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An in vitro study of the torsional properties of new and used rotary nickel-titanium files in plastic blocks
An in vitro study of the torsional properties of new and used rotary nickel-titanium files in plastic blocks Ghassan Yared, DDS, MSc,a and Gajanan K. Kulkarni, BDS, PhD,b Toronto, Ontario, Canada UNIVERSITY OF TORONTO
Objective. The purpose of this study was to compare torque (in grams per centimeter) and angle of rotation (in degrees) at the time of fracture of new and used nickel-titanium .04 ProFile rotary files. The relationship between the size of the instrument and the torque at fracture was also investigated. Study design. The torque and angle of rotation at fracture of new and used nickel-titanium .04 ProFile rotary files Nos. 15 to 40 were determined according to American National Standards Institute/American Dental Association Specification No. 28. Analysis of variance was used to compare the torque and angle of rotation at fracture of the different sizes of the new instruments and of new and used instruments of the same size (␣ ⫽ .05). The relationship between torque at fracture and the size of the instrument was determined by means of regression analysis. Results. The torque at fracture of the new instruments increased significantly with the diameter. The used instruments, Nos. 25 to 40, had significantly lower values of torque at fracture than did the new ones. There were no statistically significant differences between the angle of rotation at fracture of the new and the used instruments (P ⬎ .05). A stronger relationship was found between the size of the file and the torque at fracture for the new instruments (P ⬍ .0001) than for the used ones (P ⬍ .0001). Conclusion. The results of the present study suggest that the values of the torque at fracture of new instruments increased significantly with the diameter. The results also suggest that repeated use of ProFile .04 instruments mainly affected the torque at fracture. (Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2003;96:466-71)
Instrument separation is a serious concern in endodontic therapy. Several studies have evaluated the influence of various factors on the fracture of nickel-titanium (NiTi) endodontic rotary instruments.1-6 It is important for the clinician to have detailed research information to provide a rational basis for instrument selection. According to American National Standards Institute/ American Dental Association Specification No. 28, the torsional properties of endodontic instruments can be evaluated as the torque and angle of rotation required to cause instrument fracture.7 Previous studies have investigated the torsional properties of various brands of NiTi endodontic rotary instruments. Sattapan et al8 evaluated the torque at fracture of Quantec Series 2000 rotary NiTi instruments (Tycom Corp, Irvine, Calif). The instruments fractured at torque values varying from 23 gcm to 200.2 gcm. The torque at fracture of Lightspeed instruments (Lightspeed Technology, Inc, San Antonio, Tex) ranged from 20 g/cm to 427.92 g/cm. a
Assistant Professor and Director of Undergraduate Program of Endodontics, Department of Endodontics, Faculty of Dentistry, University of Toronto. b Assistant Professor, Department of Paediatric Dentistry, Faculty of Dentistry, University of Toronto. Received for publication Jun 28, 2002; returned for revision Oct 8, 2002; accepted for publication Nov 8, 2002. © 2003, Mosby, Inc. All rights reserved. 1079-2104/2003/$30.00 ⫹ 0 doi:10.1016/S1079-2104(03)91706-3
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The angle of rotation at fracture varied from 637.2° to 1710°.9 Silvaggio and Hicks2 investigated the torsional properties of NiTi Profile .04 Series 29 instruments (Tulsa Dentsply, Tulsa, Okla). Peters and Barbakow10 tested selected NiTi ProFile .04 rotary instruments. The torque and angle of rotation at fracture for Nos. 15, 35, and 60 files ranged from 36.6 g/cm to 323 g/cm and from 514.3° (No. 60) to 614.1° (No. 35). Instrument fracture results from cyclic fatigue or torsional stress.11 Fracture from cyclic fatigue is more likely to occur when instruments are rotated in root canals with abrupt apical curvatures.1 Although recent studies have shown that cyclic fatigue is not the main mode of NiTi rotary instrument fracture,10-13 instruments can lock into the canal3 and be subjected to high levels of torsional stress, leading to deformation and fracture. Sattapan et al8 showed that the torque at fracture was significantly higher than the torque during instrumentation. In their study, each set of instruments was used to prepare only 1 canal. Questions have since been raised about whether the repeated use of NiTi rotary instruments adversely affect their torsional properties and render them more prone to torsional fracture. Sattapan et al8 compared the torque at fracture of new Quantec Series 2000 rotary NiTi instruments (Tycom Corp) with the torque generated during root canal preparation. Peters and Barbakow10 determined the torque and angle of rotation at fracture for only 3 sizes of NiTi ProFile .04 instruments. The determination of
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Fig 1. Reversible gear motor (left) and torque-measuring device (right).
torque at the fracture of new NiTi rotary instruments can be helpful for future comparative studies on the torsional properties. This study was undertaken to compare torque (in grams per centimeter) and angle of rotation (in degrees) at the fracture of new and used NiTi .04 ProFile rotary instruments. The relationship between the size of the instrument and the torque at fracture was also investigated.
of the tip of the instrument in the chuck (Fig 2). Analysis of variance was used to compare the torque and angle of rotation at fracture among the different sizes of the new instruments. Pairwise comparisons were performed with the Duncan multiple range test to detect significant differences. The relationship between the size of the instrument and the torque at fracture was also analyzed with regression analysis. The significance was determined at the 95% confidence level.
