Basic Research—Technology
Effect of Repeated Simulated Clinical Use and Sterilization on the Cutting Efficiency and Flexibility of Hyflex CM Nickel-Titanium Rotary Files Scott T. Seago, DDS,* Brian E. Bergeron, DMD,† Timothy C. Kirkpatrick, DDS,* Mark D. Roberts, DMD,* Howard W. Roberts, DDS,* Van T. Himel, DDS,‡ and Kent A. Sabey, DDS‡ Abstract Introduction: Recent nickel-titanium manufacturing processes have resulted in an alloy that remains in a twinned martensitic phase at operating temperature. This alloy has been shown to have increased flexibility with added tolerance to cyclic and torsional fatigue. The aim of this study was to assess the effect of repeated simulated clinical use and sterilization on cutting efficiency and flexibility of Hyflex CM rotary files. Methods: Cutting efficiency was determined by measuring the load required to maintain a constant feed rate while instrumenting simulated canals. Flexibility was determined by using a 3-point bending test. Files were autoclaved after each use according to the manufacturer’s recommendations. Files were tested through 10 simulated clinical uses. For cutting efficiency, mean data were analyzed by using multiple factor analysis of variance and the Dunnett post hoc test (P < .05). For flexibility, mean data were analyzed by using Levene’s Test of Equality of Error and a general linear model (P < .05). Results: No statistically significant decrease in cutting efficiency was noted in groups 2, 5, 6, and 7. A statistically significant decrease in cutting efficiency was noted in groups 3, 4, 8, 9, and 10. No statistically significant decrease in flexibility was noted in groups 2, 3, and 7. A statistically significant decrease in flexibility was noted in groups 4, 5, 6, 8, 9, 10, and 11. Conclusions: Repeated simulated clinical use
From the *Department of Endodontics, Keesler Medical Center, Keesler Air Force Base, Biloxi, Mississippi; †Department of Endodontics, Georgia Regents University, Augusta, Georgia; and ‡Department of Endodontics, Louisiana State University, New Orleans, Louisiana. This article is the work of the United States government and may be reprinted without permission. Opinions expressed herein, unless otherwise specifically indicated, are those of the authors. They do not represent the views of the Department of the Air Force or any other department or agency of the United States government. Address requests for reprints to Dr Timothy C. Kirkpatrick, Program Director, Endodontics Residency, Keesler Medical Center, 81st Dental Squadron, 606 Fisher Street, Keesler AFB, MS 39534. E-mail address: [email protected] 0099-2399/$ - see front matter Published by Elsevier Inc. on behalf of American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2015.01.011
JOE — Volume -, Number -, - 2015
and sterilization showed no effect on cutting efficiency through 1 use and no effect on flexibility through 2 uses. (J Endod 2015;-:1–4)
Key Words Cutting efficiency, flexibility, Hyflex CM
T
he goal of endodontic therapy is to treat or prevent apical periodontitis. The primary etiology of apical periodontitis is bacterial infection of the root canal system (1), which in the apical 5 mm is polymicrobial (2). Enlargement of the root canal system through instrumentation is not only necessary to provide mechanical debridement (3) but also to facilitate effective irrigation (4). One method to obtain predictable results is when there is a continuous taper from the canal orifice to the apex, the original anatomy of the canal is followed, and the spatial relationship of the apical foramen to periapical tissues and root surface is maintained (5). This proves to be a significant challenge in curved canals and leads to the increased incidence of procedural errors such as canal transportation, zips, ledging, perforations, or separated instruments. Even minor amounts of apical transportation can lead to increased leakage of obturated canals (6). The introduction of nickel-titanium (NiTi) rotary instruments has enabled an efficient shaping process while preserving the original canal anatomy (7). Continued development in the manufacturing process of NiTi files has resulted in the production of controlled memory NiTi rotary files. This manufacturing process increases the austenite transformation temperature above 37 C, leaving the NiTi rotary file in the twinned martensitic phase at operating temperature (8). The switch to martensitic phase results in a NiTi with little to no memory, thus decreasing the tendency for the file to straighten during use. This results in a more flexible file (9), with increased resistance to both cyclic fatigue and torsional fracture (10). Cutting efficiency and flexibility are important mechanical properties to maintain because they affect the ability of the practitioner to safely shape the root canal system. Multiple sterilization cycles have been shown to significantly decrease the cutting efficiency of ProFile rotary NiTi instruments (11); however, repeated autoclave cycles have been shown to have no effect on torsional resistance of controlled memory files (12). The performance of rotary NiTi instruments regarding their ability to safely and efficiently instrument a canal is directly related to their cutting efficiency and flexibility. The manufacturer of Hyflex CM NiTi rotary files (Coltene/Whaledent, Cuyahoga Falls, OH) states that their files can be reused. However, there have been no published studies that have explored the effect that repeated clinical use and sterilization have on the mechanical properties of cutting efficiency and flexibility of Hyflex CM NiTi rotary files. Therefore, the aim of this study was to assess the effect of repeated clinical use and sterilization on the cutting efficiency and flexibility of Hyflex CM rotary files.
