SCIENTIFIC ARTICLES
T o r s i o n a l f a i l u r e of e n d o d o n t i c f i l e s Lester B. Chernick, DDS; Joshua J. Jacobs; Euqene P. Lautenschlaqer, MS, PhD; and Michael A. Heuer, DDS, MS, Chicaqo
The effects of s o m e hitherto i g n o r e d c l i n i c a l v a r i a b l e s o n the t o r s i o n a l p r o p e r t i e s of e n d o d o n t i c files w e r e d e t e r m i n e d . T o r s i o n a l tests a n d s c a n n i n g e l e c t r o n m i c r o s c o p e e x a m i n a t i o n r e v e a l e d that e n d o d o n t i c files t w i s t e d in a c o u n t e r c l o c k w i s e m a n n e r w e r e e x t r e m e l y brittle in c o m p a r i s o n to those twisted in a c l o c k w i s e m a n n e r . This p h e n o m e n o n r e p r e s e n t s a p o t e n t i a l c o n t r i b u t o r to the c l i n i c a l f r a c t u r e of t h e s e i n s t r u m e n t s . A d d i t i o n a l tests s h o w e d that e n d o d o n t i c b e a d sterilization at 425 F h a s n o a p p a r e n t effect o n t o r s i o n a l s t r e n q t h a n d d u c t i l i t y a n d that t h e s e e n d o d o n t i c files do n o t e x h i b i t s t r a i n - r a t e s e n s i t i v i t y .
The occasional fracture of a file or reamer in a root canal continues to be an inherent hazard in endodontic therapy. While many have investigated the mechanical properties of these instruments,1-7 no one has conclusively accounted for their seemingly random failure. The question remains as to whether the fracture of endodontic instruments is due to inadvertent clinical misuse or to inherent flaws in the instrument itself. This study was undertaken to investigate the effect of some hitherto ignored clinical variables on the torsional properties of one brand of endodontic file. Clockwise and counterclockwise torsional strength and ductility, the effect of hot bead sterilization, strain-rate sensitivity, and torsional fracture mode were ex-
94
amined in an effort to attribute instrument breakage to either manufacturing flaws or clinical mishandling.
MATERIALS AND METHODS Torsional Strenqth and Ductility Torsional tests were performed on size no. 10, 15, 20, 25, and 30 stainless steel files.* Although described as stainless steel, these instruments actually consisted of a carbon steel core with stainless steel plating for anticorrosive properties and tissue tolerance. Fhese files were of interest to us since, at the time of this study, they were being used by the students a~ the Northwestern University dental clinics. This particular size interval was chosen because these instruments are the most prone to fracture in clinical use. The torsional testing apparatus used
in this study (Fig 1) is a modification of that proposed by Oliet and Sorin. 6 Our apparatus differs primarily in that it is coupled to an Instron testing machine, t which provides far more sensitive data. Controlled deformation rates to _ 0 . 0 0 2 inches/min and load detection to -4-0.02 gm were made possible with the use of this instrument. The following procedure was followed when testing (Fig 1): (1) The torque apparatus was fitted onto the crosshead of the Instron testing machine in such a way that the suspended cord was perpendicular to the axis of rotation of the cylindrical shaft. This simplified the calculation of torque. (2) The cord was wrapped around an aluminum pulley in a clockwise or counterclockwise direction such that each strand was alongside the next. This ensured a constant radius throughout the test. (3) The file handle was inserted in the bore of the shaft and secured with a set screw. The file was then aligned so that it was parallel to the axis of rotation of the shaft to ensure that it was subjected to pure torsion. (4) The alumin u m pulley, which was narrower than the distance between the brass supports, was centered between the brass supports to prevent friction between
JOURNAL OF ENDODONTICS I vo~- 2, NO 4, APRIL 1976
/
RESULTS
TEST FILE
/
~1J
SUSPENDED
/
SET
~
CORD
(TO
BRASS SUPPORTS
the pulley and the support. In addition, as reported by Oliet and Sorin,~ this allowed the shaft to float longitudinally to correct for instrument elongation or contraction encountered during testing. (5) Finally, the working end of the file was clamped between two brass jaws 3 m m from the tip, as suggested by Heuer.2 The vertical motion of the testing machine crosshead was translated to rotational motion of the shaft by the pulley which was secured to the shaft with a set screw. In turn, this applied torsional stress on the attached file. Torsional strength, reported as maximum torque, and ductility, reported as number of revolutions to failure, were calculated using the following formulas: T m a x = (rp + re) • Pmax, where Tmax i s maximum torque, rp and re are the radius of the pulley and cord, respectively, and Pm~x is the maximum load recorded by the testing machine. The number of revolutions to failure (N) is
N=d C where d is the distance moved by the crosshead from the onset of initial torque to fracture, and C is the circumference of the pulley and string.
