used to optimize the microstructure and transformation behavior of NiTi alloys, which in turn has greater influence on the mechanical properties of NiTi files (3, 7–9). Some studies (10–12) have shown that NiTi instruments made of M-Wire have significantly improved fatigue resistance compared with those made of conventional superelastic NiTi alloys. Both new-generation NiTi files, ProFile Vortex (Dentsply Tulsa Dental Specialities) and Vortex Blue (Dentsply Tulsa Dental Specialities) instruments, are made out of M-Wire (10, 11). Vortex Blue instruments have a unique blue color not seen in traditional superelastic NiTi instruments. One study (13) found that Vortex Blue files have metallurgical behavior different from ProFile Vortex instruments. It was suggested that the different manufacturing processes used to produce these instruments could result in different phase transformation behaviors, which gives rise to different mechanical behaviors. ProTaper Universal (PTU, Dentsply Tulsa Dental Specialities) is a much studied NiTi rotary system manufactured with a variable taper over the length of the cutting blades, convex triangular cross sections, and noncutting tips. Recently, ProTaper Gold (PTG, Dentsply Tulsa Dental Specialities) instruments were introduced. The PTG files have a design that features identical geometries as PTU but are more flexible and have been developed with proprietary advanced metallurgy. The manufacturer claims that these instruments have fatigue resistance superior to PTU. These new methods and materials for manufacturing NiTi instruments may advance the science of endodontic rotary instrumentation. However, the properties of the PTG files have not been examined by independent research. The phase transformation behavior
From the *Division of Endodontics, Department of Oral Biological and Medical Sciences, Faculty of Dentistry, The University of British Columbia, Vancouver, Canada; and †Center for Biomedical Materials and Engineering, Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin, China. Address requests for reprints to Dr Ya Shen, Division of Endodontics, Department of Oral Biological and Medical Sciences, UBC Faculty of Dentistry, 2199 Wesbrook Mall, Vancouver, BC, Canada V6T 1Z3. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2015 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2015.02.030
JOE — Volume -, Number -, - 2015
ProTaper Gold and Universal Instruments
1
Basic Research—Technology determines the mechanical properties of NiTi alloys. The relationship between thermal behavior and fatigue properties of new PTG endodontic instruments has not been investigated. Therefore, the purpose of this study was to examine the flexibility and fatigue behavior of the PTG system and compare it with its predecessor (ie, PTU) and to evaluate the phase transformation behavior of PTG and PTU files using differential scanning calorimetry (DSC) analysis.
Materials and Methods NiTi rotary instruments of PTG and PTU sizes S1 to F3 were subjected to 3-point bending at a curvature of 40 with a 6-mm radius under deionized water at room temperature (23 C 2 C) in the laboratory (14). The instrument was then allowed to rotate at 300 rpm and torque of 150–520 gcm (as recommended by the manufacturer) until it fractured. Each group included 15 instruments. This fatigue testing protocol has been described previously (14). The fatigue life, or the number of cycles to fracture (NCF), was recorded. Detached fragments were measured for length. The fractured instrument was further cleansed in an ultrasonic bath in absolute alcohol, and the fractured surface was faced upward for a fractographic examination using a scanning electron microscope (Helios NanoLab 650; FEI, Eindhoven, Netherlands) operating at 3 kV (15). Flexibility was measured via a bending test, which was performed using a torsiometer (Sabri Dental Enterprises, Downers Grove, IL) at room temperature according to the ISO specification 3630-1 (16). The instruments were secured at a distance of 3 mm from the tip and then bent 45 about their long axis, and the moment of bending at an angular deflection of 45 was recorded. Twelve files were tested for each group. DSC analyses were performed on unused and fractured PTG and PTU instruments of sizes S1, F1, and F2. Five specimens from each group were subjected to these tests. Each specimen consisted of 2 segments 3–4 mm in length and cut from the instruments using a slowspeed water-cooled diamond saw. The DSC analyses of full cycles were conducted (PYRIS Diamond Series DSC; PerkinElmer, Shelton, CT) over a temperature range from 80 C to 80 C using liquid nitrogen cooling to achieve subambient temperatures (17, 18). The transformation temperatures were obtained from the intersection between the extrapolation of the baseline and the maximum gradient line of the lambda-type DSC curve. The austenite finish (Af) was determined. The data for bending moment, NCF, and Af were analyzed statistically using 2-way analysis of variance (SPSS for Windows 11.0; SPSS, Chicago, IL). Post hoc multiple comparison (Tukey test) was used to isolate and compare the means of the results at a significance level of P < .05.