Materials and Methods Bovine Cortical Bone Preparation Two fresh bovine femurs were obtained, and sections of cortical bone were removed from the middle third of the diaphysis. The marrow was removed, and the specimen was fixed in a 10% formalin solution. The bovine cortical bone was then sectioned by using an
Hyflex CM Cutting Efficiency and Flexibility
1
Basic Research—Technology Accutom 50 precision saw (Struers, Westlake, OH) with a 0.30-mm-thick diamond blade under continuous irrigation. The cortical samples were first sectioned longitudinally and then transversely at a standard thickness of 5 mm. The resultant bovine cortical sections were stored in individual containers containing 10% formalin. Each bovine cortical section was subjected to Vickers microhardness testing by using the Micromet 5104 (Buehler, Lake Bluff, IL) at 2 random locations. Microhardness testing was conducted at a load of 1000 g with a square-based diamond pyramid indenter with a face angle of 136 . The hardness value was calculated by using the following formula: HV = 0.1891 (F/d2) in N/mm2. Cortical samples with a microhardness value outside of the range of 46–56 N/mm2 were discarded. Each bovine cortical section had 10 canals prepared perpendicular to the cortical section by using a high-speed micro drill in a drill press under continuous irrigation. This produced canals with a standardized length of 5 mm and diameter of 0.41 mm. The cortical sections were randomly assigned to 2 groups (n = 30) for use in either the cutting efficiency or flexibility portion of the study. Each group of 30 bovine cortical sections was numbered from 1 to 30, which would ultimately correspond to the same number Hyflex CM NiTi rotary file.
Hyflex CM NiTi Rotary Files Sixty 21-mm size 35/.04 Hyflex CM NiTi rotary files were used in this study. Before being randomly assigned to an experimental group, the length of each file was measured (from D0 to the start of the Dshaped groove of the latch-type shank) with a digital caliper to the nearest 0.01 mm. Files within the range of 21.10–21.18 mm were assigned to the flexibility portion of the study. This ensured a uniform file diameter at D8 where the load cell of the Instron engaged the files. Those files outside of these parameters were assigned to the cutting efficiency portion of the study. This produced a sample size (n = 30) for each
portion of the study. The Hyflex CM NiTi rotary files in each group were then randomly assigned a number, 1–30, that corresponded to the previously numbered bovine cortical section.
