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A Method to Assess Rotary Nickel-Titanium Files
JOURNAL OF ENDODONTICS Copyright © 2000 by The American Association of Endodontists
Printed in U.S.A. VOL. 26, NO. 9, SEPTEMBER 2000
A Method to Assess Rotary Nickel-Titanium Files Timothy A. Svec, DDS, MS, and John M. Powers, PhD
curvature, size of canal, and beginning of curved portion of canal. For the instrument, these parameters were: load, rate of loading (rate of penetration), depth of penetration, and rate of rotation. Then it was determined what effect these conditions would have on torsional moment and angular deflection at failure for the files of three different sizes.
A number of studies have assessed the physical properties and durability of rotary nickel-titanium files. This study proposes a method that will permit this to be done in a controlled manner. Files of 0.04 taper; 25 mm in length; and in ISO sizes 25, 30, and 35 were each used four times in plastic blocks. The canal in the block had a working length of 17 mm; 30° curvature; 15 mm radius of curvature; size 20 at the apex; and the curve began at 8 mm from the orifice of the canal. The irrigant used was Glyde®. An electric handpiece was configured to rotate at 200 rpm and secured to the testing device, which also held the plastic block. The testing device controlled the depth of penetration at 17 mm; the load was at 8 N and the rate of penetration was at 120 mm/min. Controls were not subjected to use. Torsional moment (N-cm) and angular deflection (degrees) at failure were determined on a Torsiometer/Memocouple. Data (n ⴝ 5) were analyzed by analysis of variance. Tukey-Kramer intervals at the 0.05 significance level were determined. Torsional moment significantly increased with an increase in size. Torsional moment also significantly increased for used instruments, compared with controls. Angular deflection had a range of 480 degrees to 810 degrees for the instruments tested.
MATERIALS AND METHODS ProFile® 0.04 taper rotary files 25 mm long in ISO sizes 25, 30, and 35 (lots P062698397, PO62398911, and PO61898552, respectively) were obtained (Dentsply/Tulsa Dental, Tulsa, OK). These files were divided into two experimental groups with five of each size in each group for a total of 30. Group A was the control. Group B (used files) was exposed to use in the following manner. Twenty plastic blocks (Pecina & Associates, Waukegan, IL) were divided into five groups of four each. Each plastic block had a working length of 17 mm with a 30-degree curve that started at 8 mm from the orifice of the canal. The size of the canal at the apex was equivalent to an ISO size 20 instrument. Each canal had a smoothly tapered form that gradually increased in diameter as the orifice was approached. The radius of curvature was 15 mm. Each block was instrumented from size 25 to 35 so that a total of five instruments of each size were used four times. Glyde® (Dentsply International, Inc., Tulsa, OK) was used as an irrigant/lubricant. This irrigant was renewed periodically during the instrumentation of the blocks. An electric handpiece (Aseptico model AEU-17B; Aseptico, Inc., Woodenville, WA) was configured to have the files rotate at 200 rpm. A special clamp was fabricated to secure the handpiece to the testing machine (model MN 44, Instron Corp., Canton, MA). Another clamp was used to secure the plastic block to the testing machine. The testing machine was set up so that the load limit was 8 N. The maximum depth of penetration of the instrument was set to 17 mm (the working length of the plastic block). The crosshead speed (rate of penetration) was set to 120 mm/min. Torsional moment (N-cm) and angular deflection (degrees) at failure were measured on the Torsiometer/Memocouple (Maillefer, Ballaigues, Switzerland). Data were analyzed by analysis of variance (SuperANOVA, Abacus Concepts, Berkeley, CA). Tukey-Kramer intervals at the 0.05 significance level were calculated.
Two recent studies have stated that nickel-titanium rotary files can be used a number of times before being discarded. Zuolo and Walton (1) stated that they could be used up to 6 min without visible deterioration of the instruments. Tongbaiyai and Torabinejad (2) stated that the same type of instruments could be used 10 times before discarding. These statements do not agree with some literature that recommends discarding instruments after one use in severely curved canals, regardless of any visible changes in the instrument (3). Other studies that addressed the physical properties of the instruments did not test these instruments under conditions that simulate clinical usage (4 – 6). The purpose of this study was to subject rotary nickel-titanium files to conditions that simulated clinical usage while controlling all of the relevant parameters. For the plastic block, these parameters were length of canal, degree of canal curvature, radius of
RESULTS Means and standard deviation of torsional moment (N-cm) are reported in Table 1. Torsional moment significantly increased with an increase in size. Torsional moment was also significantly in517
518
Svec and Powers
Journal of Endodontics
TABLE 1. Torsional moment of rotary files Condition Size 25 30 35
Control
Used
0.78 (0.04) 1.06 (0.10) 1.47 (0.15)
0.93 (0.08) 1.20 (0.13) 1.44 (0.07)
Means of torsional moment in N-cm (n ⫽ 5) with SD in parentheses. Differences in means ⬎0.08 N-cm for control versus used files are significant at the 0.05 significance level.
