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Cutting ability of K type endodontic files
Printed in the U.S.A. VOL, 9, NO. 2, FEBRUARY 1983
0099-2399/83/0902-0052/$02.00/0 JOURNAL OF ENDODONTICS Copyright 9 1983 by The American Association of Endodontists
Cutting Ability of K Type Endodontic Files Richard G. Neal, DDS, MS, Robert G. Craig, PhD, and John M. Powers, PhD
The cutting ability of K type stainless steel files was measured on an instrument that controlled the force on the files and the length and number of strokes. The depth of cuts in poly(methyl methacrylate) was used to determine cutting ability. The cutting ability of a single surface of a file did not decrease as a result of 6,100 strokes. The cutting ability of individual files varied considerably, but good reproducibility was found for the same file with various numbers of strokes. The wear of files did not have a significant effect on the cutting ability of the files.
cutting efficiency between types of instruments and found that those with triangular cross-sections were sharper but lost sharpness after use, whereas those with square cross-sections retained sharpness better. Hedstrom files lost cutting efficiency very rapidly. The purpose of this investigation was to determine the variability in the cutting ability of regular endodontic files and to evaluate the effects of wear on cutting ability using an apparatus which simulated the rasplike motion used in the filing technique. MATERIALS AND METHODS Cutting Apparatus
The thorough debridement of the root canal system is generally accepted as the most important phase in endodontic therapy (1-5). Such biomechanical preparation is done to cleanse the pulp chamber and root canals of pulpal remnants, necrotic debris, infectious microorganisms, affected predentin, and dentin (1,6, 7) and to provide for enlargement and shaping of the pulpal system (2, 5, 8, 9). The dentist performing endodontics assumes that the design and properties of the instruments will assist in the attainment of these objectives. Physical characteristics of the instruments are important, and one of the most important factors in achieving these objectives is the cutting ability of endodontic instruments. Most studies which have sought to assess cutting ability directly have utilized poorly controlled force on the instrument and/or a substrate of varying hardness (10-15). Several investigators (16, 17) have studied the cutting efficiency and the effect of flute design on cutting efficiency of endodontic instruments in a rotary motion. In the former study, the time required to penetrate and enlarge predrilled holes in wet bovine femur evaluated cutting efficiency. In the latter study, the energy to remove unit volume of material was used to determine cutting ability. Reamers were significantly more efficient than files, and smaller instruments were generally more efficient than Iarger sizes. One study (18) determined the cutting ability of root canal instruments using a linear push-pull motion. The cutting ability was evaluated by measuring the weight of wet bovine femur removed in 5 min at three strokes per s. The investigators observed a wide range of
DeJongh and Willoughby (13) developed an apparatus which simulated the rasp-like motion of the filing technique. The main components were a movable platform attached to an electrical drive motor via a slotted cam with aluminum rods suspended over the platform (Fig. 1). The movable platform had plastic fittings allowing free reciprocal motion along two parallel steel rods mounted on a supporting base. The platform supported four holders for rigidly mounting endodontic instruments parallel to each other and to the steel rods upon which the platform traveled. The instruments were passed through a hole in a right-angle metal bracket and 3 to 4 mm of the tip end were clamped between two pieces of brass. The instruments were held in this manner under tension providing a rigid, saw-like instiument. The holding devices were adjustable to allow'for the proper orientation of the instrument. The material to be cut was suspended on four aluminum arms above the platform. The arms were synchronized to allow contact with the instruments only on the pull-stroke. A slotted cam between the platform and the motor provided a push-stop-pullstop-push motion for the platform. The stops allowed for a break in contact between the instruments and the material being cut except during the putl portion of the motion. The full cycle was contact-pull-stopremove from contact-push-stop-contact. A constant speed motor resulted in 50 strokes per min. The length and force of the stroke and the area of the instrument contacted during a working stroke 52
Vol. 9, No, 2, February 1983
Cutting Ability of K Type Files
A
53
B i m
El !FIG 1. Photograph and sketch of cutting instrument. A, aluminum bar retaining test sample; B, variable load; C, bearing to control movement of aluminum bar; D, jig to retain file; E, movable platform attached to a slotted cam; F, guide to restrain lateral movement of aluminum bar; G, cam-motor complex providing back-and-forth motion of platform and up-and-down motion of aluminum rods; H, oscillating bar to lift substrate and aluminum bar unit off the file; I, mechanical counter; J, length gauge.