Cutting Efficiency Thirty Hyflex CM NiTi rotary files were used in the cutting efficiency portion of the study. Initially, the 30 files were not subjected to any sterilizations or uses before testing and served as the control for all subsequent groups (group 1). The 30 files were then sterilized and tested through 9 simulated uses and sterilizations (groups 2–10). To determine the relative cutting efficiency of the files, an Instron 5566 Universal Testing Machine (Instron, Norwood, MA) monitored the force necessary to maintain a constant feed rate as the 35/.04 Hyflex CM NiTi rotary file instrumented the simulated canal. An 8:1 endodontic handpiece (Anthogyr, Sallanches, France) was fixed to the load cell of the Instron, and the bovine cortical section was fixed in place by using a customized aluminum block (Fig. 1). The load cell was initially adjusted so D0 of the Hyflex CM NiTi rotary instrument was at the superior border of the bovine cortical section. By using an AEU-20 Endo ITR motor (Aseptico, Woodinville, WA) with the torque limiting value set at 2, the files were rotated at 500 rpm with continuous water irrigation. The load necessary to maintain a constant feed rate of 10 mm/min was recorded every 0.002 seconds. After instrumentation of the simulated canal, debris was removed from each file by using a sponge soaked in 70% isopropyl alcohol. Sterilization was then accomplished according to the manufacturer’s recommendation by using the STATIM 5000 (SciCan, Toronto, Canada) cassette autoclave. Each cycle was performed for 6 minutes at 132 C. Instruments were then allowed to cool to room temperature before subsequent testing.
Figure 1. (A and B) Cutting efficiency apparatus secured by vise to Instron table. (C and D) Jig used for flexibility testing.
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Basic Research—Technology
Figure 2. Cutting efficiency results. Groups are statistically different (P < .05) if they are designated by different letters.
Flexibility Thirty Hyflex CM NiTi rotary files were used in the flexibility portion of the study. Initially, the 30 files were not subjected to any sterilizations or uses before initial testing and served as the control for all subsequent groups (group 1). The files were then repeatedly sterilized and tested through 10 simulated uses (groups 2–11). The relative flexibility of the Hyflex CM NiTi rotary files was determined by a 3-point bending test with a 10-mm span by using an Instron 5566 Universal Testing Machine. A custom polyvinylsiloxane jig was fabricated to standardize the placement of the files before testing (Fig. 1). The customized jig ensured the load cell of the Instron engaged the files at D8 throughout testing. The load necessary to maintain a constant feed rate of 1 mm/min was recorded every 0.1 seconds. To allow for repeated testing, files were not taken to failure. After flexibility testing, each file was used to instrument a single simulated canal following the manufacturer’s recommendation at 500 rpm, with a torque limiting value set at 2. Debris was removed from each file by using a sponge soaked in 70% isopropyl alcohol. Sterilization and subsequent testing were then accomplished as described above. Statistical Analysis Cutting efficiency data were analyzed by using multiple factor analysis of variance and the Dunnett post hoc test (P < .05). Flexibility data were initially analyzed by using the Levene’s Test of Equality of Error.
Flexibility data were further analyzed by using a general linear model (P < .05).
Results Cutting Efficiency When compared with the control (group 1), no statistically significant difference was noted for group 2 (P > .05). A statistically significant decrease in cutting efficiency was noted for groups 3 and 4 (P < .05). No statistically significant decrease in cutting efficiency was noted for groups 5, 6, and 7 (P > .05). A statistically significant decrease in cutting efficiency was noted for groups 8, 9, and 10 (P < .05) (Fig. 2). Flexibility When compared with the control (group 1), no statistically significant difference was noted for groups 2 and 3 (P > .05). A statistically significant decrease in flexibility was noted for groups 4, 5, and 6 (P < .05). No statistically significant decrease in flexibility was noted for group 7. A statistically significant decrease in flexibility was noted for groups 8, 9, 10, and 11 (P < .05) (Fig. 3).
Discussion To date, there have been many variations in cutting efficiency protocols. Studies have used substrates of human dentin, plastic blocks, bovine dentin, and bovine cortical bone to determine the relative cutting
Figure 3. Flexibility results. Groups are statistically different (P < .05) if they are designated by different letters.
