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The effects of various sterilization methods on the torsional strength of endodontic files
0099-2399/85/1106-0266/$02.00/0 JOURNALOF ENDODONTICS Copyright 9 1985 by The American Associationof Endodontists
Printed in U.S.A. VOL. 11, NO. 6, JUNE1985
The Effects of Various Sterilization Methods on the Torsional Strength of Endodontic Files Efectos de Varios Metodos de Esterilizacion en la Resistencia a la Fractura por Torsion de Limas Endodonticas G. W. Iverson, CD, DDS, J. A. yon Fraunhofer, MSc, PhD, and J. W. Herrmann, DMD,FACD,FICD
estufa de calor seco en comparaci6n con los controles; y aun cuando no hubo diferencia notable en los grados de rotacion necesarios' para producir fractura las limas K sobresalleron-ligeramente como mas resistentes a la torsion.
The effects of different sterilization methods on the torsional strength of two types of endodontic files have been studied. Kerr K-Flex files and Burns Unifiles were subjected to ten cycles of autoclaving, bead, dry heat, and cold chemical sterilization. A torquemeter was used to determine the degrees of revolution and torque to failure for the files after repeated sterilization. Repeated sterilization was found to have no effect on the torque resistance and degrees to failure for the Burns Unifile and no effect on degrees to failure for the K-Flex file. Dry heat and cold sterilization, however, slightly increased the torque resistance of the K-Flex files as compared with the controls and the other sterilization methods. No differences were found between the files in the degrees of rotation to failure but the KFlex files had a slightly greater torque resistance.
Fracture of root canal files and/or reamers is an occasional problem in endodontic therapy and the prognosis for a fractured file varies with its location and other factors. Fracture in the apical third with the file effectively filling the canal by being tightly lodged with dentin shavings packed ahead of it, often allows the broken file to be incorporated as part of the filling material. If the file does not fill the canal and cannot be bypassed, then endodontic surgery becomes necessary. A surgical approach may also become necessary if the file breaks in the middle third of the tooth and cannot be bypassed. Finally, if fracture occurs beyond the apical foramen, the periapical portion must be removed since the protruding portion will be a mechanical irritant under occlusal loading of the tooth (1). Fracture of an endodontic file occurs under torsional loading when it cuts and wedges into the canal wall, especially when it binds tightly against the wall. The ANSI/ADA specification no. 28 (2) requires 360 degrees of angular deflection as a minimum value for clockwise torquing on certain endodontic files and a torque strength of 65 g-cm. The torsional strength and degrees of rotation to failure of a file are instrument characteristics that are determined by the fabrication material, the method of fabrication, and any procedures that might adversely affect these properties. In particular, the elevated temperatures applied during sterilization might adversely affect endodontic files. Mitchell et al. (3) recently showed that there was no decrease in the torsional strength of Kerr K-type files until autoclaved at least ten times, sizes 35 and 40 being the most severely affected. These authors performed a simulated
En presente articulo estudia los efectos de varios metodos de esterilizacion en la resistencia a la fractura por torsion de dos diferentes tipos de limas endodonticas. Una muestra d e limas K-Flex num 35 (instrumentos fabricados por rotaci6n de una varillita metalica romboidal), y otra de limas Burns unifiles (instrumento fabricado por corte a maquina de una varillita met/,lica), tambien numero 35, fueron sometidas a diez ciclos de esterilizacion por diferentes metodos: autoclave, calor por bolitas de cuarzo, estufa de calor seco y substancias quimicas. Despues de estas exposiciones controladas y utilizando un medidor de resistencia a la torsi6n se midio el n0mero de grados de rotaci6n y fuerza de torsion requeridos para producir una fractura del instrumento. No se encontro relacion-directa causaefecto entre los methodos de esterilizacion empleados y la fractura por torsion tanto en limas K como en limas Burns. Sin embargo se observo una ligera diferencia que. Indicaba mayor resistencia ala fractura por torsion de las limas K, esterilizadas en 266
