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A method to determine the cutting efficiency of root canal instruments in linear motion
JOURNAL OF ENDODONTICS I VOL 6, NO 11, NOVEMBER 1980
A method to determine the cutting efficiency of root canal instruments in linear motion J. W e b b e r ,
BDS, MS; ]. B. M o s e r , PhD; a n d M . A. H e u e r , DDS, MS
T h e efficiency o f files a n d r e a m e r s in e n l a r g i n g g r o o v e s in m o i s t b o v i n e b o n e w a s tested in a s i m u l a t e d filing m o d e . T h e w e i g h t losses p r o d u c e d in b o n e b y a p u s h - p u l l s t r o k e of different sets of f o u r identical i n s t r u m e n t s w e r e c o m p a r e d in t y p e a n d size as well as a m o u n t of c u t t i n g time. Initially, m o s t size no. 50 i n s t r u m e n t s r e m o v e d m o r e m a t e r i a l t h a n the s m a l l e r size no. 30. T h i s difference d i s a p p e a r e d a f t e r ten m i n u t e s cutting time. I n s t r u m e n t s w i t h t r i a n g u l a r cross sections w e r e initially m o r e efficient b u t lost s h a r p n e s s m o r e r a p i d l y t h a n s q u a r e o n e s of the s a m e size. H e d s t r o e m i n s t r u m e n t s h a d l o w e r cutting efficiency t h a n m o s t o t h e r t y p e s a n d also lost t h e i r s h a r p n e s s m o r e r a p i d l y in this p a r t i c u l a r test.
T h e widespread use of standardized root canal instruments has greatly eased the instrumentation of root canals. However, in seeking international standardization of those instruments, 1 cutting efficiency and retention of sharpness have been largely ignored. Oliet and Sorin 2 studied the cutting efficiency of reamers using a drill press which held the instrument tightly in a chuck and rotated the reamer into a sample of bovine bone. Molven 3 used a similar approach, but used dentin wafers as cutting samples. The size of the prepared openings was measured by projection on a screen. The applied force was not controlled, as the instruments were held by hand. DeJong and Willoughby 4 applied dentin samples to the cutting portions of K-type
files, reamers, and Hedstroem files. The samples were mechanically lowered on the instruments, and a short push-pull stroke engaged the flutes of the instruments with the surface of the dentin. The root canal instruments were rigidly held, and the load on the dentin samples was varied. More recently, Stenman 5 used a similar method studying only Hedstroem files. A study of the geometry of the cutting portion of root canal instruments is appropriate to any consideration of cutting efficiency. Oliet and Sorin 2 concluded that the cross-sectional configuration, the n u m b e r of flutes, and the sharpness of the cutting edges were interrelating factors that would affect overall cutting efficiency. Heuer 6 speculated that a root canal instrument manufactured
from a triangular shaft would have a sharper edge and would remove dentin more efficiently than a square shaft. Heuer 7 had postulated that "somewhere there was an optimal relationship of spirals and size that would produce a m a x i m u m of tactile sensation and instrument efficiency with a m i n i m u m of breakage." Shoji s showed that reamers of triangular cross section make deeper cuts into the root canal walls than instruments with rectangular cross sectionals. In designing an in vitro experiment for an in vivo problem, certain questions must be asked concerning the relationship between the two. The most important question is how the experimental model relates to the real situation. Molven, 3 and DeJong and Willoughby* used dentin wafers as material for testing the efficiency 829
of root canal instruments. The extent of drying, or desiccation, and its effect on the physical properties of the material were given little discussion. By using acrylic blocks, Stenman a overcame the problem of lack of material standardization inherent in using dentin samples from many teeth. One manufacturer of root canal instruments used sheets of soft brass to test the efficiency of the instruments (Unitek Dental Mfg. Co., Calif, personal communication, 1977). Oliet and Sorin ~ used samples of bovine bone, briefly defending its use, but they did not discuss any of the storage problems involved. The use of bovine bone as an experimental model was suggested by Lugassy2 Its advantages are the standardization of specimen shape and orientation, the machining of a representative number of samples from the same location, and the exact selection of specimen orientat i o n with respect to the structures within the material. Several authors 1~ have shown that the properties of dead bone, especially when kept in moist condition, are comparable to the properties and behavior of living tissue. The purpose of the present study was to design a test procedure that could assess the cutting efficiency of selected root canal instruments used in linear (filing) motion. Five designs of root canal instruments were studied: files manufactured from square and triangular cross sections; reamers manufactured from both cross sections; and Hedstroem files manufactured from circular cross sections (Fig 1). Furthermore, moist bovine bone was evaluated according to its suitability as a work piece on which the root canal instruments would operate. Chemical material charac-
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Fig 2-Overall assembly used in experimental procedure.
Fig 1--Five different types of root canal instruments, from left to right: square reamer, square file, triangular file, triangular reamer, Hedstroem file.
terization of the commercially available root canal instruments was not considered at this time. MATERIALS AND METHODS The complete assembly designed for this investigation is shown in Figure 2. A saber saw engineered with a push-pull stroke approximately 12.5 mm in length, propelled the root canal instruments in linear motion. The speed was controlled by a variable rheostat and adjusted to three strokes per second. The saber saw was clamped to a vise, and the whole assembly was mounted on a wooden base. A special clear acrylic adaptor capable of holding four root canal instruments was constructed and fit directly on the shaft of the saw with the aid of a conventional saw blade (Fig 3). A rack and pinion drill press was used to apply the loads during cutting. The central rod was lowered by a lever arm that was kept at an angle by a heavy leaf-spring in the main assembly. A simple clamp, which held the bovine bone, extended from the central rod of the drill press and moved downwards when a weight was hung from the lever arm.
Fig 3--Root canal instrument adaptor with four identical instruments in place.