could be controlled. A mechanical counter (I.T.T. General Controls, Inc., Glendale, CA) allowed for accurate recording of the number of strokes. The DeJongh-Willoughby cutting apparatus was modified by incorporating a spring-loaded dial gage ( L S. Starrett Co., Athol, MA) to monitor the length of the stroke. The length of the stroke was 5.00 _+ 0.06 ram, and it was checked at the beginning of each test and after every 100 strokes. Small adjustments rarely were required to meet the tolerance. The section of the file used was approximately the middle 5 mm of the working length, and the instruments had fluted lengths of 16 mm. The load on each file was maintained at 150.00 ___0.01 g. With the exception of the wear tests, the total number of strokes was 500. Instruments
Commercially available regular K type # 3 0 endodontic stainless steel files manufactured from stainless steel wire of the AISI (American Iron and Steel Institute) 300 series were tested. The files were visually inspected for distortions, irregularities, and corrosion and were thoroughly cleaned with two separate alcohol-soaked 2- x 2-inch cotton gauze sponges before testing. Substrate
Because of the variability of the microhardness of dentin, poly(methyl methacrylate) (Plexiglas R, Rohm and Haas, Philadelphia, PA) was chosen as a more
suitable material. One centimeter-square wafers were sectioned from a single sheet of 1/16-inch poly(methyl methacrylate) with a band saw. A technique developed by DeJongh and Willoughby (13) to produce dentin discs of a uniform thickness was utilized. A glass microscope slide approximately 1.0 mm in thickness was selected, and two holes slightly greater than 1 cm in diameter were cut in it with a diamond stone at ultraspeed. The slide was cemented to a second glass slide by a thin layer of sticky wax. A large portion of wax was then attached to the back side of the supporting slide to serve as a handle. This jig was moistened with water; oversized poly(methyl methacrylate) wafers were placed in the countersunk holes and they were ground to 1.00 _+ 0.02-mm thickness using a 400-grit silicon carbide disk (Wet or Dry-mite Paper, 3M Co., St. Paul, MN) on a polishing wheel (Polimet Polish, Buehler, Ltd., Evanston, IL) with water lubrication at 400 rpm. The wafers were measured before and after testing, and all remained within the tolerance. One edge of each wafer was placed in a slot in the end of an 18-ram long section of 1/4-inch square steel rod. The wafer was ridigly held by cold curing acrylic. (Duralay, Reliance Dental Mfg., Worth, IL) The free edge of the wafer was tl~en smoothened with a 400grit silicon carbide disc using the polishing wheel. This procedure provided a smooth surface on which to begin a cut and from which to make a measurement of the depth of cut. The prepared steel rod and wafer were then rigidly fixed to the aluminum arm of the
54
Neal et al.
Journal of E n d o d o n t i c s
cutting apparatus in a screw clamping device shown in Fig. 2. Once embedded in the metal rods, the wafers were coded so that a particular endodontic file could be related to a specific poly(methyl methacrylate) wafer throughout testing. The Knoop microhardness of six randomly selected wafers was determined (Wilson Instruments Division, American Chain and Cable Co., Inc., New York, NY). Three values were obtained from divergent points on each sample wafer, and the mean for all samples was determined. To determine the effect of variation in microhardness on cutting ability, two files were used to make cuts of 500 strokes on each of five poly(methyl methacrylate) wafers under 150 g of force.
seven subgroups of four files each. Each file in a particular subgroup performed the same number of strokes on different wafers in making single cuts. Different subgroups existed for 100, 250, 500, 750, 1,000, 1,500, and 2,000 strokes.
Measurement of Cuts For a single cut, both sides of the poly(methyl methacrylate) wafers were measured for the depth of the cut and the results were averaged. A microscope (Ernst Heitz G.m.b.h. Wetzlar Co., Germany) with a calibrated-filar eyepiece was used at a magnification of 8 5 x , and the depth of the cuts was measured to +_ 0.001 mm.
RESULTS Cutting Tests One hundred eighty # 3 0 K type endodontic files (in groups of four) were used to make single cuts of 500 strokes under 150 g of force. After the cuts were completed, the wafers were indexed and separated from the metal rods, and the depth of the cut was measured. The effect of wear on the cutting ability of regular endodontic files was evaluated by two separate tests. Four and 28 # 3 0 K type files were used in the first and second test groups, respectively. In one test, each of four files was associated with three separate poly(methyl methacrylate) wafers. Cuts of 100, 250, and 500 strokes were done on the first wafer. The second wafer received cuts of 750, 1,000, and 1,500 strokes; a cut of 2,000 strokes was made on the third wafer. The files were indexed in such a fashion so that the files could be oriented in the cutting apparatus the same for all tests. In a second test, instruments were divided into
~'*
The microhardness of six randomly selected poly(methyl methacrylate)wafers with three measurements per wafer was 20.9 _+ 0.08 k g / m m 2. The range had less than 4% variation from the mean. Twenty instruments were used in each of two groups to make two sets of cuts of 500 strokes. In one group cuts were made on one side and then the opposite side of the instrument; mean depths of cut were 0.63 mm on one side and 0.70 mm on the opposite side. When the two sets of cuts from 500 strokes were made using the same side of the instrument, mean depths of cuts were 0.65 and 0.62 mm. Therefore, the remainder of the results were obtained using one side of the instrument only. Tests involving two files making five consecutive cuts on five separate poly(methyl methacrylate) wafers are presented in Table 1. All cuts were made with the same side of each file. The ranges for both files indicated less than 5% variation from the means existed for the most divergent values.
,-I
FIG 2. A file in position to make a cut in the poly(methyl methacrylate) sample.
Cutting Ability of K Type Files
Vol. 9, No. 2, February 1983
V a r i a t i o n in Cutting Ability
One hundred eighty regular endodontic files made cuts of 500 strokes under 150 g. The overall mean depth of these cuts was 0.65 mm, and the standard deviation was 0.28. The extremes were cuts of 0.21 and 1.76 mm. The distribution of all 180 cuts is presented in Fig. 3.
