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Canal master files: Scanning electron microscopic evaluation of new instruments and their wear with clinical usage
0099-2399/92/1807-0336/$03.00/0 JOURNAL OF ENDODONTICS Copyright © 1992 by The American Association of Endodontists
Printed in U.S.A. VOL. 18, NO. 7, JULY 1992
Canal Master Files: Scanning Electron Microscopic Evaluation of New Instruments and Their Wear with Clinical Usage Mario L. Zuolo, CD, Richard E. Walton, DMD, MS and Carlos A. F. Murgel, CD
instrument is innovative and, as shown in the manufacturer's diagrams, incorporates three distinct features: (a) a noncutting pilot tip; (b) a cutting (fluted) head tapered from 16 mm to 1 to 2 mm; and (c) a small, nontapered flexible shaft. The developers have proposed that the instrument would be effective in curved canals by reducing transportation and generally giving better control. A recent report (13) confirms this suggestion, although there was a tendency toward distortion and breakage of the files during canal preparation. Aside from this report (13) there have been no controlled studies of usage patterns or of the manufacturer's quality control in production of the Canal Master files. The purpose of this study was to examine Canal Master files with the scanning electron microscope. In a longitudinal analysis, the following qualities were assessed for both new and used instruments: (a) quality of the new instruments as compared with the manufacturer's diagrams; (b) wear characteristics under progressive clinical use; and (c) incidence of breakage of the various sized files with time of usage.
Canal Master instruments have a short, fluted cutting area and a small flexible shaft; they may be predisposed to rapid wear and breakage. This study examined new and used files as to tip, flute, and shaft design when new and with increasing time of canal preparation. One hundred files prepared 140 curved canals. Sizes #20, #40, and # 6 0 files were examined unused and after 1, 3, 5, and 7 min of use. Evaluation was for uniformity when new and for deterioration and breakage with usage. The smaller sizes (#20 and #40) had some inconsistencies when new and were most predisposed to wear and breakage with time. Findings suggest that smaller files could have had improved quality control by the manufacturer. Also, they should be used with caution and discarded after short times of use in small, curved canals. This should minimize instrument separation and maximize cutting efficiency.
MATERIALS AND METHODS Intracanal instruments are selected for cleaning and shaping of the root canal system according to certain perceived physical characteristics. Efficiency and efficacy during instrumentation should be achieved when the instruments employed are used with regard to their inherent qualities as well as their limitations (1). To test physical properties and failure characteristics, several brands and designs of files have been subjected to laboratory tests. Several studies have investigated the cutting efficiency of files and reamers (2-5). Factors related to damage have been examined including the amount of bending and torsional stresses causing breakage of the files (6-9). File deterioration resulting from use in canal preparation has also been studied (10, 11). Overall, these studies have shown that instruments become increasingly defective, lose their cutting efficiency, and tend to separate with progressive or excessive
Instruments New Canal Master files were obtained directly from the manufacturer. One hundred files of 25 mm (the only length available) were evaluated. This group was further divided into 10 each in sizes #20 through #60, and 5 each in #70 and #80. For scanning electron microscopic evaluation, the 10 files in each of the #20, #40, and #60 groups were examined. All 100 files were used for canal preparation and therefore were included in the determination of breakage. Before initial examination and use all files were placed in boiling water for 5 min, followed by 5-min exposure in an ultrasonic bath containing a cleaning solution. This was done to remove any plastic or other debris which frequently remain on new files as part of the manufacturing process (14). One hundred forty root canals with a 25- to 35-degree curve (15) from 70 first and second mandibular molars were used. Teeth were stored in distilled water after extraction until
use.
Recently, a new root canal instrument, the Canal Master System (Brasseler USA Incorporated, Savannah, GA), has been introduced by Wildey and Senia (12). The design of this
use.
336
Canal Master Files
Vol. 18, No. 7, July 1992
Instrument Usage Canal preparation with the Canal Master files was according to the manufacturer's recommendations (16): (a) Following straight-line access preparation, canal length was established at 1 mm from the apical foramen. Canals were instrumented to length using a K-Flex file with filing motion. (b) Next, the cervical and mid-portion of the canal was prepared with a #50 through a #100 Canal Master rotary instrument to the level of the beginning of the curvature. (c) The apical preparation was then completed with Canal Master hand files, #20 through #40, at length. The step-back preparation was at 1-mm increments with files #45 through #80. Each file was used for 30 s in each mesial canal, resulting in a total usage time of I min per file per root. In the event of an instrument separation, that canal preparation was not completed. (d) During enlargement, 30 ml of 2.6% sodium hypochlorite/ canal delivered from a 27-gauge needle was used as an irrigant. Irrigation was maintained throughout by replenishing after each rotary and hand instrument.
