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The effects of steam sterilization and usage on cutting efficiency of endodontic instruments
0099-2399/89/1509-0427/$0200/0 JOURNAL OF ENDODONTfCS Copyright 9 1989 by The American Association of Endodontists
Printed in U.S.A. MOL+ 15, NO. 9, SEPTEMBER1989
The Effects of Steam Sterilization and Usage on Cutting Efficiency of Endodontic Instruments Scott W. Morrison, DDS, MSD, Carl W. Newton, DDS, MSD, and Cecil E. Brown, Jr., DDS, MS
The effects of steam sterilization and usage on sharpness were evaluated on # 2 5 endodontic files. Files were used to instrument 1, 5, and 10 molars. Control groups determined the effect of steam sterilization alone on cutting efficiency of unused files. A cutting efficiency test was performed on an apparatus that compares sharpness of files when used in linear motion. Scanning electron microscopic analysis was performed in each group. Significant differences were found between experimental files used to instrument 1 molar and those used for 5 or 10 molars. The difference in cutting efficiency between the second and third experimental groups was not significant, indicating that most of the decrease in sharpness occurred with use between one and five molars. No significant difference was found between the control groups, indicating no decrease in cutting efficiency by sterilization alone. The scanning electron microscopic analysis supported the statistical data.
instruments to be slightly corroded after autoclaving (11, 12). Custer and Andcrsen (11) reported that autoclaving instruments did not cause significant surface changes when an amine-based oil emulsion was used for corrosion protection. Ncal ct al. (13) reported that autoclaving resulted in a slight decrease in cutting efficiency, which they attributed to corrosion of the stainless steel. The purpose of this study was to investigate the effect of usage and steam sterilization on the cutting efficiency of endodontic files. M A T E R I A L S AND M E T H O D S One-hundred sixty freshly extracted human mandibular and maxillary first and second molars were collected at Indiana University School of Dentistry and stored in sterile saline at room temperature. Prior to use, the teeth were submerged in a solution of 2.6% NaOCI for 48 h and then placed into saline for storage. This was done to remove all soft tissue. Fifty-five #25 Flexofiles (L. D. Caulk Co., Milford, DE) were used for all procedures. A #25 file was selected, as this is often the last file used that would have maximum contact with canal walls prior to flaring the preparation. All files were supplied by the manufacturer and came from the same lot number. They were divided into eight groups: group l - controls, five unused and unsterilized files; group 2--controls, five unused files sterilized one time; group 3--controls, five unused files sterilized five times; group 4--controls, five unused files sterilized 10 times; group 5--controls, five unused files sterilized 15 times; group 6--experimental, 10 files used on one molar each and sterilized one time each; group 7 - experimental, 10 files used on five molars each and sterilized five times each; and group 8--experimental, 10 files used on 10 molars each and sterilized 10 times each. Standard access openings were made after all caries were removed. Preliminary preparation was begun with a new #10 K-type file (Kerr/Sybron, Romulus, MI), followed by #15 and 20 Flexofiles. New files, supplied by the manufacturer, were used in each tooth. An apical stop was created 1.0 mm short of the length where the initial file was first observed penetrating the apical foramen. A total of 5.0 ml of 2.6% NaOC1 was used for irrigating and flooding the chamber during instrumentation. Filing was performed with #10, 15, and 20 files until each fit loosely in the prepared canal. This was followed with a #25 Flexofile, filing for 1 min in each
Usage and sterilization o f e n d o d o n t i c filcs may havc a significant effect on their cutting efficiency. Currently, there are no published reports indicating when files should be discarded becausc of wcar. Luks (l) stated that files arc expendable items and should be discarded if the slightest doubt exists as to whether they have been overused. Montgomery et al. (2) also state that files may be casily damaged during use and should be eliminated if any doubt cxists about their condition. Weine (3) suggested that #8 and lO files should be discarded after one appointmcnt and # 15 through 25 after two appointments. At present, cutting efficiency and weakness of files is determined clinically by subjcctive means only. During the past 15 yr, several studies (4-8) have evaluated the cutting efficiency of endodontic flies; however, most of these have involved new, unused, and unsterilized instruments. More recently, work has been done on used files in an attempt to evaluate wear and sharpness following simulated clinical use (9). Effects of steam sterilization on instrument efficicncy and sharpness have been reported earlier. Stenman (10) found no decrease in cutting ability o f stainless steel files after nine autoclave cyclcs. Other studies have found stainless steel
canal for standardization. An attempt was made to simulate this operator's clinical method of preparing canals.
