Properties of endodontic hand instruments used in rotary motion. Part 3. Resistance to bending and fracture

Properties of endodontic hand instruments used in rotary motion. Part 3. Resistance to bending and fracture

0099-2399/97/2303-0141 $03.(30/0 JOURNALOF ENDODONTICS Copyright © 1997 by The American Association of Endodontists Printed in U.S.A. VOL. 23, No. 3...

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0099-2399/97/2303-0141 $03.(30/0 JOURNALOF ENDODONTICS Copyright © 1997 by The American Association of Endodontists

Printed in U.S.A.

VOL. 23, No. 3, MARCH1997

SCIENTIFIC ARTICLES Properties of Endodontic Hand Instruments Used in Rotary Motion. Part 3. Resistance to Bending and Fracture Joachim Tepel, Dr. med. dent., Edgar Sch~fer, Dr. med. dent., and Wolfgang Hoppe, Univ.-Prof. Dr. med. dent.

Bending and torsional properties of 24 different types of nickel-titanium K-files, titanium-aluminium K-files and reamers, conventional stainless steel K-files and reamers, and flexible stainless steel instruments were investigated corresponding to ISO 3630-1 by determination of the bending moment on the one hand and the torque and angular deflection on the other. Numbers 15, 25, and 35 instruments were tested with a sample size of 10 instruments for each type and size. In ascending order of bending moment the instruments ranked: nickel-titanium K-files, titanium-aluminium K-files and reamers, flexible stainless steel instruments, conventional stainless steel K-files, and reamers. Nickel-titanium, titanium aluminium, and flexible stainless steel instruments displayed lower torque values than conventional stainless steel K-files and reamers. The average angular deflection ranged from 380 ° (#15 reamer) to 2370 ° (#35 K-file). Overall, the fracture risk of the instruments tested in this study was comparably low.

determined. Depending on the instrument type and size, maximum values for the bending moment are given by ISO 3630-1. Resistance to fracture of endodontic instruments was investigated more than 50 years ago (7), and ever since investigations have been done with different and more sophisticated methods to give the clinician important information about the fracture risk of the root canal instruments he uses (2, 4, 8, 9, 10, 11). In tests corresponding to ISO 3630-1 (1) the root canal instrument is fixed at its tip and at its shaft and rotated in a clockwise direction until it fractures. Depending on the instrument type and size, minimum values for the torque and for the angular deflection, i.e. the rotation angle at which the instrument fractures, are given by ISO 3630-1. In the current study the resistance to bending and the resistance to fracture of nickel-titanium K-files, titanium-aluminium K-files, and reamers and of stainless steel K-files, reamers, and flexible instruments were determined according to 1SO 3630-1 (1).

M A T E R I A L S AND M E T H O D S Both resistance to bending and resistance to fracture were determined with a testing apparatus corresponding to ISO 3630-1 (1). In the tests concerning resistance to bending, bending angle and bending moment were continuously recorded on an xy-recorder (Watanabe WX 4301, Tokyo, Japan). The bending moment at a 45 ° bending was calculated from these records. In the tests conceming resistance to fracture, rotation angle and torque were continuously recorded and the angular deflection and the torque were calculated from these records. The following root canal instruments were tested (Table 1): (a) nieket-titanium instruments: nickel-titanium K-files made by two manufacturers; (b) titanium-aluminium instruments: Microtitane instruments made by one manufacturer; (c) conventional stainless steel instruments': K-files and reamers made by seven manufacturers; and (d) flexible stainless steel instruments: five instruments made by four manufacturers. Instrument sizes #15, #25, and #35 were tested. Sample size was

The resistance to bending and the resistance to fracture in torque and angular deflection are two mechanical requirements which are described in ISO 3630-1 (1). Resistance to bending influences the instrumentation results in curved canals. To minimize undesirable changes of the curved root canal different root canal instruments with a greater flexibility have been developed during recent years (2, 3, 4, 5, 6). The increase in flexibility was either achieved by modification of conventional stainless steel instruments or by the use of new alloys like nickel-titanium (Nitinol) or titanium-aluminium (Microtitane). In tests corresponding to ISO.3630-1 (1) the root canal instrument is bent 45 ° and the bending moment is

141

142

Journal of Endodontics

Tepel et al

RESULTS

10 instruments for each type and size. Statistics were calculated with commercial software (Statgraphics 3.0, STSC, Rockville, MD) installed on an IBM compatible AT computer.