MATERIAL AND METHODS Evaluation of new instruments NiTi .04 ProFile rotary instruments (Tulsa Dentsply), Nos. 15 to 40, were evaluated. Thirty new instruments of each size were tested for resistance to fracture by twisting according to ANSI/ADA Specification No. 28.7 Two parameters were measured: torque (in grams per centimeter) at fracture and angle of rotation (in degrees) at fracture during clockwise rotation. A digital torque meter memocouple (A-Tech Instruments Limited, Scarborough, Ontario, Canada; Fig 1) was used to measure torque, with an accuracy of ⫾1 g/cm, and angle of rotation, with an accuracy of ⫾2°. Before testing, each instrument’s handle was removed with a suitable wire cutter at the point where the handle is attached to the instrument shaft.7 The shaft end was clamped in a chuck connected to a reversible geared motor revolving at 2 rpm (Aerotech, Pittsburgh, Pa). A digital display amplifier controlled the operation of the motor. Three millimeters of the tip of the instrument were clamped in another chuck with brass jaws connected to the digital torque meter memocouple and to a computer for the recording of measurements with LabView software (National Instruments, Austin, Tex). A jig was constructed to ensure reproducible positioning
Evaluation of used instruments Thirty sets of new NiTi .04 ProFile rotary instruments, Nos. 15 to 40, were prepared. Each set of instruments was used with a 6:1 reduction handpiece powered by a Micro-Mega 324 air motor (Micro-Mega, Besanc¸ on, France) to prepare 5 endodontic resin blocks (Dentsply Maillefer, Ballaigues, Switzerland) with curved canals in the crown-down position at 170 rpm. Canals were irrigated by using 2.5% NaOCl and a 27-g syringe. The canal preparations were done by the same operator experienced in the technique (G.Y.). The instruments were used according to the following criteria: ● The apical pressure exerted on each instrument was light (described as the minimum pressure required to fracture the lead of a sharpened pencil). ● Five to 7 small in-and-out movements (3-5 mm) were used with each instrument before the operator switched to the next smaller instrument. ● Four to 5 recapitulations (waves) with ProFile No. 15 to 40 instruments were required to complete canal preparation. ● The canal preparation was considered complete when a No. 30 ProFile reached the working length of 17 mm.
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Fig 2. Jig used to determine in a reproducible manner the clamping point (3 mm from the tip of the instrument).
The ProFile sets were sterilized before each use by steam autoclaving at 135°C for 5 minutes. The entire sterilization cycle lasted 35 minutes. ● The instruments were inspected for deformation with 2.5⫻ magnification after each passage in the canal and before testing. ● The torque and angle of rotation at fracture of the instruments of the different sizes were determined as for the new instruments. Analysis of variance was used to compare the torque and angle of rotation at fracture of the different-sized new and used instruments. Pairwise comparisons were performed with the Duncan multiple range test to detect significant differences between instruments of the same size. The relationship between the torque at fracture and the size of the instrument was determined by means of regression analysis. Significance was determined at the 95% confidence level. ●
RESULTS The mean torque at fracture and the SD for the new and used instruments are reported in Table I. The means between the different sizes of the new instruments were significantly different (P ⬍ .0001; Nos. 20-25: P ⫽ .0011; Nos. 25-30: P ⫽ .0064). Between new and used instruments, for the same size of instrument, the means were significantly different for Nos. 25 (P ⫽ .0004), 30, 35, and 40 (P ⬍ .0001).