creased for sizes 25 and 30 when comparing used versus control instruments. Tukey-Kramer intervals ⬎0.08 N-cm were significant at the 0.05 level of significance for control versus used files. Angular deflection had a range of 480 to 810 degrees for the control instruments and 580 to 650 degrees for the used instruments. DISCUSSION With all of the studies that have been done on the physical properties and durability of rotary nickel-titanium files, there is still no agreement on how many times or how long such a file can be used before it is discarded. Zuolo and Walton (1) used rotary files for an additional 4 min after the files had first shown evidence of wear after 6 min of use. Tongbaiyai and Torabinejad (2) stated that these files could be used up to 10 times before being discarded. They did not state when visible defects were first seen on the files. Also, it is not clear if they meant 10 canals or 10 teeth. Kazemi et al. (7) looked at the machining efficiency and wear resistance of hand nickel-titanium files. They found that these files lost up to 50% of their cutting efficiency after 5 runs (300 strokes/run) on dentin. They also noted visible defects along the cutting edges of the files; however, they did not state when these defects first appeared. The parameters selected for the instruments used in this study were based, in part, on a pilot study. A study by Kobayashi et al. (8) utilized ProFile® instruments during the development of their automatic handpiece. They determined that the load limit should be set at 8 N. If the load limit was set higher, then too many instruments fractured during use. We selected a rotational rate that is at the lower end of that recommended by the proponents of the instrumentation technique. The crosshead speed of the testing machine has a very wide range. A rate of penetration was chosen that could be duplicated in the clinic setting without difficulty. It should be kept in mind that if the crosshead speed were set too
high, then it might be possible to exceed the plastic limit for the instrument. This would happen because there is a time lag between the machine sensing the load limit being exceeded and it automatically reversed itself. As a result the instrument being tested could reach its plastic limit and fracture. One other study has looked at the torsion moment at failure for ProFile® rotary files. Silvaggio and Hicks (6) tested the Series 29 instruments. When the results are compared, considering the D3 diameter for Series 29 and the ISO instruments we tested, they are equivalent. Although our results showed statistically significant differences between control and used files for torsional moment at failure, none of the differences were ⬎1 N-cm. The clinical relevance of such statistical differences is questionable. We do not feel it is useful to report the results for angular deflection at failure other than to give the ranges. These instruments were rotating at 1200 degrees/s. Their range of angular deflection at failure demonstrated that they would fail in less than a second. Considering all the canal parameters and instrumentation parameters that can be controlled with this experimental methodology, it is felt that this method will help to provide some answers as to the usability of any rotary instrument that is tested. Dr. Svec is associate professor, Department of Stomatology, Division of Endodontics; and Dr. Powers is professor and vice chairman, Department of Restorative Dentistry & Biomaterials, and director, Houston Biomaterials Research Center, University of Texas–Houston Health Science Center, Dental Branch, Houston, TX. Address requests for reprints to Dr. Timothy A. Svec, University of Texas-Houston Health Science Center, Dental Branch, 6516 John Freeman Avenue, Division of Endodontics, Room 202, Houston, TX 77030-3402.
References 1. Zuolo ML, Walton RE. Instrument deterioration with usage: nickeltitanium versus stainless steel. Quint Int 1997;28:397– 402. 2. Tongbaiyai C, Torabinejad M. The durability of .04 taper rotary Ni-Ti files after simulated clinical usage [Abstract]. J Endodon 1999;25:292. 3. Dental Education Laboratories. Cleaning and shaping flowcharts: chart 5. Santa Barbara: Dental Education Laboratories, 1998. 4. Wolcott J, Himel VT. Torsional properties of nickel-titanium versus stainless steel endodontic files. J Endodon 1997;23:217–20. 5. Rowan MB, Nicholls JI, Steiner J. Torsional properties of stainless steel and nickel-titanium endodontic files. J Endodon 1996;22:341–5. 6. Silvaggio J, Hicks ML. Effect of heat sterilization on the torsional properties of rotary nickel-titanium files. J Endodon 1997;23:731– 4. 7. Kazemi R, Stenman E, Spangberg L. Machining efficiency and wear resistance of nickel-titanium endodontic files. Oral Surg 1996;81:596 – 602. 8. Kobayashi C, Yoshioka T, Suda H. A new engine-driven canal preparation system with electronic canal measuring capability. J Endodon 1997; 23:751– 4.
