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Torsional properties of twisted and machined endodontic files
0099-2399/90/1608-0355/$02.00/0 JOURNAL OF ENDODONTICS Copyright 9 1990 by The American Association of Endodontists
Printed in U.S.A.
VOL 16, NO. 8, AUGUST1990
SCIENTIFIC ARTICLES Torsional Properties of Twisted and Machined Endodontic Files Bradley G. Seto, DDS, MSD, Jack I. Nicholls, PhD, and Gerald W. Harrington, DDS, MSD
The machined files (Flex-R; Union Broach, New York, NY) were the experimental group (UB) and twisted files (Flex-OFile; Maillefer/L. D. Caulk Co., Milford, DE) were the control group (M). These files were selected because cross-sectional shapes were the same for each file size (#10 square and #15 to 40 triangular). All instruments were examined at x30 to ensure uniformity of cutting flutes and a defect-free surface. File diameters were then measured at three points on the cutting surface (1, 3, and 16 m m from the tip) with a vernier caliper (model 40185; Sears, Roebuck and Co., Chicago, IL) (Fig. 1). Vernier precision was 0.025 mm. The vernier was mounted in a plastic device that had three holes to reproducibly position the files for measurement at the desired levels. File measurement consistency was determined by l0 repeated measurements of the same file (three files per size) which were completed by two separate evaluators. Less than 3% variation was found. Ten files of each size and group were tested in clockwise (CW) and counterclockwise (CCW) rotation. Therefore, 140 files from each manufacturer were tested. Torsion without axial load was applied with a device (Fig. 2) attached to the crosshead o f the Instron Testing Machine (Instron Corp., Canton, MA). The files were grasped 3 m m from the tip by immobile brass jaws and the handle secured with triple set screws on the rotating shaft. A core wrapped around the rotating shaft was connected to the load cell. Rotation occurred as the crosshead was lowered and calculated to be 2.2 rpm. The crosshead and rotating shaft were returned to the original position prior to subsequent testing. The machine was calibrated with a t 00-g weight prior to each testing session. Resistance in the rotating shaft/bearing complex was determined to be less than 3 g of static load. Load and angular deflection (rotation) were recorded continuously on the chart recorder and were correlated visually at 90, 180, 360, and every 360 degrees until failure. Load was converted to torque via the formula: torque = load x (shaft radius + cord radius). Mean values of torque at yield, torque at failure, rotation at yield, and rotation at failure were compared. Yield was defined as the point at which elastic deformation ends and failure was the point at which the file breaks (Fig. 3). The number of files for each group that did not meet A N S I / A D A Specification No. 28 (8) was recorded. All files were reinspected at x30 to characterize the fracture surface and the spirals adjacent to the point of failure.
The torsional properties of conventionally twisted Ktype endodontic files and recently developed machined K-type endodontic files were compared. File sizes 10 through 40 were subjected to torsional load in clockwise and counterclockwise directions independently. Results showed that a statistically significant reduction in clockwise rotation occurred at failure with all of the machined files except size 10. Counterclockwise rotation at failure was also significantly lower for the machined files in sizes 10 through 30. There was no difference in torsional strength between the file types regardless of rotation direction. Therefore, machined files exhibit less ductility than twisted files prior to fracture and may be more susceptible to torsional failure clinically.
Conventional K-type endodontic files have been manufactured by twisting a tapered stainless steel blank. Recently, a new method was developed whereby the flutes are machined into a tapered, round stainless steel blank to create a similar triangular cross-section file. It can be hypothesized that the new "machined" files would have less strain-hardening and lower internal stresses by avoiding the twisting phase of fabrication. Therefore, the machined files should allow more rotation prior to fracture, especially in counterclockwise twisting. This would be a significant gain in physical properties since twisted endodontic files have been shown to fracture at much less angular deflection in counterclockwise torsion than clockwise (1-5). Conversely, since a machining method is also used for making Hedstrom files, the machined K-type files may have similar torsional properties as Hedstrom files. This may result in less rotation prior to fracture (6, 7) than would be expected from twisted K-type files (3-5). The purpose of this article was to compare the torsional properties of machined and twisted K-type endodontic files. MATERIALS AND METHODS Stainless steel K-type root canal files (# 10 to #40) with the same length (25 mm) and cross-sectional shape were studied.
355
356
Journal of Endodontics
Seto et al.
Data were analyzed by analysis of variance and a multiple range test (Student-Newman-Keuls). Non-parametric data were analyzed by the chi-square test. Significance was accepted when p ~ 0.05. RESULTS Rotation at Failure
FtG 1. File measuring device--vernier caliper with 0.025-mm precision. Jig-positioned files for measurements at 1,3, and 16 mm from the tip.
Rotation at failure in a CW direction was significantly greater for the M files than UB files for all sizes, except #10 (Fig. 4.4). All files exceeded A N S I / A D A Specification No. 28 for rotation at failure except UB #10 files. Rotation at failure in a CCW direction was significantly greater for the M files than UB files for sizes #10 through #30 (Fig. 4B). M files showed a decrease in rotation at failure with increasing file size. The UB files, however, had the same rotation at failure for all file sizes.