337
(primarily the smaller) showed wear with use and a tendency for breakage with time.
Scanning Electron Microscopic Evaluation NEW INSTRUMENTS (CONTROLS) The Canal Master files showed variability in their manufactured characteristics (Fig. 1). The small (#20) size in particular, and intermediate (#40) sizes had some inconsistencies in taper, uniformity, and flute characteristics. The ends of small files were round and short, in contrast with the manufacturer's diagram of a cone-shaped "pilot tip" (Fig. 1). However, the #60 consistently showed a longer, uniform, and well-designed tip (Fig. 2). In the cutting head of all sizes, the flute edges were often rounded but were generally of the specified design. Some of the #20 files did not have definitive, deep grooves and pronounced flutes. Also, there often was a variable number of flutes in each instrument size
Scanning Electron Microscopic Examination Because the files could be studied repeatedly in the scanning electron microscope without affecting their physical properties, the same file could be examined new, thereby serving as an internal control, and then after each usage period. The times each file was viewed were at 0 (new, unused), 1, 3, 5, and 7 min. For each viewing, instruments were prepared as follows: (a) Files were cleaned in an ultrasonic bath for 5 min to remove dentin and tissue debris. (b) Files were mounted on copper tape on stubs; no sputter coating was necessary because the metal is conductive. (c) Tip, flute, and shaft were viewed and photographed on an AMRAY 1820 scanning electron microscope.
Evaluation Photomicrographs of each region were studied at varying magnifications for defects created during canal preparation. All photomicrographs were coded, mixed, and analyzed blindly for the following: New instruments--Design, tapering uniformity, and length of the tip, cutting head and shaft were noted. Also examined were the consistency and shape of the cutting edges of the flutes, the absence of metal tears, and the quality of the grinding process. Used instruments--On the flutes, the presence of defects such as gouges, pits, and loss of portions of metal was noted. Cutting edges were examined for rounding off or disruption. Shafts were studied for distortions such as twisting, irregularities, or potential fracture regions as indicated by tears in the metal. All files which separated were recorded as to their usage time and size. Results were expressed as a percentage of breakdown in each file size.
Fre 1. Number 20 files new, unused. Note the inconsistency in number and sharpness of flutes. There is no "pilot tip" on any of the files. Variability in flutes and absence of a pilot tip was characteristic of small files.
RESULTS New instruments showed some variations in shape and design, particularly in the smaller sizes. In addition, the files
FIG 2. Number 60 files new. In contrast to smaller sizes, these showed more consistency with sharp, uniform flutes and a cone-shaped pilot tip.
338
Zuolo et al.
Journal of Endodontics
FiG 3. Number 40, 5-min use. The flute edges show some wear and flattening.
FiG 6. Number 20, #40 files separated in mesial canals. These separated at the shaft near the flutes.
(Fig. 1). The shafts of all sizes were generally very uniform with an absence of tears or other notable defects. Larger files showed a more regular pattern of grinding of the flutes (Fig. 2). USED INSTRUMENTS FtG 4. Number 40, 7-min use. Some show more flute flattening but are not significantly changed from 5-min use. The middle file is shaft only; the head separated during instrumentation.
All three file sizes showed similar patterns. At the first two times of use (1 or 3 min), there was little evidence of wear. The cutting heads of the flutes were similar to those of the controls without visible defects or rounding of the edges. After 5 min, alterations appeared in the flutes as small gouges, fiat spots, or irregularities on the cutting edges (Fig. 3). A visible change was the distortion and twisting of the shaft portion with definite evidence of wear. In addition, potential fracture areas appeared, particularly in the #20 and #40 sizes. After 7 min of use, the #20 files showed marked deterioration with complete loss of the rounded shape of the cutting edges and severe distortions in the shaft. #40 files had less wear than the #20's, but showed generally more than at the 5-min use (Fig. 4). The #60 file showed considerably less evidence of wear or other defects (Fig. 5).
Instrument Breakage
FIG 5. Number 60, 7-min use. These show relatively little wear, a typical finding.
