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Copyrigtii © Munk.sgaard 1997

Endodontics &Dental Traumatology

ISSN 0109-2502

Review article from invited author

Endodontic hand instruments: cuttingefficiency, instrumentation of curved canals,bending and torsional propertiesTepel J, Schafer E. Endodontic hand instruments: cuttingeflicicncy, instrumentation of curved canals, bending and torsionalproperties. Endod Dent Traumatol 1997; 13: 201 ilO.© Munksgaard, 1997.

Abstract - In an assessment of the usefulness of diffcrcnl root canalinstruments, two aspects arc of particular interest fbr the endodon-tist: the cutting efficiency of the instrunieiUs and their ability toenlarge cun^ed canals without undesirable changes of the canalshape such as the fbnriation of zips and elbows. This review pajUM"is based on own investigations on the cutting elliciency and iiistru-mentaUon of curved canals. Addilionally, two parameters whicharc described in ISO 3630-1, resistance to bending and resistanceto fracture, are discussed. With regard to cutting eflicicncy in rotaiymotion, flexible stainless steel reamers and K-filcs clearly displaythe best results and are superior to conventional stainless steel aswell as titanium-based reamers and K-files. Regarding cutting ef-ficiency in linear motion, stainless steel Hedstrom files made bycertain manufacturers are significantly superior to stainless steeland titanium-based Hedstrom files of other brands. Flexible stain-less steel instruments with modified noncutting tips clearly pro-duce the best canal shape in curved canals. With only rare excep-tions, all the instruments tested fulfilled the requirements of theISO standard concerning resistance to fracture and resistance tobending.

J. Tepel, E. SchaferZentrum fur Zahn-, Mund- und Kieferheilkunde,Poliklinik fur Zahnerhaltung, University of Munster,Munster. Germany

Key words: curved root canals; endodonticinstruments; endodontic treatment; root canalinstrumentation

Joachim Tepel, Poliklinik tiJr Zahnerhaltung,

Waldeyerstr. 30, D-48149 Munster, Germany

Accepted April 28, 1997

Root canal instruments fbr manual use have beenmodified during the last years with regard to the useof both new alloys and new design features (1).

Stainless steel root canal instruments for manualuse can be divided into conventional stainless steeland the so-called flexible stainless steel instruments.The new alloys have to a certain extent been used inother fields of dcnti.stry. 'I'his i.s the case wilh nickcl-litanium, which has been used in orthodontics innickel-Litaniuni wires (2). Moreover, in.strumentsmade of a titanium-aluminium alloy have been avail-

able since 1994. An over\aew of the variety of rootcanal instruments for manual use with respect to theirshape, alloys and certain clinical properties has beengiven in a previous article (1).

This paper presents a discussion, based on own in-x'estigations, of the cutting efficiency of various handinstruments, the instrumentation of curved canals

different instruments and instrnmentation tech-niques, and the bending and torsional properties ofthe instruments. Fhe instruments tested are fisted inTable 1.

2D1

Tepel & Schafer

Table 1. The instruments discussed in this paper

Group Manufacturer Instrument

Conventional stainless steel instruments KerrKarlsruhe, GermanyKomet (Gebr. Brasseler)Lemgo. GermanyMailleterBallaigues, SwitzerlandManiNakaakutsu, JapanMeisingerDusseldort, GermanyMicro-MegaBesangon, FranceVDWMunich, Germany

KerrKarlsruhe, GermanyMailleferBallaigues, SwitzerlandUnion BroachNew York City, NY, USAVDWMunich, Germany

From each manufacturerreamerK-fileHedstrom file

K-Flex

K-Flexofile^K-Flexoreamer'Flex-R file^

Flexicut

Flexible stainless steel instruments

Titanium-aluminium Micro-MegaBesangon, France

reamerK-fileHedstrom file

Nickel-titanium MityRidgefield, CT, USA

Hedstrom fileK-file

Available with noncutting tip in ISO sizes 15 to 40.' Available with noncutting tip in ISO sizes 10 to 140.

Design of root canal instruments and their workingmotion

In ])rinciple, root canal instruments for manual usecomprise three different types according to their de-sign: reamers, K-files and Hedstrom files. Accordingto ISO standardization (3), reamers are .symbolizedby a triangle, K-files by a square, and Hedstrom filesby a circle. However, these symbols do not necessarilyrepresent the true cross-section of the instruinents.Most reamers, for instance, are made from scjuareblanks, at least up to size 25.