Printed in U.S.A. VOL. 26, NO. 9, SEPTEMBER 2000
A Method to Assess Rotary Nickel-Titanium Files Timothy A. Svec, DDS, MS, and John M. Powers, PhD
curvature, size of canal, and beginning of curved portion of canal. For the instrument, these parameters were: load, rate of loading (rate of penetration), depth of penetration, and rate of rotation. Then it was determined what effect these conditions would have on torsional moment and angular deflection at failure for the files of three different sizes.
A number of studies have assessed the physical properties and durability of rotary nickel-titanium files. This study proposes a method that will permit this to be done in a controlled manner. Files of 0.04 taper; 25 mm in length; and in ISO sizes 25, 30, and 35 were each used four times in plastic blocks. The canal in the block had a working length of 17 mm; 30° curvature; 15 mm radius of curvature; size 20 at the apex; and the curve began at 8 mm from the orifice of the canal. The irrigant used was Glyde®. An electric handpiece was configured to rotate at 200 rpm and secured to the testing device, which also held the plastic block. The testing device controlled the depth of penetration at 17 mm; the load was at 8 N and the rate of penetration was at 120 mm/min. Controls were not subjected to use. Torsional moment (N-cm) and angular deflection (degrees) at failure were determined on a Torsiometer/Memocouple. Data (n ⴝ 5) were analyzed by analysis of variance. Tukey-Kramer intervals at the 0.05 significance level were determined. Torsional moment significantly increased with an increase in size. Torsional moment also significantly increased for used instruments, compared with controls. Angular deflection had a range of 480 degrees to 810 degrees for the instruments tested.
MATERIALS AND METHODS ProFile® 0.04 taper rotary files 25 mm long in ISO sizes 25, 30, and 35 (lots P062698397, PO62398911, and PO61898552, respectively) were obtained (Dentsply/Tulsa Dental, Tulsa, OK). These files were divided into two experimental groups with five of each size in each group for a total of 30. Group A was the control. Group B (used files) was exposed to use in the following manner. Twenty plastic blocks (Pecina & Associates, Waukegan, IL) were divided into five groups of four each. Each plastic block had a working length of 17 mm with a 30-degree curve that started at 8 mm from the orifice of the canal. The size of the canal at the apex was equivalent to an ISO size 20 instrument. Each canal had a smoothly tapered form that gradually increased in diameter as the orifice was approached. The radius of curvature was 15 mm. Each block was instrumented from size 25 to 35 so that a total of five instruments of each size were used four times. Glyde® (Dentsply International, Inc., Tulsa, OK) was used as an irrigant/lubricant. This irrigant was renewed periodically during the instrumentation of the blocks. An electric handpiece (Aseptico model AEU-17B; Aseptico, Inc., Woodenville, WA) was configured to have the files rotate at 200 rpm. A special clamp was fabricated to secure the handpiece to the testing machine (model MN 44, Instron Corp., Canton, MA). Another clamp was used to secure the plastic block to the testing machine. The testing machine was set up so that the load limit was 8 N. The maximum depth of penetration of the instrument was set to 17 mm (the working length of the plastic block). The crosshead speed (rate of penetration) was set to 120 mm/min. Torsional moment (N-cm) and angular deflection (degrees) at failure were measured on the Torsiometer/Memocouple (Maillefer, Ballaigues, Switzerland). Data were analyzed by analysis of variance (SuperANOVA, Abacus Concepts, Berkeley, CA). Tukey-Kramer intervals at the 0.05 significance level were calculated.
Two recent studies have stated that nickel-titanium rotary files can be used a number of times before being discarded. Zuolo and Walton (1) stated that they could be used up to 6 min without visible deterioration of the instruments. Tongbaiyai and Torabinejad (2) stated that the same type of instruments could be used 10 times before discarding. These statements do not agree with some literature that recommends discarding instruments after one use in severely curved canals, regardless of any visible changes in the instrument (3). Other studies that addressed the physical properties of the instruments did not test these instruments under conditions that simulate clinical usage (4 – 6). The purpose of this study was to subject rotary nickel-titanium files to conditions that simulated clinical usage while controlling all of the relevant parameters. For the plastic block, these parameters were length of canal, degree of canal curvature, radius of
RESULTS Means and standard deviation of torsional moment (N-cm) are reported in Table 1. Torsional moment significantly increased with an increase in size. Torsional moment was also significantly in517
518
Svec and Powers
Journal of Endodontics
TABLE 1. Torsional moment of rotary files Condition Size 25 30 35
Control
Used
0.78 (0.04) 1.06 (0.10) 1.47 (0.15)
0.93 (0.08) 1.20 (0.13) 1.44 (0.07)
Means of torsional moment in N-cm (n ⫽ 5) with SD in parentheses. Differences in means ⬎0.08 N-cm for control versus used files are significant at the 0.05 significance level.