Of the 100 files, 21 separated during canal preparation, invariably in the apical one third (Figs. 4 and 6), for an overall occurrence of 21% breakage. The following size files were broken with incidences (out of 10 files/size) in parentheses: #20 (4), #25 (4), #30 (3), #35 (4), #40 (5) and #60 (1). The times of breakage of the sizes are shown in Fig. 7.
Vol. 18, No. 7, July 1992
Canal Master Files
DISCUSSION The unique design characteristics are claimed in advertising to be responsible for the special behavior of the Canal Master files. However, although the majority of the instruments examined were generally shaped as advertised by the manufacturer, the flutes and tips of the files were frequently different, particularly in the smallest sizes. The tip supposedly is a noncutting pointed pilot of approximately 0.75 m m which is intended to guide the instrument in the canal as well as enable the instrument to be maintained within the canal during enlargement. Our observations were that a round blunt tip was frequently present and would not serve as a pilot. This could certainly affect the usage characteristics of the files. Another alteration from the manufacturer's diagrams was the inconsistency of the taper and length of the cutting head. This variation in flute numbers, particularly in different sizes, could result in differences in feel between files. A complication could be inefficient cutting, resulting in an underrotation or locking of the file. This condition creates potential pressure points predisposed to fracture of the shaft. We could not determine whether this was a major cause of instrument failure in our samples. The new shafts were smooth and round, showing uniformity in properties and flexibility. Presumably, this would encourage the instrument to easily follow canal curvature as advertised; this was the impression during preparations. Overall, our results showed the necessity of either better quality control by the manufacturer or a different machining system for the instruments, particularly in the smaller sizes. The evidence of wear and the relatively high incidence of breakage after 5 and 7 min of use dictates that instruments be used carefully and discarded without hesitation. This should be the rule during preparation of smaller more curved canals. An early report (12) did caution against overaggressive cutting, particularly in a clockwise direction, which would predispose the file to counterclockwise separation Operator skill is critical in this type of research. The author (M. Z.) performing the preparations was familiar with and well trained in the use of Canal Master files, both with patients and in extracted teeth. Even with this expertise, there were defects and failures of instruments. This reinforces the necessity of practicing with the instruments in extracted teeth to 12
E
~1 ~o
"5:'.S-,:' : 2
~
s
~
4
;.~;'~/j~_l:-Z:c
3
~=," a,p "~J!.~=.;i
~
o 1
3
5
7
Period of Observation (minutes) FiG 7. The number of instruments that separated at each usage time. No instruments separated at 1 rain, whereas 11 broke after 7 rain of instrumentation.
339
gain familiarity with application of their unique physical properties (13). The relatively high incidence of breakage could be a result of inherent weakness of the shaft but also because of length, which may predispose the shafts to fracture more easily than if they were shorter. However, because the files are only available in 25 mm, this cannot be determined with certainty. Perhaps this weakness would be reduced in 19- or 21-mm files. All separated instruments were wedged in the apical third of the canal. From the location and the nature of the accident, the broken tips would be difficult or impossible to remove. This further reinforces that smaller instruments (#20 to #40) should be used with care. A variation in the manufacturer's recommendation (16) would be to use smaller instruments minimally (for example #20 and #25) at length and begin step-back. Files should be readily discarded if the operator notices any "softness," twisting, or bending of the shaft; these frequently were indicators of subsequent breakage upon the next file usage. The manufacturer has introduced a modification in the instruments, renamed the Canal Master "U" file. This primarily changes the flute design and methods of use. Whether the changes result in different physical or performance characteristics has yet to be demonstrated in clinical tests. Dr. Zuolo is a professor in endodontics, Camilo Castelo Branco, School of Dentistry, University S&o Paulo, Brazil. Dr. Walton is professor and chairman, Department of Endodontics, University of Iowa, College of Dentistry, Iowa City, IA. Dr. Murgel is a professor in endodontics, Camilo Castelo Branco, School of Dentistry, University S~o Paulo, Brazil. Address requests for reprints to Dr. Richard Walton, Department of Endodontics, University of Iowa, College of Dentistry, Iowa City, IA 52242.