Sterilization Methods of Files
Vol. 11, No. 6, June 1985
clinical use procedure prior to torque testing. Chernick et al. (4) showed, in torsional tests, that files twisted counterclockwise exhibited greater brittleness than when twisted clockwise. These investigators also showed that endodontic bead sterilization had no effect on the torsional strength of Star K-type files of sizes 10, 15, 20, 25, and 30 following a 10-s glass bead immersion. Eichner et al. (5), in a photographic study, showed that sodium hypochlorite solution did not corrode stainless steel endodontic files. No studies appear to have been performed on the effects of cold sterilizing (germicidal) solutions on endodontic files and there are no data in the literature on the effect of other sterilization methods on the torsional strength of endodontic files. The present study was undertaken to assess the effects of four commonly used sterilization methods on the torsional strength of two types of endodontic files. MATERIALS AND METHODS Two types of endodontic files, Kerr K-Flex size 35 and the recently introduced Burns Unifile size 35, were used in this study. The Kerr K-Flex file is a twisted file of diamond-shaped configuration and the Burns Unifile is a machine-cut file. The commonly used size 35 file was selected for this study. Sixty files of each type were divided into five groups of 12 files. One group of 12 was used as controls, the other groups being subjected to the different sterilization regimens. The second group was subjected to ten autoclaving cycles in a general purpose electric steam generator autoclave based on the gravity air-displacement principle (Verna-Clave Sterilizer model R2038 CLZ-1, Vernitron Medical Products Inc., Carlstadt, NJ). The autoclave was operated with dry saturated steam only and without addition of a corrosion-inhibiting solution. The files were placed in an open tray and heated to 127~ at 30 psi for 30 min and dried rapidly with forced exhaust. This cycle was repeated ten times over 5 days. The third group of files was subjected to ten cycles of glass bead sterilization in a Buffalo model 62 sterilizer
(Buffalo Dental Manufacturing Co. Inc., Brooklyn, NY). Prior to use, the sterilizer was heated for 1 h to 218~ The files were placed in the sterilizer to the depth of the handle one-half inch from the sides and held for 20 s. The files were then allowed to cool to room temperature for 30 s. This cycle was repeated ten times for each file. The fourth group of files was placed in an open tray in a dry heat oven (Steri-Dent Model 200, Copiague, NY) at 171~ for 1 h. The files were then cooled to room temperature for 15 min. This cycle was repeated ten times. The last group of files was subjected to cold sterilization solution (Lehn and Fink Instrument germicide solution) for ten cycles of 30 min each. The files were then rinsed with sterile saline and air dried. Torsional testing of the five groups of files was performed with the torqumeter memocouple (2) (Les Fils d'Auguste Maillefer SA). The instrument operates at 2 rpm, automatically recording on a digital display the degrees of rotation and torque required for file fracture. The files were prepared for testing by removing the handle from each file at the point of attachment to the shaft and then inserting the shank of the file into the drive chuck. The tip of each file was inserted 3 mm into the opposing brass jaws, the file was aligned, and the chucks were tightened. As the torquemeter induced a clockwise rotation in the file, the torque applied and the number of degrees of rotation to failure were automatically recorded and displayed. RESULTS The degrees of revolution (angle) and the torque required to failure for the two types of files and four sterilization methods are given in Table 1. The reproducibility of the data was good as shown by a coefficient of variation generally less than 15%, except for the angular deflection values for the Burns Unifiles where values of up to 20% were found. All files exhibited angular deflection values in excess of that required by ADA specification no. 28 (2). Analysis of variance showed no difference between the test groups and
TABLE 1. Flexural strength of endodontic files Bums Unifile
K-Flex Test Group Control Autoclave Bead Dry Heat Cold * Mean__SD. t Coefficientof variation.