The mean angle of rotation at fracture and the standard deviation for the new and used instruments are reported in Table II. The means between the different sizes of the new instruments were significantly different (P ⬍ .0001; Nos. 20-25: P ⫽ .0003; Nos. 25-40: P ⫽ .0002) except for Nos. 25-30, 25-35, 25-40, 30-35, 30-40, and 35-40. Between new and used instruments, for the same size of instrument, the means were not significantly different (P ⬎ .05). A stronger relationship was found between the size of the file and the torque at fracture for the new instruments (r2 ⫽ 0.86; P ⬍ .0001) than for the used instruments (r2 ⫽ 0.75; P ⬍ .0001). Figures 3 and 4 illustrate the relationship between the torque at fracture and the size of new and used instruments, respectively. DISCUSSION The angle of rotation and torque provide valuable information about the torsional fracture when an instrument binding at the tip of the root canal is rotated. The torsional properties of NiTi rotary instruments can be investigated in terms of ANSI/ADA Specification No. 28.7 Sattapan et al8 considered ANSI/ADA Specification No. 28 inappropriate for NiTi rotary instruments because the tests are done in a static mode. However, recent studies have shown that NiTi rotary instrument fracture is attributable to mainly torsional stress.10,11 When an instrument binding at the tip of a canal is
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Table I. Mean torque at fracture (in g/cm) and SD Instrument type New Used
No. 15
No. 20
No. 25
No. 30
No. 35
No. 40
37.60 (SD ⫽ 11.52) 34.30 (SD ⫽ 9.29)
63.47 (SD ⫽ 7.01) 58.92 (SD ⫽ 7.96)
73.31 (SD ⫽ 6.84) 62.56 (SD ⫽ 9.56)
81.52 (SD ⫽ 13.69) 65.38 (SD ⫽ 17.93)
109.16 (SD ⫽ 10.26) 94.29 (SD ⫽ 9.01)
124.46 (SD ⫽ 11.69) 112.77 (SD ⫽ 17.54)
Table II. Mean angle of rotation at fracture (in degrees) and SD Instrument type New Used
No. 15
No. 20
No. 25
No. 30
No. 35
No. 40
617.97 (SD ⫽ 97.07) 586.63 (SD ⫽ 112.47)
771.80 (SD ⫽ 92.60 691.60 (SD ⫽ 76.32)
805.69 (SD ⫽ 33.58) 852.29 (SD ⫽ 69.57)
963.70 (SD ⫽ 157.56) 896.35 (SD ⫽ 153.85)
973.70 (SD ⫽ 199.37) 900.91 (SD ⫽ 235.83)
1043.28 (SD ⫽ 143.73) 984.92 (SD ⫽ 168.28)
Fig 3. Regression analysis with new instruments.
rotated, the tip will be subjected to high levels of torsional stress. Instrument fracture will occur when the torsional stress at the point of fracture becomes higher than the torque at fracture. The test, according to ANSI/ ADA Specification No. 28, simulates an instrument binding at 3 mm from the tip of the root canal and is therefore suitable to analyze the torsional properties of rotary instruments. The tests in the present study were conducted at 2 rpm, according to ANSI/ADA Specification No. 28, whereas in a clinical setting, the NiTi rotary instruments are used at a minimum speed of 150 rpm. As a general rule, in a torsional loading model, the torque is independent of the twisting rate (rotational speed).14 Torque at fracture increased significantly with the diameter of the new instruments. Although similar findings have been reported in previous studies,2,8,9 Peters and Barbakow10 compared ProFile .04 (Nos. 20, 35, and 60) and reported a higher torque at fracture for the Profile No. 35 instrument. The difference may be at-
tributable to the manufacturing processes of the tested instruments. However, Peters and Barbakow10 did not indicate the origin of the investigated instruments (ie, Dentsply Maillefer or Tulsa Dentsply). In the present study, instruments manufactured by Tulsa Dentsply were evaluated. Variations in the sample size and in the methodology could also have contributed to differences in the results. In the present study, 30 instruments of each size were compared, whereas only 8 were compared in the study by Peters and Barbakow.10 In addition, in our study a jig was constructed to ensure precise positioning of the instrument tip in the chuck. In a preliminary study, the positioning of the instrument tip in the chuck without the use of the jig was inconsistent. The clamped length ranged from 2 to 5 mm. An instrument clamped at 5 mm from the tip would have a higher torque at fracture compared with an instrument clamped at 3 mm. The ProFile .04 instruments were used in endodontic
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Fig 4. Regression analysis with used instruments.
resin blocks to simulate repeated clinical use. The resin blocks, in comparison with extracted teeth, reduced variations in the instrumentation technique by limiting the variability of parameters such as canal length and width, canal anatomy, angle of curvature, and the radius of curvature. Resin blocks do not simulate the use of files in dentin; consequently, these results should be interpreted with caution. Different types of motors are used in conjunction with NiTi instrumentation. Air motors, which are widely used, do not allow torque control, and, furthermore, a variation in air pressure could affect the rotational speed and torque. For instance, a drop in air pressure would lead to a decrease of torque. The instrument would become less active, and the operator would tend to force the instrument into the canal, subjecting the instrument to higher stress levels. Recently, a new generation of low-torque electric control motors has been introduced. These motors take into consideration the low values of torque at failure of NiTi rotary instruments. Electric motors that would have delivered more consistent torque and revolutions per minute could have been used in the present study instead of air motors. In a pilot study, we tried unsuccessfully to standardize the use of instruments (ie, in frequency and depth of insertions and the frequency of recapitulations). This attempt at standardization led to noticeable variation in the amount of pressure applied on the instruments by the operator. In other words, it was impossible to standardize the actual wear on the instrument in the present study. We did attempt to limit wear variations by standardizing the master apical file size, in addition to standardizing, as much as possible, the frequency and depth of instrument penetration and the frequency of recapitulations.