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Hyflex CM Cutting Efficiency and Flexibility
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Basic Research—Technology efficiency of different files. Bovine cortical bone was chosen for this study because of its physical and chemical similarity to human dentin, while having the possibility of machining many samples from the same location. This allowed better standardization throughout testing and proved a very consistent model. Only a small standard of deviation was noted for all groups, and all samples followed the same trends throughout both the cutting efficiency and flexibility portions of the study. It was interesting that in both the cutting efficiency and flexibility portions of the study, there was a return to normalcy after an initial statistically significant decrease in each mechanical property. It is hypothesized that after repeated clinical use and sterilizations, there is a change in the austenite finishing temperature closer to clinical operating temperatures. This would cause a shift in the proportions of austenite and martensite NiTi microstructural phases, ultimately affecting the mechanical properties of these files. Further research is necessary to determine the effect that repeated clinical use and sterilizations have on the austenite finishing temperature of the Hyflex CM NiTi rotary files. Although a statistically significant decrease in both cutting efficiency and flexibility was noted, no file separation occurred throughout testing. In addition, all Hyflex CM NiTi rotary files continued to enlarge the simulated canal without exceeding the torque control limiter, and all files returned to their original shape after each sterilization. This is similar to results by Thompson et al (13), which showed no significant difference in Hyflex CM NiTi files’ ability to maintain original canal anatomy through 3 uses in resin blocks. It is necessary for the clinician to take into account all the variables present during clinical use when determining the ability of an instrument to be reused for subsequent instrumentation, because statistical significance may or may not directly correlate to clinical significance.
Conclusion Repeated simulated clinical use and sterilizations showed no statistically significant effect on the cutting efficiency of Hyflex CM NiTi rotary files after the first use and sterilization. No statistically significant effect on the flexibility of Hyflex CM NiTi rotary files was noted through 2 uses and 2 sterilizations. In both the cutting efficiency and flexibility
4
Seago et al.
studies, a return to normalcy was noted before another statistically significant decrease in mechanical behavior was noted.
Acknowledgments The authors thank Coltene/Whaledent for providing the files used in this study. The authors deny any conflicts of interest related to this study.
References 1. Kakehashi S, Stanley HR, Fitzgerald RJ. The effects of surgical exposures of dental pulps in germ-free and conventional laboratory rats. Oral Surg Oral Med Oral Pathol 1965;20:340–9. 2. Baumgartner JC, Falkler WA. Bacteria in the apical 5 mm of infected root canals. J Endod 1991;17:380–3. 3. Card S, Sigurdsson A, Orstavik D, Trope M. The effectiveness of increased apical enlargement in reducing intracanal bacteria. J Endod 2002;28:779–83. 4. Khademi A, Yazdizadeh M, Feizianfard M. Determination of the minimum instrumentation size for penetration of irrigants to the apical third of root canal systems. J Endod 2006;32:417–20. 5. Schilder H. Cleaning and shaping the root canal. Dent Clin North Am 1974;18: 269–96. 6. Wu M, Fan B, Wesselink P. Leakage along apical root fillings in curved root canals: part I—effects of apical transportation on seal of root fillings. J Endod 2000;26: 210–6. 7. Glosson CR, Haller RH, Dove SB, del Rio CE. A comparison of root canal preparations using Ni-Ti hand, Ni-Ti engine-driven, and K-Flex endodontic instruments. J Endod 1995;21:146–51. 8. Shen Y, Zhou H, Zheng Y, et al. Metallurgical characterization of controlled memory wire nickel-titanium rotary instruments. J Endod 2011;37:1566–71. 9. Testarelli L, Plotino G, Al-Sudani D, et al. Bending properties of a new nickeltitanium alloy with a lower percent by weight of nickel. J Endod 2011;37:1293–5. 10. Shen Y, Qian W, Abtin H, et al. Fatigue testing of controlled memory wire nickeltitanium rotary instruments. J Endod 2011;37:997–1001. 11. Rapisarda E, Bonaccorso A, Tripi TR, Condorelli GG. Effect of sterilization on the cutting efficiency of rotary nickel-titanium endodontic files. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1999;88:343–7. 12. Casper R, Roberts H, Roberts M, et al. Comparison of autoclaving effects on torsional deformation and fracture resistance of three innovative endodontic file systems. J Endod 2011;37:1572–5. 13. Thompson M, Sidow S, Lindsey K, et al. Evaluation of a new filing system’s ability to maintain canal morphology. J Endod 2013;40:867–70.