The bovine bone was taken from the femur of a cow and cut into transverse sections approximately 50 mm long. These sections were then sliced horizontally and machined to produce smooth surfaces. The cortical bone specimens presented cutting faces exactly 5 mm wide which the flutes of the root canal instruments would move. All sectioning of the bone was done with a circular carborundum saw under a copious water spray. The blocks of bone were wrapped in pieces of gauze soaked in Ringer's solution, and stored in plastic bags in the freezer compartment of a refrigerator at a temperature of --20 C. Before testing, the bone was allowed to thaw. Throughout the investigation, the bone sections were always used in a moist condition. The cutting efficiency of the instruments was measured as a function of weight loss from the surface of the bovine bone. Each type of instrument was moved in reciprocating linear motion under a specified load applied to a block of bovine bone. A constant weight hanging from the lever arm of the drill press held the bone in contact with selected instruments. Using the Instron Universal Testing machine, preliminary work
JOURNAL OF ENDODONTICS ] VOL 6, NO 11, NOVEMBER 198o
showed that the o p t i m u m load at which the i n s t r u m e n t s could cut effectively was 600 gm. This was equivalent to a c a l i b r a t e d weight of 350 gm t h a t was s u s p e n d e d from the lever a r m of the drill press d u r i n g the test. T h e lateral force on each file was 150 gm. I n f o r m a l trials have shown t h a t this force relates well to intraoral clinical conditions. Twelve root c a n a l i n s t r u m e n t s of each design, sizes no. 30 a n d no. 50, were s e p a r a t e d into groups of four. T h e four instruments were held by the a d a p t o r p l a c e d in its most r e t r u d e d position in the saber saw. T h e root canal i n s t r u m e n t s were positioned on the five m m - w i d e cutting surface of the p r e p a r e d block of bone. T h e block was held b y the clamp, a n d a pencil line m a r k e d the position of the root c a n a l instruments. T h e block was r e m o v e d from the clamp, a n d shallow grooves were filed into the c u t t i n g surface with a saw blade. These grooves a c t e d as pilot channels to aid in g u i d i n g the instruments b a c k a n d forth. T h e block of bone was t h e n weighed on a balance to a sensitivity of 0.1 mg. T h e block was t h e n r e t u r n e d to the c l a m p , b u t with the pilot grooves facing d o w n w a r d s a n d a l i g n e d a l o n g the long axis of the four root canal instruments. This o r i e n t a t i o n of the block p e r m i t t e d the tips of the fluted portion of the root c a n a l instruments to be housed in the pilot grooves (Fig 4). T h e c a l i b r a t e d load, consisting of lead shot, h u n g from the lever a r m of the drill press w h e n the saber saw was set in m o t i o n at three strokes per second for five minutes. A t the end of the c u t t i n g period, the bone was reweighed to d e t e r m i n e the a m o u n t of bone removed. T h i s p r o c e d u r e was r e p e a t e d twice on fresh blocks of
Fig 4--Root canal instruments placed in pilot grooves before commencement of run.
bone, using the same g r o u p of instruments. T h e flutes of the i n s t r u m e n t s were cleaned after each five-minute run. A f t e r the first g r o u p o f instruments h a d been tested three times in succession, the second a n d third groups of instruments of the same design were transferred to the a d a p tor a n d the exact p r o c e d u r e r e p e a t e d again. In this way, twelve instrum e n t s o f each size a n d design were tested. T h e m e a n s a n d s t a n d a r d deviations for each successive r u n were o b t a i n e d from the i n d i v i d u a l values of weight loss. A p a i r e d t-test was used to c o m p a r e the m e a n s a f t e r the first a n d third runs of two sets of root canal instruments of the s a m e type. Similarly, the m e a n weight loss achieved by different i n s t r u m e n t types was c o m p a r e d using the same test o f significance. RESULTS
T h e b a r graphs (Fig 5, 6) show the m e a n weight loss from the surface of the b o n e a n d s t a n d a r d d e v i a t i o n s for the five designs of i n s t r u m e n t s after the first a n d third five-minute-runs. Initially, size no. 30 t r i a n g u l a r files, reamers, a n d square files h a d similar c u t t i n g abilities. All of these designs were significantly b e t t e r (P < .05) t h a n square r e a m e r s a n d
H e d s t r o e m files (Fig 5). However, after the t h i r d five-minute-run, size no. 30 square files a p p e a r e d to be more efficient t h a n a n y other design and h a d a significantly greater cutting efficiency t h a n the square r e a m ers a n d H e d s t r o e m files (P < .05). The triangular reamer remained a significantly more efficient instrument t h a t the H e d s t r o e m file. T h e square r e a m e r a n d the H e d s t r o e m file were the most inefficient instruments in this size range when used in linear motion. In most cases, d a t a scattering increased significantly between the first a n d third run. T h e pooled s t a n d a r d deviation for the size no. 30 instruments was 9.0 for the first run a n d 14.6 for the t h i r d run. M o r e v a r i a t i o n is seen a m o n g the designs in size no. 50 instruments t h a n a m o n g designs in size no. 30 instruments. T h e first run (Fig 6) showed t h a t t r i a n g u l a r files were significantly m o r e efficient t h a n a n y other design (P < .05). T r i a n g u l a r reamers, H e d s t r o e m files, a n d square files h a d a similar c u t t i n g efficiency. Square r e a m e r s showed the poorest cutting efficiency when used in linear motion. A f t e r the third run, the effect of w e a r on most of the instruments was evident. T h e size no. 50 square file was more efficient t h a n the t r i a n g u l a r reamer, the H e d stroem file, a n d the square r e a m e r (P < .05), b u t not significantly m o r e efficient t h a n the t r i a n g u l a r file. However, the c u t t i n g efficiency of the t r i a n g u l a r size no. 50 file was considerably reduced, a n d no significant difference was observed between it a n d a n y other size no. 50 instrument. As with the results o b t a i n e d with size no. 30 instruments, d a t a scattering increased considerably after the first run, with the
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JOURNAL OF ENDODONTICS I VOL 6, NO 11, NOVEMBER 1980
100
--]RUN I rt-A
80 pooled standard deviation rising from 9.8 to 16.1. W h e n different types of instruments of the same size were compared at a significance level of 0.05 (Table), the following results were obtained. At size no. 30, during the first run, triangular files were superior to square reamers and Hedstroem files; square files were superior to square reamers; and triangular reamers were superior to square reamers and Hedstroem files. After the third run, fewer differences were seen. Square files were superior to square reamers, and both square files and triangular reamers were superior to Hedstroem files. At size no. 50, triangular files showed superior cutting ability to all other instruments during the first cycle. Also, in the first cycle, square files, triangular reamers, and Hedstroem files far exceeded the cutting ability of square reamers. After the third run, only square files were clearly most effective c o m p a r e d with other instruments. At P < .05, square files showed better cutting ability t h a n either square and triangular reamers, or Hedstroem files. Analysis of the data summarized in the Table showed that only with one type of instrument, the triangular reamer, was there a significant difference in cutting ability between the smaller and larger size for the full 15 minutes of the test. However, the cutting ability was more effective in the larger size only during the first run. After the third run, the situation was reversed. T r i a n g u l a r and Hedstroem files showed significant cutting superiority in the larger size c o m p a r e d with the smaller size for the first run, but not at the end of the third run. There was no significant difference between sizes in square
832
]~RUN 3
~IA
Z
to z
I
60
STANDARD DEVIATION
C.0
~
40
20
HEDSTRC)'N FILE
TRIANGULAR TRIANGULAR FILE REAMER
SQUARE FILE
SQUARE REAMER
Fig 5--Mean weight loss using size no. 30 instruments.