2880"
-- A
2520 '
[
9
UB
218o-'
I E~ M
1800 !
,o o-
g,
2o-1 10
15"
N
V-d 20*
25*
30*
35*
40*
FILE SIZE * Statistically
B
450
z
360
~-p
270
significant (p<0.05)
FIG 2. Torsion testing device--developed 2.2 rpm during testing. O O
9
UB
"O
Failure
Ir
(3
180
Yield
o
90
o I-
0 10*
15*
20*
25*
30*
35
40
FILE SIZE
I E'as'icP"a'e I--
P'a' c "ase ROTATION
FIG 3. Diagrammatic torque/rotation curve--yield is the end of elastic deformation and beginning of plastic deformation.
* Statistically significant (p<0.05)
FtG 4. A, Clockwise rotation at fracture--twisted files (M) were significantly greater than machined files (UB) except for file size #10 (p < 0.05). B, Counterclosewise rotation at failure--twisted flies (M) were significantly greater than machined flies (UB) except sizes 30 to 40 (p < 0.05).
Vol. 16, No. 8, August 1990
Torsional Properties of Files
Comparing CW versus CCW rotation for both M and UB files, there was a significantly greater mean CW rotation at failure (Fig. 4).
A 40
Torque at Failure
30
Comparing files of the same size, there was no significant difference in torque at failure for both groups, regardless of direction of rotation (Fig. 5). Torque at failure increased with file size (Fig. 5).
357
#20
D
O o
E= 20
#15
.10 &"
10 Yield Properties
For both CW and CCW rotation, torque at yield increased with file size. There was no difference in torque at yield between M and UB files, except for #25 and #40 in CW rotation and #30 in CCW rotation. Rotation at yield, however, was significantly greater for M files than UB files in CW and c c w rotation except for files #15, #30, and #40 in CCW direction (Figs. 6 and 7).
150
i
0
360
I
I
I
!
1440
1800
2160
2520
(deg rees)
B 140 120
r ......
100
m. ~
#40 .-.-4
• .---
#35
--~
9 uB ]
#
. . . . .
"- - -
[] M
l
1080
CW ROTATION
09 80 n- E O~m 6O
I
I
720
I
0
3
0
~ -
--"
I
I uB---
20
1 ~
0
E ~ !~ ~~
10
15
20
B
25
30
35
360
I
9
J
"
I
"
720 1080 1440 CW ROTATION (degrees)
|
1800
"
I
2160
FIG 6. A, Clockwise torque-rotation curve for sizes # 1 0 to 20. Only the plastic portion of the graph from yield point (left) to failure (right) was depicted for clarity. B, Clockwise torque-rotation curve for sizes # 2 5 to 40. Only the plastic portion of the graph from yield point (left) to failure (right) was depicted for clarity.
0 40
FILE SIZE 200
File Diameters
100
At the 1-mm level, the M files were significantly larger than the UB files for sizes #20 through #40 (Table 1). At the 3m m level, the M files were significantly larger for #30 through #40. At the 16-ram level, M files were larger for #25 through #40.
o
I,uBI
Or n- E O o I- E
I
0
Visual Inspection at Failure
0 10
15
20
25
30
35
40
FILE SIZE FiG 5. A, Clockwise torque at failure--no significant difference between twisted (M) and machined (UB) files (p > 0.05). B, Counterclockwise torque at failure--no significant difference between twisted (M) and machined (UB) files (p > 0.05).
All files exhibited permanent deformation of the cutting shaft with a smooth fractured surface, regardless of rotation direction or group (Figs. 8 and 9). In clockwise rotation, there was a distinct reversal point of the cutting spirals and progressive tightening of these reversed spirals closer to the fractured edge (opposite direction to the normal fluting pattern) (Figs. 10 and 11). The number of tightened spirals toward the tip of the file reflected the number of revolutions at fracture after permanent deformation. Machined files (UB) had less
358
Seto et al.
Joumal of Endodontics
A 40. UB " - "
r ~
30
#20
20
o,o
#15 #10
10
i
i
180
9O
38'0
270
CCW ROTATION (degrees)
B
FiG 8. Fractured surface for twisted (M) files--CW
140
(right) and CCW
(left) rotations (original magnification x63).
"~ #40
I M
120
UB . . . . . 100 e__.~=--.-=-:4.., m #35
80
O hOE=
9
60
-- #30 - #25
e
40 20 0 0
i
i
i
i
90
180
270
360
CCW
ROTATION
(degrees) FiG 7. A, Counterclockwise torque-rotation curve for sizes #10 to 20. Oniy the plastic portion of the graph from yield point (left) to failure (right) was depicted for clarity. B, Counterclockwise torquerotation curve for sizes #25 to 40. Only the plastic portion of the graph from yield point (left) to failure (right) was depicted for clarity. FiG 9. Fractured surface for machined (UB) files--CW CCW (left) rotations (original magnification x63).