427
428
Journal of Endodontics
Morrison et ah
All files were stored in 1-dram screw cap glass vials (Kimberly-Clark Corp., Roswcll, GA) numbered from I to 55. Control groups 2, 3, 4, and 5 were sterilized 1, 5, 10, and 15 times, respectively, in a steam autoclave (Amsco Medalist 200, American Sterilizer Co., Erie, PA) for 15 rain at 121~ under 20 psi. The files were cleaned ultrasonically for 3 min in a 1/40 dilution of multipurpose ultrasonic solution (HS Health Sonics, San Ramon, CA) before each sterilization cycle. This was followed by a thorough rinsing with 25 ml of distilled water. A vapor phase corrosion inhibitor (Lorvic Corp., St. Louis, MO) was used during each sterilization. The same procedure was used on all experimental files after each molar was treated. Files again were placed into the glass vials. The cutting efficiency test was performed on an apparatus (Quality Dental Products, Johnson City, TN) that compares sharpness of files when used in linear motion (Fig. 1). The apparatus was calibrated before this investigation. The length of stroke, number of strokes per minute, and load on the file were kept constant; the length of stroke was 12 ram, at 175 strokes per minute, with a load of 250 g. The number of strokes was automatically controlled by a counter set for 25 strokes. The files were clamped in a screw vise on the machine and supported at both ends to minimize flexure during the test. Each file was oriented in the vise with the number 25 on the handle at a 12 o'clock position. The area on the file tested was the apical 12 mm. The substrate for measuring sharpness was a strip of fiberfilled phenolic resin 102-mm long, 19-ram wide, and 0.5-mm thick (Fig. 2). All strips were obtained from the same larger piece of resin. The Knoop Hardness Number of the resin was 33 kg per m m as determined on a Leco M400 hardness tester (Leco Corp., St. Joseph, MI). Each test groove on the strip was numbered, corresponding to the file number (1 through 55). The strips were rigidly held in place by two screw clamps on the apparatus. The depth of each groove was measured with a micrometer slide measuring microscope (Gaertner Scientific Corp., Chicago, IL) to the hundredth of a millimeter. All files were cleaned ultrasonically for 3 rain following the sharpness test to remove any substratc debris.
FIG 2. Two phenolic resin strips showing the grooves made during the cutting efficiency test. The numbers correspond to the control and experimental files.
One file was randomly chosen from each control group and two files from each experimental group for scanning electron microscopic (SEM) analysis. The plastic handles were cut off using a diamond separating disc in a slow-speed handpiece. Each file to be viewed was cleaned ultrasonically for 2 min in acetone to remove any surl~ace contamination, followed by 3 min in the ultrasonic cleaner using distilled water. The files were placed in small metal scanning electron microscope vises and numbered to identify the group. A model S 450 scanning electron microscope (Hitachi, Tokyo, Japan) was used. The working distance was 35 m m and the angle of viewing was 15 degrees. Each file was viewed at a magnification of 60 and photographed using Polaroid PN 55 film. Areas of interest then were magnified 300 times and photographed. Films were washed, coated, dried, and permanently fixed, and the negatives were stored for future reference. An unused #25 Flexofile was photographed at a magnification of 60 and 300 with the scanning electron microscope. The file was used to instrument five molars in the same manner as the experimental files. The file then was photographed at 60 and 300 magnification with the scanning electron microscope. The same file then was used to instrument five additional molars, followed by SEM analysis at 60 and 300 magnification. The file was notched on the shank below the handle to ensure the same orientation in the scanning electron microscope during each photographic examination. The photographs were compared for evidence of wear or damage. Statistical tests of homogeneity of variances and the analysis of variance F tests of equality of means were performed. The Student-Newman-Keuls multiple range test also was done. RESUI,TS
FIG 1. A photograph of the cutting efficiency test apparatus provided by Quality Dental Products. It was used to test the cutting efficiency of files in linear motion.