100
T
'--] RUN I
I I. /IA
80
[~RUN 3
Z
I
Lo _z o ~
60
40
20
/I/ '1/
STANDARD DEVIATION
T
/[A
/
/ / /
/ /
/ /
,4
/ / / /
TRIANGULAR TRIANGULAR FILE REAMER
HEDSTROM FILE
SQUARE FILE
SQUARE
REAMER
Fig 6--Mean weight loss using size no. 50 instruments.
Table 9 Cutting ability expressed in weight loss (mg/5 min).
First run (SD) Instrument designation Triangular files Square files Triangular reamers Square reamers Hedstroem files
Third run (SD)
#30
#50
~30
#50
58(1)* 58(15) 56(12)* 35(2)
97(13)* 58(9) 67(6)* 40(10) 66(9)*
52(22) 79(16) 62(2)t 50(16) 32(8)
62(17) 83(15) 49(14)r 42(15) 33(19)
38(5)*
SD = standard deviation. *Significant difference (P < 0.05) between :#'30 and # 5 0 instruments in first run. **Significant difference (P--< 0.05) between # 3 0 and # 5 0 instruments in third run.
JOURNAL OF ENDODONTICS i VOL 6, NO 11, NOVEMBER 1980
instruments at a n y time d u r i n g the experiment. DISCUSSION T h e e x p e r i m e n t a l p r o c e d u r e used the back a n d forth action o f a commercial saber saw. T h e s i m u l a t i o n of pure linear m o t i o n closely resembled the filing action used in root canals. T h e length of the stroke was constant, a n d the speed was carefully controlled by the v a r i a b l e rheostat. A speed of three cycles p e r second, p e r h a p s excessive b y clinical standards, was necessary to overcome the load a p p l i e d to the bone by the rack a n d pinion drill press. T h e drill press was found to be a suitable device for the a p p l i c a t i o n of loads. Excessive friction in the loading mechanisms was e l i m i n a t e d , resulting in a close correlation between lever a r m load a n d pressure on the root canal instruments. T h e saber saw allowed four root canal instruments to o p e r a t e at one time, a n d was easy to disassemble a n d reassemble with new groups of instruments. M e a s u r i n g weight loss from the bone specimens after opera t i n g four root canal i n s t r u m e n t s was found to be more c o n v e n i e n t than m e a s u r i n g the smaller values of weight loss o b t a i n e d after o p e r a t i n g only one root c a n a l instrument. A five-minute o p e r a t i n g time was long enough to give m e a s u r a b l e a m o u n t s of weight loss t h a t could be reprod u c e d a n d repeated. Moist bovine bone for testing i n s t r u m e n t c u t t i n g efficiency was superior to dry bone. P r e l i m i n a r y work showed that d e h y d r a t e d bone d i d not allow a d e q u a t e surface penetration by root canal instruments because of increased bone hardness. F o r this reason, a freshly thawed, moist bone section was used for every
test run. In contrast to the excellent c h a r a c teristics o f moist bovine bone, acrylic was erratic as a workpiece a n d it was difficult to assess the c u t t i n g efficiency of root canal instruments. C h i p s of acrylic c o m p l e t e l y coated the flutes of the instruments in such excessive a m o u n t s t h a t at times the instruments were unable to move back a n d forth. T h e m e a s u r e m e n t of weight loss from the acrylic was also difficult because at times the a m o u n t was so small. T h e inability to s t a n d a r d i z e a representative n u m b e r of d e n t i n samples p r e c l u d e d their use in this study. T h e results of this research showed that, w i t h i n the time limits of the e x p e r i m e n t , wear was associated with certain root canal instruments; only i n s t r u m e n t s with c u t t i n g edges of less t h a n 90 degrees, t h a t is t r i a n g u l a r files, reamers, a n d H e d s t r o e m files, showed this kind of wear. G u t i e r r e z a n d others 1:~ studied the wear of root canal instruments as a m u l t i f a c e t e d p r o b l e m , but were unable clearly to show a n y p r e d i c t a b l e changes in the instruments' edges after used. T h e w e a r shown by the H e d stroem files was surprising in view of the favorable reports of these instruments, in previous research 4 a n d by m a n y endodontists. This study f o u n d the H e d s t r o e m file inferior to most other types of instruments in r e m o v ing m a t e r i a l from a root canal or its c o u n t e r p a r t . T h e H e d s t r o e m file is only o p e r a t i o n a l on the o u t w a r d stroke; therefore, an i n s t r u m e n t such as a file or t r i a n g u l a r reamer, which removed dentin on both an o u t w a r d a n d i n w a r d stroke, would be a m o r e effective c u t t i n g instrument. T h e 60 degree c u t t i n g edge of the t r i a n g u l a r i n s t r u m e n t was initially the sharpest, and the results of this