None of the instruments tested exceeded the maximum bending moments given in ISO 3630-1 (1) (Table 2). Conventional stainless steel instruments showed the greatest average resistance to bending in all three lSO-sizes tested. Among conventional stainless steel K-files and reamers, there were no consistent differences concerning the average resistance to bending (Fig. 1). In some cases K-files showed a greater resistance to bending than reamers of the same brand, in other cases--depending on the cross section of the instrument--the findings were the opposite. The average resistance to bending of flexible stainless steel instruments was less than that of conventional stainless steel instruments. Except for one #15 instrument, the resistance to bending of flexible stainless steel instruments (#15 and #25 instruments) was about 20 to 40% less than that of the conventional stainless steel K-files and reamers. Nickel-titanium K-files showed a resistance to bending that was about half to a quarter of the average resistance to bending of conventional stainless steel K-files. The findings with titaniumaluminium K-files and reamers lay in the middle; these instruments showed less resistance to bending than conventional K-files and reamers but the #15 instruments clearly displayed greater resistance to bending than nickel-titanium K-files.

TABLE 1. Tested root canal instruments Alloy

Manufacturer/Distributor

Nickel-titanium

Mity, Ridgefield, CT Texceed, Costa Mesa, CA Titanium-Aluminium Micro M~ga, Besan?on, France Antaeos, Munich, Germany Stainless steel

Resistance to Bending

Instrument K-file K-file K-filereamer

K-filereamer Flexicut~) Brasseler, Lemgo, Germany K-filereamer Kerr, Karlsruhe, Germany K-filereamer K-Flex1) Maillefer, Ballaigues, K-filereamer Switzerland Ftexoreamer~) K-Flexofile1) Meisinger, D~esseldorf, K-fiiereamer Germany Micro Mega, Besancon, K-filereamer France Roeko, UIm, Germany2) K-filereamer Union Broach, New York, Flex-R file 1) NY

1) Flexible stainless steel instruments. 2t Manufacturer: Mani, Nakaakutsu, Japan.

TABLE 2. Resistance to bending bending moment [gcm] instrument

nickeltitanium

K-file

Mity

Texceed titaniumK-file Micro M~ga aluminium reamer Micro Mega stainless conventional K-files and reamers steel K-file Antaeos Kerr Komet Maillefer Meisinger Micro Mega Roeko reamer Antaeos Kerr Komet Maillefer Meisinger Micro Mega Roeko flexible instruments Antaeos: Fiexicut Kerr: K-Flex Maiilefer: Flexoreamer Maillefer: K-Flexofile Union Broach: FlexR-file

#15

#25

#35

mean

SD

maximum

mean

SD

maximum

mean

SD

maximum

13.4

7.5

25

21.2

4.1

27

48.4

6.9

55

8.5 10.5

0.71 1.4

9 12

24.0 43.2

3.2 3.6

29 50

42.5 78.0

3.7 9.2

47 98

9.1

1.3

12

30.2

3.4

36

66.7

4.9

75

36.2 28.5 34.4 28.7 30.2 18.0 34.6 36.2 27.9 36.0 30.4 32.6 17.3 57.4

2.8 3.2 2.1 1.8 2.8 1.6 3.0 2.3 3.6 2.2 3.1 3.6 1.8 12.9

42 33 38 32 35 20 40 40 32 39 36 38 20 74

101.1 69.8 101.1 89.9 87.2 83.0 90.7 100.8 60.8 99.0 81.5 84.5 59.7 94.5

4.1 7.4 6.4 5.0 3.7 7.0 6.4 4.0 13.1 3.0 2.5 4.4 2.8 9.5

109 81 110 98 94 96 102 107 76 103 87 93 64 110

126.8 171.0 120.1 187.0 194.3 122.2 199.4 217.6 110.2 221.5 189.9 180.9 114.4 203.5

4.7 26.8 4.3 8.9 10.0 8.6 9.2 8.9 10.0 9.7 6.2 5.4 9.3 6.8

132 203 128 200 205 134 212 237 130 239 203 191 129 210

19.9 26.8 15.1 14.0 36.4

1.9 5.4 2.7 1.5 5.I

25 42 20 17 45

57.7 49.4 54.5 52.9 59.5

2.5 3.3 3.9 1.7 5.8

61 54 61 55 69

119.8 126.6 103.5 106.3 117.4

5.7 12.0 6.1 4.2 10.3

133 147 113 113 135

Bending moment (n = 10). The following maximum values, which the instruments should not exceed, are given by ISO 3630-1 (1): #15 K-files and reamers 50 gcm; #25 K-files and reamers 120 gcm; #35 K-files 190 gcm and #35 reamers 220 gcm.