Results The PTG file had a significantly higher NCF than the PTU file (P < .001) (Table 1). S1 and S2 files were more resistant to fatigue
failure compared with F1 to F3 files in both PTG and PTU systems (P < .001). PTG S1 had the highest NCF among all files (P < .001), whereas PTU F3 had the lowest NCF. The fragment length ranged from 3.7–4.8 mm. The scanning electron microscopic topographic appearance of the fracture surfaces of PTG and PTU showed typical features of cyclic fatigue, including 1 or more crack initiation areas, the presence of fatigue striations, and a fast fracture zone with dimples (Fig. 1A–H). The bending moments of the files tested are shown in Table 1. The bending load values were significantly lower for PTG than for PTU (P < .05). There was a significant difference among files (P < .0001) within each file system. DSC plots for both the heating and cooling cycles of different sizes of unused instruments and instruments subjected to the fatigue process are shown in Figure 2A–C. In all DSC plots, the heating curve is shown at the top of the figure, and the cooling curve is shown at the bottom of the figure. The typical DSC curve for PTU instruments exhibited a single and defined peak on cooling and heating, respectively. The Af temperatures for unused PTU files were 21.2 C 1.9 C. Two endothermic peaks (1 weak and 1 intensive peak) were observed on the heating curve of PTG files. The Af temperatures for unused PTG files (50.1 C 1.7 C) were significantly higher than those for PTU files (P < .001). There was no difference in Af temperatures between unused and fractured instruments (P > .05).
Discussion Despite the identical architecture and operation of the PTG and PTU systems, the different manufacturing processes of the instruments clearly affect their stress-strain distribution patterns and fatigue resistance behaviors. Cyclic failure is more common than failure by torque for rotary ProTaper files (19), whereas torsional failure is more common for hand ProTaper files (20). It is important for clinicians to understand the differences between the PTG and PTU files to take advantage of the latest technology and facilitate good choices to meet anatomic challenges. Thermomechanical treatment of NiTi alloys has a strong impact on their transformation behavior. In near-equiatomic NiTi alloys, the martensitic transformation can occur as either a single-stage transformation (austenite [A] martensite [M]) or a 2-stage transformation (A-R-M) depending on the thermomechanical treatments (21). Usually a 1-stage transformation A-to-M occurs in Ni-rich NiTi alloys, and a 2stage transformation A-R-M occurs after additional heat treatment, which creates finely dispersed Ti3Ni4 precipitates in the austenitic matrix (21, 22). The change from 1-stage transformation to 2-stage transformation can be understood by considering that R-phase is another potential martensite phase and by considering the relative preference of the R-phase over martensite in the presence of fine particles. This is because Ti3Ni4 particles strongly resist the formation of martensite, which is associated with a large lattice deformation, but they have much less resistance to the formation of R-phase, which is associated
TABLE 1. Bending Moment (gcm) and Number of Cycles to Fracture (NCF) of ProTaper Gold (PTG) and ProTaper Universal (PTU) at a Curvature 40 with a 6-mm Radius Bending moment (gcm) Files S1 S2 F1 F2 F3
Fatigue test (NCF)
PTGa
PTUb
PTGi
PTUj
4.8 0.8c 8.8 1.3d 14.8 2.9e 34.0 5.0g 39.5 4.3go
9.0 1.9d 21.7 2.3fu 24.4 2.6f 47.1 4.0h 57.2 5.3p
1750.4 129.1k 1388.8 166.5s 1168.2 126.1l 985.2 135.5m 835.5 119.3q
1074.2 168.7lm 813.3 112.8 744.0 151.9 677.6 172.5 564.8 90.7r
Different superscript letters indicate statistically significant difference between groups (P < .05).