study c o r r o b o r a t e similar observations of Shoji, 8 a n d Heuer, ~ and Oliet a n d Sorin. ~ However, o u r observation was that these instruments were unable to retain their sharpness, whereas the 90 degree c u t t i n g edge of the square file a n d r e a m e r m a i n tained a high degree of sharpness after r e p e a t e d trials. A n o t h e r factor was the v a r i a t i o n in the helical angle (the angle f o r m e d by the cutting edge a n d the long axis of the instrument). A square r e a m e r has a small helical angle, with the edges turned toward the long axis of the instrument. This results in a root canal instrument of m i n i m a l c u t t i n g efficiency when used in a filing action. However, when the edges are more p e r p e n d i c u l a r to the long axis of the instrument, as in a highly fluted file, the force d i s t r i b u t i o n along the edge would be highly c o n c e n t r a t e d , making the i n s t r u m e n t an effective dentin remover w h e n used in linear motion. However, the results suggest that this force m a y exceed the strength of the i n s t r u m e n t ' s edge; consequently, an initially sharp root canal i n s t r u m e n t could become dull very quickly. T h e t r i a n g u l a r r e a m e r m a y be closer to the ideal than a n y other root canal i n s t r u m e n t when used in linear motion. T h e sharp 60 degree edge enables it to p e n e t r a t e deeply into the root c a n a l wall. T h e edge also has a helical angle smaller t h a n a t r i a n g u l a r file b u t larger t h a n a square reamer. These characteristics would tend to d i s t r i b u t e the c u t t i n g force along a greater length of edge, m a k i n g it b e t t e r able to absorb the stress w i t h o u t w e a r i n g out. By varying the helical angles to values somewhere between the most fluted file a n d the least fluted reamer, it should be possible to m a n u f a c t u r e a root
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JOURNAL OF ENDODONTICS [ VOL 6, NO 11, NOVEMBER 1980
c a n a l i n s t r u m e n t o f t r i a n g u l a r cross s e c t i o n w h i c h , w h e n u s e d in a filing action, would achieve superior cutt i n g efficiency w i t h m i n i m u m w e a r .
SUMMARY AND CONCLUSIONS T h e p u r p o s e o f t h i s s t u d y w a s to d e s i g n a n d e v a l u a t e a test p r o c e d u r e w h e r e b y t h e c u t t i n g e f f i c i e n c y o f five d i f f e r e n t l y d e s i g n e d r o o t c a n a l ins t r u m e n t s , w h e n o p e r a t e d in a l i n e a r (filing) m o t i o n , c o u l d b e assessed. Linear motion was supplied by the back and forth action of a commercial s a b e r saw. The following conclusions can be d r a w n f r o m this s t u d y : t h e e x p e r i m e n t a l p r o c e d u r e d e s c r i b e d w a s suita b l e for d e t e r m i n i n g t h e c u t t i n g effic i e n c y o f root c a n a l i n s t r u m e n t s used in l i n e a r m o t i o n . M o i s t b o v i n e b o n e w a s a s u i t a b l e m a t e r i a l for assessment of root canal instrument cutt i n g efficiency. T h e r e w a s w i d e r a n g e o f c u t t i n g efficiency b e t w e e n e a c h t y p e o f root c a n a l i n s t r u m e n t , b o t h i n i t i a l l y a n d a f t e r s u c c e s s i v e use. Instruments manufactured from
834
t r i a n g u l a r cross s e c t i o n s w e r e initially m o r e efficient b u t lost s h a r p ness a f t e r successive use. I n s t r u m e n t s m a n u f a c t u r e d f r o m s q u a r e cross sections had greater retention of sharpness. H e d s t r o e m files lost c u t t i n g efficiency very rapidly. This work was supported in part by a grant from the Endowment and Memorial Foundation of the American Association of Endodontists. Dr. Webber is a practicing endodontist in London, England. Dr. Moser is associate professor of biological materials; l)r. tleuer is professor and chairman, department of end()dontics, Northwestern University Dental School. Requests for reprints should be directed to Dr. J. B. Moser, Department of Biological Materials, Northwestern University Dental School, 311 E Chicago Ave, Chicagn, 60611.
References I. American National Standards Institute: Dentistry, report of meeting of ISO/TC 196, London, Sept. 16-20, Chicago, ADA. 2. Oliet, S., and Sorin, S.M. Cutting efficiency of endodontic reamers. Oral Surg 36:243-252, 1973. 3. Molven, O. A comparison of the dentinremoving ability of five root canal instruments. Scand J Dent Res 78:500-511, 1970.
4. DeJong, L.C., and Willoughby, J.W. Endodontic instruments, an evaluation of cutting ability. Thesis. University of Michigan, 1975. 5. Stenman, E. Effects of sterilization and endodontic medicaments on mechanical propeyries of root canal instruments. Thesis. University of Ume~, Sweden, 1977. 6. Heuer, M.A. The biomechanics of endodontic therapy. Dent Clin N Am 1963, pp 341-359. 7. Heuer, M.A. A study of the structural, dimensional and physical characteristics of root canal instruments. Thesis. University of Michigan, 1959. 8. Shoji, R. Studies on the mechanism of the mechanical enlargement of root canals. J Nihon Univ School Dent 7(2):71-78, 1965. 9. Luga~sy, A. Mechanical and viscoelastic properties of cow bone and sperm whale dentine studied under compression. A dissertation in metallurgy and materials science. Thesis. University of Pennsylvania, 1968. 10. Amprino, R. Investigatinn on some physical properties of bone tissue. Acta Anat 34(3): 161-186, 1958. t l. Sedlin, E.D. A rheologieal model for cortical bone. Acta Orthop Stand, Supp183:177, 1965. 12. Stevens, J., and Ray, R.D. An experimental comparison of living and dead bone in rats. J Bone Joint Surg 44(B):412-413, 1962. 13. Guttierrez, J.H.; Gigoux, C.; and Sanhueza, J. Physical and chemical deterioration of endodontic reamers during mechanical preparation. Oral Surg 28:394-403, 1969.