Strain-Rate Sensitivity To determine whether the rate of torque application has an effect on strength and ductility, torsional tests were repeated on size no. 10 through 30 files at 10 and 0.1 times the original crosshead speed of 5.0 inches/ min. B e a d Sterilization T e s t s The effect of hot bead sterilization on torsional strength and ductility also was examined. A n endodontic hot bead sterilizers was preheated for 15 minutes to 425 F. Files (size no. 15 through 30) were placed in the sterilizer 89 inch from the sides and to the depth of the handle for ten seconds, as prescribed by the manufacturer. Each instrument was then allowed to cool at room temperature for 30 seconds. This cycle was repeated 100 times for each instrument. Torsional tests were then performed on the sterilized files. T o r s i o n a l Fracture M o d e To provide additional information on the nature of instrument failure, fracture surfaces were analyzed with a scanning electron microscope (SEM).w Micrographs obtained by the SEM were used as an aid in interpreting the results of torsion,a1 tests.
T o r s i o n a l S t r e n q t h a n d Ductility The torsional properties of the files are reported in the Table. F o r a given size, there is surprisingly little variation in maximum torque and revolutions to failure. As anticipated, maximum torque increases with increasing file size. Also, with the exception of size no. 30, there is no significant difference in clockwise and counterclockwise torsional strength for a given size. The most striking feature about these data is the difference in clockwise and counterclockwise ductility. In the clockwise mode, files could generally sustain better than one complete revolution before fracture. In sharp contrast, files tested in the counterclockwise mode failed in less than half a revolution. Values in the Table show no correlation between file size and ductility. Figure 2 shows the typical shape of a torque-revolution curve of ,a no. 20 file, as recorded by the testing machine. This figure emphasizes the notable difference in clockwise and counterclockwise ductility. Notice that torque rises linearly with the number of revolutions up to a certain point and. then, for clockwise torsion, increases slowly as the file unflutes. This continues for over three revolutions. In counterclockwise torsion, the torque at the end of the linear region is greater than that for clockwise torsion; but the file fractures shortly after the linear region has been exceeded. Subsequent tests in our laboratory, with the same equipment and procedure, indicate that counterclockwise brittleness is not unique to the product tested in this report. Strain-Rate Sensitivity Differences in torsional strength and ductility observed for the three different crosshead speeds were corn95
JOURNAL OF ENDODONTICS [ VOL 2, NO 4, APRIL 1976
Table 9 Tonfoncd Properties of files.
Clockwise Counterclockwise Maximum Maximum torque Revolutions torque Revolutions Size (no.) (gin-era) to failure (gm-cm) to failure 10
15 20 25 30
11.7
1.02
(1.8) 18.4 (3.7) 38.2 (2.9) 50.7 (7.2)
(0.25) 1.50 (0.11) 3.25 (0.48) 3.58 (1.01)
70.6
1.11
(4.9)
(0.38)
8.07 (0.81) 19.5 (1.3) 36.7 (3.2) 55.7 (8.0)
0.45 (0.08) 0.28 (0.03) 0.43 (0.07) 0.38 (0.08)
96.3
0.46
(9.0)
(0.04)
Note: Data are reported as the mean of five samples with the standard deviation in parenthesis.