No visible damage occurred to the files during the testing procedure, nor did file separation occur during instrumentation. Groups 1 through 5 were control groups and groups 6 through 8 were experimental groups.
File Wear
Vol. 15, No. 9, September 1989
429
TABLE 1. Mean, standard deviation, and standard error for control and experimental groups Group Control 1 2 3 4 5 Experimental 6 7 8
Mean
SD
SE
9.03 8.63 8.77 8.72 8.09
0.14 0.58 0.47 1.29 0.45
0.06 0.26 0.21 0.58 0.20
8.34 7.07 6.84
0.77 0.83 0.69
0.24 0.26 0.22
TABLE 2, Analysis of variance df
Mean Square
F Ratio
Between control groups* 4 Within control groups 20 Between experimentalgroups 2 Within experimental groups 27
0.0012 0.0010 6.4769 0.5841
1.25
Source
11.09
P <0.32 <0.0003
9 A f l e r log l r a n s f o r m a t i o n .
TABLE 3. Student-Newman-Keuls multiple range test Multiple Comparisons
Significance
Group 6 versus 7 Group 6 versus 8 Group 7 versus 8
p < 0.05 p < 0.05 Not significant
Tablc 1 shows the mean, standard deviation, and standard error for the control and experimental groups. It is evident that the mean depth of cuts in the control groups is close in value. The only variable in these groups was the number of sterilization cycles. In the experimental groups, the average depth of cut decreased proportionately with an increase in usage. No significant difference was found between the control groups. The analysis of variance shows that the diff~rcncc in cutting efficiency between experimental groups was highly significant at p < 0.0003 (Table 2). To compare differences among the experimental groups, the Student-Newman-Keuls multiple range test was done, with the results shown in Table 3. Using multiple comparisons, a significant difference was found between group 6 (one molar and one sterilization) and group 7 (five molars and 5 sterilization cycles) and between group 6 and group 8 (10 molars and 10 sterilization cycles). The difference in cutting efficiency of files between group 7 and group 8 was not significant, indicating that most of the decrease in sharpness occurs with use in one to five molars. Since the difference between groups 7 and 8 was not significant, usage after five molars does not appear to further reduce the cutting efficiency compared with the reduction which occurs in usage in one to five molars. Figure 3 is a SEM photograph of an unused, unsterilized #25 Flexofile at 300 times magnification. The area indicated by the a r r o w in Fig. 3 is the cutting edge on the lburth flute from the tip of the file. Figure 4 shows the same file after the instrumentation of five molars, with the file being sterilized after each tooth. The a r r o w in Fig. 4 points to a flattened area
FIG 3. SEM photograph of an unused and unsterilized file. Arrow points to the cutting edge of the fourth flute (original magnification •
FiG 4. SEM photograph of file illustrated in Fig. 3. This file was used to instrument five molars and sterilized five times. Arrow indicates change in cutting edge of the fourth flute (original magnification •
430
Journal of Endodontics
Morrison et al.