2
Hieawy et al.
JOE — Volume -, Number -, - 2015
Basic Research—Technology
Figure 1. Scanning electron micrographs of the fracture surfaces showing the pattern of fatigue failure. (A) PTG F1 with 2 crack origins at the cutting edge (arrows). (B) A higher magnification view of 1 crack origin (arrow). (C) PTU F1 with 1 crack origin (arrow). (D) A higher magnification view of C. (E) PTG F2 with 1 crack origin (arrow). (F) A higher magnification view of F. (G) PTU F2 with 1 crack origin (arrow). (H) A higher magnification view of G.
JOE — Volume -, Number -, - 2015
ProTaper Gold and Universal Instruments
3
Basic Research—Technology
Figure 2. DSC of unused and fractured (A) S1, (B) F1, and (C) F2 PTG and PTU NiTi instruments.
with a significantly smaller lattice deformation. The presence of Ti3Ni4 particles favors the formation of R-phase, but the alloy requires further cooling to form martensite. Therefore, martensitic transformation occurs in 2 steps: A-R-M (21). Recently, 1 study (18) found that only a single and defined peak (A-to-M) on heating and cooling was detected in ProFile Vortex instruments. Vortex Blue had a 2-stage transformation (13) although Vortex Blue files are also made out of M-Wire. Interestingly, the Af temperatures of Vortex Blue (38 C) were lower than in ProFile Vortex (50 C) instruments (13, 18). Superelasticity or pseudo-elasticity is associated with the occurrence of a phase transformation of the NiTi alloy upon application of stress above a critical level, which takes place when the ambient temperature is above the so-called Af temperature of the material. Therefore, the working temperature for conventional superelastic NiTi files must be above the Af to allow for the use of the pseudo-elasticity. Indeed, superelastic NiTi files, namely ProFile and ProTaper, have Af temperatures below body temperature (23–25). The thermomechanically treated CM Wire files do not have the rebound effect after unloading. Their behavior may be explained by the presence of stable martensite; therefore, the working temperature is below the Af. In the present study, the DSC results showed that PTG instruments had a 2-stage specific transformation behavior, indicating that reverse transformation of the alloy passes through the intermediated R-phase, which reflected the complex phase transformation behavior tracking back to the manufacturing process. Interestingly, the metallurgical characteristics of PTG files had not only a 2-stage specific transformation behavior but also had high Af temperatures, similar to CM Wire (25). The presence of these martensite variants, which can be related to the high transformation temperatures found in PTG files, explains the differences in fatigue resistance between the PTG and PTU files. Figueiredo et al (26) showed that the number of cycles to failure in martensitic NiTi wires can be as much as 100 times higher than in stable and superelastic austenitic NiTi. In the present study, PTG was more flexible than PTU. For a given strain, a more flexible file would experience less stress, allowing for a longer fatigue lifetime if all other factors (cross section, design, and so on) are the same. The fatigue life of a component can be expressed as the number of loading cycles required to initiate a fatigue crack and to propagate the crack to a critical size. The crack propagation mechanism in martensite presents a large number of highly branched cracks that propagate very slowly. In superelastic NiTi, only a few fatigue cracks nucleate, and the propagation is faster (27). A previous study (14) found that CM Wire NiTi files had multiple crack origins on the fracture surface. Although, in the present study, thermally treated PTG instruments exhibited superior cyclic fatigue resistance compared with superelastic PTU instruments, and there was no difference in the fractographic features between the 2 types of instruments. It is generally agreed that the radius of curvature and the size of the instrument affect an instrument’s fatigue life (28–30). In fact, 4
Hieawy et al.