pared by the statistical t test. Results of this test show that at the 95% confidence level there is no significant difference in maximum torque or revolutions to failure for the crosshead speeds of 50, 5, and 0.5 inches/min. Bead
Sterilization
Tests
The statistical t test revealed that at the 95% confidence level there is no significant difference between sterilized and nonsterilized files in either torsional strength or ductility. Torsional
Fracture Mode
Figures 3 and 4 represent fracture surfaces of files that have failed in torsion. Figure 3 is typical of the fracture surface of a file that has failed in clockwise torsion. Note the extensive plastic deformation that occurred before fracture. Figure 4 is typical of the cleavage-type fracture surface which resulted from failure of a file in counterclockwise torsion. This photomicrograph indicates that little plastic deformation has occurred before fracture. These findings are in keeping with the results of the tor-
96
sional tests--ductile fracture observed in clockwise torsion and brittle fracture observed in counterclockwise torsion would predict that these instruments would sustain a greater number of revolutions in clockwise torsion. Of additional interest, these photomicrographs indicate that the stainless steel plating has a tendency to break away from the carbon steel core. DISCUSSION
All previous studies that have examined the torsional properties of endodontic instruments have been concerned solely with clockwise torsion, since this is the direction of clinical force application. However, if an instrument is inadvertently lodged in a canal, the normal clinical response is a counterclockwise motion. Therefore, we thought that counterclockwise torsional properties should be studied. Indeed, the results of the counterclockwise torsion tests proved to be the most interesting. Both torsional tests and SEM examination showed that files twisted in a counterclockwise manner were extremely brittle in
~40
COUNTERCLOCKWISE
I
~o ta
2o
o n. 10 o i0
I
I
I
I
2
3
REVOLUTIONS
Fig 2--Torque-revolution curves /or no. 20 file delineating difference in clockwise and counterclockwise ductility. Standard deviation of five samples in maximum torque and revolutions to failure is indicated by boxes.
comparison with those twisted in a clockwise manner. This finding may have far reaching implications for clinical practice and instrument design. The practitioner should be warned that his files could be very brittle when placed in counterclockwise torsion. Therefore, great caution should be exercised when backing off an embedded instrument during the preparation of a root canal. This phenomenon is most likely a consequence of the fabrication technique. Normally, endodontic instruments are made from rectangular or triangular carbon or stainless s t e e l wire blanks that have been ground to the proper taper and twisted in a counterclockwise f a s h i o n to produce either a file or reamer. Perhaps the twisting procedure locks in residual stresses that act to decrease the instrument's ductility in counterclockwise torsion. This effect would probably be more pronounced in files, which are twisted more tightly than reamers when fabricated. (A file has a greater number of flutes per unit length than does a reamer.) As a
JOURNAL OF ENDODONTICS [ VOL 2, NO 4, APRIL 1976
had a tendency to break away from the core.
Fig 3---Ductile fracture encountered during clockwise torsion o/ no. 25 file (orig mag X 100).
result of this finding, we strongly recommend that counterclockwise torsional properties be included in pending performance specifications for root canal files and reamers. The effect of hot bead sterilization was investigated because we suspected that the recurrent high temperatures encountered during this procedure could change the temper of the instrument and, therefore, deleteriously affect the mechanical properties. However, for an instrument sterilized as prescribed by the American Dental Association Council on Dental Therapeutics, this was found not to be the case. In addition, these instruments are not strain-rate sensitive within the deformation rates used in this study. Clinically, this suggests that neither hot bead sterilization nor the rate of torque application will affect the torsional strength or ductility of these instruments. In light of SEM studies, the efficacy of plating stainless steel over a carbon steel core to produce endodontic instruments must be questioned. The tendency of the plating to break away from the core may have deleterious effects on the cutting efficiency and corrosion resistance and also may present the possibility that metal chips could become lodged in the canal. Subsequent to the completion of this investigation the manufac-
Fig 4--Brittle fracture encountered during counterclockwise torsion o/ no. 15 file. Note that stainless steel plating is breaking away from carbon steel core (orig mag X300).