F~G 5. SEM photograph of file illustrated in Fig 3. This file was used to instrument 10 molars and sterilized 10 times. Arrow indicates cutting edge ol the fourth flute (original magn,ficatlon •
on thc cutting edge. prcsumabl~ caused h \ ~ear. Figurc 5 shows the samc instrun~ent alter filing 10 molars and Ix.'ing sterilized each time. In Fig. 5 (~+rr,m). the appearance of the edge o f the flute is similar to that in i:ig. 4. elhis corresponds; to thc statistical data ~hich indicatc that the p r i m e d decrease in cutting cllicicnc.~ occurs with file ns4lge in one tO live molars.
l ) l ~ ( ' t ",%%1( ) N
l h e sharpness of cndodtmtic files is an important consideration during root canal preparation. ('urrentlx. there arc no standards for shaqmes.s of cndodontic files. .Man\ factors m e \ allect cutting abilitx of cndodontic instruments. These include cross-sectional contiguration of the instrument. Ihc material from v, hich it x~as nlanufacttlred, the manul~acturcr, lhe size of the inslrt, mcnt, electropolishing. n u m b e r of tlt, tcs. talx+r, and sharpness el'the cutting cdgc (4). +lhe trend is to~ard a decrease in cutting ahilit\ as size increascs. Fch et al. (6) sho~+cd that smallcr instruments x~cre more efficient than larger ones o1 the same flute design. The phenolic resin strip t,scd in this investigation ~as an cxcellent substratc lot use in the sharpness test. ['sing this material eliminated the hardness variabilit\ of dentin and bone. Since this studs was a con~parison in\estigati(tn of decreased cutting~etllcienc.x of tilcs, it ~as not neccssara, to use a test material that simulated dentin.
This stud,, conlirmed that measuring thc depth of cut is a viaNe method of comparing culling ellicienc.x of rids...\s expected, it shm~cd that as the numher of teeth inslrunlentcd ~ ith a
References "~ Lul,,s S E~asiC reel canal insfronlentb a99, ~ho.lrcorrect dse NY Der:l J 7 9 5 3 2 3 114 7 2 \4{-I'~lOorT'e*y S Car:ales M L de' R o CF F,,o cleft'age dtJr,rl~ roe! (:anal preparer,on J Err;eden 1984 10 45 7 3 We,no FS ['ndoclont,c the,apy 3to ecJ SI Lo~e5 CV Most)y 1982 266 4 Delongh LC WhtOUt.~"Dy JW Endtxlontlc iil~,lrumonlb-- ~r" cvaluahop of cuttlnq ab,hty JMaster >, ! qes;s I Uqlvers ly of M,cf', san Schoo o t Oe,"l s t r y 1975 5 Vh d+o:',os I l L Moher JIB fleuer MA A r~elh(xl, to clete-",,nu file ctlttlng off Cler'cy of tool canal iiis i1 rotary met,c~ J En(:cx~oq1980.6 6 6 7 /1
Vol. 15, No. 9, September 1989 6. Felt RA, Moser J13. Heuer MA. Flute design of endodontic instruments: its influence on cutting efficiency. J Endodon 1982;8:253-9 7. Machian GR, Peters DD, Lorton L. The comparative efficiency of four types of endodontic instruments. J Endodon 1982;8:398-402. 8. Neal RG, Craig RG, Powers JM. Cutting ability of K-type endodontic files. J Endodon 1983;9:52-7. 9. Chenail BL, Brantley WA, Gerstein H. Clockwise torsional properties of new and used root canal files. J Endodon 1986;12:59-63. 10. Stenman E. Effects of sterilization and endodontic medicaments on mechanical properties of root canal instruments IMaster's "l-hesisl. Umea,
File Wear
431
Sweden: University of Umea. I977:25. 11. Custer F, Andersen H. A study of surface changes in instruments during sterilization procedures. NY J Dent 1968;38:8-17. 12. Sandrik JL, Wragg LE. Corrosion effects of three sterilization methods on experimental surfaces. J Biomed Mater Res 1970;4:275-7. 13. Neal RG, Craig RG, Powers J M Effect of sterilization and irrigants on the cutting ability of stainless steel files. J Endodon 1983;9:93-6. 14. Felhaber "TB. The effects of repeated use and sterilization of certain properties of stainless steel endodontic files IMaster's Thesisl. Indianapolis, Indiana: Indiana University School of s 1969:22-8.