Vol. 23, No. 3, March 1997

Bending and Torsional Properties

143

TABLE 3*6,. Resistance to fracture torque [gcm] instrument

#15 mean SD

nickel-titanium titanium-aluminium stainless steel

K-file

Mity Texceed K-file Micro Mega reamer Micro Mega conventional K-files and reamers K-file Antaeos Kerr Komet Maillefer Meisinger Micro Mega Roeko reamer Antaeos Kerr Komet Maillefer Meisinger Micro Mega Roeko flexible instruments Antaeos: Flexicut Kerr: K-Flex Maillefer: Flexoreamer Maillefer: K-Flexofile Union Broach: Flex-R-file

#25

minimum

mean SD

#35

minimum

mean

SD

minimum

16.4 14.0 15.4 12.4

1.0 1.4 5.4 2.3

15.0 11.0 10.0 8.0

35.4 35.5 33.5 23.0

5.8 1.7 5.2 1.8

28.0 30.3 24.0 20.0

81.2 72.3 62.0 48.3

10.3 7.6 8.7 3.7

66.4 54.0 53.0 45.0

29.2 21.1 28.7 17.6 18.8 14.3 17.4 28.6 19.2 26.0 38.9 21.2 11.8 27.4

2.6 6.8 3.5 3.0 4.3 1.8 4.4 3.1 2.7 2.9 5.3 2.5 2.7 6.5

25.0 12.0 24.0 14.0 11.0 11.0 13.0 24.0 15.0 23.0 30.0 17.0 8.0 19.0

77.0 8.7 76.7 8.4 69.0 6.7 64.5 14.9 64.5 6.8 54.5 24.1 82.8 8.0 76.1 6.9 41.4 5.2 81.6 8.2 62.3 6.4 63.3 7.0 42.0 4.3 89.1 5.0

58.0 62.0 58.0 39.0 54.0 36.0 71.0 60.0 32.0 69.0 54.0 54.0 35.0 79.0

99.6 122.0 96.2 120.8 112.2 100.0 149.0 150.3 79.5 144.2 115.3 115.0 82.1 147.8

9.7 12.4 11.1 6.4 8.2 14.0 13.3 8.4 10.9 8.3 11.0 8.0 10.2 10.3

88.6 103.0 75.4 114.0 104.1 82.0 131.0 131.2 66.0 133.3 92.3 104.1 68.1 121.0

18.7 19.3 11.6 12.8 28.6

2.1 2.9 1.8 2.4 1.4

16.0 15.0 8.0 9.0 26.0

45.0 44.8 41.0 40.6 42.8

40.0 35.0 35.0 35.0 30.0

98.2 90.3 74.9 78.7 87.0

10.8 14.7 6.7 7.0 8.3

82.8 71.3 67.2 68.9 73.4

5.0 4.9 4.9 4.2 6.3

TABLE 3B. angular deflection [°] instrument

nickel-titanium titanium-aluminium stainless steel

K-file

Mity Texceed K-file Micro Mega reamer Micro M~ga conventional K-files and reamers K-file Antaeos Kerr Komet Maillefer Meisinger Micro Mega Roeko reamer Antaeos Kerr Komet Maillefer Meisinger Micro Mega Roeko flexible instruments Antaeos: Ftexicut Kerr: K-Flex Maillefer: Flexoreamer Maillefer: K-Flexofile Union Broach: Flex-R-file

#15

#25

#35

mean SD

minimum

mean

SD

mean

SD

940 102 790 82 1094 127 663 137

736 712 910 460

786 879 680 711

108 113 106 119

648 686 537 574

659 834 729 772

159 157 138 110

431 481 475 481

735 179 719 97 756 54 475 73 447 53 542 88 1057 302 500 59 498 59 548 36 380 64 385 33 480 58 587 96

542 617 686 405 346 406 455 390 360 465 251 331 376 481

1393 219 856 130 944 177 679 62 575 85 584 68 1290 150 633 73 963 164 678 171 441 50 496 63 582 72 1283 150