55
100 to 2,000 strokes. The depths of the cut of these seven subgroups are presented in Fig. 5, which shows a reasonably consistent increase in the depth of cut as the number of strokes increased. The marks on the vertical lines in Fig. 5 also indicate the variability in cutting ability of four similar files at an identical number of working strokes. DISCUSSION
Effect of Wear on Cutting Ability The effect of wear on four files making seven consecutive cuts is presented in Fig. 4. Cuts of 100, 250, 500, 750, 1,000, 1,500, and 2,000 strokes were made with the same side of each of four files. Each file was used for a total of 6,100 strokes. Each file showed a fairly uniform increase in the depth of cut as the number of strokes increased, and the curve for the mean depth of cut versus number of strokes was essentially linear. Four files of each of seven subgroups made only one cut at the same number of strokes ranging from TABLE 1. Depth of five consecutive cuts made by t w o files on five poly(methyl methacrylate) samples Depth of Cut (ram) Sample File 1
File 2
0.579 0_550 0.575 0.578 0.553
0,426 0.413 0,417 0.448 0.444
0.567 +_ 0.017
0.430 ___0.01 8
1 2 3 4 5 Mean +_ range
K type files were tested because they may be used in pathfinding, enlarging, and smoothing in most techniques. The # 3 0 instrument was chosen because that size is used in most canal preparations, and the brand is representative of the stainless steel instruments available. An effort was made to utilize testing procedures as analogous to clinical practice as possible, and thus the files were used in a rasping mode which is an accepted clinical usage. The experimental design allowed for adequate control of the force applied, of the length and number of strokes, of the portion of the instrument used, and of the substrate being cut. Molven (10) used hand power to control the force and had no mechanism to control the number or length of strokes. In addition, the variability of the microhardness of the dentin substrate added another uncontrolled factor. Studies indirectly evaluating the cutting ability of endodontic instruments by comparing the results of various preparation techniques have the error that hand-controlled instrumentation does not allow for the accurate control of force applied to the instrument nor does it provide for the elimination of operator bias.
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56
Journal of Endodontics
Neal et al.
55 50
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No. STROKES IN CUT FIG 4. Effect of wear on the depth of cut of four files after consecutive cuts of varying number of strokes. 5.5 5.0 45 i= 4.0 E
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FIG 5. E f f e c t o f w e a r o n t h e d e p t h o f c u t o f s e v e n s u b g r o u p s o f f o u r files each atdifferentnumberofstrokes.
The force, length and number of strokes, and area of instrument used were adapted from the ranges set forth by DeJongh and Willoughby (13) and Stenman (19). Both the DeJongh-Willoughby (13) and Stenman (19) studies, as well as microhardness measurements on dentin (20, 21), indicated that dentin was a poor substrate for evaluating cutting ability. Also, the microhardness of about 21 k g / m m 2 for poly(methyl methacrylate) was not greatly different from the values of 35 k g / m m 2 for dentin near the pulp (20, 21), and both are dramatically lower than 578 +_ 28 kg/mm 2 for the files. Poly(methyl methacrylate) was selected as the substrate because of its availability, cutting characteristics, and uniformity. Hardness measurements verified that less than 5% variation from the mean existed throughout the samples. Also, five sep-
arate cuts of 500 strokes were made by two files on five separate poly(methyl methacrylate) wafers, and all cuts had less than 5% variation from the mean depth of cut. These results also demonstrated it was unnecessary to heat treat or to control the storage humidity of the poly(methyl methacrylate) wafers as had Stenman (19). However, careful handling and storage of the poly(methyl methacrylate) was done in this study. The present study showed the tremendous variability in the cutting ability among instruments of a single size and manufacturer. This fact is shown by the depth of cut at the upper extreme (1.752 mm) being 800% greater than the minimum (0.210 mm) and more than 250% greater than the mean depth of cut (0.649 mm). Variation was also noted for cuts using different sides of the same instrument. Thus, orientation of instruments in any cutting test is important if variance is to be controlled. The effect of wear on the cutting ability of regular endodontic files was shown to be insignificant. The results demonstrated that through 6,100 strokes under a force of 150 g, the cutting ability of the files was not adversely affected and beyond 6,000 strokes the instruments remained effective. Oliet and Sorin (11, 12) reported that metal wear was not a factor in reducing cutting ability. They concluded that significant decreases in the cutting ability of reamers occurred only after permanent deformation of the instru'rnent. The present study supports the concept that wear alone is not a significant factor in reducing the cutting ability of regular endodontic files. The effects of permanent deformation of these files on their cutting ability has not been assessed, although Gutierrez et al. (22), in a study on used reamers, assumed that physical alterations of
Vol. 9, No. 2, February 1 9 8 3