ture of plated instruments has, to our knowledge, been discontinued. SUMMARY
AND
CONCLUSIONS
A potential contributor to the clinical fracture of endodontic instruments was found. Torsional tests and SEM examination revealed that files twisted in a counterclockwise manner were extremely brittle in comparison to those twisted in a clockwise manner. These findings lead us to suggest the following: - - E n d o d o n t i c practitioners should exercise caution when backing off embedded root canal instruments during the cleansing and shaping of the root canal. --Counterclockwise torsional properties should be considered in pending specifications for root canal files and reamers. Other findings on the product tested include the following: - - B e a d sterilization at 425 F has no apparent effect on torsional strength and ductility. - - E n d o d o n t i c files are not strainrate sensitive. Therefore, the rate of clinical torque application will not affect strength and ductility. ----The efficacy of plating stainless steel over carbon steel in producing endodontic instruments is questioned. SEM studies revealed that the plating
*Star micrograin K-type 25-mm files, S~ar Dental Mfg. Co., Inc., Philadelphia. tlnstron Corp., Canton, Mass. *Pulpdent Corporation of America, Brookline, Mass. w Stereoscan S - 4 , Kent Cambridge Instruments Ltd., Cambridge, Eng. The authors thank Dr. B. F. Gurney and Dr. B. K. Moore for their assistance in experimental design and method. Dr. Chernick is assistant professor, department of endodontics; Mr. Jacobs is an undergraduate research student in the department of materials science and engineering; Dr. Lautenschlager is professor, department of biological materials; and Dr. Heuer is professor and chairman, department of endodontics at Northwestern University, Chicago. Reprint requests should be directed to Dr. E. P. Lautenschlager, Department of Biological Materials, Dental School, Northwestern University, Chicago, Ill 60611. References
1. Bucher, J. A preliminary investigation of the feasibility of stainless steel files as root canal instruments and as a root canal filling material. Master's thesis, Ann Arbor, Mich, School of Dentistry, University of Michigan, 1958. 2. Heuer, M.A. A study of the structural, dimensional and physical characteristics of root canal instruments. Master's thesis, Ann Arbor, Mich, School of Dentistry, University of Michigan, 1959. 3. Craig, R.G., and Peyton, F.A. Physical properties of carbon steel root canal files and reamers. Oral Surg 15:213 Feb 1962. 4. Craig, R.G., and Peyton, F.A. Physical properties of stainless steel endodontic files and reamers. Oral Surg 16:206 Feb 1963. 5. Craig, R.G.; Mcllwain, E.D.; and Peyton, F.A. Bending and torsion properties of endodontic instruments. Oral Surg 25:239 Feb 1968. 6. Oliet, S., and Sorin, S.M. Torsional tester for root canal instruments. Oral Surg 20:654 Nov 1965. 7. Lilley, J.D., and Smith, D.C. An investigation of the fracture of root canal reamers. Br Dent J 120:364, April 19, 1966. 97
T o r s i o n a l f a i l u r e of e n d o d o n t i c f i l e s Lester B. Chernick, DDS; Joshua J. Jacobs; Euqene P. Lautenschlaqer, MS, PhD; and Michael A. Heuer, DDS, MS, Chicaqo
The effects of s o m e hitherto i g n o r e d c l i n i c a l v a r i a b l e s o n the t o r s i o n a l p r o p e r t i e s of e n d o d o n t i c files w e r e d e t e r m i n e d . T o r s i o n a l tests a n d s c a n n i n g e l e c t r o n m i c r o s c o p e e x a m i n a t i o n r e v e a l e d that e n d o d o n t i c files t w i s t e d in a c o u n t e r c l o c k w i s e m a n n e r w e r e e x t r e m e l y brittle in c o m p a r i s o n to those twisted in a c l o c k w i s e m a n n e r . This p h e n o m e n o n r e p r e s e n t s a p o t e n t i a l c o n t r i b u t o r to the c l i n i c a l f r a c t u r e of t h e s e i n s t r u m e n t s . A d d i t i o n a l tests s h o w e d that e n d o d o n t i c b e a d sterilization at 425 F h a s n o a p p a r e n t effect o n t o r s i o n a l s t r e n q t h a n d d u c t i l i t y a n d that t h e s e e n d o d o n t i c files do n o t e x h i b i t s t r a i n - r a t e s e n s i t i v i t y .
The occasional fracture of a file or reamer in a root canal continues to be an inherent hazard in endodontic therapy. While many have investigated the mechanical properties of these instruments,1-7 no one has conclusively accounted for their seemingly random failure. The question remains as to whether the fracture of endodontic instruments is due to inadvertent clinical misuse or to inherent flaws in the instrument itself. This study was undertaken to investigate the effect of some hitherto ignored clinical variables on the torsional properties of one brand of endodontic file. Clockwise and counterclockwise torsional strength and ductility, the effect of hot bead sterilization, strain-rate sensitivity, and torsional fracture mode were ex-
94
amined in an effort to attribute instrument breakage to either manufacturing flaws or clinical mishandling.