(right) and
TABLE 1. Comparison of file diameters (mm) at 1, 3, and 16 mm from the tip Twisted (M) Files File Size 1 mm_+SD
3mm-SD
16mm_+SD
10 15 20 25 30 35 40
0.107 _+ 0.005 0.152 - 0.008 0.201 ___0.005* 0.249 _ 0.005* 0.292 _+ 0.008* 0 . 3 4 8 _ 0.005* 0.409_+ 0.005*
0.150 _ 0.008 0.190 _+ 0.005 0.241 _+ 0.008 0.295 _ 0.008 0.338 -+ 0.008 0.389-+ 0.005* 0.442 + 0.008*
0.389 -+ 0.003 0.462 _ 0.008 0.495 ___0.005 0.579 _+ 0.013" 0.635 _ 0.005* 0.688-+ 0.008* 0.696-+ 0.010"
10 15 20 25 30 35 40
0.119 _ 0.005 0.145_+0.013 0.185 _+ 0.010 0.239 _+ 0.010 0.274 _+ 0.010 0.300 _+ 0.010 0.368 _+ 0.015
Machined (UB) files 0.155 _+ 0.005 0.193_+0.013 0.241 _+ 0.008 0.297 _+ 0.010 0.328 _+ 0.008 0.368 _+ 0.013 0.424 _+ 0.015
0.437 _ 0.003 0.460___0.003 0.498 _+ 0.008 0.561 _+ 0.003 0.577 _+ 0.010 0.622 _+ 0.025 0.706 _+ 0.020
'~Statistically larger than comparable sizes of Union Broach files.
deformation than the twisted files (M). In counterclockwise rotation, there was a progressive tightening o f the spirals as the fractured tip was approached, which was less pronounced for the UB files (Figs. 10 and 11). DISCUSSION The testing apparatus used in this study is a modification of a device described previously by Oliet and Sorin (9) but similar to a more recent derivation used by Chernick et al. (l) and Lautenschlager et al. (2). This method eliminated operator bias by directly monitoring data on the chart recorder of the Instron Testing Machine. This test sequence differed from the A N S I / A D A Specification No. 28 in that the handles were left on during testing. Following testing, no evidence of slippage was seen between the triple set screws and the handle nor the file shaft and the handle.
Vol. 16, No. 8, August 1990
FIG 10. Fractured shaft for twisted (M) files. CW rotation (top) showing "reversal point" (arrow) of spirals and marked deformation. CCW rotation (bottom) showing minimal deformation but subjectively more than Union Broach CCW fractures (original magnification x20).
FIG 11. Fractured shaft for machined (UB) files. CW rotation (top) showing reversal point (arrow) of spirals and marked deformation. CCW rotation (bottom) showing minimal to no deformation (original magnification x20),
For the files tested, there was no demonstrable benefit to the torsional properties derived from the machining method of K-type root canal file fabrication over the conventional twisting manufacturing technique. In these torsional tests, the machined files did not perform as well as the twisted files. This is demonstrated by a lower rotation at failure in clockwise and counterclockwise twisting and less visible deformation of the spirals prior to fracture. The ductility of an endodontic file is measured by the amount of rotation it can withstand prior to failure and can be considered a "safety factor" for endodontic files. The greater this ductility, the more detectable is the deformation of the cutting spirals and the more likely these files will be discarded prior to breakage. At failure, the machined files exhibited less clockwise and counterclockwise rotation than
Torsional Properties of Files
359
the twisted files. Clockwise rotation at failure for the machined files is more comparable to results from other studies obtained for machined Hedstrom files (6, 7). The machined K-type files, however, do exceed minimum ANSI/ADA standards (8) for clockwise rotation. In counterclockwise rotation, there was significantly lower rotation at failure for the machined files in the smaller file sizes (#10 through 30). Thus, the ductility of the machined files is lower than the twisted files tested in this study and offer less of a safety factor against breakage when used clinically. The mechanism of instrument failure was the same for both file types tested in this study. During clockwise torque application, the cutting spirals unraveled. The outer portion of these spirals would then experience longitudinal compression as the spiral direction begani reversing. After the spirals reversed direction, tension would be created in these spirals as they were tightened. Then smooth-ended fracture would result. The fractured surfaces appeared to be similar to the brittle counterclockwise fractures described previously by Chernick et al. (1). During counterclockwise torque application, the cutting spirals tightened further. The outer portion of these spirals experienced immediate longitudinal tension. Then fracture would result. The appearance of the counterclockwise fracture surface was also smooth ended. At failure, both machined and twisted files demonstrated significantly greater clockwise rotation than counterclockwise rotation for the same file size. This can be explained by the above-mentioned mechanism whereby tension in the outer portion of the shaft occurs much earlier in counterclockwise rotation. Clockwise rotation must first overcome the unraveling or compression of the outer aspect of the shaft and develop tension prior to failure, and counterclockwise rotation develops tension in the outer aspect of the shaft immediately. Therefore, lower ductility is expected and seen in counterclockwise rotation. The direction of rotation could not be determined from observations of the fracture surface but could be distinguished by observing the direction of the cutting spirals immediately adjacent to the fractured edge. In the clinically oriented report by Roane and Sabala (10), clockwise rotation caused fractured ends to split and separate (ductile fractures), whereas counterclockwise rotation resulted in smooth-ended fractures (brittle fractures). This frayed appearance was first described as ductile clockwise fracture in an in vitro study by Chernick et al. (1). Only smooth-ended fractures were seen with the two brands of files tested in the present study (Figs. 8 and 9). Therefore, one cannot use the appearance of the fractured end as the sole criterion for determining the direction of rotation causing fracture. The relative flexibility of files can be deduced from the slope of the torque/rotation curve up to the end of elastic deformation (4, 5). The steeper the slope of the curve, the stiffer the file. As expected, the stiffness of the files increased with file size. However, the machined files appeared to be stiffer than their twist manufactured counterparts (Figs. 6 and 7). The reason for this difference in stiffness is not known but it may reflect the difference in the alloy or the difference in the manufacturing method. The number of spirals per length of cutting shaft did not appear to influence flexibility since the machined files were consistently stiffer, despite having more spirals per unit length in the smaller sizes and fewer in the larger sizes.