Printed in U.S.A. MOL+ 15, NO. 9, SEPTEMBER1989
The Effects of Steam Sterilization and Usage on Cutting Efficiency of Endodontic Instruments Scott W. Morrison, DDS, MSD, Carl W. Newton, DDS, MSD, and Cecil E. Brown, Jr., DDS, MS
The effects of steam sterilization and usage on sharpness were evaluated on # 2 5 endodontic files. Files were used to instrument 1, 5, and 10 molars. Control groups determined the effect of steam sterilization alone on cutting efficiency of unused files. A cutting efficiency test was performed on an apparatus that compares sharpness of files when used in linear motion. Scanning electron microscopic analysis was performed in each group. Significant differences were found between experimental files used to instrument 1 molar and those used for 5 or 10 molars. The difference in cutting efficiency between the second and third experimental groups was not significant, indicating that most of the decrease in sharpness occurred with use between one and five molars. No significant difference was found between the control groups, indicating no decrease in cutting efficiency by sterilization alone. The scanning electron microscopic analysis supported the statistical data.
instruments to be slightly corroded after autoclaving (11, 12). Custer and Andcrsen (11) reported that autoclaving instruments did not cause significant surface changes when an amine-based oil emulsion was used for corrosion protection. Ncal ct al. (13) reported that autoclaving resulted in a slight decrease in cutting efficiency, which they attributed to corrosion of the stainless steel. The purpose of this study was to investigate the effect of usage and steam sterilization on the cutting efficiency of endodontic files. M A T E R I A L S AND M E T H O D S One-hundred sixty freshly extracted human mandibular and maxillary first and second molars were collected at Indiana University School of Dentistry and stored in sterile saline at room temperature. Prior to use, the teeth were submerged in a solution of 2.6% NaOCI for 48 h and then placed into saline for storage. This was done to remove all soft tissue. Fifty-five #25 Flexofiles (L. D. Caulk Co., Milford, DE) were used for all procedures. A #25 file was selected, as this is often the last file used that would have maximum contact with canal walls prior to flaring the preparation. All files were supplied by the manufacturer and came from the same lot number. They were divided into eight groups: group l - controls, five unused and unsterilized files; group 2--controls, five unused files sterilized one time; group 3--controls, five unused files sterilized five times; group 4--controls, five unused files sterilized 10 times; group 5--controls, five unused files sterilized 15 times; group 6--experimental, 10 files used on one molar each and sterilized one time each; group 7 - experimental, 10 files used on five molars each and sterilized five times each; and group 8--experimental, 10 files used on 10 molars each and sterilized 10 times each. Standard access openings were made after all caries were removed. Preliminary preparation was begun with a new #10 K-type file (Kerr/Sybron, Romulus, MI), followed by #15 and 20 Flexofiles. New files, supplied by the manufacturer, were used in each tooth. An apical stop was created 1.0 mm short of the length where the initial file was first observed penetrating the apical foramen. A total of 5.0 ml of 2.6% NaOC1 was used for irrigating and flooding the chamber during instrumentation. Filing was performed with #10, 15, and 20 files until each fit loosely in the prepared canal. This was followed with a #25 Flexofile, filing for 1 min in each
Usage and sterilization o f e n d o d o n t i c filcs may havc a significant effect on their cutting efficiency. Currently, there are no published reports indicating when files should be discarded becausc of wcar. Luks (l) stated that files arc expendable items and should be discarded if the slightest doubt exists as to whether they have been overused. Montgomery et al. (2) also state that files may be casily damaged during use and should be eliminated if any doubt cxists about their condition. Weine (3) suggested that #8 and lO files should be discarded after one appointmcnt and # 15 through 25 after two appointments. At present, cutting efficiency and weakness of files is determined clinically by subjcctive means only. During the past 15 yr, several studies (4-8) have evaluated the cutting efficiency of endodontic flies; however, most of these have involved new, unused, and unsterilized instruments. More recently, work has been done on used files in an attempt to evaluate wear and sharpness following simulated clinical use (9). Effects of steam sterilization on instrument efficicncy and sharpness have been reported earlier. Stenman (10) found no decrease in cutting ability o f stainless steel files after nine autoclave cyclcs. Other studies have found stainless steel
canal for standardization. An attempt was made to simulate this operator's clinical method of preparing canals.