A method to determine the cutting efficiency of root canal instruments in linear motion J. W e b b e r ,
BDS, MS; ]. B. M o s e r , PhD; a n d M . A. H e u e r , DDS, MS
T h e efficiency o f files a n d r e a m e r s in e n l a r g i n g g r o o v e s in m o i s t b o v i n e b o n e w a s tested in a s i m u l a t e d filing m o d e . T h e w e i g h t losses p r o d u c e d in b o n e b y a p u s h - p u l l s t r o k e of different sets of f o u r identical i n s t r u m e n t s w e r e c o m p a r e d in t y p e a n d size as well as a m o u n t of c u t t i n g time. Initially, m o s t size no. 50 i n s t r u m e n t s r e m o v e d m o r e m a t e r i a l t h a n the s m a l l e r size no. 30. T h i s difference d i s a p p e a r e d a f t e r ten m i n u t e s cutting time. I n s t r u m e n t s w i t h t r i a n g u l a r cross sections w e r e initially m o r e efficient b u t lost s h a r p n e s s m o r e r a p i d l y t h a n s q u a r e o n e s of the s a m e size. H e d s t r o e m i n s t r u m e n t s h a d l o w e r cutting efficiency t h a n m o s t o t h e r t y p e s a n d also lost t h e i r s h a r p n e s s m o r e r a p i d l y in this p a r t i c u l a r test.
T h e widespread use of standardized root canal instruments has greatly eased the instrumentation of root canals. However, in seeking international standardization of those instruments, 1 cutting efficiency and retention of sharpness have been largely ignored. Oliet and Sorin 2 studied the cutting efficiency of reamers using a drill press which held the instrument tightly in a chuck and rotated the reamer into a sample of bovine bone. Molven 3 used a similar approach, but used dentin wafers as cutting samples. The size of the prepared openings was measured by projection on a screen. The applied force was not controlled, as the instruments were held by hand. DeJong and Willoughby 4 applied dentin samples to the cutting portions of K-type
files, reamers, and Hedstroem files. The samples were mechanically lowered on the instruments, and a short push-pull stroke engaged the flutes of the instruments with the surface of the dentin. The root canal instruments were rigidly held, and the load on the dentin samples was varied. More recently, Stenman 5 used a similar method studying only Hedstroem files. A study of the geometry of the cutting portion of root canal instruments is appropriate to any consideration of cutting efficiency. Oliet and Sorin 2 concluded that the cross-sectional configuration, the n u m b e r of flutes, and the sharpness of the cutting edges were interrelating factors that would affect overall cutting efficiency. Heuer 6 speculated that a root canal instrument manufactured
from a triangular shaft would have a sharper edge and would remove dentin more efficiently than a square shaft. Heuer 7 had postulated that "somewhere there was an optimal relationship of spirals and size that would produce a m a x i m u m of tactile sensation and instrument efficiency with a m i n i m u m of breakage." Shoji s showed that reamers of triangular cross section make deeper cuts into the root canal walls than instruments with rectangular cross sectionals. In designing an in vitro experiment for an in vivo problem, certain questions must be asked concerning the relationship between the two. The most important question is how the experimental model relates to the real situation. Molven, 3 and DeJong and Willoughby* used dentin wafers as material for testing the efficiency 829
of root canal instruments. The extent of drying, or desiccation, and its effect on the physical properties of the material were given little discussion. By using acrylic blocks, Stenman a overcame the problem of lack of material standardization inherent in using dentin samples from many teeth. One manufacturer of root canal instruments used sheets of soft brass to test the efficiency of the instruments (Unitek Dental Mfg. Co., Calif, personal communication, 1977). Oliet and Sorin ~ used samples of bovine bone, briefly defending its use, but they did not discuss any of the storage problems involved. The use of bovine bone as an experimental model was suggested by Lugassy2 Its advantages are the standardization of specimen shape and orientation, the machining of a representative number of samples from the same location, and the exact selection of specimen orientat i o n with respect to the structures within the material. Several authors 1~ have shown that the properties of dead bone, especially when kept in moist condition, are comparable to the properties and behavior of living tissue. The purpose of the present study was to design a test procedure that could assess the cutting efficiency of selected root canal instruments used in linear (filing) motion. Five designs of root canal instruments were studied: files manufactured from square and triangular cross sections; reamers manufactured from both cross sections; and Hedstroem files manufactured from circular cross sections (Fig 1). Furthermore, moist bovine bone was evaluated according to its suitability as a work piece on which the root canal instruments would operate. Chemical material charac-
830
Fig 2-Overall assembly used in experimental procedure.
Fig 1--Five different types of root canal instruments, from left to right: square reamer, square file, triangular file, triangular reamer, Hedstroem file.
terization of the commercially available root canal instruments was not considered at this time. MATERIALS AND METHODS The complete assembly designed for this investigation is shown in Figure 2. A saber saw engineered with a push-pull stroke approximately 12.5 mm in length, propelled the root canal instruments in linear motion. The speed was controlled by a variable rheostat and adjusted to three strokes per second. The saber saw was clamped to a vise, and the whole assembly was mounted on a wooden base. A special clear acrylic adaptor capable of holding four root canal instruments was constructed and fit directly on the shaft of the saw with the aid of a conventional saw blade (Fig 3). A rack and pinion drill press was used to apply the loads during cutting. The central rod was lowered by a lever arm that was kept at an angle by a heavy leaf-spring in the main assembly. A simple clamp, which held the bovine bone, extended from the central rod of the drill press and moved downwards when a weight was hung from the lever arm.
Fig 3--Root canal instrument adaptor with four identical instruments in place.