360
Journal of Endodontics
Seto et al.
The file diameters were fairly consistent within each manufacturer's sizes but there was some variation between the two manufacturers. For example, at 1 m m from the tip, #20 through #40 files from the machined group were as much as one size (0.05 mm) smaller than the same-sized twisted files, whereas the smaller files (#10 and #15) corresponded closely in size. This size difference at the 1-mm level did not reflect the modified tip design of the machined instruments since similar findings were seen at the 16-mm level (Table 1). The diameter variation between the manufacturers did not result in different torsional strengths (Fig. 5) and did not apparently influence the rotation at failure. SUMMARY Torsional strength and rotation at failure of twisted (Maillefer Flex-O-Files) and machined (Union Broach Flex-R) Ktype root canal files were compared. The conclusions are as follows: Twisted files had significantly greater rotation at failure in clockwise and counterclockwise directions than the comparable machined files. Both machined and twisted files exhibited significantly greater rotation at failure in clockwise than counterclockwise torsion. There was no difference in torsional strength between the twisted and machined files. All files exceeded ANSI/ADA Specification No. 28 for clockwise torsional strength and rotation except the machined # 10 files. Machined files showed less visible deformation prior to failure than the twisted files in both rotation directions. Machined files exhibited less ductility than twisted files prior to fracture.
The mechanism of failure was similar for twisted and machined files regardless of direction of torsion. The direction of cutting spirals adjacent to the fracture end can be used to determine the direction of rotation which caused failure for the two files tested in this study. The authors would like to thank the Union Broach and L. D. Caulk/Dentsply companies for donation of their instruments for this study. The authors would also like to thank Dr. Robert Oswald for his input and critique of this research project. Dr. Seto is a former graduate student in endodontics and is now in private practice in Santa Monica, CA. Dr. Nicholls is professor, Department of Restorative Dentistry, School of Dentistry, University of Washington, Seattle, WA. Dr. Harrington is professor, Department of Endodontics, and director, Graduate Endedontics Program, School of Dentistry, University of Washington, Seattle, WA.
9
References 1. Chemick LB, Jacobs JJ, Lautenschlager EP, Heuer MA. Torsional failure of endodontic files. J Endodon 1976;2:94-7. 2. Lautenschlager EP, Jacobs JJ, Marshall GW, Heuer MA. Brittle and ductile torsional failures of endodontic instruments. J Endodon 1977;3:175-8. 3. Lentine, FN. A study of torsional and angular deflection of endodontic files and reamers. J Endodon 1979;5:181-91. 4. Dolan DW, Craig RG. Bending and torsion of endodontic files with rhombus cross-sections. J Endodon 1982;8:260-4. 5. Krupp JD, Brantley WA, Gerstein H. An investigation of the torsional and bending properties of seven brands of endodontic files, J Endodon 1984;10:372-80. 6. Mueller HJ, Suchak AJ, Stanford WB, Stanford JW, Stanford SK. Comparison of some root canal instruments in bending and torsion to newly formed or draft specifications. J Endodon 1984; 10:182-7. 7. Bolger WL, Gough RW, Foster CD. A comparsion of the potential for breakage: the Bums Unifile versus Hedstrom files. J Endodon 1985;11:110-6. 8. Council on Dental Materials, Instruments and Equipment, Revised American National Standards Institute/American Dental Association Specification No. 28 for root canal files and reamers, type K (revised 1988). 9. Oliet S, Sorin SM. Torsional tester for root canal instruments. Oral Surg Oral Med Oral Pathol 1965;20:654-62. 10. Roane JB, Sabala C. Clockwise or counterclockwise. J Endodon 1984; 10:349-53.