A fundamental difference between the three differ-ent instrument tyj cs is the angle of their cutting edgesto the long axis of the instruments. This angle deter-mines the most effective working motion of the instru-ments (Fig. 1). For reamers and K-files the angle loe-tween the cutting edge and the long axis is less than45°. 1 hus, these instruments are primarily designedto be used with a rotary motion. On the other hand,Hedstrom files show an angle of the cutting edges tothe long axis of about 60° to 65°, and therefore theseinstruments are primarily designed to be used with alinear, filing motion.

Cutting efficiency

The investigation of the cutting efficiency in a rotaryas well as in a linear motion was condticted understandardized conditions, using two specially designedtesting devices.

Rotary working motion

Glutting efficiency in a rotaiy motion was determinedusing specimens with a cylindrical canal. Flie speci-mens were made of a polyester resin (Alpolit UP 004,Hocchst, Hamburg, Germany) to which 5% wtquartz powder was added (4, 5). 'Fhe abrasion charac-teristics and the microhardncss of this resin were simi-lar to the dentin of the human root canal wall andoffered uniform and reproclucilDle al^rasive character-istics (6). A specially designed computer-driven testingdevice which simulated manual instrumentation anal-ogous to the reaming working motion was used (Fig.2). Furthermore, different instrumentation techniquescould be simulated with this testing device so that theal^ility ofdilTerent techniques to maintain the originalcanal shape and cui'vature during enlargement could

2D2

Endodontic hand instruments

reamer K-file Hedstrom file

angle between long axis and cutting edges

< 45° < 45° > 45°

/<)». /. S1''.M ph()l(),()ra|)li of clilVcrciH l-ool canal insirumciiis iI.SOsv/.v '.Vi, original magnilicalion X4()). A lunclanuMUal clinciriuc 1K--twccn reamers., K-liles and Hedsurim files concerns the angle ofthe cutlmg edges to the long axis ol the I'oot eanal instrnnient. l*'orreamers and K-lilcs this angle is less than -I.')", lor Hedstrom files itis more ihan 4-5°. Therefore reamers and K-liles are primarily de-signed lor a rotary, reaming motion whereas Hedstrom liles are|irimarily designed for a linear, filing motion.

Eig. 2. Computer-driven testing device for .standardized imesti-gatioiis on the cutting efliciency of rool canal instruments and onthe changes occuring in the sha|)e of c ur\'ed canals dnc to instru-mentation, i he testing de\ i( e generates a rotary working motion,which is controlled by the specially designed software and can bemodilied in order to simulate dillerent instrumentation technic|ues.On the computer screen, some original ctitting efliciency cur\e.sare displayed.

be compared. The construction and the function ofthe testing device has been described in detail in pre-vious papers (4, 5, 7, 8).

To determine ctitting efficiency, each experimentwas carried otU tmtil the test instrument was \'erifiablyblunt and did not remo\e any more material. At thistime, the maximum penetration depth of the instrti-ment into the cylindrical lumen w is reached. Themaximtim penetration depth ser\'ed as the criterionfor cutting efficiency and therefore was used as thefoasis for comparison (4).

The investigations on cutting efficiency were per-formed with conx'entional stainless instruments, flex-ible stainless steel instruments with conventional ormodified tips, instruments made of titanium-alumin-ium and nickel-titanitim alloys (Table 1). Instrtimentsizes 25 and 35 were tested.

Fhe results were similar fbr size 25 and size 35instruments and can be stiminarized as follows:• Instrtiments made of the titanium-aluminium alloy

showed nearly the same cutting efliciency as corre-sponding conventional stainless steel instruments.K-files made of the nickel-titanium alloy showedthe poorest cutting efficiency ol all instrtimentstested (Fig. 3a).

• All stainless steel instrtiments had a greater cuttingefficiency than nickel-titanium instruments. Withfew exceptions, stainless steel K-files displayed agreater cutting efTiciency than stainless steelreamers of the same brand.

• The fiexible stainless steel instruments showed thegreatest cutting efficiency, and were signifieantlysuperior to all other instruments (Fig. 3b).

• In the group of fiexible stainless steel instruments,the K-Flexofiles and the K-Flexoreamers showedthe greatest cutting efficiency (Fig. 4).