The bovine bone was taken from the femur of a cow and cut into transverse sections approximately 50 mm long. These sections were then sliced horizontally and machined to produce smooth surfaces. The cortical bone specimens presented cutting faces exactly 5 mm wide which the flutes of the root canal instruments would move. All sectioning of the bone was done with a circular carborundum saw under a copious water spray. The blocks of bone were wrapped in pieces of gauze soaked in Ringer's solution, and stored in plastic bags in the freezer compartment of a refrigerator at a temperature of --20 C. Before testing, the bone was allowed to thaw. Throughout the investigation, the bone sections were always used in a moist condition. The cutting efficiency of the instruments was measured as a function of weight loss from the surface of the bovine bone. Each type of instrument was moved in reciprocating linear motion under a specified load applied to a block of bovine bone. A constant weight hanging from the lever arm of the drill press held the bone in contact with selected instruments. Using the Instron Universal Testing machine, preliminary work
JOURNAL OF ENDODONTICS ] VOL 6, NO 11, NOVEMBER 198o
showed that the o p t i m u m load at which the i n s t r u m e n t s could cut effectively was 600 gm. This was equivalent to a c a l i b r a t e d weight of 350 gm t h a t was s u s p e n d e d from the lever a r m of the drill press d u r i n g the test. T h e lateral force on each file was 150 gm. I n f o r m a l trials have shown t h a t this force relates well to intraoral clinical conditions. Twelve root c a n a l i n s t r u m e n t s of each design, sizes no. 30 a n d no. 50, were s e p a r a t e d into groups of four. T h e four instruments were held by the a d a p t o r p l a c e d in its most r e t r u d e d position in the saber saw. T h e root canal i n s t r u m e n t s were positioned on the five m m - w i d e cutting surface of the p r e p a r e d block of bone. T h e block was held b y the clamp, a n d a pencil line m a r k e d the position of the root c a n a l instruments. T h e block was r e m o v e d from the clamp, a n d shallow grooves were filed into the c u t t i n g surface with a saw blade. These grooves a c t e d as pilot channels to aid in g u i d i n g the instruments b a c k a n d forth. T h e block of bone was t h e n weighed on a balance to a sensitivity of 0.1 mg. T h e block was t h e n r e t u r n e d to the c l a m p , b u t with the pilot grooves facing d o w n w a r d s a n d a l i g n e d a l o n g the long axis of the four root canal instruments. This o r i e n t a t i o n of the block p e r m i t t e d the tips of the fluted portion of the root c a n a l instruments to be housed in the pilot grooves (Fig 4). T h e c a l i b r a t e d load, consisting of lead shot, h u n g from the lever a r m of the drill press w h e n the saber saw was set in m o t i o n at three strokes per second for five minutes. A t the end of the c u t t i n g period, the bone was reweighed to d e t e r m i n e the a m o u n t of bone removed. T h i s p r o c e d u r e was r e p e a t e d twice on fresh blocks of
Fig 4--Root canal instruments placed in pilot grooves before commencement of run.
bone, using the same g r o u p of instruments. T h e flutes of the i n s t r u m e n t s were cleaned after each five-minute run. A f t e r the first g r o u p o f instruments h a d been tested three times in succession, the second a n d third groups of instruments of the same design were transferred to the a d a p tor a n d the exact p r o c e d u r e r e p e a t e d again. In this way, twelve instrum e n t s o f each size a n d design were tested. T h e m e a n s a n d s t a n d a r d deviations for each successive r u n were o b t a i n e d from the i n d i v i d u a l values of weight loss. A p a i r e d t-test was used to c o m p a r e the m e a n s a f t e r the first a n d third runs of two sets of root canal instruments of the s a m e type. Similarly, the m e a n weight loss achieved by different i n s t r u m e n t types was c o m p a r e d using the same test o f significance. RESULTS
T h e b a r graphs (Fig 5, 6) show the m e a n weight loss from the surface of the b o n e a n d s t a n d a r d d e v i a t i o n s for the five designs of i n s t r u m e n t s after the first a n d third five-minute-runs. Initially, size no. 30 t r i a n g u l a r files, reamers, a n d square files h a d similar c u t t i n g abilities. All of these designs were significantly b e t t e r (P < .05) t h a n square r e a m e r s a n d
H e d s t r o e m files (Fig 5). However, after the t h i r d five-minute-run, size no. 30 square files a p p e a r e d to be more efficient t h a n a n y other design and h a d a significantly greater cutting efficiency t h a n the square r e a m ers a n d H e d s t r o e m files (P < .05). The triangular reamer remained a significantly more efficient instrument t h a t the H e d s t r o e m file. T h e square r e a m e r a n d the H e d s t r o e m file were the most inefficient instruments in this size range when used in linear motion. In most cases, d a t a scattering increased significantly between the first a n d third run. T h e pooled s t a n d a r d deviation for the size no. 30 instruments was 9.0 for the first run a n d 14.6 for the t h i r d run. M o r e v a r i a t i o n is seen a m o n g the designs in size no. 50 instruments t h a n a m o n g designs in size no. 30 instruments. T h e first run (Fig 6) showed t h a t t r i a n g u l a r files were significantly m o r e efficient t h a n a n y other design (P < .05). T r i a n g u l a r reamers, H e d s t r o e m files, a n d square files h a d a similar c u t t i n g efficiency. Square r e a m e r s showed the poorest cutting efficiency when used in linear motion. A f t e r the third run, the effect of w e a r on most of the instruments was evident. T h e size no. 50 square file was more efficient t h a n the t r i a n g u l a r reamer, the H e d stroem file, a n d the square r e a m e r (P < .05), b u t not significantly m o r e efficient t h a n the t r i a n g u l a r file. However, the c u t t i n g efficiency of the t r i a n g u l a r size no. 50 file was considerably reduced, a n d no significant difference was observed between it a n d a n y other size no. 50 instrument. As with the results o b t a i n e d with size no. 30 instruments, d a t a scattering increased considerably after the first run, with the
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JOURNAL OF ENDODONTICS I VOL 6, NO 11, NOVEMBER 1980
100
--]RUN I rt-A
80 pooled standard deviation rising from 9.8 to 16.1. W h e n different types of instruments of the same size were compared at a significance level of 0.05 (Table), the following results were obtained. At size no. 30, during the first run, triangular files were superior to square reamers and Hedstroem files; square files were superior to square reamers; and triangular reamers were superior to square reamers and Hedstroem files. After the third run, fewer differences were seen. Square files were superior to square reamers, and both square files and triangular reamers were superior to Hedstroem files. At size no. 50, triangular files showed superior cutting ability to all other instruments during the first cycle. Also, in the first cycle, square files, triangular reamers, and Hedstroem files far exceeded the cutting ability of square reamers. After the third run, only square files were clearly most effective c o m p a r e d with other instruments. At P < .05, square files showed better cutting ability t h a n either square and triangular reamers, or Hedstroem files. Analysis of the data summarized in the Table showed that only with one type of instrument, the triangular reamer, was there a significant difference in cutting ability between the smaller and larger size for the full 15 minutes of the test. However, the cutting ability was more effective in the larger size only during the first run. After the third run, the situation was reversed. T r i a n g u l a r and Hedstroem files showed significant cutting superiority in the larger size c o m p a r e d with the smaller size for the first run, but not at the end of the third run. There was no significant difference between sizes in square
832
]~RUN 3
~IA
Z
to z
I
60
STANDARD DEVIATION
C.0
~
40
20
HEDSTRC)'N FILE
TRIANGULAR TRIANGULAR FILE REAMER
SQUARE FILE
SQUARE REAMER
Fig 5--Mean weight loss using size no. 30 instruments.