Printed in U.S.A.
VOL 16, NO. 8, AUGUST1990
SCIENTIFIC ARTICLES Torsional Properties of Twisted and Machined Endodontic Files Bradley G. Seto, DDS, MSD, Jack I. Nicholls, PhD, and Gerald W. Harrington, DDS, MSD
The machined files (Flex-R; Union Broach, New York, NY) were the experimental group (UB) and twisted files (Flex-OFile; Maillefer/L. D. Caulk Co., Milford, DE) were the control group (M). These files were selected because cross-sectional shapes were the same for each file size (#10 square and #15 to 40 triangular). All instruments were examined at x30 to ensure uniformity of cutting flutes and a defect-free surface. File diameters were then measured at three points on the cutting surface (1, 3, and 16 m m from the tip) with a vernier caliper (model 40185; Sears, Roebuck and Co., Chicago, IL) (Fig. 1). Vernier precision was 0.025 mm. The vernier was mounted in a plastic device that had three holes to reproducibly position the files for measurement at the desired levels. File measurement consistency was determined by l0 repeated measurements of the same file (three files per size) which were completed by two separate evaluators. Less than 3% variation was found. Ten files of each size and group were tested in clockwise (CW) and counterclockwise (CCW) rotation. Therefore, 140 files from each manufacturer were tested. Torsion without axial load was applied with a device (Fig. 2) attached to the crosshead o f the Instron Testing Machine (Instron Corp., Canton, MA). The files were grasped 3 m m from the tip by immobile brass jaws and the handle secured with triple set screws on the rotating shaft. A core wrapped around the rotating shaft was connected to the load cell. Rotation occurred as the crosshead was lowered and calculated to be 2.2 rpm. The crosshead and rotating shaft were returned to the original position prior to subsequent testing. The machine was calibrated with a t 00-g weight prior to each testing session. Resistance in the rotating shaft/bearing complex was determined to be less than 3 g of static load. Load and angular deflection (rotation) were recorded continuously on the chart recorder and were correlated visually at 90, 180, 360, and every 360 degrees until failure. Load was converted to torque via the formula: torque = load x (shaft radius + cord radius). Mean values of torque at yield, torque at failure, rotation at yield, and rotation at failure were compared. Yield was defined as the point at which elastic deformation ends and failure was the point at which the file breaks (Fig. 3). The number of files for each group that did not meet A N S I / A D A Specification No. 28 (8) was recorded. All files were reinspected at x30 to characterize the fracture surface and the spirals adjacent to the point of failure.
The torsional properties of conventionally twisted Ktype endodontic files and recently developed machined K-type endodontic files were compared. File sizes 10 through 40 were subjected to torsional load in clockwise and counterclockwise directions independently. Results showed that a statistically significant reduction in clockwise rotation occurred at failure with all of the machined files except size 10. Counterclockwise rotation at failure was also significantly lower for the machined files in sizes 10 through 30. There was no difference in torsional strength between the file types regardless of rotation direction. Therefore, machined files exhibit less ductility than twisted files prior to fracture and may be more susceptible to torsional failure clinically.
Conventional K-type endodontic files have been manufactured by twisting a tapered stainless steel blank. Recently, a new method was developed whereby the flutes are machined into a tapered, round stainless steel blank to create a similar triangular cross-section file. It can be hypothesized that the new "machined" files would have less strain-hardening and lower internal stresses by avoiding the twisting phase of fabrication. Therefore, the machined files should allow more rotation prior to fracture, especially in counterclockwise twisting. This would be a significant gain in physical properties since twisted endodontic files have been shown to fracture at much less angular deflection in counterclockwise torsion than clockwise (1-5). Conversely, since a machining method is also used for making Hedstrom files, the machined K-type files may have similar torsional properties as Hedstrom files. This may result in less rotation prior to fracture (6, 7) than would be expected from twisted K-type files (3-5). The purpose of this article was to compare the torsional properties of machined and twisted K-type endodontic files. MATERIALS AND METHODS Stainless steel K-type root canal files (# 10 to #40) with the same length (25 mm) and cross-sectional shape were studied.
355
356
Journal of Endodontics
Seto et al.
Data were analyzed by analysis of variance and a multiple range test (Student-Newman-Keuls). Non-parametric data were analyzed by the chi-square test. Significance was accepted when p ~ 0.05. RESULTS Rotation at Failure
FtG 1. File measuring device--vernier caliper with 0.025-mm precision. Jig-positioned files for measurements at 1,3, and 16 mm from the tip.
Rotation at failure in a CW direction was significantly greater for the M files than UB files for all sizes, except #10 (Fig. 4.4). All files exceeded A N S I / A D A Specification No. 28 for rotation at failure except UB #10 files. Rotation at failure in a CCW direction was significantly greater for the M files than UB files for sizes #10 through #30 (Fig. 4B). M files showed a decrease in rotation at failure with increasing file size. The UB files, however, had the same rotation at failure for all file sizes.