Torque (g-cm)
267
Angle (deg)
Torque (g-cm)
Angle (deg)
841.92 __.157.63 (18.72%) 836.50 __.164.55 (19.67%) 728.58 _ 151.74 (20.83%) 777.33 _.+135.53 (17.44%) 845.75 _+153.95 (18.20%)
94.92 _ 12.21 * (12.86%)t 97.83 _.+ 8.48
807.83 _ 99.37 (12.30%) 791.25 _ 83.63
83.75 _ 8.84 (10.56%) 81.83 __ 7.00
(8.67%) 91.33 _ 8.87 (9.71%) 107.00 _.+11.96 (11.18%) 104.08 _ 12.62 (12.13%)
(10.57%) 745.42 _ 71.20 (9.55%) 757.17 _ 112.66 (14.88%) 798.33 _ 74.27 (9.30%)
(8.55%) 84.75 __.4.96 (5.85%) 86.00 __7.42 (8.63%) 85.92 __9.68 (11.27%)
268
Journal of Endodontics
Iverson et al.
TABLE 2. Statistical comparison of K-Flex file torque strengths Control Control Autoclave Bead Dry heat
Autoclave
Bead
Dry Heat
Cold
NS*
NS PS
PS PS S
PS NS S NS
* NS, not significant(p > 0.05); PS, probably significant(p < 0.05); S, significant(p < 0.01).
controls for the torque and angular deflection for the Burns Unifiles and for angular deflection for the Kerr KFlex files. Significant differences (p < 0.01) were found for the Kerr K-Flex torque values. The individual differences between these values were identified by means of Student's t test and are summarized in Table 2. The data indicate that dry heat and cold sterilization ap, peared to effect a small increase in torque strength, with no difference being found among the torque values for the controls, autoclaved, and bead-sterilized files. Comparison of the mean values for both parameters using Student's t test showed no difference between the angular deflections for the K-Flex and Unifiles. Significant differences were found, however, between the torque values of the two types of files, the Kerr K-Flex files having the greater value. The difference was highly significant (p < 0.001) for the autoclaved, dry heat, and cold-sterilized specimens, significant (p < 0.01) for the controls, and probably significant (p < 0.05) following bead sterilization. DISCUSSION Sterilization of dental instruments and particularly endodontic files is central to preventing infection. The ADA Council on dental materials, instruments, and equipment recently reviewed sterilization procedures (6). Cold sterilization was not discussed as a recommended procedure but some clinicians use cold sterilizing solutions for holding or storing files and reamers during endodontic procedures. Cold sterilization was included in this study to determine the effect, if any, of such a solution on the mechanical properties.of files. Both the K-Flex and Burns Unifiles were able to withstand significantly greater angular deflection than the 360 degrees required by the ANSI/ADA specification no. 28. Furthermore, the data reported in Table 1 showed that neither type of file was significantly affected by the four sterilization regimens as compared with controls (p > 0.05). These findings are in agreement with those of Chernick et al. (4). Mitchell et al. (3), however, found that sterilization effected a significant decrease in angular deflection following autoclaving, although these workers incorporated a simulated clinical usage procedure in their test regimen. Sterilization also had no effect upon the torque resistance of the Burns Unffile. In contrast, a significant increase (p < 0.05 or 0.01) in torque strength was found for the dry
heat and cold-sterilized K-Flex files compared with that of the controls and other sterilization methods. The reason for the observed increases in torque strength, 12.7% and 9.7%, respectively, is not clear but it is doubtful whether they are clinically significant. No statistically significant differences were found in the angular deflection for the two types of file in the asreceived state or after sterilization. The torque resistance of the K-Flex files, however, was significantly greater (p < 0.01) than that of the Burns Unifile under all test conditions. This difference is unlikely to have any clinical significance and may be attributable to their method of manufacture. The K-Flex files are made by twisting diamond-shaped blanks while the Burns Unifile is manufactured by machine cutting of blanks. The torque strength of both types of file was markedly greater than the minimum value of 65 g-cm required by specification no. 28. CONCLUSIONS The findings of this study indicate that torque strength and angular deflection are not affected by the sterilization procedure. Differences between the two types of file are unlikely to be clinically significant. Breakage of files continues to be an occasional clinical problem and future research efforts should be devoted to the relationship between the degree of file usage and its failure rate. Commercial material and equipment are identified in this communication to specify the experimental procedure. Such identification does not imply official recommendation or endorsement or that the materials are necessarily the best available for the purpose. Furthermore, the opinions expressed are those of the authors and are not to be construed as those of the Canadian Forces Dental Services or the Army Medical Department. The components of Lehn and Fink instrument germicide solution are: ethyl alcohol, soap, o-phenylphenol, o-benzyl-p-chlorophenol, isopropyl alcohol, and tetrasodium ethylenediamine tetraacetate. We would like to express our appreciation to the American Dental Association's Council on Dental Materials, Instruments, and Equipment for making the torqumeter memcouple available for this study and to Mr. Zaker I. Sabri for his expert technical assistance. Dr. Iverson is a major in the Canadian Army and is presently a general practice resident at the U.S. Army Dental Facility, Fort Knox, KY. Dr. von Fraunhofer is professor of biomaterials science, University of Louisville, School of Dentistry, Louisville, KY. Dr. Herrmann is a colonel in the U.S. Army Dental Corps at Fort Meade, MD.