Linear working motion

Cutting efficiency in a linear, filing motion was deter-mined by a specially designed test apparatus, whichhas been described in detail previously (9). The speci-mens w ere made of polyester resin (Alpolit UP 004,Hoechst) to which 5"() wt quartz powder was added,l l i e specimen thickness was 1 mm. The instrtimentto be tested was fixed in the apparatus and placed onthe specimen, and every test run was continued untilthe instrtiment was bltmt, i.e., until the instrument nolonger removed any material, thereby creating maxi-mum gToo\e depth. Fhe depth of the groove was usedas the measure of cutting efficiency (10).

W'ith regard to ctitting efficiency in a linear motion,the Hedstrom files removed far more material thanK-filcs or reamers (Fig. 5) (10) conlirming- the resultso( pre\ious studies (f f, 12).

However, Hedstrom files made by diflerent manu-facturers showed significant dilferences in their cut-ting efTiciency (Fig. 6) (10).

Modifications of the traditional Hedstrom file arethe S- and U-liles which have a double-helix design.

2D3

Tepel & Schafer

apenetration depth [mm]

5-

4 -

3-

2-

1-

titanium-aluminiuminstruments

nickel-titaniumK-file

I I I I I I I I0 60 120 180 240

t[s]

penetration depth [mm]

240

Fig. 3. O\'ei-\'ie\v of the cutting elTiciency of root canal instru-ments used in a rotary motif)n (ISC) size 2." ). The ma.\imumpenetration depth which was reached at the end o[ each experi-ment served as the measure Ibr cutting efficieney. All instrumentsin each group showed maximum penetration depth.s within thehatched area, a) Titanium Ijascd instruments, b) Stainless steelinstruments. The group of flexible stainle.s.s steel instrumentsreached a significantly greater maximum penetration depth thanall other instruments.

Compared with most Hedstrom files, these instru-ments displayed significantly lower cutting efficiency(9). Hedstrom files made of stainless steel displayedgreater cutting efficiency than fifes made of nickel-titanium or titanium-aluminium alloys.

penetration depth [mm]

5-

4 -

3 -

2 -

1-

K-FlexoreamerK-Flexofile

K-FlexFlex-R fileFlexicut

60— I —120

t [ s ]

180 240

Fig. 4. Cut t ing efFuiene)' of flexil)le stainless steel instruments used

in a rotai~v motion m detail (ISO size 2. ), mean , //= 12).

2.5-1

2.0-

1.5-

1.0-

0.5-

groove depth [mm]

0K-file Hedstrom filereamer

Fig. 5. Maximum groove depth (mean, SD, «=12) of new rootcatial instruments (ISO size 25, VDW, Munich, (jcrmany) used ina linear, filing motion. In this working motion Hedstriim files re-move far more material than reamers or K-files.

Instrumentation of curved canals

Undesirable shapes of instrumented curved canalsmay jeopardize bacteria-tight filling of the canals,lliis is especially the case wlien the smallest cfianieterof the enlarged canal is not located at the apical con-striction but more coronally. Undesirable shaping ef-fects, such as ledging, zip and elbow configurations

204

Endodontic hand instruments

2.5 -

2.0 -

1.5 -

1.0 -

0.5 -

groove depth [mm]

stainless steel Hedstrom files titanium-aluminium

nickel-titanium

/ 7.<;. 6. Maximum groo\e de])ths of nev\' Hedstrcim liles used in alinear, filing motion (ISO size 25). The horizontal line shows themean of each test group («=12), the bar represents the result ofthe statistical analysis (ANOVA, Sehefle, a = 0.05). The diflerencebetween two grou]3S is significant if the bars of the groups do noto\erlap.

and straightening, are re]3orted to occur frequently incun-ed canals (13). It wxjtild be desirable, therefore,to have root canal instruments which would allowadequate centrical enlargement of curved canals, ifpossible without manipulating" the instruments by pre-curving or removing the fiutes on one side of the pre-cuwed instrument (13).

Certainly, Hedstrom files used in a linear workingmotion take off a maximum of root canal dentin in aminimum of time, but using them in a filing motiondown to the working length resulted in a straighteningof the inner canal wall as well as excessive materialremoval (rom the outer side of the cun'ature (If, 14,15). Hence, instrumentation of the apical part ofcviiA-ed canals using Hedstrom files seems not to berecommendable.