2880"
-- A
2520 '
[
9
UB
218o-'
I E~ M
1800 !
,o o-
g,
2o-1 10
15"
N
V-d 20*
25*
30*
35*
40*
FILE SIZE * Statistically
B
450
z
360
~-p
270
significant (p<0.05)
FIG 2. Torsion testing device--developed 2.2 rpm during testing. O O
9
UB
"O
Failure
Ir
(3
180
Yield
o
90
o I-
0 10*
15*
20*
25*
30*
35
40
FILE SIZE
I E'as'icP"a'e I--
P'a' c "ase ROTATION
FIG 3. Diagrammatic torque/rotation curve--yield is the end of elastic deformation and beginning of plastic deformation.
* Statistically significant (p<0.05)
FtG 4. A, Clockwise rotation at fracture--twisted files (M) were significantly greater than machined files (UB) except for file size #10 (p < 0.05). B, Counterclosewise rotation at failure--twisted flies (M) were significantly greater than machined flies (UB) except sizes 30 to 40 (p < 0.05).
Vol. 16, No. 8, August 1990
Torsional Properties of Files
Comparing CW versus CCW rotation for both M and UB files, there was a significantly greater mean CW rotation at failure (Fig. 4).
A 40
Torque at Failure
30
Comparing files of the same size, there was no significant difference in torque at failure for both groups, regardless of direction of rotation (Fig. 5). Torque at failure increased with file size (Fig. 5).
357
#20
D
O o
E= 20
#15
.10 &"
10 Yield Properties
For both CW and CCW rotation, torque at yield increased with file size. There was no difference in torque at yield between M and UB files, except for #25 and #40 in CW rotation and #30 in CCW rotation. Rotation at yield, however, was significantly greater for M files than UB files in CW and c c w rotation except for files #15, #30, and #40 in CCW direction (Figs. 6 and 7).
150
i
0
360
I
I
I
!
1440
1800
2160
2520
(deg rees)
B 140 120
r ......
100
m. ~
#40 .-.-4
• .---
#35
--~
9 uB ]
#
. . . . .
"- - -
[] M
l
1080
CW ROTATION
09 80 n- E O~m 6O
I
I
720
I
0
3
0
~ -
--"
I
I uB---
20
1 ~
0
E ~ !~ ~~
10
15
20
B
25
30
35
360
I
9
J
"
I
"
720 1080 1440 CW ROTATION (degrees)
|
1800
"
I
2160
FIG 6. A, Clockwise torque-rotation curve for sizes # 1 0 to 20. Only the plastic portion of the graph from yield point (left) to failure (right) was depicted for clarity. B, Clockwise torque-rotation curve for sizes # 2 5 to 40. Only the plastic portion of the graph from yield point (left) to failure (right) was depicted for clarity.
0 40
FILE SIZE 200
File Diameters
100
At the 1-mm level, the M files were significantly larger than the UB files for sizes #20 through #40 (Table 1). At the 3m m level, the M files were significantly larger for #30 through #40. At the 16-ram level, M files were larger for #25 through #40.
o
I,uBI
Or n- E O o I- E
I
0
Visual Inspection at Failure
0 10
15
20
25
30
35
40
FILE SIZE FiG 5. A, Clockwise torque at failure--no significant difference between twisted (M) and machined (UB) files (p > 0.05). B, Counterclockwise torque at failure--no significant difference between twisted (M) and machined (UB) files (p > 0.05).
All files exhibited permanent deformation of the cutting shaft with a smooth fractured surface, regardless of rotation direction or group (Figs. 8 and 9). In clockwise rotation, there was a distinct reversal point of the cutting spirals and progressive tightening of these reversed spirals closer to the fractured edge (opposite direction to the normal fluting pattern) (Figs. 10 and 11). The number of tightened spirals toward the tip of the file reflected the number of revolutions at fracture after permanent deformation. Machined files (UB) had less
358
Seto et al.
Joumal of Endodontics
A 40. UB " - "
r ~
30
#20
20
o,o
#15 #10
10
i
i
180
9O
38'0
270
CCW ROTATION (degrees)
B
FiG 8. Fractured surface for twisted (M) files--CW
140
(right) and CCW
(left) rotations (original magnification x63).
"~ #40
I M
120
UB . . . . . 100 e__.~=--.-=-:4.., m #35
80
O hOE=
9
60
-- #30 - #25
e
40 20 0 0
i
i
i
i
90
180
270
360
CCW
ROTATION
(degrees) FiG 7. A, Counterclockwise torque-rotation curve for sizes #10 to 20. Oniy the plastic portion of the graph from yield point (left) to failure (right) was depicted for clarity. B, Counterclockwise torquerotation curve for sizes #25 to 40. Only the plastic portion of the graph from yield point (left) to failure (right) was depicted for clarity. FiG 9. Fractured surface for machined (UB) files--CW CCW (left) rotations (original magnification x63).