In a previous study,13 we noted that deformations went undetected if magnification was not used. The instruments were inspected for deformation at 2.5⫻ magnification after each passage in the canal and before testing. When in doubt about deformation, the operator used as controls both brand new and deformed (unwound) instruments of the same size and taper as the instrument being inspected. None of the used instruments was deformed or damaged. The use of a light microscope could have decreased the incidence of false negatives (undetected deformations). The used instruments, Nos. 25 to 40, had lower torque at failure, a significant difference compared with the new ones. An SEM study of NiTi endodontic rotary instruments revealed a high incidence of surface defects where cracks are usually initiated.15 Cyclic fatigue, flexural or torsional, caused by the use of the instruments in a curved canal and by the repeated locking of the instruments in the canal3 could have facilitated the initiation and propagation of a crack15 and, therefore, affected the torque at fracture of the instruments. Peters and Barbakow10 demonstrated that higher torque levels were generated in canals in resin blocks than in canals in extracted teeth. The use of resin blocks probably subjected the instruments to higher levels of torsional stress, contributing to a higher incidence of crack initiation and propagation and lower values of torque at fracture for the used instruments than for the new ones. The canals in the endodontic resin blocks were of size 20 at the working length. Consequently, instrument Nos. 15 and 20 were not subjected to high levels of stress during root canal preparation, limiting crack formation. Thus, torque at fracture of used No. 15 and 20 instruments did not change significantly. Our regression analysis, in accordance with the results of Camps and Pertot,16 revealed a linear relation-
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ship between torque at fracture and instrument diameter (r2 ⫽ 0.86). The data of Marsicovetere et al9 suggest an exponential relation. Direct comparisons cannot be made because different instruments were investigated in the 3 studies. A weaker relationship observed with the used instruments (r2 ⫽ 0.75) than with the new instruments (r2 ⫽ 0.86) reflected the effect of repeated use on the torsional properties. A trend toward a higher angle of rotation at fracture was observed with the larger new instruments. This finding was in agreement with data from a previous study2 and in contradiction to other documented results.9,10 In addition, Peters and Barbakow10 evaluated ProFile .04 instruments, Nos. 20 and 35, and reported lower angle of rotation values at fracture than were found in the present study. Differences among the studies could be attributed to variations in the type of instrument, sample size, and methodology. There was also a trend toward a lower angle of rotation at fracture for the used instruments than for the new ones. Interestingly, the differences were not statistically significant. Therefore, it seemed that torsional and flexural stress did not significantly affect the angle of rotation at fracture. CONCLUSIONS The results of the present study suggest that the torque at fracture of new instruments increased significantly with the diameter. The used instruments, Nos. 25 to 40, had lower torque at fracture than did the new ones. The results also suggest that repeated use of ProFile .04 instruments in resin blocks affected mainly the torque at fracture.
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speed, torque and operator’s proficiency on ProFile failures. Int Endod J 2001;34:47-53. Barbakow F, Lutz F. The Lightspeed preparation technique evaluated by Swiss clinicians after attending continuing education courses. Int Endod J 2001;34:47-53. Mandel E, Adib-Yazdi M, Benhamou L-M, Lachkar T, Mesgouez T, Sobel M. Rotary Ni-Ti ProFile systems for preparing curved canals in resin blocks: influence of operator on instrument breakage. Int Endod J 1999;32:436-43. Yared GM, Bou Dagher FE, Machtou P. Failure of ProFile instruments used with high and low torque motors. Int Endod J 2001;34:471-5. ANSI/ADA. Specification No. 28 for root canal files and reamers, Type K. New York: American National Standards Institute; 1988. Sattapan B, Palamara JEA, Messer HH. Torque during canal instrumentation using rotary nickel-titanium files. J Endod 2000; 26:156-60. Marsicovetere ES, Burgess JO, Clement DJ, del Rio CE. Torsional testing of the Lightspeed nickel-titanium instrument system. J Endod 1996;22:681-4. Peters OA, Barbakow F. Dynamic torque and apical forces of ProFile .04 rotary instruments during preparation of curved canals. Int Endod J 2002;35:379-89. Sattapan B, Nervo GJ, Palamara JE, Messer HH. Defects in rotary nickel-titanium files after clinical use. J Endod 2000;26: 161-5. Yared GM, Bou Dagher FE, Machtou P. Cyclic fatigue of ProFile rotary instruments after simulated clinical use. Int Endod J 1999;32:115-9. Yared GM, Bou Dagher FE, Machtou P. Cyclic fatigue of ProFile rotary instruments after clinical use. Int Endod J 2000;33: 204-7. Bailey J. Fundamental aspects of torsional loading. In: Mechanical testing. 9th edition, Volume 8. American Society for Metals: Ohio; 1985. p. 140-4. Kuhn G, Tavernier B, Jordan L. Influence of structure on nickeltitanium endodontic instruments failure. J Endod 2001;27:51620. Camps J, Pertot WJ. Torsional and stiffness properties of Canal Master U stainless steel and nitinol instruments. J Endod 1994; 20:395-8.