100
T
'--] RUN I
I I. /IA
80
[~RUN 3
Z
I
Lo _z o ~
60
40
20
/I/ '1/
STANDARD DEVIATION
T
/[A
/
/ / /
/ /
/ /
,4
/ / / /
TRIANGULAR TRIANGULAR FILE REAMER
HEDSTROM FILE
SQUARE FILE
SQUARE
REAMER
Fig 6--Mean weight loss using size no. 50 instruments.
Table 9 Cutting ability expressed in weight loss (mg/5 min).
First run (SD) Instrument designation Triangular files Square files Triangular reamers Square reamers Hedstroem files
Third run (SD)
#30
#50
~30
#50
58(1)* 58(15) 56(12)* 35(2)
97(13)* 58(9) 67(6)* 40(10) 66(9)*
52(22) 79(16) 62(2)t 50(16) 32(8)
62(17) 83(15) 49(14)r 42(15) 33(19)
38(5)*
SD = standard deviation. *Significant difference (P < 0.05) between :#'30 and # 5 0 instruments in first run. **Significant difference (P--< 0.05) between # 3 0 and # 5 0 instruments in third run.
JOURNAL OF ENDODONTICS i VOL 6, NO 11, NOVEMBER 1980
instruments at a n y time d u r i n g the experiment. DISCUSSION T h e e x p e r i m e n t a l p r o c e d u r e used the back a n d forth action o f a commercial saber saw. T h e s i m u l a t i o n of pure linear m o t i o n closely resembled the filing action used in root canals. T h e length of the stroke was constant, a n d the speed was carefully controlled by the v a r i a b l e rheostat. A speed of three cycles p e r second, p e r h a p s excessive b y clinical standards, was necessary to overcome the load a p p l i e d to the bone by the rack a n d pinion drill press. T h e drill press was found to be a suitable device for the a p p l i c a t i o n of loads. Excessive friction in the loading mechanisms was e l i m i n a t e d , resulting in a close correlation between lever a r m load a n d pressure on the root canal instruments. T h e saber saw allowed four root canal instruments to o p e r a t e at one time, a n d was easy to disassemble a n d reassemble with new groups of instruments. M e a s u r i n g weight loss from the bone specimens after opera t i n g four root canal i n s t r u m e n t s was found to be more c o n v e n i e n t than m e a s u r i n g the smaller values of weight loss o b t a i n e d after o p e r a t i n g only one root c a n a l instrument. A five-minute o p e r a t i n g time was long enough to give m e a s u r a b l e a m o u n t s of weight loss t h a t could be reprod u c e d a n d repeated. Moist bovine bone for testing i n s t r u m e n t c u t t i n g efficiency was superior to dry bone. P r e l i m i n a r y work showed that d e h y d r a t e d bone d i d not allow a d e q u a t e surface penetration by root canal instruments because of increased bone hardness. F o r this reason, a freshly thawed, moist bone section was used for every
test run. In contrast to the excellent c h a r a c teristics o f moist bovine bone, acrylic was erratic as a workpiece a n d it was difficult to assess the c u t t i n g efficiency of root canal instruments. C h i p s of acrylic c o m p l e t e l y coated the flutes of the instruments in such excessive a m o u n t s t h a t at times the instruments were unable to move back a n d forth. T h e m e a s u r e m e n t of weight loss from the acrylic was also difficult because at times the a m o u n t was so small. T h e inability to s t a n d a r d i z e a representative n u m b e r of d e n t i n samples p r e c l u d e d their use in this study. T h e results of this research showed that, w i t h i n the time limits of the e x p e r i m e n t , wear was associated with certain root canal instruments; only i n s t r u m e n t s with c u t t i n g edges of less t h a n 90 degrees, t h a t is t r i a n g u l a r files, reamers, a n d H e d s t r o e m files, showed this kind of wear. G u t i e r r e z a n d others 1:~ studied the wear of root canal instruments as a m u l t i f a c e t e d p r o b l e m , but were unable clearly to show a n y p r e d i c t a b l e changes in the instruments' edges after used. T h e w e a r shown by the H e d stroem files was surprising in view of the favorable reports of these instruments, in previous research 4 a n d by m a n y endodontists. This study f o u n d the H e d s t r o e m file inferior to most other types of instruments in r e m o v ing m a t e r i a l from a root canal or its c o u n t e r p a r t . T h e H e d s t r o e m file is only o p e r a t i o n a l on the o u t w a r d stroke; therefore, an i n s t r u m e n t such as a file or t r i a n g u l a r reamer, which removed dentin on both an o u t w a r d a n d i n w a r d stroke, would be a m o r e effective c u t t i n g instrument. T h e 60 degree c u t t i n g edge of the t r i a n g u l a r i n s t r u m e n t was initially the sharpest, and the results of this