References 1. Moreinis SA. The K-type file and reamers: mainstays of the endodontic armamentarium. N Y State Dent J 1979;45:390-3. 2. Newman JG, Brantley WA, Gerstein H. A study of the cutting efficency of seven brands of endodontic files in linear motion. J Endodon 1983;9:31622. 3. Miserendino LJ, Moser JB, Heuer MA, Setek EM. Cutting efficiency of endodontic instruments. Part L A quantitative comparison of the tip and fluted region. J Endodon 1985;11:435-41. 4. Miserendino LJ, Moser JB, Heuer MA, Osetek EM. Cutting of endodontic instruments. Part II. Analysis of tip design. J Endodon 1986;12:8-12. 5. Stenman E, Spangberg L. Machining efficiency of Flex-R, K-Flex, TrioCut and S Files. J Endodon 1990;16:575-9. 6. Craig RG, Mcllwain ED, Peyton FA. Bending and torsion properties of endodontics instruments. Oral Surg 1968;25:239-54. 7. Lautenschlager EP, Jacobs JJ, Marshall GW Jr, Heuer MA. Brittle and ductile torsional failures of endodontic instruments. J Endodon 1977;3:175-8. 8. Dolan DW, Craig RG. Bending and torsion of endodontic files with rhombus cross-sections. J Endodon 1982;8:260-4. 9. Roth WC, Gouch RW, Grandich RA, Walker WA. A study of the strength of endodontic files: potential for torsional breakage and relative flexibility. J Endodon 1983;9:228-32. 10. Montgomery S, Canales ML, del Rio CE. File damage during root canal preparation. J Endodon 1984;10:45-7. 11. Scott GL, Walton RE. Ultrasonic endodontics: the wear of instruments with usage. J Endodon 1986;12:279-83. 12. Wildey WL, Senia ES. A new root canal instrument and instrumentation technique. A preliminary report. Oral Surg 1989;67:198-207. 13. Gilles JA, del Rio CE. A comparison of the Canal Master endodontic instrument and K-type files for enlargement of curved root canals. J Endodon 1990;16:561-5. 14. Murgel CAF, Walton RE, Rittman B, Pecora JD. A comparison of techniques for cleaning endo~ontic files after usage: a quantitative scanning electron microscopic study. J Endodon 1990;16:214-7. 15. Schneider SW. A comparison of canal preparations in straight and curved root canals. Oral Surg 1971 ;32:271-5. 16. Canal Master: recommended technique guide. Brasseler USA, Incorporated, 1989.
Printed in U.S.A. VOL. 18, NO. 7, JULY 1992
Canal Master Files: Scanning Electron Microscopic Evaluation of New Instruments and Their Wear with Clinical Usage Mario L. Zuolo, CD, Richard E. Walton, DMD, MS and Carlos A. F. Murgel, CD
instrument is innovative and, as shown in the manufacturer's diagrams, incorporates three distinct features: (a) a noncutting pilot tip; (b) a cutting (fluted) head tapered from 16 mm to 1 to 2 mm; and (c) a small, nontapered flexible shaft. The developers have proposed that the instrument would be effective in curved canals by reducing transportation and generally giving better control. A recent report (13) confirms this suggestion, although there was a tendency toward distortion and breakage of the files during canal preparation. Aside from this report (13) there have been no controlled studies of usage patterns or of the manufacturer's quality control in production of the Canal Master files. The purpose of this study was to examine Canal Master files with the scanning electron microscope. In a longitudinal analysis, the following qualities were assessed for both new and used instruments: (a) quality of the new instruments as compared with the manufacturer's diagrams; (b) wear characteristics under progressive clinical use; and (c) incidence of breakage of the various sized files with time of usage.
Canal Master instruments have a short, fluted cutting area and a small flexible shaft; they may be predisposed to rapid wear and breakage. This study examined new and used files as to tip, flute, and shaft design when new and with increasing time of canal preparation. One hundred files prepared 140 curved canals. Sizes #20, #40, and # 6 0 files were examined unused and after 1, 3, 5, and 7 min of use. Evaluation was for uniformity when new and for deterioration and breakage with usage. The smaller sizes (#20 and #40) had some inconsistencies when new and were most predisposed to wear and breakage with time. Findings suggest that smaller files could have had improved quality control by the manufacturer. Also, they should be used with caution and discarded after short times of use in small, curved canals. This should minimize instrument separation and maximize cutting efficiency.