the instrument would be detrimental to its cutting ability. However, in the present study, using only one cutting surface of many available on the circumference, a file maintained its cutting ability for more than 6,000 strokes. It would, therefore, seem quite unlikely that abrasive wear alone would be a major factor in decreased cutting ability before the instrument was damaged in another manner. The papers by Felt et al. (17) and Villalobos et al. (16) are difficult to relate to the present study since cutting ability was determined using a rotary motion. This motion is not generally used with K type files, which is usually a quarter turn only and withdraw, where the main part of the cutting takes place on the withdrawal stroke. Also, considering the design of triangular versus square cross-sectional files, it is not surprising they observed higher cutting efficiency with the former in a rotary motion. The study of cutting efficiency of root canal instruments in a linear push and pull motion by Webber et al. (18) is in general agreement with the present study since # 3 0 square files did not show a significant difference in cutting ability from the first to the third run. CONCLUSIONS The cutting ability of regular K type stainless steel endodontic files was evaluated using an instrument that simulated the rasping motion used in filing root canals and using poly(methyl methacrylate) to simulate dentin. The force on the file and the length and number of strokes were controlled, and the cutting ability was measured by the depth of the cut in poly(methyl methacrylate). The cutting ability of a single surface of files did not decrease as a result of 6 , 1 0 0 strokes with a length of a stroke being 5 mm. The cutting ability of individual files varied considerably as indicated by the average depth of cut of 0.65 mm and the extreme of the range being 0.21 and 1.75 mm. Reproducibility of the cutting ability of the same surface of a single file, however, was good. Thus, the effects of clinical handling of files on the cutting ability should be evaluated on the same instrument and on the same cutting surface. The values for the depth of cut as a function of the number of strokes demonstrated that wear alone does
Cutting Ability of K Type Files
57
not have a significant role in decreasing the cutting ability of regular K type stainless steel files. This investigation was based on a thesis submitted in partial fulfillment of
the requirements for the M.S. degree in the Horace H. Rackham School of Graduate Studies at The University of Michigan, 1980. Dr. Neal is now in private practice in Gastonia, NC. Dr. Craig is Professor and Chairman, and Dr. Powers is Professor of the Dental Materials Department at the University of Michigan, School of Dentistry, Ann Arbor, M148109. Requests for reprints should be directed to Dr. Craig_
References 1. Grossman LI. Endodontic practice. Philadelphia: Lea and Febiger, 1974. 2. Heuer MA. The biomechanics of endodontic therapy. Dent Clin North Am 1963;344-59. 3. Schilder H. Cleaning and shaping the root canal. Dent Clin North Am t 974;18:269-96. 4. Stewart GG, Kapsimalas P, Rappaport H. EDTA and urea peroxide for root canal preparation. J Am Dent Assoc 1969;78:335-8. 5. Weine FS. Endodontic therapy. 2nd ed. St Louis: CV Mosby, 1976. 6. Gutierrez JH, Garcia J. Microscopic and macroscopic investigation on results of mechanical preparation of root canals. Oral Surg 1968;25:10816. 7_ Jungman CL, Uchin RA, Bucher JF. Effect of instrumentation on the shape of the roof canal. J Endodon t 975;1:66-9. 8. Mizrahi SJ, Tucker JW, Seltzer S. A scanning electron microscopic study of the efficacy of various endodontic instruments. J Endodon 1975;1:324-33. 9. Curson I. Endodontic techniques. VII. Preparation of root canals. Br Dent J 1966;121:329-34. 10. Molven O. A comparison of the dentin removing ability of five root canal instruments. Scand J Dent Res 1970;76:500-11. 11. Oliet S, Sorin SM. Cutting efficiency of endodontic reamers. Oral Surg 1973;36:243-52. 12. Oliet S, Sorin SM. Evaluation of the cutting efficiency of endodontic reamers. Microfilmed Paper No. 606. International Association for Dental Research, Program and abstract of papers, 49th general session, 1971. 13. DeJongh LC, Willoughby JW. Endodontic instruments: an evaluation of cutting ability. Master's Thesis, Ann Arbor, University of Michigan, School of Dentistry, 1975. 14. Shoji Y. Studies of the mechanism of the mechanical enlargement of root canals. Nihon Univ School of Dent J 1965;7:71-8. 15. Ochiai H. Studies on dental hand reamer. I. Automatic measurement of the reamer diameter and cutting torque. II. Diameter and cutting sharpness of commercial reamers. Jpn J Conserv Dent 1976; 19:41-73. 16. Villalobos RL, Moser JB, Heuer MA. A method to determine the cutting efficiency of root canal instruments in rotary motion. J Endodon 1980;6:667-71. 17. Felt RA, Moser JB, Heuer MA. Flute design of endodontic instruments: its influence on cutting efficiency. J Endodon 1982;8:253-9. 18. Webber J, Moser JB, Heuer MA. A method to determine the cutting efficiency of root canal instruments in linear motion. J Endodon 1980;6:82934. 19. Stenman E. Effects of sterilization and endodontic medicaments on mechanical properties of root canal instruments. Doctoral Dissertation, Umea, University of Umea, Sweden, 1977. 20. Craig RG, Gehring PE, Peyton FA. Relation of structure to microhardness of human dentin. J Dent Res 1959;38:624-30 21. Craig RG, Peyton FA. The microhardness of enamel and dentin. J Dent Res 1958;37:661-8. 22. Gutierrez JH, Gigoux C, Sanhueza I. Physical and chemical deterioration of endodontic reamers during mechanical preparation. Oral Surg 1969;26:394-403.