(right) and
TABLE 1. Comparison of file diameters (mm) at 1, 3, and 16 mm from the tip Twisted (M) Files File Size 1 mm_+SD
3mm-SD
16mm_+SD
10 15 20 25 30 35 40
0.107 _+ 0.005 0.152 - 0.008 0.201 ___0.005* 0.249 _ 0.005* 0.292 _+ 0.008* 0 . 3 4 8 _ 0.005* 0.409_+ 0.005*
0.150 _ 0.008 0.190 _+ 0.005 0.241 _+ 0.008 0.295 _ 0.008 0.338 -+ 0.008 0.389-+ 0.005* 0.442 + 0.008*
0.389 -+ 0.003 0.462 _ 0.008 0.495 ___0.005 0.579 _+ 0.013" 0.635 _ 0.005* 0.688-+ 0.008* 0.696-+ 0.010"
10 15 20 25 30 35 40
0.119 _ 0.005 0.145_+0.013 0.185 _+ 0.010 0.239 _+ 0.010 0.274 _+ 0.010 0.300 _+ 0.010 0.368 _+ 0.015
Machined (UB) files 0.155 _+ 0.005 0.193_+0.013 0.241 _+ 0.008 0.297 _+ 0.010 0.328 _+ 0.008 0.368 _+ 0.013 0.424 _+ 0.015
0.437 _ 0.003 0.460___0.003 0.498 _+ 0.008 0.561 _+ 0.003 0.577 _+ 0.010 0.622 _+ 0.025 0.706 _+ 0.020
'~Statistically larger than comparable sizes of Union Broach files.
deformation than the twisted files (M). In counterclockwise rotation, there was a progressive tightening o f the spirals as the fractured tip was approached, which was less pronounced for the UB files (Figs. 10 and 11). DISCUSSION The testing apparatus used in this study is a modification of a device described previously by Oliet and Sorin (9) but similar to a more recent derivation used by Chernick et al. (l) and Lautenschlager et al. (2). This method eliminated operator bias by directly monitoring data on the chart recorder of the Instron Testing Machine. This test sequence differed from the A N S I / A D A Specification No. 28 in that the handles were left on during testing. Following testing, no evidence of slippage was seen between the triple set screws and the handle nor the file shaft and the handle.
Vol. 16, No. 8, August 1990
FIG 10. Fractured shaft for twisted (M) files. CW rotation (top) showing "reversal point" (arrow) of spirals and marked deformation. CCW rotation (bottom) showing minimal deformation but subjectively more than Union Broach CCW fractures (original magnification x20).
FIG 11. Fractured shaft for machined (UB) files. CW rotation (top) showing reversal point (arrow) of spirals and marked deformation. CCW rotation (bottom) showing minimal to no deformation (original magnification x20),
For the files tested, there was no demonstrable benefit to the torsional properties derived from the machining method of K-type root canal file fabrication over the conventional twisting manufacturing technique. In these torsional tests, the machined files did not perform as well as the twisted files. This is demonstrated by a lower rotation at failure in clockwise and counterclockwise twisting and less visible deformation of the spirals prior to fracture. The ductility of an endodontic file is measured by the amount of rotation it can withstand prior to failure and can be considered a "safety factor" for endodontic files. The greater this ductility, the more detectable is the deformation of the cutting spirals and the more likely these files will be discarded prior to breakage. At failure, the machined files exhibited less clockwise and counterclockwise rotation than
Torsional Properties of Files
359
the twisted files. Clockwise rotation at failure for the machined files is more comparable to results from other studies obtained for machined Hedstrom files (6, 7). The machined K-type files, however, do exceed minimum ANSI/ADA standards (8) for clockwise rotation. In counterclockwise rotation, there was significantly lower rotation at failure for the machined files in the smaller file sizes (#10 through 30). Thus, the ductility of the machined files is lower than the twisted files tested in this study and offer less of a safety factor against breakage when used clinically. The mechanism of instrument failure was the same for both file types tested in this study. During clockwise torque application, the cutting spirals unraveled. The outer portion of these spirals would then experience longitudinal compression as the spiral direction begani reversing. After the spirals reversed direction, tension would be created in these spirals as they were tightened. Then smooth-ended fracture would result. The fractured surfaces appeared to be similar to the brittle counterclockwise fractures described previously by Chernick et al. (1). During counterclockwise torque application, the cutting spirals tightened further. The outer portion of these spirals experienced immediate longitudinal tension. Then fracture would result. The appearance of the counterclockwise fracture surface was also smooth ended. At failure, both machined and twisted files demonstrated significantly greater clockwise rotation than counterclockwise rotation for the same file size. This can be explained by the above-mentioned mechanism whereby tension in the outer portion of the shaft occurs much earlier in counterclockwise rotation. Clockwise rotation must first overcome the unraveling or compression of the outer aspect of the shaft and develop tension prior to failure, and counterclockwise rotation develops tension in the outer aspect of the shaft immediately. Therefore, lower ductility is expected and seen in counterclockwise rotation. The direction of rotation could not be determined from observations of the fracture surface but could be distinguished by observing the direction of the cutting spirals immediately adjacent to the fractured edge. In the clinically oriented report by Roane and Sabala (10), clockwise rotation caused fractured ends to split and separate (ductile fractures), whereas counterclockwise rotation resulted in smooth-ended fractures (brittle fractures). This frayed appearance was first described as ductile clockwise fracture in an in vitro study by Chernick et al. (1). Only smooth-ended fractures were seen with the two brands of files tested in the present study (Figs. 8 and 9). Therefore, one cannot use the appearance of the fractured end as the sole criterion for determining the direction of rotation causing fracture. The relative flexibility of files can be deduced from the slope of the torque/rotation curve up to the end of elastic deformation (4, 5). The steeper the slope of the curve, the stiffer the file. As expected, the stiffness of the files increased with file size. However, the machined files appeared to be stiffer than their twist manufactured counterparts (Figs. 6 and 7). The reason for this difference in stiffness is not known but it may reflect the difference in the alloy or the difference in the manufacturing method. The number of spirals per length of cutting shaft did not appear to influence flexibility since the machined files were consistently stiffer, despite having more spirals per unit length in the smaller sizes and fewer in the larger sizes.