these 2 factors determine the maximum strain on the surface of the instrument. A power function relationship between the lowcycle fatigue life and the surface strain amplitude, which agrees with the Coffin-Mason equation, has been shown for various brands of NiTi files (31). Thus, the larger the strain (generally given by the ratio of the radius of instrument at the breakage point to the radius of curvature), the shorter the fatigue life. Previous studies (29, 30) found that the S1 PTU files had statistically significantly better cyclic fatigue resistance than the F1, F2, and F3 files. This was also confirmed by the present study in which both PTG and PTU showed lower fatigue resistance with an increase in instrument diameter. On the basis of the results in this study, the 2 types of NiTi instruments should be recommended for selective applications according to the canal conditions. For instance, PTG may be more suited for preparing canals with more abrupt curvature because of its good fatigue resistance.
Conclusions Under the limitations of this study, PTG files were significantly more flexible and resistant to fatigue than PTU files. The fatigue life of size S1 and S2 was significantly longer than that of sizes F1–F3 files. PTG exhibited different phase transformation behavior than PTU. The Af temperature of PTG instruments (50.1 C 1.7 C) was higher than the PTU instruments (21.2 C 1.9 C). Furthermore, PTG instruments showed a 2-stage transformation behavior.
Acknowledgments The authors thank Dentsply Tulsa for donating the files used in this study. Supported in part by American Association of Endodontists, the Canadian Academy of Endodontics, and start-up funds provided by the Faculty of Dentistry, University of British Columbia, Canada and Canada Foundation for Innovation (CFI fund; project number 32623). The authors deny any conflicts of interest related to this study.
References 1. Cheung GS, Liu CS. A retrospective study of endodontic treatment outcome between nickel-titanium rotary and stainless steel hand filing techniques. J Endod 2009;35: 938–43. 2. Sattapan B, Nervo GJ, Palamara JE, Messer HH. Defects in rotary nickel-titanium files after clinical use. J Endod 2000;26:161–5. 3. Gambarini G, Grande NM, Plotino G, et al. Fatigue resistance of engine-driven rotary nickel-titanium instruments produced by new manufacturing methods. J Endod 2008;34:1003–5. 4. Gutmann JL, Gao Y. Alteration in the inherent metallic and surface properties of nickeltitanium root canal instruments to enhance performance, durability and safety: a focused review. Int Endod J 2012;45:113–28.
JOE — Volume -, Number -, - 2015
Basic Research—Technology 5. Shen Y, Zhou HM, Zheng YF, et al. Current challenges and concepts of the thermomechanical treatment of nickel-titanium instruments. J Endod 2013;39:163–72. 6. Kahan RS. Summary of: a survey of adoption of endodontic nickel-titanium rotary instrumentation part 1: general dental practitioners in Wales. Braz Dent J 2013; 214:114–5. 7. Bardsley S, Peters CI, Peters OA. The effect of three rotational speed settings on torque and apical force with vortex rotary instruments in vitro. J Endod 2011;37: 860–4. 8. Gao Y, Gutmann JL, Wilkinson K, et al. Evaluation of the impact of raw materials on the fatigue and mechanical properties of ProFile Vortex rotary instruments. J Endod 2012;38:398–401. 9. Shen Y, Coil JM, Zhou HM, et al. ProFile Vortex instruments after clinical use: a metallurgical properties study. J Endod 2012;38:1613–7. 