1014 683 705 570 465 478 1032 519 589 501 393 394 424 1063

1854 621 2370 658 828 605 1541 771 868 709 597 664 831 655

303 56 502 92 104 84 232 137 89 116 86 172 131 111

1394 514 1747 521 650 515 1186 639 783 592 429 519 562 538

799 101 701 124 721 71 801 98 649 251

706 573 625 711 415

725 625 570 616 420

1861 739 1234 1589 1021

369 132 100 364 128

1412 501 1095 1244 709

856 872 688 959 488

111 129 98 257 44

minimum

A. Torque (n - 10). The following minimum values are given by ISO 3630-1 (1): #15 K-files and reamers 8 gcm, #25 K-files 30 gcm, #25 reamers 20 gcm, reamers 50 gcm. B. Angular deflection (n = 10). The minimum value given by ISO 3630-1 (1) is 360 ° for all instruments tested in this study.

minimum

#35 K-files65 gcm and #35

144

Tepel et al

Journal of Endodontics

Ni Ti

Ti AI

K-ftieS

] K f~Je learner

stainless K files

A

steel ~eamers

Ni Ti

ftexibte instt~me~ls

K files

1 2 0 - - -

120

IO0

100

_~

80-

~

eo4o-

Ti AI K.file

reamer

stainless K fitS$

steel learners

l~exible ~nSl[l,~tl~nl~

ao

J,

40 2

2o-

a

-

2O 0

FIG 1. Resistance to bending: Notched boxplot of the bending moments of the tested #25 root canal instruments (n = 10). The maximum value given by ISO 3630-1 (1), which the instruments should not exceed, is indicated by the bold line.

B Ni Ti K files

stainless

T i AI K-file reamer

K files

steel reamers

flexible instruments

Resistance to Fracture

~ The results concerning torque and angular deflection are given in detail in Table 3. With a few exceptions, conventional stainless steel K-files and reamers showed about the same torque (Fig. 2A). In most cases K-files reached greater angular deflection than reamers of the same brand (Fig. 2B). Flexible stainless steel instruments displayed less torque than most conventional stainless steel K-files and reamers. The angular deflection of these instruments was similar to conventional stainless steel K-files. Nickeltitanium and titanium-aluminium instruments also showed less torque than conventional K-files and reamers and reached about the same angular deflection as conventional stainless steel reamers.

DISCUSSION As previously described in the literature (3, 4, 6, 12), resistance to bending depended on the instrument types and alloys: flexible stainless steel instruments as well as titanium-aluminium instruments, and especially nickel-titanium instruments, clearly showed less resistance to bending than conventional stainless steel K-files and reamers. Therefore, the tendency of flexible stainless steel, titanium-aluminium, and especially nickel-titanium instruments to straighten themselves in the curved root canal is much less compared to conventional stainless steel K-files and reamers. However, the question arises whether the widely held assumption, that the more flexible a root instrument the less the danger of undesirable changes in the curved root canal, is accurate. In a previous study (13) it has been shown that nickel-titanium K-files caused few undesirable changes in the curved root canal shape, but this was because almost no material was removed from the canal walls by these instruments because they wore very rapidly. Flexible stainless steel instruments, especially those with a modified, no,cutting tip (13, 14) performed better than nickeltitanium or titanium-aluminium instruments. Resistance to bending allows conclusions about whether an instrument will follow the curved root canal. However, this parameter allows no prediction on whether undesirable changes to the curved root canal will appear. Therefore, resistance to bending has only a limited'clinical impact, and this parameter alone is not an appropriate selection criterion for root canal instruments. Resistance to fracture, on the other hand, has a direct clinical impact. Under clinical conditions, the angular deflection gives some information about the risk that an instrument that is binding

14aO 1080- _

i

7zo

_

+

FIG 2. Resistance to fracture: Notched boxplots of (A) the torque and (B) the angular deflection (n = 10). The minimum values given by ISO 3630-1 (1), which the instruments should exceed, are indicated by the bold lines.

at its tip will fracture if it is rotated any further. In the literature, rotation angles between 90 ° (15) and 180 ° (16, 17) are suggested for the reaming motion. None of the instruments tested in our study fractured after a 180 ° rotation. Except for the nickel-titanium instruments, visible nonelastic deformation occurred before fracture, which lead to an obvious unwinding of the twisted instruments. Differences in torque may become significant if one is going to change from an instrument type with high torque values to an instrument type with lower torque values because the latter instrument will unwind easier (i.e. visible nonelastic deformation occurs at lower torque values). In previous investigations (14, 18), flexible stainless steel instruments displayed higher cutting efficiencies than conventional stainless steel K-files and reamers, especially higher than nickel-titanium and titanium-aluminium instruments. Moreover, flexible stainless steel instruments with modified noncutting tips caused less undesirable changes of the curved root canal shape due to instrumentation (13). Therefore, from a clinical point of view, flexible stainless steel instruments offer two major advantages. According to the results of this study, compared with conventional stainless steel K-files and reamers, flexible stainless steel instruments display lower torque values. Therefore the forces needed to unwind these instruments, if they bind in the canal, are less. Clinically, this aspect should not be overemphasized, however. If root canal instruments are carefully checked relative to nonelastic deformation during instrumentation and thrown away as soon as unwinding has occurred, the advantages of the flexible stainless steel instruments with modified noncutting tip can be used without any increased risk of instrument fracture.