360
Journal of Endodontics
Seto et al.
The file diameters were fairly consistent within each manufacturer's sizes but there was some variation between the two manufacturers. For example, at 1 m m from the tip, #20 through #40 files from the machined group were as much as one size (0.05 mm) smaller than the same-sized twisted files, whereas the smaller files (#10 and #15) corresponded closely in size. This size difference at the 1-mm level did not reflect the modified tip design of the machined instruments since similar findings were seen at the 16-mm level (Table 1). The diameter variation between the manufacturers did not result in different torsional strengths (Fig. 5) and did not apparently influence the rotation at failure. SUMMARY Torsional strength and rotation at failure of twisted (Maillefer Flex-O-Files) and machined (Union Broach Flex-R) Ktype root canal files were compared. The conclusions are as follows: Twisted files had significantly greater rotation at failure in clockwise and counterclockwise directions than the comparable machined files. Both machined and twisted files exhibited significantly greater rotation at failure in clockwise than counterclockwise torsion. There was no difference in torsional strength between the twisted and machined files. All files exceeded ANSI/ADA Specification No. 28 for clockwise torsional strength and rotation except the machined # 10 files. Machined files showed less visible deformation prior to failure than the twisted files in both rotation directions. Machined files exhibited less ductility than twisted files prior to fracture.
The mechanism of failure was similar for twisted and machined files regardless of direction of torsion. The direction of cutting spirals adjacent to the fracture end can be used to determine the direction of rotation which caused failure for the two files tested in this study. The authors would like to thank the Union Broach and L. D. Caulk/Dentsply companies for donation of their instruments for this study. The authors would also like to thank Dr. Robert Oswald for his input and critique of this research project. Dr. Seto is a former graduate student in endodontics and is now in private practice in Santa Monica, CA. Dr. Nicholls is professor, Department of Restorative Dentistry, School of Dentistry, University of Washington, Seattle, WA. Dr. Harrington is professor, Department of Endodontics, and director, Graduate Endedontics Program, School of Dentistry, University of Washington, Seattle, WA.
9
References 1. Chemick LB, Jacobs JJ, Lautenschlager EP, Heuer MA. Torsional failure of endodontic files. J Endodon 1976;2:94-7. 2. Lautenschlager EP, Jacobs JJ, Marshall GW, Heuer MA. Brittle and ductile torsional failures of endodontic instruments. J Endodon 1977;3:175-8. 3. Lentine, FN. A study of torsional and angular deflection of endodontic files and reamers. J Endodon 1979;5:181-91. 4. Dolan DW, Craig RG. Bending and torsion of endodontic files with rhombus cross-sections. J Endodon 1982;8:260-4. 5. Krupp JD, Brantley WA, Gerstein H. An investigation of the torsional and bending properties of seven brands of endodontic files, J Endodon 1984;10:372-80. 6. Mueller HJ, Suchak AJ, Stanford WB, Stanford JW, Stanford SK. Comparison of some root canal instruments in bending and torsion to newly formed or draft specifications. J Endodon 1984; 10:182-7. 7. Bolger WL, Gough RW, Foster CD. A comparsion of the potential for breakage: the Bums Unifile versus Hedstrom files. J Endodon 1985;11:110-6. 8. Council on Dental Materials, Instruments and Equipment, Revised American National Standards Institute/American Dental Association Specification No. 28 for root canal files and reamers, type K (revised 1988). 9. Oliet S, Sorin SM. Torsional tester for root canal instruments. Oral Surg Oral Med Oral Pathol 1965;20:654-62. 10. Roane JB, Sabala C. Clockwise or counterclockwise. J Endodon 1984; 10:349-53.