427
428
Journal of Endodontics
Morrison et ah
All files were stored in 1-dram screw cap glass vials (Kimberly-Clark Corp., Roswcll, GA) numbered from I to 55. Control groups 2, 3, 4, and 5 were sterilized 1, 5, 10, and 15 times, respectively, in a steam autoclave (Amsco Medalist 200, American Sterilizer Co., Erie, PA) for 15 rain at 121~ under 20 psi. The files were cleaned ultrasonically for 3 min in a 1/40 dilution of multipurpose ultrasonic solution (HS Health Sonics, San Ramon, CA) before each sterilization cycle. This was followed by a thorough rinsing with 25 ml of distilled water. A vapor phase corrosion inhibitor (Lorvic Corp., St. Louis, MO) was used during each sterilization. The same procedure was used on all experimental files after each molar was treated. Files again were placed into the glass vials. The cutting efficiency test was performed on an apparatus (Quality Dental Products, Johnson City, TN) that compares sharpness of files when used in linear motion (Fig. 1). The apparatus was calibrated before this investigation. The length of stroke, number of strokes per minute, and load on the file were kept constant; the length of stroke was 12 ram, at 175 strokes per minute, with a load of 250 g. The number of strokes was automatically controlled by a counter set for 25 strokes. The files were clamped in a screw vise on the machine and supported at both ends to minimize flexure during the test. Each file was oriented in the vise with the number 25 on the handle at a 12 o'clock position. The area on the file tested was the apical 12 mm. The substrate for measuring sharpness was a strip of fiberfilled phenolic resin 102-mm long, 19-ram wide, and 0.5-mm thick (Fig. 2). All strips were obtained from the same larger piece of resin. The Knoop Hardness Number of the resin was 33 kg per m m as determined on a Leco M400 hardness tester (Leco Corp., St. Joseph, MI). Each test groove on the strip was numbered, corresponding to the file number (1 through 55). The strips were rigidly held in place by two screw clamps on the apparatus. The depth of each groove was measured with a micrometer slide measuring microscope (Gaertner Scientific Corp., Chicago, IL) to the hundredth of a millimeter. All files were cleaned ultrasonically for 3 rain following the sharpness test to remove any substratc debris.
FIG 2. Two phenolic resin strips showing the grooves made during the cutting efficiency test. The numbers correspond to the control and experimental files.
One file was randomly chosen from each control group and two files from each experimental group for scanning electron microscopic (SEM) analysis. The plastic handles were cut off using a diamond separating disc in a slow-speed handpiece. Each file to be viewed was cleaned ultrasonically for 2 min in acetone to remove any surl~ace contamination, followed by 3 min in the ultrasonic cleaner using distilled water. The files were placed in small metal scanning electron microscope vises and numbered to identify the group. A model S 450 scanning electron microscope (Hitachi, Tokyo, Japan) was used. The working distance was 35 m m and the angle of viewing was 15 degrees. Each file was viewed at a magnification of 60 and photographed using Polaroid PN 55 film. Areas of interest then were magnified 300 times and photographed. Films were washed, coated, dried, and permanently fixed, and the negatives were stored for future reference. An unused #25 Flexofile was photographed at a magnification of 60 and 300 with the scanning electron microscope. The file was used to instrument five molars in the same manner as the experimental files. The file then was photographed at 60 and 300 magnification with the scanning electron microscope. The same file then was used to instrument five additional molars, followed by SEM analysis at 60 and 300 magnification. The file was notched on the shank below the handle to ensure the same orientation in the scanning electron microscope during each photographic examination. The photographs were compared for evidence of wear or damage. Statistical tests of homogeneity of variances and the analysis of variance F tests of equality of means were performed. The Student-Newman-Keuls multiple range test also was done. RESUI,TS
FIG 1. A photograph of the cutting efficiency test apparatus provided by Quality Dental Products. It was used to test the cutting efficiency of files in linear motion.