Bending and Torsional Properties

Vol. 23, No. 3, March 1997

When the four parameters investigated in the current and the previous studies (13, 18), namely resistance to bending, resistance to fracture, cutting efficiency, and instrumentation of curved canals, are evaluated with regard to their usefulness for the clinician, the conclusion can be drawn that resistance to bending is the least relevant parameter during clinical usage. Compared to this, resistance to fracture has a greater clinical importance, which, however, is qualified by the fact that for the instruments tested in this study fracture should not be a complication if the instruments are used correctly. Hence, in our opinion, the most important parameters are cutting efficiency and instrumentation of curved canals. These two parameters allow the evaluation of root canal instruments from a clinical perspective and, therefore, can serve as a useful complement to the already existing international standards. We thank Professor J. L. Gutmann (Dallas, TX) for reviewing the article. Drs. Tepel, Sch&fer, and Hoppe are affiliated with the Poliklinikfor Zahnerhaltung A, MOnster, Germany. Address requests for reprints to Dr. Joachim Tepel, Poliklinik for Zahnerhaltung A, Waldeyerstr. 30, D-48149 MOnster, Germany.

References 1. International Standard Organization: ISO 3630-1. Dental Root canal instruments--Part h files, reamers, barbed broaches, rasps, paste carriers, explorers and cotton broaches. Genf: International Standard Organization, 1992. 2. Brankin GW, Wenckus CS, Ashrafi SH. Flexibility/torsional properties of nickel-titanium and stainless steel files. J Endodon 1993;19:193. 3. Dolan DW, Craig RC. Bending and torsion of endodontic files with rhombus crossection. J Endodon 1982;8:260-64.

145

4. Krupp JD, BrantleyWA, Gerstein H. An investigation ofthetorsionaland bending properties of seven brands of endodontic files. J Endodon 1984;10: 372-80. 5. Roth WC, Gough 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. 6. Walia H, Brantley WA, Gerstein H. An initial investigation of the bending and torsional properties of Nitinol root canal files. J Endodon 1988;14:346-51. 7. Schultz H. Der EinfluB der HeiBluftsterilisation auf die Torsionsfestigkeit der Wurzelkanalinstrumente. Schweiz Mschr Zahnheilk 1944;54:239-65. 8. Lentfne FN. A study of torsional and angular deflection of endodontic files and reamers. J Endodon 1979;5:181-91. 9. Lilley JD, Smith DC. An investigation of the fracture of root canal reamers. Br Dent J 1966;120:364-72. 10. Massa GR, Nicholls JI, Harringtion GW. Torsional properties of the Canal Master instrument. J Endodon 1992;18:222-7. 11. Rowan MB, Nichols J, Steiner J. Torsional properties of stainless steel and nickel-titanium endodontic files. J Endodon 1995;21:216. 12. St~dler P, Jeglitzsch M. Elastizit~itund MaBtreue endodontischer Aufbereitungsinstrumente. Endod 1993;2:25-31. 13. Sch&fer E, Tepel J, Hoppe W. Properties of endodontic hand instruments used in rotary motion. Part 2. Instrumentation of curved canals. J Endodon 1995;21:493-7. 14. Tepel J, Sch&fer E. Effizienz von Wurzelkanalinstrumenten for die manuelle Aufbereitung. In: Heidemann D (ed). Deutscher Zahn~irztekalender 1996. M0nchen: Hanser: 47-75. 15. Wildey WL, Senia EE. A new root canal instrument and instrumentation technique: a preliminary report. Oral Surg Oral Med Oral Pathol 1989;67:198207. 16. RoaneJB, Sabala CL, Duncanson MG. The "balanced force" concept for instrumentation of curved canals. J Endodon 1985;11:203-11. 17. Sch~fer E. Effects of four instrumentation techniques on curved canals: a comparison study. J Endodon (in press). 18. Tepel J, Sch~fer E, Hoppe W. Properties of endodontic hand instruments used in rotary motion. Part 1. Cutting efficiency. J Endodon 1995;21: 418-21.

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