MATERIALS AND METHODS Torsional Strenqth and Ductility Torsional tests were performed on size no. 10, 15, 20, 25, and 30 stainless steel files.* Although described as stainless steel, these instruments actually consisted of a carbon steel core with stainless steel plating for anticorrosive properties and tissue tolerance. Fhese files were of interest to us since, at the time of this study, they were being used by the students a~ the Northwestern University dental clinics. This particular size interval was chosen because these instruments are the most prone to fracture in clinical use. The torsional testing apparatus used
in this study (Fig 1) is a modification of that proposed by Oliet and Sorin. 6 Our apparatus differs primarily in that it is coupled to an Instron testing machine, t which provides far more sensitive data. Controlled deformation rates to _ 0 . 0 0 2 inches/min and load detection to -4-0.02 gm were made possible with the use of this instrument. The following procedure was followed when testing (Fig 1): (1) The torque apparatus was fitted onto the crosshead of the Instron testing machine in such a way that the suspended cord was perpendicular to the axis of rotation of the cylindrical shaft. This simplified the calculation of torque. (2) The cord was wrapped around an aluminum pulley in a clockwise or counterclockwise direction such that each strand was alongside the next. This ensured a constant radius throughout the test. (3) The file handle was inserted in the bore of the shaft and secured with a set screw. The file was then aligned so that it was parallel to the axis of rotation of the shaft to ensure that it was subjected to pure torsion. (4) The alumin u m pulley, which was narrower than the distance between the brass supports, was centered between the brass supports to prevent friction between
JOURNAL OF ENDODONTICS I vo~- 2, NO 4, APRIL 1976
/
RESULTS
TEST FILE
/
~1J
SUSPENDED
/
SET
~
CORD
(TO
BRASS SUPPORTS
the pulley and the support. In addition, as reported by Oliet and Sorin,~ this allowed the shaft to float longitudinally to correct for instrument elongation or contraction encountered during testing. (5) Finally, the working end of the file was clamped between two brass jaws 3 m m from the tip, as suggested by Heuer.2 The vertical motion of the testing machine crosshead was translated to rotational motion of the shaft by the pulley which was secured to the shaft with a set screw. In turn, this applied torsional stress on the attached file. Torsional strength, reported as maximum torque, and ductility, reported as number of revolutions to failure, were calculated using the following formulas: T m a x = (rp + re) • Pmax, where Tmax i s maximum torque, rp and re are the radius of the pulley and cord, respectively, and Pm~x is the maximum load recorded by the testing machine. The number of revolutions to failure (N) is
N=d C where d is the distance moved by the crosshead from the onset of initial torque to fracture, and C is the circumference of the pulley and string.
Strain-Rate Sensitivity To determine whether the rate of torque application has an effect on strength and ductility, torsional tests were repeated on size no. 10 through 30 files at 10 and 0.1 times the original crosshead speed of 5.0 inches/ min. B e a d Sterilization T e s t s The effect of hot bead sterilization on torsional strength and ductility also was examined. A n endodontic hot bead sterilizers was preheated for 15 minutes to 425 F. Files (size no. 15 through 30) were placed in the sterilizer 89 inch from the sides and to the depth of the handle for ten seconds, as prescribed by the manufacturer. Each instrument was then allowed to cool at room temperature for 30 seconds. This cycle was repeated 100 times for each instrument. Torsional tests were then performed on the sterilized files. T o r s i o n a l Fracture M o d e To provide additional information on the nature of instrument failure, fracture surfaces were analyzed with a scanning electron microscope (SEM).w Micrographs obtained by the SEM were used as an aid in interpreting the results of torsion,a1 tests.