References 1. Zeigler PE, Serene TP. Failures in therapy. In: Cohen S, Bums RC, eds. Pathways of the pulp. St. Louis: CV Mosby Co., 1984. 2. American National Standards Institute/American Dental Association Specification No. 28 for Root Canal Files and Reamers, Type K. October, 1981. 3. Mitchell BF, James MS, Nelson RC. The effect of autoclave sterilization on endodontic files. J Oral Surg 1983;55:204. 4. Chernick LB, Jacobs JJ, Lautenschlager EP, Heuer MA. Torsional failure of endodontic files. J Endodon 1976;2:94. 5. Eichner MA, Schoen DM, Goldman M, Kronman JH. Effect of protein and sodium hypochlorite on endodontic instruments. J Endodon 1976;2:335. 8. American Dental Association Council on Dental Materials, Instruments, and Equipment. Current status of sterilization instruments, devices and methods for the dental office. J Am Dent Assoc 1981 ;102:683.
Printed in U.S.A. VOL. 11, NO. 6, JUNE1985
The Effects of Various Sterilization Methods on the Torsional Strength of Endodontic Files Efectos de Varios Metodos de Esterilizacion en la Resistencia a la Fractura por Torsion de Limas Endodonticas G. W. Iverson, CD, DDS, J. A. yon Fraunhofer, MSc, PhD, and J. W. Herrmann, DMD,FACD,FICD
estufa de calor seco en comparaci6n con los controles; y aun cuando no hubo diferencia notable en los grados de rotacion necesarios' para producir fractura las limas K sobresalleron-ligeramente como mas resistentes a la torsion.
The effects of different sterilization methods on the torsional strength of two types of endodontic files have been studied. Kerr K-Flex files and Burns Unifiles were subjected to ten cycles of autoclaving, bead, dry heat, and cold chemical sterilization. A torquemeter was used to determine the degrees of revolution and torque to failure for the files after repeated sterilization. Repeated sterilization was found to have no effect on the torque resistance and degrees to failure for the Burns Unifile and no effect on degrees to failure for the K-Flex file. Dry heat and cold sterilization, however, slightly increased the torque resistance of the K-Flex files as compared with the controls and the other sterilization methods. No differences were found between the files in the degrees of rotation to failure but the KFlex files had a slightly greater torque resistance.