Ihe investigations on tlie instrumentation ofcurved canals were pcrlbrmed tinder standardizedconditions, using selfinade specimens of clear poly-ester resin (Alpolit UP 004, Hoechst) with simulatedcolored root canals. The simulated canals, wdth diam-eter and conicity equivalent to ISO size 15, were14 mm long, the straight part being 6.5 mm and thecun-ed ]3art 7.5 mm. The curvature was mathemat-ically defined with a radius of 5.5 mm and an angleof 42° in accordance with the method described bySchneider (16). Changes in canal shape were meas-ured and quantified at X40 tiiagnification at sevenmeasuring points at 1 nim intervals ]:)eginning at theapical end of the canals. The canals were enlargedsize by size from ISO size 15 to ISO size 35. Theinstrumentation was carried out using the same test-

ing de\dce as for the investigations on cutting ef-ficiency in a rotaiy motion on straight canals. A 110°clockwise rotaiy motion was tised.

Different root canal instruments

All instruments caused undesirable instrumentationeffects in the curved canals (17, 18). None of the in-struments tised gave a centrical enlargement in termsof eqtial material removal from the inner and outerside of the cui'vature. In fact, no instrument removedmaterial from the whole length of the inner side ofthe ctiivattire. On the other hand, all instruments re-moved material from the otiter side of the ctm'attire.The resulting canal shape depended on the t\pe ofinstrument used (Fig. 7).• Conventional stainless steel reamers and K-files

caused severe bulging of the outer side of the cur-vature, whereas on the inner side, no material wasremoved from the 3.5 mm of the canal wall closestto the apex. Coronal to this uninstrumented part,the inner cuives of the canals were clearlystraightened.

• Reamers and K-files made of titanium-aluminiumalloy created nearly the same undesirable shapingefTects as the corresponding stainless steel instrti-ments.

• Instrtimentation with nickel-titanium K-files didnot have any undesirable shaping effects, but theydid not remove material fVom the inner side alongthe 3.5 mm closest to the apex.

• Flexible stainless steel instruments with conven-

conventional stainless steelreamer or K-file

titanium-aluminium K-file nickel-titanium K-tile

flexible stainless steelinstrument withconventtonat tip

flexible stainless steelinstrument withnoncutttng tip

Fig. 7. Mean changes in the canal shape (angle eonesponding toSchneider's method (16): 42°. raditis: .')..') mm, ;/=12) a.s a result ofinstrumentation with ditVerent root eanal instruments used in a to-taiy, reaming motion (110° clockwise rotation).

205

Tepel & Schafer

Table 2. The difterent instrumentation techniques tested. Each working cyclegenerated by the computer-driven testing device consisted ot three consecu-tive phases. First, the instrument was inserted into the canal; second, theinstrument was rotated; and third, the instrument was removed trom the ca-nal. The ditterences between the instrumentation techniques mainly concernedthe rotation ot the root canal instrument

Instrumentation technique Rotation

Reaming motion 110° clockwise rotation

Wildey & Senia (21) 90° clockwise rotation tollowed by90° counterclockwise rotation

"Balanced torce" technique(19)

180° clockwise rotation tollowed by180° counterclockwise rotation

"Step-back" technique (22) 110° clockwise rotation. From ISO size 25 on,each instrument was inserted 1 mm shorterinto the canal than the preceding one

"Combined technique" (23) ISO sizes 15 and 20; reaming motionISO sizes 25 to 35: "balanced force" technique

tional ti]:)s created a moderate straightening of theinner side of the cun^atuiw On average, no ma-terial was removed fVom the inner side of the last1.5 mm from the apical end.

• Flexible stainless steel instruments with modifiedtips (KrFlexofile Batt-tip, K-Flexoreamer Batt-tipand Flex-R file) removed material nearly equallywell from the inner and outer side of the curvature,exce]3t at the 1.0 mm of the inner side nearest theapex where no material was removed.Straightening of the canal was very slight.The results showed that fiexible stainless steel in-

struments are a substantial improvement comparedwith conventional stainless steel reamers and K-filesand, somewhat surprisingly, that they perform betterin ciined canals than the fiexible titanium-based in-struments tested. Moreover, the results pointed outthe great importance of the instrtiments' tip designas tlie instruments with noncutting tips were clearlysuperior to those with conventional tips.