0099-2399/83/0902-0052/$02.00/0 JOURNAL OF ENDODONTICS Copyright 9 1983 by The American Association of Endodontists
Cutting Ability of K Type Endodontic Files Richard G. Neal, DDS, MS, Robert G. Craig, PhD, and John M. Powers, PhD
The cutting ability of K type stainless steel files was measured on an instrument that controlled the force on the files and the length and number of strokes. The depth of cuts in poly(methyl methacrylate) was used to determine cutting ability. The cutting ability of a single surface of a file did not decrease as a result of 6,100 strokes. The cutting ability of individual files varied considerably, but good reproducibility was found for the same file with various numbers of strokes. The wear of files did not have a significant effect on the cutting ability of the files.
cutting efficiency between types of instruments and found that those with triangular cross-sections were sharper but lost sharpness after use, whereas those with square cross-sections retained sharpness better. Hedstrom files lost cutting efficiency very rapidly. The purpose of this investigation was to determine the variability in the cutting ability of regular endodontic files and to evaluate the effects of wear on cutting ability using an apparatus which simulated the rasplike motion used in the filing technique. MATERIALS AND METHODS Cutting Apparatus
The thorough debridement of the root canal system is generally accepted as the most important phase in endodontic therapy (1-5). Such biomechanical preparation is done to cleanse the pulp chamber and root canals of pulpal remnants, necrotic debris, infectious microorganisms, affected predentin, and dentin (1,6, 7) and to provide for enlargement and shaping of the pulpal system (2, 5, 8, 9). The dentist performing endodontics assumes that the design and properties of the instruments will assist in the attainment of these objectives. Physical characteristics of the instruments are important, and one of the most important factors in achieving these objectives is the cutting ability of endodontic instruments. Most studies which have sought to assess cutting ability directly have utilized poorly controlled force on the instrument and/or a substrate of varying hardness (10-15). Several investigators (16, 17) have studied the cutting efficiency and the effect of flute design on cutting efficiency of endodontic instruments in a rotary motion. In the former study, the time required to penetrate and enlarge predrilled holes in wet bovine femur evaluated cutting efficiency. In the latter study, the energy to remove unit volume of material was used to determine cutting ability. Reamers were significantly more efficient than files, and smaller instruments were generally more efficient than Iarger sizes. One study (18) determined the cutting ability of root canal instruments using a linear push-pull motion. The cutting ability was evaluated by measuring the weight of wet bovine femur removed in 5 min at three strokes per s. The investigators observed a wide range of
DeJongh and Willoughby (13) developed an apparatus which simulated the rasp-like motion of the filing technique. The main components were a movable platform attached to an electrical drive motor via a slotted cam with aluminum rods suspended over the platform (Fig. 1). The movable platform had plastic fittings allowing free reciprocal motion along two parallel steel rods mounted on a supporting base. The platform supported four holders for rigidly mounting endodontic instruments parallel to each other and to the steel rods upon which the platform traveled. The instruments were passed through a hole in a right-angle metal bracket and 3 to 4 mm of the tip end were clamped between two pieces of brass. The instruments were held in this manner under tension providing a rigid, saw-like instiument. The holding devices were adjustable to allow'for the proper orientation of the instrument. The material to be cut was suspended on four aluminum arms above the platform. The arms were synchronized to allow contact with the instruments only on the pull-stroke. A slotted cam between the platform and the motor provided a push-stop-pullstop-push motion for the platform. The stops allowed for a break in contact between the instruments and the material being cut except during the putl portion of the motion. The full cycle was contact-pull-stopremove from contact-push-stop-contact. A constant speed motor resulted in 50 strokes per min. The length and force of the stroke and the area of the instrument contacted during a working stroke 52
Vol. 9, No, 2, February 1983
Cutting Ability of K Type Files
A
53
B i m
El !FIG 1. Photograph and sketch of cutting instrument. A, aluminum bar retaining test sample; B, variable load; C, bearing to control movement of aluminum bar; D, jig to retain file; E, movable platform attached to a slotted cam; F, guide to restrain lateral movement of aluminum bar; G, cam-motor complex providing back-and-forth motion of platform and up-and-down motion of aluminum rods; H, oscillating bar to lift substrate and aluminum bar unit off the file; I, mechanical counter; J, length gauge.