No visible damage occurred to the files during the testing procedure, nor did file separation occur during instrumentation. Groups 1 through 5 were control groups and groups 6 through 8 were experimental groups.
File Wear
Vol. 15, No. 9, September 1989
429
TABLE 1. Mean, standard deviation, and standard error for control and experimental groups Group Control 1 2 3 4 5 Experimental 6 7 8
Mean
SD
SE
9.03 8.63 8.77 8.72 8.09
0.14 0.58 0.47 1.29 0.45
0.06 0.26 0.21 0.58 0.20
8.34 7.07 6.84
0.77 0.83 0.69
0.24 0.26 0.22
TABLE 2, Analysis of variance df
Mean Square
F Ratio
Between control groups* 4 Within control groups 20 Between experimentalgroups 2 Within experimental groups 27
0.0012 0.0010 6.4769 0.5841
1.25
Source
11.09
P <0.32 <0.0003
9 A f l e r log l r a n s f o r m a t i o n .
TABLE 3. Student-Newman-Keuls multiple range test Multiple Comparisons
Significance
Group 6 versus 7 Group 6 versus 8 Group 7 versus 8
p < 0.05 p < 0.05 Not significant
Tablc 1 shows the mean, standard deviation, and standard error for the control and experimental groups. It is evident that the mean depth of cuts in the control groups is close in value. The only variable in these groups was the number of sterilization cycles. In the experimental groups, the average depth of cut decreased proportionately with an increase in usage. No significant difference was found between the control groups. The analysis of variance shows that the diff~rcncc in cutting efficiency between experimental groups was highly significant at p < 0.0003 (Table 2). To compare differences among the experimental groups, the Student-Newman-Keuls multiple range test was done, with the results shown in Table 3. Using multiple comparisons, a significant difference was found between group 6 (one molar and one sterilization) and group 7 (five molars and 5 sterilization cycles) and between group 6 and group 8 (10 molars and 10 sterilization cycles). The difference in cutting efficiency of files between group 7 and group 8 was not significant, indicating that most of the decrease in sharpness occurs with use in one to five molars. Since the difference between groups 7 and 8 was not significant, usage after five molars does not appear to further reduce the cutting efficiency compared with the reduction which occurs in usage in one to five molars. Figure 3 is a SEM photograph of an unused, unsterilized #25 Flexofile at 300 times magnification. The area indicated by the a r r o w in Fig. 3 is the cutting edge on the lburth flute from the tip of the file. Figure 4 shows the same file after the instrumentation of five molars, with the file being sterilized after each tooth. The a r r o w in Fig. 4 points to a flattened area
FIG 3. SEM photograph of an unused and unsterilized file. Arrow points to the cutting edge of the fourth flute (original magnification •
FiG 4. SEM photograph of file illustrated in Fig. 3. This file was used to instrument five molars and sterilized five times. Arrow indicates change in cutting edge of the fourth flute (original magnification •
430
Journal of Endodontics
Morrison et al.