I'he inffuence of tip design on canal sliape couldalso be shown l)y comparing the edect of fiexiblestainless ste( I instruments with conventional tips andthe same instruments with noncutting tips (5). l"heinstrtiments witli noncutting tips maintained the orig-inal canal curvature far better than the instrtimentswith conventional tips, apparently owing to a bettercentrical guidance in the curved canal, and thus re-sulting in almost centrical enlargement (19, 20).

Different instrumentation techniques

As ajopears from the above, flexible stainless steel in-struments with nonculting tips tiscd in a rotary mo-tion perform better in cuiA ed canals tfiaii other in-struments. The qticstion then arises of whether the

instrumentation technique itself can infiuence thesha]3e of a curved c anal when these instruments areused. The reaming motion and the techniqties de-scribed by Wildey & Senia (21), the ''balanced Ibrce''technique (19), the "step-back" technique (22) and apreviously described combination of the reaming mo-tion and the "balanced fbrce" technique (23) weretested.

All these techniques had an element of rotaiyworking motion.

Independent of the technique, the root canal in-strument was first introclticed into the canal tintil itcame in contact with the canal wall. I'hen a rotaiymovement, which depended on the specific- tech-nique, followed, and finally the instrument was re-moved iVom the canal (Table 2). The experimentswere done using the simtilated ctirved canals de-scribed above. The canals were enlarged with flexiblestainless steel instruments with noncutting tips size ])ysize from ISO size 15 to ISO size 35 u.sing the com-puter-driven testing device, which generated the par-ticular worlving motion, lliese were our findings (Fig.8) (23):• Curved canals sha]3ed with a simj^le reaming work-

ing motion (110° clockwise rotation) showed onlya .slight straightening. On the inner side of ihc- cur-vature no material was removed fiom the 1 mmof the canal nearest the apex.

• Using the technic|tie introduced by Wildey & Senia(90° clockwise rotafion followed by a 90° c-otinter-clockwise rotation) (21), the canal showed only aslight straightening. No material was removedfrom the inner side of the curvattire at the 3.5 mmol the canal nearest the apex.

reaming motion Wildey & Senia "balanced force" technique

"step-back" technique "combined" technique

Fig. H. Mean clianp,es in the canal shape (anj le corresponding to

Schneifler's method (IG): 42°, radiu.s: .5..") mm, ii=\2) as a resiill of

inslrumeniation wilh Ilexible stainless steel instruments tisinu, diller-

ent in.slriimenlalioii lecliniqucs.

206

Endodontic hand instruments

bending moment [gem]

140 -

120 -

100 -

80 -

60 -

40 -

20 -

0

stainless steel

conventional

ii

i5

flexible

i

titanium-aluminium

i

nickel-titanium

M0) CD

l E ^ >flj O

k—

> o

CD \

til \

CD

— ^ f" ^>~ fll , ^ ^>D • 1 "J "

ZZ O ">

'cD m

O'cD

CDCT

1Oo

roCT-Qi

O

o

CDCT

O

o

[^reamer ^K-file •Hedstrom file/•/ '. .'A B e n d i n g m o m e n t s of the inx'est igated root c ana l i n s i r u m e n i s ( I S O s i /e 2.'). m e a n . S I ) , / ; = 1 0 ! . T h e g r e a t e r t he b e n d i n g m o m e n t of

an i n s l r u m e n i . ihe m o r e il will s i r a igh l en inside a < tu \ ec i c a n a l . T h e m a x i m u m b e n d i n g m o m e n l s w h i c h , a c c o r d i n g to I S O s t a n d a r d !-5(i!-)0-

1 (3) the i n s t r u m e n l s s h o u l d not e x c e e d , a r e i n d i c a t e d h\ the h o r i z o n t a l l ines.