T o r s i o n a l S t r e n q t h a n d Ductility The torsional properties of the files are reported in the Table. F o r a given size, there is surprisingly little variation in maximum torque and revolutions to failure. As anticipated, maximum torque increases with increasing file size. Also, with the exception of size no. 30, there is no significant difference in clockwise and counterclockwise torsional strength for a given size. The most striking feature about these data is the difference in clockwise and counterclockwise ductility. In the clockwise mode, files could generally sustain better than one complete revolution before fracture. In sharp contrast, files tested in the counterclockwise mode failed in less than half a revolution. Values in the Table show no correlation between file size and ductility. Figure 2 shows the typical shape of a torque-revolution curve of ,a no. 20 file, as recorded by the testing machine. This figure emphasizes the notable difference in clockwise and counterclockwise ductility. Notice that torque rises linearly with the number of revolutions up to a certain point and. then, for clockwise torsion, increases slowly as the file unflutes. This continues for over three revolutions. In counterclockwise torsion, the torque at the end of the linear region is greater than that for clockwise torsion; but the file fractures shortly after the linear region has been exceeded. Subsequent tests in our laboratory, with the same equipment and procedure, indicate that counterclockwise brittleness is not unique to the product tested in this report. Strain-Rate Sensitivity Differences in torsional strength and ductility observed for the three different crosshead speeds were corn95
JOURNAL OF ENDODONTICS [ VOL 2, NO 4, APRIL 1976
Table 9 Tonfoncd Properties of files.
Clockwise Counterclockwise Maximum Maximum torque Revolutions torque Revolutions Size (no.) (gin-era) to failure (gm-cm) to failure 10
15 20 25 30
11.7
1.02
(1.8) 18.4 (3.7) 38.2 (2.9) 50.7 (7.2)
(0.25) 1.50 (0.11) 3.25 (0.48) 3.58 (1.01)
70.6
1.11
(4.9)
(0.38)
8.07 (0.81) 19.5 (1.3) 36.7 (3.2) 55.7 (8.0)
0.45 (0.08) 0.28 (0.03) 0.43 (0.07) 0.38 (0.08)
96.3
0.46
(9.0)
(0.04)
Note: Data are reported as the mean of five samples with the standard deviation in parenthesis.
pared by the statistical t test. Results of this test show that at the 95% confidence level there is no significant difference in maximum torque or revolutions to failure for the crosshead speeds of 50, 5, and 0.5 inches/min. Bead
Sterilization
Tests
The statistical t test revealed that at the 95% confidence level there is no significant difference between sterilized and nonsterilized files in either torsional strength or ductility. Torsional
Fracture Mode
Figures 3 and 4 represent fracture surfaces of files that have failed in torsion. Figure 3 is typical of the fracture surface of a file that has failed in clockwise torsion. Note the extensive plastic deformation that occurred before fracture. Figure 4 is typical of the cleavage-type fracture surface which resulted from failure of a file in counterclockwise torsion. This photomicrograph indicates that little plastic deformation has occurred before fracture. These findings are in keeping with the results of the tor-
96
sional tests--ductile fracture observed in clockwise torsion and brittle fracture observed in counterclockwise torsion would predict that these instruments would sustain a greater number of revolutions in clockwise torsion. Of additional interest, these photomicrographs indicate that the stainless steel plating has a tendency to break away from the carbon steel core. DISCUSSION
All previous studies that have examined the torsional properties of endodontic instruments have been concerned solely with clockwise torsion, since this is the direction of clinical force application. However, if an instrument is inadvertently lodged in a canal, the normal clinical response is a counterclockwise motion. Therefore, we thought that counterclockwise torsional properties should be studied. Indeed, the results of the counterclockwise torsion tests proved to be the most interesting. Both torsional tests and SEM examination showed that files twisted in a counterclockwise manner were extremely brittle in
~40
COUNTERCLOCKWISE
I
~o ta
2o
o n. 10 o i0
I
I
I
I
2
3
REVOLUTIONS
Fig 2--Torque-revolution curves /or no. 20 file delineating difference in clockwise and counterclockwise ductility. Standard deviation of five samples in maximum torque and revolutions to failure is indicated by boxes.
comparison with those twisted in a clockwise manner. This finding may have far reaching implications for clinical practice and instrument design. The practitioner should be warned that his files could be very brittle when placed in counterclockwise torsion. Therefore, great caution should be exercised when backing off an embedded instrument during the preparation of a root canal. This phenomenon is most likely a consequence of the fabrication technique. Normally, endodontic instruments are made from rectangular or triangular carbon or stainless s t e e l wire blanks that have been ground to the proper taper and twisted in a counterclockwise f a s h i o n to produce either a file or reamer. Perhaps the twisting procedure locks in residual stresses that act to decrease the instrument's ductility in counterclockwise torsion. This effect would probably be more pronounced in files, which are twisted more tightly than reamers when fabricated. (A file has a greater number of flutes per unit length than does a reamer.) As a
JOURNAL OF ENDODONTICS [ VOL 2, NO 4, APRIL 1976
had a tendency to break away from the core.