F~G 5. SEM photograph of file illustrated in Fig 3. This file was used to instrument 10 molars and sterilized 10 times. Arrow indicates cutting edge ol the fourth flute (original magn,ficatlon •
on thc cutting edge. prcsumabl~ caused h \ ~ear. Figurc 5 shows the samc instrun~ent alter filing 10 molars and Ix.'ing sterilized each time. In Fig. 5 (~+rr,m). the appearance of the edge o f the flute is similar to that in i:ig. 4. elhis corresponds; to thc statistical data ~hich indicatc that the p r i m e d decrease in cutting cllicicnc.~ occurs with file ns4lge in one tO live molars.
l ) l ~ ( ' t ",%%1( ) N
l h e sharpness of cndodtmtic files is an important consideration during root canal preparation. ('urrentlx. there arc no standards for shaqmes.s of cndodontic files. .Man\ factors m e \ allect cutting abilitx of cndodontic instruments. These include cross-sectional contiguration of the instrument. Ihc material from v, hich it x~as nlanufacttlred, the manul~acturcr, lhe size of the inslrt, mcnt, electropolishing. n u m b e r of tlt, tcs. talx+r, and sharpness el'the cutting cdgc (4). +lhe trend is to~ard a decrease in cutting ahilit\ as size increascs. Fch et al. (6) sho~+cd that smallcr instruments x~cre more efficient than larger ones o1 the same flute design. The phenolic resin strip t,scd in this investigation ~as an cxcellent substratc lot use in the sharpness test. ['sing this material eliminated the hardness variabilit\ of dentin and bone. Since this studs was a con~parison in\estigati(tn of decreased cutting~etllcienc.x of tilcs, it ~as not neccssara, to use a test material that simulated dentin.
This stud,, conlirmed that measuring thc depth of cut is a viaNe method of comparing culling ellicienc.x of rids...\s expected, it shm~cd that as the numher of teeth inslrunlentcd ~ ith a
References "~ Lul,,s S E~asiC reel canal insfronlentb a99, ~ho.lrcorrect dse NY Der:l J 7 9 5 3 2 3 114 7 2 \4{-I'~lOorT'e*y S Car:ales M L de' R o CF F,,o cleft'age dtJr,rl~ roe! (:anal preparer,on J Err;eden 1984 10 45 7 3 We,no FS ['ndoclont,c the,apy 3to ecJ SI Lo~e5 CV Most)y 1982 266 4 Delongh LC WhtOUt.~"Dy JW Endtxlontlc iil~,lrumonlb-- ~r" cvaluahop of cuttlnq ab,hty JMaster >, ! qes;s I Uqlvers ly of M,cf', san Schoo o t Oe,"l s t r y 1975 5 Vh d+o:',os I l L Moher JIB fleuer MA A r~elh(xl, to clete-",,nu file ctlttlng off Cler'cy of tool canal iiis i1 rotary met,c~ J En(:cx~oq1980.6 6 6 7 /1
Vol. 15, No. 9, September 1989 6. Felt RA, Moser J13. Heuer MA. Flute design of endodontic instruments: its influence on cutting efficiency. J Endodon 1982;8:253-9 7. Machian GR, Peters DD, Lorton L. The comparative efficiency of four types of endodontic instruments. J Endodon 1982;8:398-402. 8. Neal RG, Craig RG, Powers JM. Cutting ability of K-type endodontic files. J Endodon 1983;9:52-7. 9. Chenail BL, Brantley WA, Gerstein H. Clockwise torsional properties of new and used root canal files. J Endodon 1986;12:59-63. 10. Stenman E. Effects of sterilization and endodontic medicaments on mechanical properties of root canal instruments IMaster's "l-hesisl. Umea,
File Wear
431
Sweden: University of Umea. I977:25. 11. Custer F, Andersen H. A study of surface changes in instruments during sterilization procedures. NY J Dent 1968;38:8-17. 12. Sandrik JL, Wragg LE. Corrosion effects of three sterilization methods on experimental surfaces. J Biomed Mater Res 1970;4:275-7. 13. Neal RG, Craig RG, Powers J M Effect of sterilization and irrigants on the cutting ability of stainless steel files. J Endodon 1983;9:93-6. 14. Felhaber "TB. The effects of repeated use and sterilization of certain properties of stainless steel endodontic files IMaster's Thesisl. Indianapolis, Indiana: Indiana University School of s 1969:22-8.