JOE — Volume -, Number -, - 2015
Effect of Repeated Simulated Clinical Use and Sterilization on the Cutting Efficiency and Flexibility of Hyflex CM Nickel-Titanium Rotary Files Scott T. Seago, DDS,* Brian E. Bergeron, DMD,† Timothy C. Kirkpatrick, DDS,* Mark D. Roberts, DMD,* Howard W. Roberts, DDS,* Van T. Himel, DDS,‡ and Kent A. Sabey, DDS‡ Abstract Introduction: Recent nickel-titanium manufacturing processes have resulted in an alloy that remains in a twinned martensitic phase at operating temperature. This alloy has been shown to have increased flexibility with added tolerance to cyclic and torsional fatigue. The aim of this study was to assess the effect of repeated simulated clinical use and sterilization on cutting efficiency and flexibility of Hyflex CM rotary files. Methods: Cutting efficiency was determined by measuring the load required to maintain a constant feed rate while instrumenting simulated canals. Flexibility was determined by using a 3-point bending test. Files were autoclaved after each use according to the manufacturer’s recommendations. Files were tested through 10 simulated clinical uses. For cutting efficiency, mean data were analyzed by using multiple factor analysis of variance and the Dunnett post hoc test (P < .05). For flexibility, mean data were analyzed by using Levene’s Test of Equality of Error and a general linear model (P < .05). Results: No statistically significant decrease in cutting efficiency was noted in groups 2, 5, 6, and 7. A statistically significant decrease in cutting efficiency was noted in groups 3, 4, 8, 9, and 10. No statistically significant decrease in flexibility was noted in groups 2, 3, and 7. A statistically significant decrease in flexibility was noted in groups 4, 5, 6, 8, 9, 10, and 11. Conclusions: Repeated simulated clinical use
From the *Department of Endodontics, Keesler Medical Center, Keesler Air Force Base, Biloxi, Mississippi; †Department of Endodontics, Georgia Regents University, Augusta, Georgia; and ‡Department of Endodontics, Louisiana State University, New Orleans, Louisiana. This article is the work of the United States government and may be reprinted without permission. Opinions expressed herein, unless otherwise specifically indicated, are those of the authors. They do not represent the views of the Department of the Air Force or any other department or agency of the United States government. Address requests for reprints to Dr Timothy C. Kirkpatrick, Program Director, Endodontics Residency, Keesler Medical Center, 81st Dental Squadron, 606 Fisher Street, Keesler AFB, MS 39534. E-mail address: [email protected] 0099-2399/$ - see front matter Published by Elsevier Inc. on behalf of American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2015.01.011
JOE — Volume -, Number -, - 2015
and sterilization showed no effect on cutting efficiency through 1 use and no effect on flexibility through 2 uses. (J Endod 2015;-:1–4)
Key Words Cutting efficiency, flexibility, Hyflex CM
T
he goal of endodontic therapy is to treat or prevent apical periodontitis. The primary etiology of apical periodontitis is bacterial infection of the root canal system (1), which in the apical 5 mm is polymicrobial (2). Enlargement of the root canal system through instrumentation is not only necessary to provide mechanical debridement (3) but also to facilitate effective irrigation (4). One method to obtain predictable results is when there is a continuous taper from the canal orifice to the apex, the original anatomy of the canal is followed, and the spatial relationship of the apical foramen to periapical tissues and root surface is maintained (5). This proves to be a significant challenge in curved canals and leads to the increased incidence of procedural errors such as canal transportation, zips, ledging, perforations, or separated instruments. Even minor amounts of apical transportation can lead to increased leakage of obturated canals (6). The introduction of nickel-titanium (NiTi) rotary instruments has enabled an efficient shaping process while preserving the original canal anatomy (7). Continued development in the manufacturing process of NiTi files has resulted in the production of controlled memory NiTi rotary files. This manufacturing process increases the austenite transformation temperature above 37 C, leaving the NiTi rotary file in the twinned martensitic phase at operating temperature (8). The switch to martensitic phase results in a NiTi with little to no memory, thus decreasing the tendency for the file to straighten during use. This results in a more flexible file (9), with increased resistance to both cyclic fatigue and torsional fracture (10). Cutting efficiency and flexibility are important mechanical properties to maintain because they affect the ability of the practitioner to safely shape the root canal system. Multiple sterilization cycles have been shown to significantly decrease the cutting efficiency of ProFile rotary NiTi instruments (11); however, repeated autoclave cycles have been shown to have no effect on torsional resistance of controlled memory files (12). The performance of rotary NiTi instruments regarding their ability to safely and efficiently instrument a canal is directly related to their cutting efficiency and flexibility. The manufacturer of Hyflex CM NiTi rotary files (Coltene/Whaledent, Cuyahoga Falls, OH) states that their files can be reused. However, there have been no published studies that have explored the effect that repeated clinical use and sterilization have on the mechanical properties of cutting efficiency and flexibility of Hyflex CM NiTi rotary files. Therefore, the aim of this study was to assess the effect of repeated clinical use and sterilization on the cutting efficiency and flexibility of Hyflex CM rotary files.