could be controlled. A mechanical counter (I.T.T. General Controls, Inc., Glendale, CA) allowed for accurate recording of the number of strokes. The DeJongh-Willoughby cutting apparatus was modified by incorporating a spring-loaded dial gage ( L S. Starrett Co., Athol, MA) to monitor the length of the stroke. The length of the stroke was 5.00 _+ 0.06 ram, and it was checked at the beginning of each test and after every 100 strokes. Small adjustments rarely were required to meet the tolerance. The section of the file used was approximately the middle 5 mm of the working length, and the instruments had fluted lengths of 16 mm. The load on each file was maintained at 150.00 ___0.01 g. With the exception of the wear tests, the total number of strokes was 500. Instruments
Commercially available regular K type # 3 0 endodontic stainless steel files manufactured from stainless steel wire of the AISI (American Iron and Steel Institute) 300 series were tested. The files were visually inspected for distortions, irregularities, and corrosion and were thoroughly cleaned with two separate alcohol-soaked 2- x 2-inch cotton gauze sponges before testing. Substrate
Because of the variability of the microhardness of dentin, poly(methyl methacrylate) (Plexiglas R, Rohm and Haas, Philadelphia, PA) was chosen as a more
suitable material. One centimeter-square wafers were sectioned from a single sheet of 1/16-inch poly(methyl methacrylate) with a band saw. A technique developed by DeJongh and Willoughby (13) to produce dentin discs of a uniform thickness was utilized. A glass microscope slide approximately 1.0 mm in thickness was selected, and two holes slightly greater than 1 cm in diameter were cut in it with a diamond stone at ultraspeed. The slide was cemented to a second glass slide by a thin layer of sticky wax. A large portion of wax was then attached to the back side of the supporting slide to serve as a handle. This jig was moistened with water; oversized poly(methyl methacrylate) wafers were placed in the countersunk holes and they were ground to 1.00 _+ 0.02-mm thickness using a 400-grit silicon carbide disk (Wet or Dry-mite Paper, 3M Co., St. Paul, MN) on a polishing wheel (Polimet Polish, Buehler, Ltd., Evanston, IL) with water lubrication at 400 rpm. The wafers were measured before and after testing, and all remained within the tolerance. One edge of each wafer was placed in a slot in the end of an 18-ram long section of 1/4-inch square steel rod. The wafer was ridigly held by cold curing acrylic. (Duralay, Reliance Dental Mfg., Worth, IL) The free edge of the wafer was tl~en smoothened with a 400grit silicon carbide disc using the polishing wheel. This procedure provided a smooth surface on which to begin a cut and from which to make a measurement of the depth of cut. The prepared steel rod and wafer were then rigidly fixed to the aluminum arm of the
54
Neal et al.
Journal of E n d o d o n t i c s
cutting apparatus in a screw clamping device shown in Fig. 2. Once embedded in the metal rods, the wafers were coded so that a particular endodontic file could be related to a specific poly(methyl methacrylate) wafer throughout testing. The Knoop microhardness of six randomly selected wafers was determined (Wilson Instruments Division, American Chain and Cable Co., Inc., New York, NY). Three values were obtained from divergent points on each sample wafer, and the mean for all samples was determined. To determine the effect of variation in microhardness on cutting ability, two files were used to make cuts of 500 strokes on each of five poly(methyl methacrylate) wafers under 150 g of force.
seven subgroups of four files each. Each file in a particular subgroup performed the same number of strokes on different wafers in making single cuts. Different subgroups existed for 100, 250, 500, 750, 1,000, 1,500, and 2,000 strokes.
Measurement of Cuts For a single cut, both sides of the poly(methyl methacrylate) wafers were measured for the depth of the cut and the results were averaged. A microscope (Ernst Heitz G.m.b.h. Wetzlar Co., Germany) with a calibrated-filar eyepiece was used at a magnification of 8 5 x , and the depth of the cuts was measured to +_ 0.001 mm.
RESULTS Cutting Tests One hundred eighty # 3 0 K type endodontic files (in groups of four) were used to make single cuts of 500 strokes under 150 g of force. After the cuts were completed, the wafers were indexed and separated from the metal rods, and the depth of the cut was measured. The effect of wear on the cutting ability of regular endodontic files was evaluated by two separate tests. Four and 28 # 3 0 K type files were used in the first and second test groups, respectively. In one test, each of four files was associated with three separate poly(methyl methacrylate) wafers. Cuts of 100, 250, and 500 strokes were done on the first wafer. The second wafer received cuts of 750, 1,000, and 1,500 strokes; a cut of 2,000 strokes was made on the third wafer. The files were indexed in such a fashion so that the files could be oriented in the cutting apparatus the same for all tests. In a second test, instruments were divided into
~'*
The microhardness of six randomly selected poly(methyl methacrylate)wafers with three measurements per wafer was 20.9 _+ 0.08 k g / m m 2. The range had less than 4% variation from the mean. Twenty instruments were used in each of two groups to make two sets of cuts of 500 strokes. In one group cuts were made on one side and then the opposite side of the instrument; mean depths of cut were 0.63 mm on one side and 0.70 mm on the opposite side. When the two sets of cuts from 500 strokes were made using the same side of the instrument, mean depths of cuts were 0.65 and 0.62 mm. Therefore, the remainder of the results were obtained using one side of the instrument only. Tests involving two files making five consecutive cuts on five separate poly(methyl methacrylate) wafers are presented in Table 1. All cuts were made with the same side of each file. The ranges for both files indicated less than 5% variation from the means existed for the most divergent values.
,-I
FIG 2. A file in position to make a cut in the poly(methyl methacrylate) sample.
Cutting Ability of K Type Files
Vol. 9, No. 2, February 1983
V a r i a t i o n in Cutting Ability
One hundred eighty regular endodontic files made cuts of 500 strokes under 150 g. The overall mean depth of these cuts was 0.65 mm, and the standard deviation was 0.28. The extremes were cuts of 0.21 and 1.76 mm. The distribution of all 180 cuts is presented in Fig. 3.