MATERIALS AND METHODS Intracanal instruments are selected for cleaning and shaping of the root canal system according to certain perceived physical characteristics. Efficiency and efficacy during instrumentation should be achieved when the instruments employed are used with regard to their inherent qualities as well as their limitations (1). To test physical properties and failure characteristics, several brands and designs of files have been subjected to laboratory tests. Several studies have investigated the cutting efficiency of files and reamers (2-5). Factors related to damage have been examined including the amount of bending and torsional stresses causing breakage of the files (6-9). File deterioration resulting from use in canal preparation has also been studied (10, 11). Overall, these studies have shown that instruments become increasingly defective, lose their cutting efficiency, and tend to separate with progressive or excessive
Instruments New Canal Master files were obtained directly from the manufacturer. One hundred files of 25 mm (the only length available) were evaluated. This group was further divided into 10 each in sizes #20 through #60, and 5 each in #70 and #80. For scanning electron microscopic evaluation, the 10 files in each of the #20, #40, and #60 groups were examined. All 100 files were used for canal preparation and therefore were included in the determination of breakage. Before initial examination and use all files were placed in boiling water for 5 min, followed by 5-min exposure in an ultrasonic bath containing a cleaning solution. This was done to remove any plastic or other debris which frequently remain on new files as part of the manufacturing process (14). One hundred forty root canals with a 25- to 35-degree curve (15) from 70 first and second mandibular molars were used. Teeth were stored in distilled water after extraction until
use.
Recently, a new root canal instrument, the Canal Master System (Brasseler USA Incorporated, Savannah, GA), has been introduced by Wildey and Senia (12). The design of this
use.
336
Canal Master Files
Vol. 18, No. 7, July 1992
Instrument Usage Canal preparation with the Canal Master files was according to the manufacturer's recommendations (16): (a) Following straight-line access preparation, canal length was established at 1 mm from the apical foramen. Canals were instrumented to length using a K-Flex file with filing motion. (b) Next, the cervical and mid-portion of the canal was prepared with a #50 through a #100 Canal Master rotary instrument to the level of the beginning of the curvature. (c) The apical preparation was then completed with Canal Master hand files, #20 through #40, at length. The step-back preparation was at 1-mm increments with files #45 through #80. Each file was used for 30 s in each mesial canal, resulting in a total usage time of I min per file per root. In the event of an instrument separation, that canal preparation was not completed. (d) During enlargement, 30 ml of 2.6% sodium hypochlorite/ canal delivered from a 27-gauge needle was used as an irrigant. Irrigation was maintained throughout by replenishing after each rotary and hand instrument.
337
(primarily the smaller) showed wear with use and a tendency for breakage with time.
Scanning Electron Microscopic Evaluation NEW INSTRUMENTS (CONTROLS) The Canal Master files showed variability in their manufactured characteristics (Fig. 1). The small (#20) size in particular, and intermediate (#40) sizes had some inconsistencies in taper, uniformity, and flute characteristics. The ends of small files were round and short, in contrast with the manufacturer's diagram of a cone-shaped "pilot tip" (Fig. 1). However, the #60 consistently showed a longer, uniform, and well-designed tip (Fig. 2). In the cutting head of all sizes, the flute edges were often rounded but were generally of the specified design. Some of the #20 files did not have definitive, deep grooves and pronounced flutes. Also, there often was a variable number of flutes in each instrument size
Scanning Electron Microscopic Examination Because the files could be studied repeatedly in the scanning electron microscope without affecting their physical properties, the same file could be examined new, thereby serving as an internal control, and then after each usage period. The times each file was viewed were at 0 (new, unused), 1, 3, 5, and 7 min. For each viewing, instruments were prepared as follows: (a) Files were cleaned in an ultrasonic bath for 5 min to remove dentin and tissue debris. (b) Files were mounted on copper tape on stubs; no sputter coating was necessary because the metal is conductive. (c) Tip, flute, and shaft were viewed and photographed on an AMRAY 1820 scanning electron microscope.
Evaluation Photomicrographs of each region were studied at varying magnifications for defects created during canal preparation. All photomicrographs were coded, mixed, and analyzed blindly for the following: New instruments--Design, tapering uniformity, and length of the tip, cutting head and shaft were noted. Also examined were the consistency and shape of the cutting edges of the flutes, the absence of metal tears, and the quality of the grinding process. Used instruments--On the flutes, the presence of defects such as gouges, pits, and loss of portions of metal was noted. Cutting edges were examined for rounding off or disruption. Shafts were studied for distortions such as twisting, irregularities, or potential fracture regions as indicated by tears in the metal. All files which separated were recorded as to their usage time and size. Results were expressed as a percentage of breakdown in each file size.