Instrumentation tising the "balanced force" tech-nique (180° clockwise rotation followed by a 180°counterclockwise lotation) (19) restilted in only aslight straightening of the curved canal. No ma-terial was removed from the inner side of the cur-vature at the 2 mm clc:)sest to the apex.Canals enlarged by the "step-back" technique(110° clockwise rotation, each instrument from size25 was withdrawn 1 mm short of the precedinginstrument) (22) showed moderate bulging of theouter side of the curved canal. No material wasremoved from the inner side of the ctir\ ature atthe 2.5 mm of the canal closest tc:) the apex.The most ap]3ropriate canal shape restilted from acombination of a reaming motion (1 10° clockwiserotation) with size 15 and 20 instrtiments followedby the "balanced fbrcc" technique (180° clc)ck\viserotation followed by a 180° counterclockwise ro-tation) with the subsequent instrument sizes tip tosize 35 (23). Tlie canals were not straightened andmaterial was removed from the inner curve of thecanal except in some instances in the f mm closestto the a

In accordance with otir results, several atithorshave reported better instrtimeiUatic:)n of curved canalswith the "balanced force-" techniqtie as compared tothe "step-back" technicitie (24, 25).

Bending properties

In the ISO standard 3630-1 (3), several mechanicalrequirements for root canal instriinients are listed.One of these requirements concerns the resistance tobending. In order to determine the resistance tobending of a root canal instrtiment., the instrument isfixed at its tip over 3 mm and bent. The bendingmoment at a bending angle of 45° is determined.

The experiments on the resistance to bending ofthe instrtiments of this sttidy gave the following restilts(Fig. 9) (26):• In the group of stainless steel instruments, the flex-

ible Stainless steel instrtiments dis]3layed 20 40"()less resistance to bending than com etitional stain-less steel reamers and R-filcs of the same lu'and,which showed the greatest resistance to bending a[all instrumctils tested. Stainless steel reamers and

207

Tepel & Schafer

aangular deflection [°]

1800 -

1440 -

1080 -

720 -

360 -

0

i

c 05

'05 O

stainless steel

conventional flexible

titanium-aluminium

05 O

Q3 5 c 5 S,:^

llilfg^^ li

0 ro Q= 2>ro DQ

O'c

-05

O

o

-05

oo

CD- 0

oo

nickel-titanium

torque [gem]

140 -

120 -

100 -

80

60

40

20

0

iI J

i

I reamer ^K-fi le • Hedstrom file

stainless steel

conventional flexible

i

f^.n M M I I M i l I ta la la Eg ta Kg lia i n i i i i i i i i i i i

III! pi III! pi I ill pi^ r o .^ ^'m .^ ^ - S .«

^ 0 5 0 • « 5 0 5 O ^ 0 5 0

^.y ^.y ^ .2

titanium-aluminium

^ 05 '^ ' - J

ro CQ

o'c3

'0 M

eg

a

o

0 M

eg

a

o

0 M

eg

a

o

nickel-titanium

dreamer ^K-file nHedstrom fileFig. to. a) Angular dcficcticjn and b) torque of the investigated lool (anal in.struments (ISO .size 2.'), mean, SD, n=10). In icsLs accordingto ISO siandard 3630-1 (3) the in.stniment was fixed at its lip and rotated clockwise until it fractured. The minimum requirements givenby ISO standard 3630-1 (3) are indicated by the hori/ontal lines.

208

Endodontic hand instruments

Fig. 11. l inwot ind K-lile (rotation angle: !^()()°:. In

ing nonelaslic delornial ion should be disc arded.

enis show-

K-files displayed a greater resistance to bendingthan stainless steel Hedstrom files. Ihere were noconsistent difTerences between con\-entional stain-less steel reamers and K-files.

• Instruments made of a titanium-altiminium alloydisplayed a lower resistance to bending than bothcorres]5onding conventional and flexible stainlesssteel instruments, but a greater resistance to bend-ing than nickel-titanium instruments.

• Nickel-titanium instruments showed far less resist-ance to bending than all other instrtiments. Forexami)le, thc-se instruments displayed .50 75% lessresistance to bending than corres])oiiding conxen-tional stainless steel instrtiments.Similar results hax'c been reported in prexious

studies (27 .30).The qtiestion then arises of whethei- the low resist-

ance to bending of the niekel-titaniuni instrumentsmakes them superior in the preparation ol ciu'xcd ca-nals. The results of our studies do not necessarilyshow that. Certainly, nickel-titanium K-files causedfew tmdesirable changes in the shaj^e of ctirved ca-nals, I)ut this was mainly because almost no materialwas removed from the canal walls fDeeatise of therapid wear of these instrtiments. Clearly, the flexiblestainless steel instruments which showed a greater re-sistance to liending than the nickel-titanium instru-ments performed better in cun'ed canals. Thus, itmay apj^ear that resistance to bending is not necess-arily a good criterion for the clinical selection of rootcanal instruments.