Fracture of root canal files and/or reamers is an occasional problem in endodontic therapy and the prognosis for a fractured file varies with its location and other factors. Fracture in the apical third with the file effectively filling the canal by being tightly lodged with dentin shavings packed ahead of it, often allows the broken file to be incorporated as part of the filling material. If the file does not fill the canal and cannot be bypassed, then endodontic surgery becomes necessary. A surgical approach may also become necessary if the file breaks in the middle third of the tooth and cannot be bypassed. Finally, if fracture occurs beyond the apical foramen, the periapical portion must be removed since the protruding portion will be a mechanical irritant under occlusal loading of the tooth (1). Fracture of an endodontic file occurs under torsional loading when it cuts and wedges into the canal wall, especially when it binds tightly against the wall. The ANSI/ADA specification no. 28 (2) requires 360 degrees of angular deflection as a minimum value for clockwise torquing on certain endodontic files and a torque strength of 65 g-cm. The torsional strength and degrees of rotation to failure of a file are instrument characteristics that are determined by the fabrication material, the method of fabrication, and any procedures that might adversely affect these properties. In particular, the elevated temperatures applied during sterilization might adversely affect endodontic files. Mitchell et al. (3) recently showed that there was no decrease in the torsional strength of Kerr K-type files until autoclaved at least ten times, sizes 35 and 40 being the most severely affected. These authors performed a simulated
En presente articulo estudia los efectos de varios metodos de esterilizacion en la resistencia a la fractura por torsion de dos diferentes tipos de limas endodonticas. Una muestra d e limas K-Flex num 35 (instrumentos fabricados por rotaci6n de una varillita metalica romboidal), y otra de limas Burns unifiles (instrumento fabricado por corte a maquina de una varillita met/,lica), tambien numero 35, fueron sometidas a diez ciclos de esterilizacion por diferentes metodos: autoclave, calor por bolitas de cuarzo, estufa de calor seco y substancias quimicas. Despues de estas exposiciones controladas y utilizando un medidor de resistencia a la torsi6n se midio el n0mero de grados de rotaci6n y fuerza de torsion requeridos para producir una fractura del instrumento. No se encontro relacion-directa causaefecto entre los methodos de esterilizacion empleados y la fractura por torsion tanto en limas K como en limas Burns. Sin embargo se observo una ligera diferencia que. Indicaba mayor resistencia ala fractura por torsion de las limas K, esterilizadas en 266
Sterilization Methods of Files
Vol. 11, No. 6, June 1985
clinical use procedure prior to torque testing. Chernick et al. (4) showed, in torsional tests, that files twisted counterclockwise exhibited greater brittleness than when twisted clockwise. These investigators also showed that endodontic bead sterilization had no effect on the torsional strength of Star K-type files of sizes 10, 15, 20, 25, and 30 following a 10-s glass bead immersion. Eichner et al. (5), in a photographic study, showed that sodium hypochlorite solution did not corrode stainless steel endodontic files. No studies appear to have been performed on the effects of cold sterilizing (germicidal) solutions on endodontic files and there are no data in the literature on the effect of other sterilization methods on the torsional strength of endodontic files. The present study was undertaken to assess the effects of four commonly used sterilization methods on the torsional strength of two types of endodontic files. MATERIALS AND METHODS Two types of endodontic files, Kerr K-Flex size 35 and the recently introduced Burns Unifile size 35, were used in this study. The Kerr K-Flex file is a twisted file of