Materials and Methods Bovine Cortical Bone Preparation Two fresh bovine femurs were obtained, and sections of cortical bone were removed from the middle third of the diaphysis. The marrow was removed, and the specimen was fixed in a 10% formalin solution. The bovine cortical bone was then sectioned by using an
Hyflex CM Cutting Efficiency and Flexibility
1
Basic Research—Technology Accutom 50 precision saw (Struers, Westlake, OH) with a 0.30-mm-thick diamond blade under continuous irrigation. The cortical samples were first sectioned longitudinally and then transversely at a standard thickness of 5 mm. The resultant bovine cortical sections were stored in individual containers containing 10% formalin. Each bovine cortical section was subjected to Vickers microhardness testing by using the Micromet 5104 (Buehler, Lake Bluff, IL) at 2 random locations. Microhardness testing was conducted at a load of 1000 g with a square-based diamond pyramid indenter with a face angle of 136 . The hardness value was calculated by using the following formula: HV = 0.1891 (F/d2) in N/mm2. Cortical samples with a microhardness value outside of the range of 46–56 N/mm2 were discarded. Each bovine cortical section had 10 canals prepared perpendicular to the cortical section by using a high-speed micro drill in a drill press under continuous irrigation. This produced canals with a standardized length of 5 mm and diameter of 0.41 mm. The cortical sections were randomly assigned to 2 groups (n = 30) for use in either the cutting efficiency or flexibility portion of the study. Each group of 30 bovine cortical sections was numbered from 1 to 30, which would ultimately correspond to the same number Hyflex CM NiTi rotary file.
Hyflex CM NiTi Rotary Files Sixty 21-mm size 35/.04 Hyflex CM NiTi rotary files were used in this study. Before being randomly assigned to an experimental group, the length of each file was measured (from D0 to the start of the Dshaped groove of the latch-type shank) with a digital caliper to the nearest 0.01 mm. Files within the range of 21.10–21.18 mm were assigned to the flexibility portion of the study. This ensured a uniform file diameter at D8 where the load cell of the Instron engaged the files. Those files outside of these parameters were assigned to the cutting efficiency portion of the study. This produced a sample size (n = 30) for each
portion of the study. The Hyflex CM NiTi rotary files in each group were then randomly assigned a number, 1–30, that corresponded to the previously numbered bovine cortical section.