study c o r r o b o r a t e similar observations of Shoji, 8 a n d Heuer, ~ and Oliet a n d Sorin. ~ However, o u r observation was that these instruments were unable to retain their sharpness, whereas the 90 degree c u t t i n g edge of the square file a n d r e a m e r m a i n tained a high degree of sharpness after r e p e a t e d trials. A n o t h e r factor was the v a r i a t i o n in the helical angle (the angle f o r m e d by the cutting edge a n d the long axis of the instrument). A square r e a m e r has a small helical angle, with the edges turned toward the long axis of the instrument. This results in a root canal instrument of m i n i m a l c u t t i n g efficiency when used in a filing action. However, when the edges are more p e r p e n d i c u l a r to the long axis of the instrument, as in a highly fluted file, the force d i s t r i b u t i o n along the edge would be highly c o n c e n t r a t e d , making the i n s t r u m e n t an effective dentin remover w h e n used in linear motion. However, the results suggest that this force m a y exceed the strength of the i n s t r u m e n t ' s edge; consequently, an initially sharp root canal i n s t r u m e n t could become dull very quickly. T h e t r i a n g u l a r r e a m e r m a y be closer to the ideal than a n y other root canal i n s t r u m e n t when used in linear motion. T h e sharp 60 degree edge enables it to p e n e t r a t e deeply into the root c a n a l wall. T h e edge also has a helical angle smaller t h a n a t r i a n g u l a r file b u t larger t h a n a square reamer. These characteristics would tend to d i s t r i b u t e the c u t t i n g force along a greater length of edge, m a k i n g it b e t t e r able to absorb the stress w i t h o u t w e a r i n g out. By varying the helical angles to values somewhere between the most fluted file a n d the least fluted reamer, it should be possible to m a n u f a c t u r e a root
833
JOURNAL OF ENDODONTICS [ VOL 6, NO 11, NOVEMBER 1980
c a n a l i n s t r u m e n t o f t r i a n g u l a r cross s e c t i o n w h i c h , w h e n u s e d in a filing action, would achieve superior cutt i n g efficiency w i t h m i n i m u m w e a r .
SUMMARY AND CONCLUSIONS T h e p u r p o s e o f t h i s s t u d y w a s to d e s i g n a n d e v a l u a t e a test p r o c e d u r e w h e r e b y t h e c u t t i n g e f f i c i e n c y o f five d i f f e r e n t l y d e s i g n e d r o o t c a n a l ins t r u m e n t s , w h e n o p e r a t e d in a l i n e a r (filing) m o t i o n , c o u l d b e assessed. Linear motion was supplied by the back and forth action of a commercial s a b e r saw. The following conclusions can be d r a w n f r o m this s t u d y : t h e e x p e r i m e n t a l p r o c e d u r e d e s c r i b e d w a s suita b l e for d e t e r m i n i n g t h e c u t t i n g effic i e n c y o f root c a n a l i n s t r u m e n t s used in l i n e a r m o t i o n . M o i s t b o v i n e b o n e w a s a s u i t a b l e m a t e r i a l for assessment of root canal instrument cutt i n g efficiency. T h e r e w a s w i d e r a n g e o f c u t t i n g efficiency b e t w e e n e a c h t y p e o f root c a n a l i n s t r u m e n t , b o t h i n i t i a l l y a n d a f t e r s u c c e s s i v e use. Instruments manufactured from
834
t r i a n g u l a r cross s e c t i o n s w e r e initially m o r e efficient b u t lost s h a r p ness a f t e r successive use. I n s t r u m e n t s m a n u f a c t u r e d f r o m s q u a r e cross sections had greater retention of sharpness. H e d s t r o e m files lost c u t t i n g efficiency very rapidly. This work was supported in part by a grant from the Endowment and Memorial Foundation of the American Association of Endodontists. Dr. Webber is a practicing endodontist in London, England. Dr. Moser is associate professor of biological materials; l)r. tleuer is professor and chairman, department of end()dontics, Northwestern University Dental School. Requests for reprints should be directed to Dr. J. B. Moser, Department of Biological Materials, Northwestern University Dental School, 311 E Chicago Ave, Chicagn, 60611.
References I. American National Standards Institute: Dentistry, report of meeting of ISO/TC 196, London, Sept. 16-20, Chicago, ADA. 2. Oliet, S., and Sorin, S.M. Cutting efficiency of endodontic reamers. Oral Surg 36:243-252, 1973. 3. Molven, O. A comparison of the dentinremoving ability of five root canal instruments. Scand J Dent Res 78:500-511, 1970.
4. DeJong, L.C., and Willoughby, J.W. Endodontic instruments, an evaluation of cutting ability. Thesis. University of Michigan, 1975. 5. Stenman, E. Effects of sterilization and endodontic medicaments on mechanical propeyries of root canal instruments. Thesis. University of Ume~, Sweden, 1977. 6. Heuer, M.A. The biomechanics of endodontic therapy. Dent Clin N Am 1963, pp 341-359. 7. Heuer, M.A. A study of the structural, dimensional and physical characteristics of root canal instruments. Thesis. University of Michigan, 1959. 8. Shoji, R. Studies on the mechanism of the mechanical enlargement of root canals. J Nihon Univ School Dent 7(2):71-78, 1965. 9. Luga~sy, A. Mechanical and viscoelastic properties of cow bone and sperm whale dentine studied under compression. A dissertation in metallurgy and materials science. Thesis. University of Pennsylvania, 1968. 10. Amprino, R. Investigatinn on some physical properties of bone tissue. Acta Anat 34(3): 161-186, 1958. t l. Sedlin, E.D. A rheologieal model for cortical bone. Acta Orthop Stand, Supp183:177, 1965. 12. Stevens, J., and Ray, R.D. An experimental comparison of living and dead bone in rats. J Bone Joint Surg 44(B):412-413, 1962. 13. Guttierrez, J.H.; Gigoux, C.; and Sanhueza, J. Physical and chemical deterioration of endodontic reamers during mechanical preparation. Oral Surg 28:394-403, 1969.