Fig 3---Ductile fracture encountered during clockwise torsion o/ no. 25 file (orig mag X 100).
result of this finding, we strongly recommend that counterclockwise torsional properties be included in pending performance specifications for root canal files and reamers. The effect of hot bead sterilization was investigated because we suspected that the recurrent high temperatures encountered during this procedure could change the temper of the instrument and, therefore, deleteriously affect the mechanical properties. However, for an instrument sterilized as prescribed by the American Dental Association Council on Dental Therapeutics, this was found not to be the case. In addition, these instruments are not strain-rate sensitive within the deformation rates used in this study. Clinically, this suggests that neither hot bead sterilization nor the rate of torque application will affect the torsional strength or ductility of these instruments. In light of SEM studies, the efficacy of plating stainless steel over a carbon steel core to produce endodontic instruments must be questioned. The tendency of the plating to break away from the core may have deleterious effects on the cutting efficiency and corrosion resistance and also may present the possibility that metal chips could become lodged in the canal. Subsequent to the completion of this investigation the manufac-
Fig 4--Brittle fracture encountered during counterclockwise torsion o/ no. 15 file. Note that stainless steel plating is breaking away from carbon steel core (orig mag X300).
ture of plated instruments has, to our knowledge, been discontinued. SUMMARY
AND
CONCLUSIONS
A potential contributor to the clinical fracture of endodontic instruments was found. Torsional tests and SEM examination revealed that files twisted in a counterclockwise manner were extremely brittle in comparison to those twisted in a clockwise manner. These findings lead us to suggest the following: - - E n d o d o n t i c practitioners should exercise caution when backing off embedded root canal instruments during the cleansing and shaping of the root canal. --Counterclockwise torsional properties should be considered in pending specifications for root canal files and reamers. Other findings on the product tested include the following: - - B e a d sterilization at 425 F has no apparent effect on torsional strength and ductility. - - E n d o d o n t i c files are not strainrate sensitive. Therefore, the rate of clinical torque application will not affect strength and ductility. ----The efficacy of plating stainless steel over carbon steel in producing endodontic instruments is questioned. SEM studies revealed that the plating
*Star micrograin K-type 25-mm files, S~ar Dental Mfg. Co., Inc., Philadelphia. tlnstron Corp., Canton, Mass. *Pulpdent Corporation of America, Brookline, Mass. w Stereoscan S - 4 , Kent Cambridge Instruments Ltd., Cambridge, Eng. The authors thank Dr. B. F. Gurney and Dr. B. K. Moore for their assistance in experimental design and method. Dr. Chernick is assistant professor, department of endodontics; Mr. Jacobs is an undergraduate research student in the department of materials science and engineering; Dr. Lautenschlager is professor, department of biological materials; and Dr. Heuer is professor and chairman, department of endodontics at Northwestern University, Chicago. Reprint requests should be directed to Dr. E. P. Lautenschlager, Department of Biological Materials, Dental School, Northwestern University, Chicago, Ill 60611. References
1. Bucher, J. A preliminary investigation of the feasibility of stainless steel files as root canal instruments and as a root canal filling material. Master's thesis, Ann Arbor, Mich, School of Dentistry, University of Michigan, 1958. 2. Heuer, M.A. A study of the structural, dimensional and physical characteristics of root canal instruments. Master's thesis, Ann Arbor, Mich, School of Dentistry, University of Michigan, 1959. 3. Craig, R.G., and Peyton, F.A. Physical properties of carbon steel root canal files and reamers. Oral Surg 15:213 Feb 1962. 4. Craig, R.G., and Peyton, F.A. Physical properties of stainless steel endodontic files and reamers. Oral Surg 16:206 Feb 1963. 5. Craig, R.G.; Mcllwain, E.D.; and Peyton, F.A. Bending and torsion properties of endodontic instruments. Oral Surg 25:239 Feb 1968. 6. Oliet, S., and Sorin, S.M. Torsional tester for root canal instruments. Oral Surg 20:654 Nov 1965. 7. Lilley, J.D., and Smith, D.C. An investigation of the fracture of root canal reamers. Br Dent J 120:364, April 19, 1966. 97