Cutting Efficiency Thirty Hyflex CM NiTi rotary files were used in the cutting efficiency portion of the study. Initially, the 30 files were not subjected to any sterilizations or uses before testing and served as the control for all subsequent groups (group 1). The 30 files were then sterilized and tested through 9 simulated uses and sterilizations (groups 2–10). To determine the relative cutting efficiency of the files, an Instron 5566 Universal Testing Machine (Instron, Norwood, MA) monitored the force necessary to maintain a constant feed rate as the 35/.04 Hyflex CM NiTi rotary file instrumented the simulated canal. An 8:1 endodontic handpiece (Anthogyr, Sallanches, France) was fixed to the load cell of the Instron, and the bovine cortical section was fixed in place by using a customized aluminum block (Fig. 1). The load cell was initially adjusted so D0 of the Hyflex CM NiTi rotary instrument was at the superior border of the bovine cortical section. By using an AEU-20 Endo ITR motor (Aseptico, Woodinville, WA) with the torque limiting value set at 2, the files were rotated at 500 rpm with continuous water irrigation. The load necessary to maintain a constant feed rate of 10 mm/min was recorded every 0.002 seconds. After instrumentation of the simulated canal, debris was removed from each file by using a sponge soaked in 70% isopropyl alcohol. Sterilization was then accomplished according to the manufacturer’s recommendation by using the STATIM 5000 (SciCan, Toronto, Canada) cassette autoclave. Each cycle was performed for 6 minutes at 132 C. Instruments were then allowed to cool to room temperature before subsequent testing.
Figure 1. (A and B) Cutting efficiency apparatus secured by vise to Instron table. (C and D) Jig used for flexibility testing.
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Basic Research—Technology
Figure 2. Cutting efficiency results. Groups are statistically different (P < .05) if they are designated by different letters.
Flexibility Thirty Hyflex CM NiTi rotary files were used in the flexibility portion of the study. Initially, the 30 files were not subjected to any sterilizations or uses before initial testing and served as the control for all subsequent groups (group 1). The files were then repeatedly sterilized and tested through 10 simulated uses (groups 2–11). The relative flexibility of the Hyflex CM NiTi rotary files was determined by a 3-point bending test with a 10-mm span by using an Instron 5566 Universal Testing Machine. A custom polyvinylsiloxane jig was fabricated to standardize the placement of the files before testing (Fig. 1). The customized jig ensured the load cell of the Instron engaged the files at D8 throughout testing. The load necessary to maintain a constant feed rate of 1 mm/min was recorded every 0.1 seconds. To allow for repeated testing, files were not taken to failure. After flexibility testing, each file was used to instrument a single simulated canal following the manufacturer’s recommendation at 500 rpm, with a torque limiting value set at 2. Debris was removed from each file by using a sponge soaked in 70% isopropyl alcohol. Sterilization and subsequent testing were then accomplished as described above. Statistical Analysis Cutting efficiency data were analyzed by using multiple factor analysis of variance and the Dunnett post hoc test (P < .05). Flexibility data were initially analyzed by using the Levene’s Test of Equality of Error.
Flexibility data were further analyzed by using a general linear model (P < .05).
Results Cutting Efficiency When compared with the control (group 1), no statistically significant difference was noted for group 2 (P > .05). A statistically significant decrease in cutting efficiency was noted for groups 3 and 4 (P < .05). No statistically significant decrease in cutting efficiency was noted for groups 5, 6, and 7 (P > .05). A statistically significant decrease in cutting efficiency was noted for groups 8, 9, and 10 (P < .05) (Fig. 2). Flexibility When compared with the control (group 1), no statistically significant difference was noted for groups 2 and 3 (P > .05). A statistically significant decrease in flexibility was noted for groups 4, 5, and 6 (P < .05). No statistically significant decrease in flexibility was noted for group 7. A statistically significant decrease in flexibility was noted for groups 8, 9, 10, and 11 (P < .05) (Fig. 3).
Discussion To date, there have been many variations in cutting efficiency protocols. Studies have used substrates of human dentin, plastic blocks, bovine dentin, and bovine cortical bone to determine the relative cutting
Figure 3. Flexibility results. Groups are statistically different (P < .05) if they are designated by different letters.