Torsional properties

Investigations on resistance of root eanal instrumc-ntsto fracture have been performed for more than 50years (31) and are part of the ISO standard 3630-1 (3).

In these tests the root canal instrtiment is fixed at itstip and at its shaft and then rotated in a clockwise direc-tion until it fractures. During the tests., the rotationangle and torque are continuously reeorded in order tocalculate the angular deflection, i.e., the torsional angleat whieh the instrument fraettires, and the toi^que fromthese records. The angular deflection is meant to gi\ eiiifbrmation afoout the risk of torsionaf fraetnre if an in-sfrumcnt binds af its fip in fhc root canal and i.s rotated

further. The torque has a clinical impact with respectto nonelastic deformation since instrtiments with low-torque N'alues tend to unwind at lower forces than thosewith high torque xalties.

Exjjeriinents on the resistance to fraeture of~ the in-struments in this study ga\c^ the following restilts (Fig.10) (26):• In the group of con\ entional stainless steel instrti-

ments, the Hedstrom files on average fracturedafter one to two revolutions. A few Hedstrom filese\ en fractured afier about half a re\ olution. Thea\ erage angular deflection of conventional reamersand K-files was greater than that of Hedstrom files,and they fractured after two to three rexoltitions.Nex'ertheless, in isolated cases, reamers and K-filesfractured after a little more than one single re\x)l-tition. K-files mostly reached greater angular de-flection than reamers of the same brand. MostHedstrom files showed lower \alues than reamersand K-files of the same brand.

• The average angular defiection of fiexible stainlesssteel instrtiments was similar to that of com en-tional reamers and K-liles. The flexible stainlesssteel instruments displayed less torque than thecc:)nventional stainless steel instrtiments.

• Instruments made of titanitim-altiminium alloys(Hedstrom files, reamers and K-files) reachedabout the same angular defiection as correspond-itig stainless steel instruments. Their averagetorque was less than with the corresponding stain-less steel instruments.

• Nickel-titanium instruments (Hedstrom files, K-files) reached abotU the same angular defiection ascorresponding stainless steel instrtiments. Their av-erage torque was less than witfi the correspondingstainless steel in.struments.Tfie fbflovxing c finical implications may be drawn

from these results:• Commonly used instrumentation techniques re-

quire rotation angles between 90° and 180° fbr tlierotai-y movement of an instrument in the root ca-nal (19, 21, 23). Even if the instrument binds at itstip at the beginning of this rotation, twisted instru-ments (reamers and K-files) will not break becausethe rotation angle where the instrument fiacture islikely to occtir is much greater than 180°. Never-theless, they will undergo nc:)nelastic deformation.Twisted stainless steel instruments will show an ol)-vious unwinding which is a warning signal that theinstrument should fx" disearded (Fig. f f). Milledinstruments, stich as Hedstrom files, do not givetfiis warning signal since they are macfiined into around blank. They should therefore not be usedwith rotary working motions (26, 32-34).

• 1 he observed differences in torque valties of thevarious instruments wiU be clinically visible sinceinstrtiments with low torqtic \-alucs will tinwind

209

Tepel & Schafer

more easily than those with high torque values.However, the low torque instruments can be usedwithout any increased risk of instrument fracture ifthey are checked during instrumentation for non-clastic deformation and discarded as soon a.s anunwinding of the flutes has occurred.

• All in.strumcnts tested fulfilled the minimum re-quirements of the ISO standard 3630-1 (3). This isimportant, but in our opinion, the most importantparameters for evaltiating root canal instrumentsfrom a clinical point of view are cutting efficiencyand behavior during the instrumentation of cui'vedcanals. These two parameters .shotild serve as usefulcomplements to existing international standards.

Conclusions

• With regard to cutting efficiency in a rotaiy motion,flexible stainless steel instruments are superior toconventional stainless steel reamers and K-files aswell as nickel-titanium instruments.

• With regard to cutting efficiency in a linear motion,stainless steel Hedstrom files display the best results.

• Concerning the instrumentation of eurved canals,flexible stainless steel instruments with noncuttingtips cause far less transportation of the canal thanother instruments, even instruments made of ti-tanium-based alloys.

• The risk of torsional fraeture of flexible stainle.ss steelreamers and K-files is no higher than that of con-ventional stainless steel reamers and K-files.

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