diamond-shaped configuration and the Burns Unifile is a machine-cut file. The commonly used size 35 file was selected for this study. Sixty files of each type were divided into five groups of 12 files. One group of 12 was used as controls, the other groups being subjected to the different sterilization regimens. The second group was subjected to ten autoclaving cycles in a general purpose electric steam generator autoclave based on the gravity air-displacement principle (Verna-Clave Sterilizer model R2038 CLZ-1, Vernitron Medical Products Inc., Carlstadt, NJ). The autoclave was operated with dry saturated steam only and without addition of a corrosion-inhibiting solution. The files were placed in an open tray and heated to 127~ at 30 psi for 30 min and dried rapidly with forced exhaust. This cycle was repeated ten times over 5 days. The third group of files was subjected to ten cycles of glass bead sterilization in a Buffalo model 62 sterilizer
(Buffalo Dental Manufacturing Co. Inc., Brooklyn, NY). Prior to use, the sterilizer was heated for 1 h to 218~ The files were placed in the sterilizer to the depth of the handle one-half inch from the sides and held for 20 s. The files were then allowed to cool to room temperature for 30 s. This cycle was repeated ten times for each file. The fourth group of files was placed in an open tray in a dry heat oven (Steri-Dent Model 200, Copiague, NY) at 171~ for 1 h. The files were then cooled to room temperature for 15 min. This cycle was repeated ten times. The last group of files was subjected to cold sterilization solution (Lehn and Fink Instrument germicide solution) for ten cycles of 30 min each. The files were then rinsed with sterile saline and air dried. Torsional testing of the five groups of files was performed with the torqumeter memocouple (2) (Les Fils d'Auguste Maillefer SA). The instrument operates at 2 rpm, automatically recording on a digital display the degrees of rotation and torque required for file fracture. The files were prepared for testing by removing the handle from each file at the point of attachment to the shaft and then inserting the shank of the file into the drive chuck. The tip of each file was inserted 3 mm into the opposing brass jaws, the file was aligned, and the chucks were tightened. As the torquemeter induced a clockwise rotation in the file, the torque applied and the number of degrees of rotation to failure were automatically recorded and displayed. RESULTS The degrees of revolution (angle) and the torque required to failure for the two types of files and four sterilization methods are given in Table 1. The reproducibility of the data was good as shown by a coefficient of variation generally less than 15%, except for the angular deflection values for the Burns Unifiles where values of up to 20% were found. All files exhibited angular deflection values in excess of that required by ADA specification no. 28 (2). Analysis of variance showed no difference between the test groups and
TABLE 1. Flexural strength of endodontic files Bums Unifile
K-Flex Test Group Control Autoclave Bead Dry Heat Cold * Mean__SD. t Coefficientof variation.
Torque (g-cm)
267
Angle (deg)
Torque (g-cm)
Angle (deg)
841.92 __.157.63 (18.72%) 836.50 __.164.55 (19.67%) 728.58 _ 151.74 (20.83%) 777.33 _.+135.53 (17.44%) 845.75 _+153.95 (18.20%)
94.92 _ 12.21 * (12.86%)t 97.83 _.+ 8.48
807.83 _ 99.37 (12.30%) 791.25 _ 83.63
83.75 _ 8.84 (10.56%) 81.83 __ 7.00
(8.67%) 91.33 _ 8.87 (9.71%) 107.00 _.+11.96 (11.18%) 104.08 _ 12.62 (12.13%)
(10.57%) 745.42 _ 71.20 (9.55%) 757.17 _ 112.66 (14.88%) 798.33 _ 74.27 (9.30%)
(8.55%) 84.75 __.4.96 (5.85%) 86.00 __7.42 (8.63%) 85.92 __9.68 (11.27%)
268
Journal of Endodontics
Iverson et al.
TABLE 2. Statistical comparison of K-Flex file torque strengths Control Control Autoclave Bead Dry heat
Autoclave
Bead
Dry Heat
Cold
NS*
NS PS
PS PS S
PS NS S NS
* NS, not significant(p > 0.05); PS, probably significant(p < 0.05); S, significant(p < 0.01).