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REFERENCES 1. Pruett JP, Clement DJ, Carnes DL. Cyclic fatigue of nickeltitanium endodontic instruments. J Endod 1997;23:77-85. 2. Silvaggio J, Hicks ML. Effect of heat sterilization on the torsional properties of rotary nickel-titanium endodontic files. J Endod 1997;23:731-4. 3. Yared GM, Bou Dagher FE, Machtou P. Influence of rotational
Ghassan Yared, DDS, MSc Department of Endodontics (Room 348) Faculty of Dentistry University of Toronto Toronto, ON M5G 1G6, Canada [email protected]
Objective. The purpose of this study was to compare torque (in grams per centimeter) and angle of rotation (in degrees) at the time of fracture of new and used nickel-titanium .04 ProFile rotary files. The relationship between the size of the instrument and the torque at fracture was also investigated. Study design. The torque and angle of rotation at fracture of new and used nickel-titanium .04 ProFile rotary files Nos. 15 to 40 were determined according to American National Standards Institute/American Dental Association Specification No. 28. Analysis of variance was used to compare the torque and angle of rotation at fracture of the different sizes of the new instruments and of new and used instruments of the same size (␣ ⫽ .05). The relationship between torque at fracture and the size of the instrument was determined by means of regression analysis. Results. The torque at fracture of the new instruments increased significantly with the diameter. The used instruments, Nos. 25 to 40, had significantly lower values of torque at fracture than did the new ones. There were no statistically significant differences between the angle of rotation at fracture of the new and the used instruments (P ⬎ .05). A stronger relationship was found between the size of the file and the torque at fracture for the new instruments (P ⬍ .0001) than for the used ones (P ⬍ .0001). Conclusion. The results of the present study suggest that the values of the torque at fracture of new instruments increased significantly with the diameter. The results also suggest that repeated use of ProFile .04 instruments mainly affected the torque at fracture. (Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2003;96:466-71)
Instrument separation is a serious concern in endodontic therapy. Several studies have evaluated the influence of various factors on the fracture of nickel-titanium (NiTi) endodontic rotary instruments.1-6 It is important for the clinician to have detailed research information to provide a rational basis for instrument selection. According to American National Standards Institute/ American Dental Association Specification No. 28, the torsional properties of endodontic instruments can be evaluated as the torque and angle of rotation required to cause instrument fracture.7 Previous studies have investigated the torsional properties of various brands of NiTi endodontic rotary instruments. Sattapan et al8 evaluated the torque at fracture of Quantec Series 2000 rotary NiTi instruments (Tycom Corp, Irvine, Calif). The instruments fractured at torque values varying from 23 gcm to 200.2 gcm. The torque at fracture of Lightspeed instruments (Lightspeed Technology, Inc, San Antonio, Tex) ranged from 20 g/cm to 427.92 g/cm. a
Assistant Professor and Director of Undergraduate Program of Endodontics, Department of Endodontics, Faculty of Dentistry, University of Toronto. b Assistant Professor, Department of Paediatric Dentistry, Faculty of Dentistry, University of Toronto. Received for publication Jun 28, 2002; returned for revision Oct 8, 2002; accepted for publication Nov 8, 2002. © 2003, Mosby, Inc. All rights reserved. 1079-2104/2003/$30.00 ⫹ 0 doi:10.1016/S1079-2104(03)91706-3
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The angle of rotation at fracture varied from 637.2° to 1710°.9 Silvaggio and Hicks2 investigated the torsional properties of NiTi Profile .04 Series 29 instruments (Tulsa Dentsply, Tulsa, Okla). Peters and Barbakow10 tested selected NiTi ProFile .04 rotary instruments. The torque and angle of rotation at fracture for Nos. 15, 35, and 60 files ranged from 36.6 g/cm to 323 g/cm and from 514.3° (No. 60) to 614.1° (No. 35). Instrument fracture results from cyclic fatigue or torsional stress.11 Fracture from cyclic fatigue is more likely to occur when instruments are rotated in root canals with abrupt apical curvatures.1 Although recent studies have shown that cyclic fatigue is not the main mode of NiTi rotary instrument fracture,10-13 instruments can lock into the canal3 and be subjected to high levels of torsional stress, leading to deformation and fracture. Sattapan et al8 showed that the torque at fracture was significantly higher than the torque during instrumentation. In their study, each set of instruments was used to prepare only 1 canal. Questions have since been raised about whether the repeated use of NiTi rotary instruments adversely affect their torsional properties and render them more prone to torsional fracture. Sattapan et al8 compared the torque at fracture of new Quantec Series 2000 rotary NiTi instruments (Tycom Corp) with the torque generated during root canal preparation. Peters and Barbakow10 determined the torque and angle of rotation at fracture for only 3 sizes of NiTi ProFile .04 instruments. The determination of
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Fig 1. Reversible gear motor (left) and torque-measuring device (right).
torque at the fracture of new NiTi rotary instruments can be helpful for future comparative studies on the torsional properties. This study was undertaken to compare torque (in grams per centimeter) and angle of rotation (in degrees) at the fracture of new and used NiTi .04 ProFile rotary instruments. The relationship between the size of the instrument and the torque at fracture was also investigated.
of the tip of the instrument in the chuck (Fig 2). Analysis of variance was used to compare the torque and angle of rotation at fracture among the different sizes of the new instruments. Pairwise comparisons were performed with the Duncan multiple range test to detect significant differences. The relationship between the size of the instrument and the torque at fracture was also analyzed with regression analysis. The significance was determined at the 95% confidence level.