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Hyflex CM Cutting Efficiency and Flexibility
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Basic Research—Technology efficiency of different files. Bovine cortical bone was chosen for this study because of its physical and chemical similarity to human dentin, while having the possibility of machining many samples from the same location. This allowed better standardization throughout testing and proved a very consistent model. Only a small standard of deviation was noted for all groups, and all samples followed the same trends throughout both the cutting efficiency and flexibility portions of the study. It was interesting that in both the cutting efficiency and flexibility portions of the study, there was a return to normalcy after an initial statistically significant decrease in each mechanical property. It is hypothesized that after repeated clinical use and sterilizations, there is a change in the austenite finishing temperature closer to clinical operating temperatures. This would cause a shift in the proportions of austenite and martensite NiTi microstructural phases, ultimately affecting the mechanical properties of these files. Further research is necessary to determine the effect that repeated clinical use and sterilizations have on the austenite finishing temperature of the Hyflex CM NiTi rotary files. Although a statistically significant decrease in both cutting efficiency and flexibility was noted, no file separation occurred throughout testing. In addition, all Hyflex CM NiTi rotary files continued to enlarge the simulated canal without exceeding the torque control limiter, and all files returned to their original shape after each sterilization. This is similar to results by Thompson et al (13), which showed no significant difference in Hyflex CM NiTi files’ ability to maintain original canal anatomy through 3 uses in resin blocks. It is necessary for the clinician to take into account all the variables present during clinical use when determining the ability of an instrument to be reused for subsequent instrumentation, because statistical significance may or may not directly correlate to clinical significance.
Conclusion Repeated simulated clinical use and sterilizations showed no statistically significant effect on the cutting efficiency of Hyflex CM NiTi rotary files after the first use and sterilization. No statistically significant effect on the flexibility of Hyflex CM NiTi rotary files was noted through 2 uses and 2 sterilizations. In both the cutting efficiency and flexibility
4
Seago et al.
studies, a return to normalcy was noted before another statistically significant decrease in mechanical behavior was noted.
Acknowledgments The authors thank Coltene/Whaledent for providing the files used in this study. The authors deny any conflicts of interest related to this study.
References 1. Kakehashi S, Stanley HR, Fitzgerald RJ. The effects of surgical exposures of dental pulps in germ-free and conventional laboratory rats. Oral Surg Oral Med Oral Pathol 1965;20:340–9. 2. Baumgartner JC, Falkler WA. Bacteria in the apical 5 mm of infected root canals. J Endod 1991;17:380–3. 3. Card S, Sigurdsson A, Orstavik D, Trope M. The effectiveness of increased apical enlargement in reducing intracanal bacteria. J Endod 2002;28:779–83. 4. Khademi A, Yazdizadeh M, Feizianfard M. Determination of the minimum instrumentation size for penetration of irrigants to the apical third of root canal systems. J Endod 2006;32:417–20. 5. Schilder H. Cleaning and shaping the root canal. Dent Clin North Am 1974;18: 269–96. 6. Wu M, Fan B, Wesselink P. Leakage along apical root fillings in curved root canals: part I—effects of apical transportation on seal of root fillings. J Endod 2000;26: 210–6. 7. Glosson CR, Haller RH, Dove SB, del Rio CE. A comparison of root canal preparations using Ni-Ti hand, Ni-Ti engine-driven, and K-Flex endodontic instruments. J Endod 1995;21:146–51. 8. Shen Y, Zhou H, Zheng Y, et al. Metallurgical characterization of controlled memory wire nickel-titanium rotary instruments. J Endod 2011;37:1566–71. 9. Testarelli L, Plotino G, Al-Sudani D, et al. Bending properties of a new nickeltitanium alloy with a lower percent by weight of nickel. J Endod 2011;37:1293–5. 10. Shen Y, Qian W, Abtin H, et al. Fatigue testing of controlled memory wire nickeltitanium rotary instruments. J Endod 2011;37:997–1001. 11. Rapisarda E, Bonaccorso A, Tripi TR, Condorelli GG. Effect of sterilization on the cutting efficiency of rotary nickel-titanium endodontic files. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1999;88:343–7. 12. Casper R, Roberts H, Roberts M, et al. Comparison of autoclaving effects on torsional deformation and fracture resistance of three innovative endodontic file systems. J Endod 2011;37:1572–5. 13. Thompson M, Sidow S, Lindsey K, et al. Evaluation of a new filing system’s ability to maintain canal morphology. J Endod 2013;40:867–70.
JOE — Volume -, Number -, - 2015