creased for sizes 25 and 30 when comparing used versus control instruments. Tukey-Kramer intervals ⬎0.08 N-cm were significant at the 0.05 level of significance for control versus used files. Angular deflection had a range of 480 to 810 degrees for the control instruments and 580 to 650 degrees for the used instruments. DISCUSSION With all of the studies that have been done on the physical properties and durability of rotary nickel-titanium files, there is still no agreement on how many times or how long such a file can be used before it is discarded. Zuolo and Walton (1) used rotary files for an additional 4 min after the files had first shown evidence of wear after 6 min of use. Tongbaiyai and Torabinejad (2) stated that these files could be used up to 10 times before being discarded. They did not state when visible defects were first seen on the files. Also, it is not clear if they meant 10 canals or 10 teeth. Kazemi et al. (7) looked at the machining efficiency and wear resistance of hand nickel-titanium files. They found that these files lost up to 50% of their cutting efficiency after 5 runs (300 strokes/run) on dentin. They also noted visible defects along the cutting edges of the files; however, they did not state when these defects first appeared. The parameters selected for the instruments used in this study were based, in part, on a pilot study. A study by Kobayashi et al. (8) utilized ProFile® instruments during the development of their automatic handpiece. They determined that the load limit should be set at 8 N. If the load limit was set higher, then too many instruments fractured during use. We selected a rotational rate that is at the lower end of that recommended by the proponents of the instrumentation technique. The crosshead speed of the testing machine has a very wide range. A rate of penetration was chosen that could be duplicated in the clinic setting without difficulty. It should be kept in mind that if the crosshead speed were set too
high, then it might be possible to exceed the plastic limit for the instrument. This would happen because there is a time lag between the machine sensing the load limit being exceeded and it automatically reversed itself. As a result the instrument being tested could reach its plastic limit and fracture. One other study has looked at the torsion moment at failure for ProFile® rotary files. Silvaggio and Hicks (6) tested the Series 29 instruments. When the results are compared, considering the D3 diameter for Series 29 and the ISO instruments we tested, they are equivalent. Although our results showed statistically significant differences between control and used files for torsional moment at failure, none of the differences were ⬎1 N-cm. The clinical relevance of such statistical differences is questionable. We do not feel it is useful to report the results for angular deflection at failure other than to give the ranges. These instruments were rotating at 1200 degrees/s. Their range of angular deflection at failure demonstrated that they would fail in less than a second. Considering all the canal parameters and instrumentation parameters that can be controlled with this experimental methodology, it is felt that this method will help to provide some answers as to the usability of any rotary instrument that is tested. Dr. Svec is associate professor, Department of Stomatology, Division of Endodontics; and Dr. Powers is professor and vice chairman, Department of Restorative Dentistry & Biomaterials, and director, Houston Biomaterials Research Center, University of Texas–Houston Health Science Center, Dental Branch, Houston, TX. Address requests for reprints to Dr. Timothy A. Svec, University of Texas-Houston Health Science Center, Dental Branch, 6516 John Freeman Avenue, Division of Endodontics, Room 202, Houston, TX 77030-3402.
References 1. Zuolo ML, Walton RE. Instrument deterioration with usage: nickeltitanium versus stainless steel. Quint Int 1997;28:397– 402. 2. Tongbaiyai C, Torabinejad M. The durability of .04 taper rotary Ni-Ti files after simulated clinical usage [Abstract]. J Endodon 1999;25:292. 3. Dental Education Laboratories. Cleaning and shaping flowcharts: chart 5. Santa Barbara: Dental Education Laboratories, 1998. 4. Wolcott J, Himel VT. Torsional properties of nickel-titanium versus stainless steel endodontic files. J Endodon 1997;23:217–20. 5. Rowan MB, Nicholls JI, Steiner J. Torsional properties of stainless steel and nickel-titanium endodontic files. J Endodon 1996;22:341–5. 6. Silvaggio J, Hicks ML. Effect of heat sterilization on the torsional properties of rotary nickel-titanium files. J Endodon 1997;23:731– 4. 7. Kazemi R, Stenman E, Spangberg L. Machining efficiency and wear resistance of nickel-titanium endodontic files. Oral Surg 1996;81:596 – 602. 8. Kobayashi C, Yoshioka T, Suda H. A new engine-driven canal preparation system with electronic canal measuring capability. J Endodon 1997; 23:751– 4.