MATERIAL AND METHODS Evaluation of new instruments NiTi .04 ProFile rotary instruments (Tulsa Dentsply), Nos. 15 to 40, were evaluated. Thirty new instruments of each size were tested for resistance to fracture by twisting according to ANSI/ADA Specification No. 28.7 Two parameters were measured: torque (in grams per centimeter) at fracture and angle of rotation (in degrees) at fracture during clockwise rotation. A digital torque meter memocouple (A-Tech Instruments Limited, Scarborough, Ontario, Canada; Fig 1) was used to measure torque, with an accuracy of ⫾1 g/cm, and angle of rotation, with an accuracy of ⫾2°. Before testing, each instrument’s handle was removed with a suitable wire cutter at the point where the handle is attached to the instrument shaft.7 The shaft end was clamped in a chuck connected to a reversible geared motor revolving at 2 rpm (Aerotech, Pittsburgh, Pa). A digital display amplifier controlled the operation of the motor. Three millimeters of the tip of the instrument were clamped in another chuck with brass jaws connected to the digital torque meter memocouple and to a computer for the recording of measurements with LabView software (National Instruments, Austin, Tex). A jig was constructed to ensure reproducible positioning
Evaluation of used instruments Thirty sets of new NiTi .04 ProFile rotary instruments, Nos. 15 to 40, were prepared. Each set of instruments was used with a 6:1 reduction handpiece powered by a Micro-Mega 324 air motor (Micro-Mega, Besanc¸ on, France) to prepare 5 endodontic resin blocks (Dentsply Maillefer, Ballaigues, Switzerland) with curved canals in the crown-down position at 170 rpm. Canals were irrigated by using 2.5% NaOCl and a 27-g syringe. The canal preparations were done by the same operator experienced in the technique (G.Y.). The instruments were used according to the following criteria: ● The apical pressure exerted on each instrument was light (described as the minimum pressure required to fracture the lead of a sharpened pencil). ● Five to 7 small in-and-out movements (3-5 mm) were used with each instrument before the operator switched to the next smaller instrument. ● Four to 5 recapitulations (waves) with ProFile No. 15 to 40 instruments were required to complete canal preparation. ● The canal preparation was considered complete when a No. 30 ProFile reached the working length of 17 mm.
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Fig 2. Jig used to determine in a reproducible manner the clamping point (3 mm from the tip of the instrument).
The ProFile sets were sterilized before each use by steam autoclaving at 135°C for 5 minutes. The entire sterilization cycle lasted 35 minutes. ● The instruments were inspected for deformation with 2.5⫻ magnification after each passage in the canal and before testing. ● The torque and angle of rotation at fracture of the instruments of the different sizes were determined as for the new instruments. Analysis of variance was used to compare the torque and angle of rotation at fracture of the different-sized new and used instruments. Pairwise comparisons were performed with the Duncan multiple range test to detect significant differences between instruments of the same size. The relationship between the torque at fracture and the size of the instrument was determined by means of regression analysis. Significance was determined at the 95% confidence level. ●
RESULTS The mean torque at fracture and the SD for the new and used instruments are reported in Table I. The means between the different sizes of the new instruments were significantly different (P ⬍ .0001; Nos. 20-25: P ⫽ .0011; Nos. 25-30: P ⫽ .0064). Between new and used instruments, for the same size of instrument, the means were significantly different for Nos. 25 (P ⫽ .0004), 30, 35, and 40 (P ⬍ .0001).
The mean angle of rotation at fracture and the standard deviation for the new and used instruments are reported in Table II. The means between the different sizes of the new instruments were significantly different (P ⬍ .0001; Nos. 20-25: P ⫽ .0003; Nos. 25-40: P ⫽ .0002) except for Nos. 25-30, 25-35, 25-40, 30-35, 30-40, and 35-40. Between new and used instruments, for the same size of instrument, the means were not significantly different (P ⬎ .05). A stronger relationship was found between the size of the file and the torque at fracture for the new instruments (r2 ⫽ 0.86; P ⬍ .0001) than for the used instruments (r2 ⫽ 0.75; P ⬍ .0001). Figures 3 and 4 illustrate the relationship between the torque at fracture and the size of new and used instruments, respectively. DISCUSSION The angle of rotation and torque provide valuable information about the torsional fracture when an instrument binding at the tip of the root canal is rotated. The torsional properties of NiTi rotary instruments can be investigated in terms of ANSI/ADA Specification No. 28.7 Sattapan et al8 considered ANSI/ADA Specification No. 28 inappropriate for NiTi rotary instruments because the tests are done in a static mode. However, recent studies have shown that NiTi rotary instrument fracture is attributable to mainly torsional stress.10,11 When an instrument binding at the tip of a canal is
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Table I. Mean torque at fracture (in g/cm) and SD Instrument type New Used
No. 15
No. 20
No. 25
No. 30
No. 35
No. 40
37.60 (SD ⫽ 11.52) 34.30 (SD ⫽ 9.29)
63.47 (SD ⫽ 7.01) 58.92 (SD ⫽ 7.96)
73.31 (SD ⫽ 6.84) 62.56 (SD ⫽ 9.56)
81.52 (SD ⫽ 13.69) 65.38 (SD ⫽ 17.93)
109.16 (SD ⫽ 10.26) 94.29 (SD ⫽ 9.01)
124.46 (SD ⫽ 11.69) 112.77 (SD ⫽ 17.54)
Table II. Mean angle of rotation at fracture (in degrees) and SD Instrument type New Used
No. 15
No. 20
No. 25
No. 30
No. 35
No. 40
617.97 (SD ⫽ 97.07) 586.63 (SD ⫽ 112.47)
771.80 (SD ⫽ 92.60 691.60 (SD ⫽ 76.32)
805.69 (SD ⫽ 33.58) 852.29 (SD ⫽ 69.57)
963.70 (SD ⫽ 157.56) 896.35 (SD ⫽ 153.85)
973.70 (SD ⫽ 199.37) 900.91 (SD ⫽ 235.83)
1043.28 (SD ⫽ 143.73) 984.92 (SD ⫽ 168.28)
Fig 3. Regression analysis with new instruments.