55
100 to 2,000 strokes. The depths of the cut of these seven subgroups are presented in Fig. 5, which shows a reasonably consistent increase in the depth of cut as the number of strokes increased. The marks on the vertical lines in Fig. 5 also indicate the variability in cutting ability of four similar files at an identical number of working strokes. DISCUSSION
Effect of Wear on Cutting Ability The effect of wear on four files making seven consecutive cuts is presented in Fig. 4. Cuts of 100, 250, 500, 750, 1,000, 1,500, and 2,000 strokes were made with the same side of each of four files. Each file was used for a total of 6,100 strokes. Each file showed a fairly uniform increase in the depth of cut as the number of strokes increased, and the curve for the mean depth of cut versus number of strokes was essentially linear. Four files of each of seven subgroups made only one cut at the same number of strokes ranging from TABLE 1. Depth of five consecutive cuts made by t w o files on five poly(methyl methacrylate) samples Depth of Cut (ram) Sample File 1
File 2
0.579 0_550 0.575 0.578 0.553
0,426 0.413 0,417 0.448 0.444
0.567 +_ 0.017
0.430 ___0.01 8
1 2 3 4 5 Mean +_ range
K type files were tested because they may be used in pathfinding, enlarging, and smoothing in most techniques. The # 3 0 instrument was chosen because that size is used in most canal preparations, and the brand is representative of the stainless steel instruments available. An effort was made to utilize testing procedures as analogous to clinical practice as possible, and thus the files were used in a rasping mode which is an accepted clinical usage. The experimental design allowed for adequate control of the force applied, of the length and number of strokes, of the portion of the instrument used, and of the substrate being cut. Molven (10) used hand power to control the force and had no mechanism to control the number or length of strokes. In addition, the variability of the microhardness of the dentin substrate added another uncontrolled factor. Studies indirectly evaluating the cutting ability of endodontic instruments by comparing the results of various preparation techniques have the error that hand-controlled instrumentation does not allow for the accurate control of force applied to the instrument nor does it provide for the elimination of operator bias.
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FiG 3. Frequency of instruments producing depth of cuts at intervals of 0.05 mm.
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56
Journal of Endodontics
Neal et al.
55 50
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No. STROKES IN CUT FIG 4. Effect of wear on the depth of cut of four files after consecutive cuts of varying number of strokes. 5.5 5.0 45 i= 4.0 E
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FIG 5. E f f e c t o f w e a r o n t h e d e p t h o f c u t o f s e v e n s u b g r o u p s o f f o u r files each atdifferentnumberofstrokes.
The force, length and number of strokes, and area of instrument used were adapted from the ranges set forth by DeJongh and Willoughby (13) and Stenman (19). Both the DeJongh-Willoughby (13) and Stenman (19) studies, as well as microhardness measurements on dentin (20, 21), indicated that dentin was a poor substrate for evaluating cutting ability. Also, the microhardness of about 21 k g / m m 2 for poly(methyl methacrylate) was not greatly different from the values of 35 k g / m m 2 for dentin near the pulp (20, 21), and both are dramatically lower than 578 +_ 28 kg/mm 2 for the files. Poly(methyl methacrylate) was selected as the substrate because of its availability, cutting characteristics, and uniformity. Hardness measurements verified that less than 5% variation from the mean existed throughout the samples. Also, five sep-
arate cuts of 500 strokes were made by two files on five separate poly(methyl methacrylate) wafers, and all cuts had less than 5% variation from the mean depth of cut. These results also demonstrated it was unnecessary to heat treat or to control the storage humidity of the poly(methyl methacrylate) wafers as had Stenman (19). However, careful handling and storage of the poly(methyl methacrylate) was done in this study. The present study showed the tremendous variability in the cutting ability among instruments of a single size and manufacturer. This fact is shown by the depth of cut at the upper extreme (1.752 mm) being 800% greater than the minimum (0.210 mm) and more than 250% greater than the mean depth of cut (0.649 mm). Variation was also noted for cuts using different sides of the same instrument. Thus, orientation of instruments in any cutting test is important if variance is to be controlled. The effect of wear on the cutting ability of regular endodontic files was shown to be insignificant. The results demonstrated that through 6,100 strokes under a force of 150 g, the cutting ability of the files was not adversely affected and beyond 6,000 strokes the instruments remained effective. Oliet and Sorin (11, 12) reported that metal wear was not a factor in reducing cutting ability. They concluded that significant decreases in the cutting ability of reamers occurred only after permanent deformation of the instru'rnent. The present study supports the concept that wear alone is not a significant factor in reducing the cutting ability of regular endodontic files. The effects of permanent deformation of these files on their cutting ability has not been assessed, although Gutierrez et al. (22), in a study on used reamers, assumed that physical alterations of
Vol. 9, No. 2, February 1 9 8 3