controls for the torque and angular deflection for the Burns Unifiles and for angular deflection for the Kerr KFlex files. Significant differences (p < 0.01) were found for the Kerr K-Flex torque values. The individual differences between these values were identified by means of Student's t test and are summarized in Table 2. The data indicate that dry heat and cold sterilization ap, peared to effect a small increase in torque strength, with no difference being found among the torque values for the controls, autoclaved, and bead-sterilized files. Comparison of the mean values for both parameters using Student's t test showed no difference between the angular deflections for the K-Flex and Unifiles. Significant differences were found, however, between the torque values of the two types of files, the Kerr K-Flex files having the greater value. The difference was highly significant (p < 0.001) for the autoclaved, dry heat, and cold-sterilized specimens, significant (p < 0.01) for the controls, and probably significant (p < 0.05) following bead sterilization. DISCUSSION Sterilization of dental instruments and particularly endodontic files is central to preventing infection. The ADA Council on dental materials, instruments, and equipment recently reviewed sterilization procedures (6). Cold sterilization was not discussed as a recommended procedure but some clinicians use cold sterilizing solutions for holding or storing files and reamers during endodontic procedures. Cold sterilization was included in this study to determine the effect, if any, of such a solution on the mechanical properties.of files. Both the K-Flex and Burns Unifiles were able to withstand significantly greater angular deflection than the 360 degrees required by the ANSI/ADA specification no. 28. Furthermore, the data reported in Table 1 showed that neither type of file was significantly affected by the four sterilization regimens as compared with controls (p > 0.05). These findings are in agreement with those of Chernick et al. (4). Mitchell et al. (3), however, found that sterilization effected a significant decrease in angular deflection following autoclaving, although these workers incorporated a simulated clinical usage procedure in their test regimen. Sterilization also had no effect upon the torque resistance of the Burns Unffile. In contrast, a significant increase (p < 0.05 or 0.01) in torque strength was found for the dry
heat and cold-sterilized K-Flex files compared with that of the controls and other sterilization methods. The reason for the observed increases in torque strength, 12.7% and 9.7%, respectively, is not clear but it is doubtful whether they are clinically significant. No statistically significant differences were found in the angular deflection for the two types of file in the asreceived state or after sterilization. The torque resistance of the K-Flex files, however, was significantly greater (p < 0.01) than that of the Burns Unifile under all test conditions. This difference is unlikely to have any clinical significance and may be attributable to their method of manufacture. The K-Flex files are made by twisting diamond-shaped blanks while the Burns Unifile is manufactured by machine cutting of blanks. The torque strength of both types of file was markedly greater than the minimum value of 65 g-cm required by specification no. 28. CONCLUSIONS The findings of this study indicate that torque strength and angular deflection are not affected by the sterilization procedure. Differences between the two types of file are unlikely to be clinically significant. Breakage of files continues to be an occasional clinical problem and future research efforts should be devoted to the relationship between the degree of file usage and its failure rate. Commercial material and equipment are identified in this communication to specify the experimental procedure. Such identification does not imply official recommendation or endorsement or that the materials are necessarily the best available for the purpose. Furthermore, the opinions expressed are those of the authors and are not to be construed as those of the Canadian Forces Dental Services or the Army Medical Department. The components of Lehn and Fink instrument germicide solution are: ethyl alcohol, soap, o-phenylphenol, o-benzyl-p-chlorophenol, isopropyl alcohol, and tetrasodium ethylenediamine tetraacetate. We would like to express our appreciation to the American Dental Association's Council on Dental Materials, Instruments, and Equipment for making the torqumeter memcouple available for this study and to Mr. Zaker I. Sabri for his expert technical assistance. Dr. Iverson is a major in the Canadian Army and is presently a general practice resident at the U.S. Army Dental Facility, Fort Knox, KY. Dr. von Fraunhofer is professor of biomaterials science, University of Louisville, School of Dentistry, Louisville, KY. Dr. Herrmann is a colonel in the U.S. Army Dental Corps at Fort Meade, MD.
References 1. Zeigler PE, Serene TP. Failures in therapy. In: Cohen S, Bums RC, eds. Pathways of the pulp. St. Louis: CV Mosby Co., 1984. 2. American National Standards Institute/American Dental Association Specification No. 28 for Root Canal Files and Reamers, Type K. October, 1981. 3. Mitchell BF, James MS, Nelson RC. The effect of autoclave sterilization on endodontic files. J Oral Surg 1983;55:204. 4. Chernick LB, Jacobs JJ, Lautenschlager EP, Heuer MA. Torsional failure of endodontic files. J Endodon 1976;2:94. 5. Eichner MA, Schoen DM, Goldman M, Kronman JH. Effect of protein and sodium hypochlorite on endodontic instruments. J Endodon 1976;2:335. 8. American Dental Association Council on Dental Materials, Instruments, and Equipment. Current status of sterilization instruments, devices and methods for the dental office. J Am Dent Assoc 1981 ;102:683.