rotated, the tip will be subjected to high levels of torsional stress. Instrument fracture will occur when the torsional stress at the point of fracture becomes higher than the torque at fracture. The test, according to ANSI/ ADA Specification No. 28, simulates an instrument binding at 3 mm from the tip of the root canal and is therefore suitable to analyze the torsional properties of rotary instruments. The tests in the present study were conducted at 2 rpm, according to ANSI/ADA Specification No. 28, whereas in a clinical setting, the NiTi rotary instruments are used at a minimum speed of 150 rpm. As a general rule, in a torsional loading model, the torque is independent of the twisting rate (rotational speed).14 Torque at fracture increased significantly with the diameter of the new instruments. Although similar findings have been reported in previous studies,2,8,9 Peters and Barbakow10 compared ProFile .04 (Nos. 20, 35, and 60) and reported a higher torque at fracture for the Profile No. 35 instrument. The difference may be at-
tributable to the manufacturing processes of the tested instruments. However, Peters and Barbakow10 did not indicate the origin of the investigated instruments (ie, Dentsply Maillefer or Tulsa Dentsply). In the present study, instruments manufactured by Tulsa Dentsply were evaluated. Variations in the sample size and in the methodology could also have contributed to differences in the results. In the present study, 30 instruments of each size were compared, whereas only 8 were compared in the study by Peters and Barbakow.10 In addition, in our study a jig was constructed to ensure precise positioning of the instrument tip in the chuck. In a preliminary study, the positioning of the instrument tip in the chuck without the use of the jig was inconsistent. The clamped length ranged from 2 to 5 mm. An instrument clamped at 5 mm from the tip would have a higher torque at fracture compared with an instrument clamped at 3 mm. The ProFile .04 instruments were used in endodontic
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Fig 4. Regression analysis with used instruments.
resin blocks to simulate repeated clinical use. The resin blocks, in comparison with extracted teeth, reduced variations in the instrumentation technique by limiting the variability of parameters such as canal length and width, canal anatomy, angle of curvature, and the radius of curvature. Resin blocks do not simulate the use of files in dentin; consequently, these results should be interpreted with caution. Different types of motors are used in conjunction with NiTi instrumentation. Air motors, which are widely used, do not allow torque control, and, furthermore, a variation in air pressure could affect the rotational speed and torque. For instance, a drop in air pressure would lead to a decrease of torque. The instrument would become less active, and the operator would tend to force the instrument into the canal, subjecting the instrument to higher stress levels. Recently, a new generation of low-torque electric control motors has been introduced. These motors take into consideration the low values of torque at failure of NiTi rotary instruments. Electric motors that would have delivered more consistent torque and revolutions per minute could have been used in the present study instead of air motors. In a pilot study, we tried unsuccessfully to standardize the use of instruments (ie, in frequency and depth of insertions and the frequency of recapitulations). This attempt at standardization led to noticeable variation in the amount of pressure applied on the instruments by the operator. In other words, it was impossible to standardize the actual wear on the instrument in the present study. We did attempt to limit wear variations by standardizing the master apical file size, in addition to standardizing, as much as possible, the frequency and depth of instrument penetration and the frequency of recapitulations.