Fre 1. Number 20 files new, unused. Note the inconsistency in number and sharpness of flutes. There is no "pilot tip" on any of the files. Variability in flutes and absence of a pilot tip was characteristic of small files.
RESULTS New instruments showed some variations in shape and design, particularly in the smaller sizes. In addition, the files
FIG 2. Number 60 files new. In contrast to smaller sizes, these showed more consistency with sharp, uniform flutes and a cone-shaped pilot tip.
338
Zuolo et al.
Journal of Endodontics
FiG 3. Number 40, 5-min use. The flute edges show some wear and flattening.
FiG 6. Number 20, #40 files separated in mesial canals. These separated at the shaft near the flutes.
(Fig. 1). The shafts of all sizes were generally very uniform with an absence of tears or other notable defects. Larger files showed a more regular pattern of grinding of the flutes (Fig. 2). USED INSTRUMENTS FtG 4. Number 40, 7-min use. Some show more flute flattening but are not significantly changed from 5-min use. The middle file is shaft only; the head separated during instrumentation.
All three file sizes showed similar patterns. At the first two times of use (1 or 3 min), there was little evidence of wear. The cutting heads of the flutes were similar to those of the controls without visible defects or rounding of the edges. After 5 min, alterations appeared in the flutes as small gouges, fiat spots, or irregularities on the cutting edges (Fig. 3). A visible change was the distortion and twisting of the shaft portion with definite evidence of wear. In addition, potential fracture areas appeared, particularly in the #20 and #40 sizes. After 7 min of use, the #20 files showed marked deterioration with complete loss of the rounded shape of the cutting edges and severe distortions in the shaft. #40 files had less wear than the #20's, but showed generally more than at the 5-min use (Fig. 4). The #60 file showed considerably less evidence of wear or other defects (Fig. 5).
Instrument Breakage
FIG 5. Number 60, 7-min use. These show relatively little wear, a typical finding.
Of the 100 files, 21 separated during canal preparation, invariably in the apical one third (Figs. 4 and 6), for an overall occurrence of 21% breakage. The following size files were broken with incidences (out of 10 files/size) in parentheses: #20 (4), #25 (4), #30 (3), #35 (4), #40 (5) and #60 (1). The times of breakage of the sizes are shown in Fig. 7.
Vol. 18, No. 7, July 1992
Canal Master Files
DISCUSSION The unique design characteristics are claimed in advertising to be responsible for the special behavior of the Canal Master files. However, although the majority of the instruments examined were generally shaped as advertised by the manufacturer, the flutes and tips of the files were frequently different, particularly in the smallest sizes. The tip supposedly is a noncutting pointed pilot of approximately 0.75 m m which is intended to guide the instrument in the canal as well as enable the instrument to be maintained within the canal during enlargement. Our observations were that a round blunt tip was frequently present and would not serve as a pilot. This could certainly affect the usage characteristics of the files. Another alteration from the manufacturer's diagrams was the inconsistency of the taper and length of the cutting head. This variation in flute numbers, particularly in different sizes, could result in differences in feel between files. A complication could be inefficient cutting, resulting in an underrotation or locking of the file. This condition creates potential pressure points predisposed to fracture of the shaft. We could not determine whether this was a major cause of instrument failure in our samples. The new shafts were smooth and round, showing uniformity in properties and flexibility. Presumably, this would encourage the instrument to easily follow canal curvature as advertised; this was the impression during preparations. Overall, our results showed the necessity of either better quality control by the manufacturer or a different machining system for the instruments, particularly in the smaller sizes. The evidence of wear and the relatively high incidence of breakage after 5 and 7 min of use dictates that instruments be used carefully and discarded without hesitation. This should be the rule during preparation of smaller more curved canals. An early report (12) did caution against overaggressive cutting, particularly in a clockwise direction, which would predispose the file to counterclockwise separation Operator skill is critical in this type of research. The author (M. Z.) performing the preparations was familiar with and well trained in the use of Canal Master files, both with patients and in extracted teeth. Even with this expertise, there were defects and failures of instruments. This reinforces the necessity of practicing with the instruments in extracted teeth to 12
E
~1 ~o
"5:'.S-,:' : 2
~
s
~
4
;.~;'~/j~_l:-Z:c
3
~=," a,p "~J!.~=.;i
~
o 1
3
5
7
Period of Observation (minutes) FiG 7. The number of instruments that separated at each usage time. No instruments separated at 1 rain, whereas 11 broke after 7 rain of instrumentation.