10. Al-Hadlaq SM, AIJarbou FA, AIThumairy RI. Evaluation of cyclic flexural fatigue of MWire nickel-titanium rotary instruments. J Endod 2010;36:305–7. 11. Gao Y, Shotton V, Wilkinson K, et al. Effects of raw material and rotational speed on the cyclic fatigue of ProFile Vortex rotary instruments. J Endod 2010;36:1205–9. 12. Pereira ES, Gomes RO, Leroy AM, et al. Mechanical behavior of M-Wire and conventional NiTi wire used to manufacture rotary endodontic instruments. Dent Mater 2013;29:e318–24. 13. Tsujimoto M, Irifune Y, Tsujimoto Y, et al. Comparison of conventional and newgeneration nickel-titanium files in regard to their physical properties. J Endod 2014;40:1824–9. 14. Shen Y, Qian W, Abtin H, et al. Fatigue testing of controlled memory wire nickeltitanium rotary instruments. J Endod 2011;37:997–1001. 15. Cheung GS, Peng B, Bian Z, et al. Defects in ProTaper S1 instruments after clinical use: fractographic examination. Int Endod J 2005;38:802–9. 16. International Organization for Standardization ISO 3630-1. Dentistry-Root Canal Instruments: Part 1—General Requirements and Test Methods. Geneva, Switzerland: International Organization for Standardization; 2008. 17. Hou X, Yahata Y, Hayashi Y, et al. Phase transformation behaviour and bending property of twisted nickel-titanium endodontic instruments. Int Endod J 2011;44: 253–8.
JOE — Volume -, Number -, - 2015
18. Shen Y, Zhou HM, Coil JM, et al. ProFile Vortex and Vortex Blue nickel-titanium rotary instruments after clinical use. J Endod. http://dx.doi.org/10.1016/j.joen. 2015.02.003. 19. Wei X, Ling J, Jiang J, et al. Modes of failure of ProTaper nickel-titanium rotary instruments after clinical use. J Endod 2007;33:276–9. 20. Shen Y, Bian Z, Cheung GS, et al. Analysis of defects in ProTaper hand-operated instruments after clinical use. J Endod 2007;33:287–90. 21. Otsuka K, Ren X. Physical metallurgy of Ti-Ni-based shape memory alloys. Prog Mater Sci 2005;50:511–678. 22. Duerig TW, Melton KN, Stockel D, Wayman CM, eds. Engineering Aspects of Shape Memory Alloys. London: Butterworth-Heinemann; 1990:3–35. 23. Alapati SB, Brantley WA, Iijima M, et al. Metallurgical characterization of a new nickel-titanium wire for rotary endodontic instruments. J Endod 2009;35: 1589–93. 24. Miyai K, Ebihara A, Hayashi Y, et al. Influence of phase transformation on the torsional and bending properties of nickel-titanium rotary endodontic instruments. Int Endod J 2006;39:119–26. 25. Shen Y, Zhou HM, Zheng YF, et al. Metallurgical characterization of controlled memory wire nickel-titanium rotary instruments. J Endod 2011;37:1566–71. 26. Figueiredo AM, Modenesi P, Buono V. Low-cycle fatigue life of superelastic NiTi wires. Int J Fatigue 2009;31:751–8. 27. Mckelvey AL, Ritchie RO. Fatigue-crack growth behavior in the superelastic and shape-memory alloy Nitinol. Metall Mater Trans A 2001;32A:731–43. 28. Pruett JP, Clement DJ, Carnes DL Jr. Cyclic fatigue testing of nickel-titanium endodontic instruments. J Endod 1997;23:77–85. 29. Nguyen HH, Fong H, Paranjpe A, et al. Evaluation of the resistance to cyclic fatigue among ProTaper Next, ProTaper Universal, and Vortex Blue rotary instruments. J Endod 2014;40:1190–3. 30. Perez-Higueras JJ, Arias A, de la Macorra JC, Peters OA. Differences in cyclic fatigue resistance between ProTaper Next and ProTaper Universal instruments at different levels. J Endod 2014;40:1477–81. 31. Cheung GS, Darvell BW. Fatigue testing of a NiTi rotary instrument. Part 1: strain-life relationship. Int Endod J 2007;40:612–8.
ProTaper Gold and Universal Instruments
5