In a previous study,13 we noted that deformations went undetected if magnification was not used. The instruments were inspected for deformation at 2.5⫻ magnification after each passage in the canal and before testing. When in doubt about deformation, the operator used as controls both brand new and deformed (unwound) instruments of the same size and taper as the instrument being inspected. None of the used instruments was deformed or damaged. The use of a light microscope could have decreased the incidence of false negatives (undetected deformations). The used instruments, Nos. 25 to 40, had lower torque at failure, a significant difference compared with the new ones. An SEM study of NiTi endodontic rotary instruments revealed a high incidence of surface defects where cracks are usually initiated.15 Cyclic fatigue, flexural or torsional, caused by the use of the instruments in a curved canal and by the repeated locking of the instruments in the canal3 could have facilitated the initiation and propagation of a crack15 and, therefore, affected the torque at fracture of the instruments. Peters and Barbakow10 demonstrated that higher torque levels were generated in canals in resin blocks than in canals in extracted teeth. The use of resin blocks probably subjected the instruments to higher levels of torsional stress, contributing to a higher incidence of crack initiation and propagation and lower values of torque at fracture for the used instruments than for the new ones. The canals in the endodontic resin blocks were of size 20 at the working length. Consequently, instrument Nos. 15 and 20 were not subjected to high levels of stress during root canal preparation, limiting crack formation. Thus, torque at fracture of used No. 15 and 20 instruments did not change significantly. Our regression analysis, in accordance with the results of Camps and Pertot,16 revealed a linear relation-
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ship between torque at fracture and instrument diameter (r2 ⫽ 0.86). The data of Marsicovetere et al9 suggest an exponential relation. Direct comparisons cannot be made because different instruments were investigated in the 3 studies. A weaker relationship observed with the used instruments (r2 ⫽ 0.75) than with the new instruments (r2 ⫽ 0.86) reflected the effect of repeated use on the torsional properties. A trend toward a higher angle of rotation at fracture was observed with the larger new instruments. This finding was in agreement with data from a previous study2 and in contradiction to other documented results.9,10 In addition, Peters and Barbakow10 evaluated ProFile .04 instruments, Nos. 20 and 35, and reported lower angle of rotation values at fracture than were found in the present study. Differences among the studies could be attributed to variations in the type of instrument, sample size, and methodology. There was also a trend toward a lower angle of rotation at fracture for the used instruments than for the new ones. Interestingly, the differences were not statistically significant. Therefore, it seemed that torsional and flexural stress did not significantly affect the angle of rotation at fracture. CONCLUSIONS The results of the present study suggest that the torque at fracture of new instruments increased significantly with the diameter. The used instruments, Nos. 25 to 40, had lower torque at fracture than did the new ones. The results also suggest that repeated use of ProFile .04 instruments in resin blocks affected mainly the torque at fracture.
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speed, torque and operator’s proficiency on ProFile failures. Int Endod J 2001;34:47-53. Barbakow F, Lutz F. The Lightspeed preparation technique evaluated by Swiss clinicians after attending continuing education courses. Int Endod J 2001;34:47-53. Mandel E, Adib-Yazdi M, Benhamou L-M, Lachkar T, Mesgouez T, Sobel M. Rotary Ni-Ti ProFile systems for preparing curved canals in resin blocks: influence of operator on instrument breakage. Int Endod J 1999;32:436-43. Yared GM, Bou Dagher FE, Machtou P. Failure of ProFile instruments used with high and low torque motors. Int Endod J 2001;34:471-5. ANSI/ADA. Specification No. 28 for root canal files and reamers, Type K. New York: American National Standards Institute; 1988. Sattapan B, Palamara JEA, Messer HH. Torque during canal instrumentation using rotary nickel-titanium files. J Endod 2000; 26:156-60. Marsicovetere ES, Burgess JO, Clement DJ, del Rio CE. Torsional testing of the Lightspeed nickel-titanium instrument system. J Endod 1996;22:681-4. Peters OA, Barbakow F. Dynamic torque and apical forces of ProFile .04 rotary instruments during preparation of curved canals. Int Endod J 2002;35:379-89. Sattapan B, Nervo GJ, Palamara JE, Messer HH. Defects in rotary nickel-titanium files after clinical use. J Endod 2000;26: 161-5. Yared GM, Bou Dagher FE, Machtou P. Cyclic fatigue of ProFile rotary instruments after simulated clinical use. Int Endod J 1999;32:115-9. Yared GM, Bou Dagher FE, Machtou P. Cyclic fatigue of ProFile rotary instruments after clinical use. Int Endod J 2000;33: 204-7. Bailey J. Fundamental aspects of torsional loading. In: Mechanical testing. 9th edition, Volume 8. American Society for Metals: Ohio; 1985. p. 140-4. Kuhn G, Tavernier B, Jordan L. Influence of structure on nickeltitanium endodontic instruments failure. J Endod 2001;27:51620. Camps J, Pertot WJ. Torsional and stiffness properties of Canal Master U stainless steel and nitinol instruments. J Endod 1994; 20:395-8.
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REFERENCES 1. Pruett JP, Clement DJ, Carnes DL. Cyclic fatigue of nickeltitanium endodontic instruments. J Endod 1997;23:77-85. 2. Silvaggio J, Hicks ML. Effect of heat sterilization on the torsional properties of rotary nickel-titanium endodontic files. J Endod 1997;23:731-4. 3. Yared GM, Bou Dagher FE, Machtou P. Influence of rotational
Ghassan Yared, DDS, MSc Department of Endodontics (Room 348) Faculty of Dentistry University of Toronto Toronto, ON M5G 1G6, Canada [email protected]