339
gain familiarity with application of their unique physical properties (13). The relatively high incidence of breakage could be a result of inherent weakness of the shaft but also because of length, which may predispose the shafts to fracture more easily than if they were shorter. However, because the files are only available in 25 mm, this cannot be determined with certainty. Perhaps this weakness would be reduced in 19- or 21-mm files. All separated instruments were wedged in the apical third of the canal. From the location and the nature of the accident, the broken tips would be difficult or impossible to remove. This further reinforces that smaller instruments (#20 to #40) should be used with care. A variation in the manufacturer's recommendation (16) would be to use smaller instruments minimally (for example #20 and #25) at length and begin step-back. Files should be readily discarded if the operator notices any "softness," twisting, or bending of the shaft; these frequently were indicators of subsequent breakage upon the next file usage. The manufacturer has introduced a modification in the instruments, renamed the Canal Master "U" file. This primarily changes the flute design and methods of use. Whether the changes result in different physical or performance characteristics has yet to be demonstrated in clinical tests. Dr. Zuolo is a professor in endodontics, Camilo Castelo Branco, School of Dentistry, University S&o Paulo, Brazil. Dr. Walton is professor and chairman, Department of Endodontics, University of Iowa, College of Dentistry, Iowa City, IA. Dr. Murgel is a professor in endodontics, Camilo Castelo Branco, School of Dentistry, University S~o Paulo, Brazil. Address requests for reprints to Dr. Richard Walton, Department of Endodontics, University of Iowa, College of Dentistry, Iowa City, IA 52242.
References 1. Moreinis SA. The K-type file and reamers: mainstays of the endodontic armamentarium. N Y State Dent J 1979;45:390-3. 2. Newman JG, Brantley WA, Gerstein H. A study of the cutting efficency of seven brands of endodontic files in linear motion. J Endodon 1983;9:31622. 3. Miserendino LJ, Moser JB, Heuer MA, Setek EM. Cutting efficiency of endodontic instruments. Part L A quantitative comparison of the tip and fluted region. J Endodon 1985;11:435-41. 4. Miserendino LJ, Moser JB, Heuer MA, Osetek EM. Cutting of endodontic instruments. Part II. Analysis of tip design. J Endodon 1986;12:8-12. 5. Stenman E, Spangberg L. Machining efficiency of Flex-R, K-Flex, TrioCut and S Files. J Endodon 1990;16:575-9. 6. Craig RG, Mcllwain ED, Peyton FA. Bending and torsion properties of endodontics instruments. Oral Surg 1968;25:239-54. 7. Lautenschlager EP, Jacobs JJ, Marshall GW Jr, Heuer MA. Brittle and ductile torsional failures of endodontic instruments. J Endodon 1977;3:175-8. 8. Dolan DW, Craig RG. Bending and torsion of endodontic files with rhombus cross-sections. J Endodon 1982;8:260-4. 9. Roth WC, Gouch RW, Grandich RA, Walker WA. A study of the strength of endodontic files: potential for torsional breakage and relative flexibility. J Endodon 1983;9:228-32. 10. Montgomery S, Canales ML, del Rio CE. File damage during root canal preparation. J Endodon 1984;10:45-7. 11. Scott GL, Walton RE. Ultrasonic endodontics: the wear of instruments with usage. J Endodon 1986;12:279-83. 12. Wildey WL, Senia ES. A new root canal instrument and instrumentation technique. A preliminary report. Oral Surg 1989;67:198-207. 13. Gilles JA, del Rio CE. A comparison of the Canal Master endodontic instrument and K-type files for enlargement of curved root canals. J Endodon 1990;16:561-5. 14. Murgel CAF, Walton RE, Rittman B, Pecora JD. A comparison of techniques for cleaning endo~ontic files after usage: a quantitative scanning electron microscopic study. J Endodon 1990;16:214-7. 15. Schneider SW. A comparison of canal preparations in straight and curved root canals. Oral Surg 1971 ;32:271-5. 16. Canal Master: recommended technique guide. Brasseler USA, Incorporated, 1989.