the instrument would be detrimental to its cutting ability. However, in the present study, using only one cutting surface of many available on the circumference, a file maintained its cutting ability for more than 6,000 strokes. It would, therefore, seem quite unlikely that abrasive wear alone would be a major factor in decreased cutting ability before the instrument was damaged in another manner. The papers by Felt et al. (17) and Villalobos et al. (16) are difficult to relate to the present study since cutting ability was determined using a rotary motion. This motion is not generally used with K type files, which is usually a quarter turn only and withdraw, where the main part of the cutting takes place on the withdrawal stroke. Also, considering the design of triangular versus square cross-sectional files, it is not surprising they observed higher cutting efficiency with the former in a rotary motion. The study of cutting efficiency of root canal instruments in a linear push and pull motion by Webber et al. (18) is in general agreement with the present study since # 3 0 square files did not show a significant difference in cutting ability from the first to the third run. CONCLUSIONS The cutting ability of regular K type stainless steel endodontic files was evaluated using an instrument that simulated the rasping motion used in filing root canals and using poly(methyl methacrylate) to simulate dentin. The force on the file and the length and number of strokes were controlled, and the cutting ability was measured by the depth of the cut in poly(methyl methacrylate). The cutting ability of a single surface of files did not decrease as a result of 6 , 1 0 0 strokes with a length of a stroke being 5 mm. The cutting ability of individual files varied considerably as indicated by the average depth of cut of 0.65 mm and the extreme of the range being 0.21 and 1.75 mm. Reproducibility of the cutting ability of the same surface of a single file, however, was good. Thus, the effects of clinical handling of files on the cutting ability should be evaluated on the same instrument and on the same cutting surface. The values for the depth of cut as a function of the number of strokes demonstrated that wear alone does
Cutting Ability of K Type Files
57
not have a significant role in decreasing the cutting ability of regular K type stainless steel files. This investigation was based on a thesis submitted in partial fulfillment of
the requirements for the M.S. degree in the Horace H. Rackham School of Graduate Studies at The University of Michigan, 1980. Dr. Neal is now in private practice in Gastonia, NC. Dr. Craig is Professor and Chairman, and Dr. Powers is Professor of the Dental Materials Department at the University of Michigan, School of Dentistry, Ann Arbor, M148109. Requests for reprints should be directed to Dr. Craig_
References 1. Grossman LI. Endodontic practice. Philadelphia: Lea and Febiger, 1974. 2. Heuer MA. The biomechanics of endodontic therapy. Dent Clin North Am 1963;344-59. 3. Schilder H. Cleaning and shaping the root canal. Dent Clin North Am t 974;18:269-96. 4. Stewart GG, Kapsimalas P, Rappaport H. EDTA and urea peroxide for root canal preparation. J Am Dent Assoc 1969;78:335-8. 5. Weine FS. Endodontic therapy. 2nd ed. St Louis: CV Mosby, 1976. 6. Gutierrez JH, Garcia J. Microscopic and macroscopic investigation on results of mechanical preparation of root canals. Oral Surg 1968;25:10816. 7_ Jungman CL, Uchin RA, Bucher JF. Effect of instrumentation on the shape of the roof canal. J Endodon t 975;1:66-9. 8. Mizrahi SJ, Tucker JW, Seltzer S. A scanning electron microscopic study of the efficacy of various endodontic instruments. J Endodon 1975;1:324-33. 9. Curson I. Endodontic techniques. VII. Preparation of root canals. Br Dent J 1966;121:329-34. 10. Molven O. A comparison of the dentin removing ability of five root canal instruments. Scand J Dent Res 1970;76:500-11. 11. Oliet S, Sorin SM. Cutting efficiency of endodontic reamers. Oral Surg 1973;36:243-52. 12. Oliet S, Sorin SM. Evaluation of the cutting efficiency of endodontic reamers. Microfilmed Paper No. 606. International Association for Dental Research, Program and abstract of papers, 49th general session, 1971. 13. DeJongh LC, Willoughby JW. Endodontic instruments: an evaluation of cutting ability. Master's Thesis, Ann Arbor, University of Michigan, School of Dentistry, 1975. 14. Shoji Y. Studies of the mechanism of the mechanical enlargement of root canals. Nihon Univ School of Dent J 1965;7:71-8. 15. Ochiai H. Studies on dental hand reamer. I. Automatic measurement of the reamer diameter and cutting torque. II. Diameter and cutting sharpness of commercial reamers. Jpn J Conserv Dent 1976; 19:41-73. 16. Villalobos RL, Moser JB, Heuer MA. A method to determine the cutting efficiency of root canal instruments in rotary motion. J Endodon 1980;6:667-71. 17. Felt RA, Moser JB, Heuer MA. Flute design of endodontic instruments: its influence on cutting efficiency. J Endodon 1982;8:253-9. 18. Webber J, Moser JB, Heuer MA. A method to determine the cutting efficiency of root canal instruments in linear motion. J Endodon 1980;6:82934. 19. Stenman E. Effects of sterilization and endodontic medicaments on mechanical properties of root canal instruments. Doctoral Dissertation, Umea, University of Umea, Sweden, 1977. 20. Craig RG, Gehring PE, Peyton FA. Relation of structure to microhardness of human dentin. J Dent Res 1959;38:624-30 21. Craig RG, Peyton FA. The microhardness of enamel and dentin. J Dent Res 1958;37:661-8. 22. Gutierrez JH, Gigoux C, Sanhueza I. Physical and chemical deterioration of endodontic reamers during mechanical preparation. Oral Surg 1969;26:394-403.