Endodontics

Instrument deterioration with usage: Nickel-titanium versusstainless steel

Mario L, Zuolo*/Richard E. Walton**

Abstract Nickel-titanium inslrumenls purpoi'tedly resist deformation and loss of sharpnessbelter ¡han do stainless steel insiruments but may be more susceptible to breakage.The processes of wear and breakage of nickel-titanium and stainless steelinstruments were examined. Sixty files of five t}'pes ( ¡2 each) and threemanufacturers were used. All were used repeatedly In curved canals until failure orfor a maximum of 22 minutes. Each instrument was examined with scanningelectron ¡nicroscopy both new (control) and at spaced intervals for evidence of wearand fatigue. All new instnonents were of good quality. Stainiess steei instrumentstended to wear the most rapidly, aitd next were nickel-titanium rotary instruments;the ¡nost resistant to wear were nickel-titanium hand instruments. The¡-e were fewinstrument .separations. In general, nickel-tltanlum (particularly hand) instrumentsresisted deterioration belter than did stainless steel. Nickel-tiianium rotary-instruments ( 2 of 12) had the most btvakagc. (Quintessence Int 1997;2S:397-402.)

Clinical relevance

Nickel-titanium instruments, both hand and rotary,exhibit flexibility and resistance to wear. Of concern isa possible tendency toward breakage. Nickei-titaniumhand files did resist wear more than did nickel-titanium rotary files, which were soinewhat better thanwere stainless steel hand fiies. There was breakage,primarily with the rotary files.

Introduction

A relatively new metal alloy, nickel-titanium (Ni-Ti)has been used to produce endodonlic instruments. Themanufacturers claim that Ni-Ti files are very clasticand fiexible and resist fracture. Nickel-titanium alsosupposedly absorbs stress and resists wear better than

" Professor of Endodontics, Ensino Odontológico tnstilute, Sao Paulo,SR Brazil

" Professor, DeparlmertofEndodontics.CollegeofDenlislri'. Univer-

sity of [owa. [ova City, leva.Reprint requests: Dr Riehard E. Walton, Professor, Depanmenl ofEndodohtics, University of Iowa, College of Dentistry, Iowa City. IowaS2342. Fas: 319-3.15-7155.

does Stainless steel. As yet, definitive scientificevidence of such advantages, as well as of clinicalsuperiority, is lacking because not ali aspects ofnickel-titanium files have been evaluated. Studies andreports are available on Ni-Ti.'"'* Usage tests andbenchtop evaluation have indicated that Ni-Ti isindeed more flexible*"- and behaves similady to-'' ' '^or somewhat better' ' than stainless steel in minimiz-ing transportation during instrumentation. One study^found Ni-Ti files to be as aggressive as or moreaggressive than stainless steel in cutting and moreresistant to wear than stainiess steel.

Disadvantages of Ni-Ti instruments include theirmuch greater cost as well as evidence of a greatertendency to fracture,^ at least in the hand instruments.

Nick el-titan i um files are manufactured in bolhengine-driven rotary and hand instrument styles. Theremay be differences in the deterioration patterns (asreflected by wear and breakage ) of tiie two types. Handinstruments would permit tactile sensation and detec-tion if the instrument is weakening or losing sharpness.In contrast, because the rotary file is engine-driven, thefile may wear and/or break without early warning.There have been no reports comparing these factors inhand and rotary nickel-titanium files.

Quintessence International Volume 28, Number 6/1997 397

Zuolo/Walton

There also is uncertainty about differences in thewear characteristics of nickel-titanium (both hand androtary) and stainless steel files. The present study wasdesigned to measure and compare deterioration ofthese types during clinical usage conditions.'" "Specificalty determined were the quality of ttie unusedinstruments (internal controts}, the wear character-istics under progressive clinical usage, and the in-cidence and time of breakage.

Method and materials

In.struments

Alt fites were obtained directly from three manufac-turers. Sixty 21-mm fites were selected from differentboxes, then divided in five groups of 12. There weretwo files of each size in each group.

Stainless steel hand

Group A: K-nex file (Kerr/Sybron): Sizes No. 15through No. 40

Group B: ProFile Series 29 reamer (Tulsa Dental):Sizes No. 2 through No. 7

Nickel-titanium hand

Group C: Ultra-Flex files (Texceed): Sizes No. 15tlirough No. 40

Group D: ProFile Series 29 reamer tTulsa Dental):Sizes No, 2 through No. 7

Nickel-titanium rotary

Group E: ProFile .04 Tapers Series 29 rotar>' instru-ments (Tuisa Dental): Sizes No. 2 throughNo, 7

Before initial scanning electron microscopic (SFM)examination and use. the files were placed in boilingwater for 5 minutes. This was followed by 5 minutes inan ultrasonic bath containing a cleaning solution. Thisremoved plastic or other debris, which frequentlyremains on new files as pan of the manufacturingprocess.'-•'•'

S peel in en s

Mesial canats were setected from extracted mandibularfirst and second motars with a moderate curvature ( 15to 39 degrees). These were stored in distilted wateruntit use.

¡nstnnnent usage

One operator famitiar with the use of each instrumenttype performed att preparations. Following creation of

standard access preparation, canal length was estab-lished at t mm from the apical foramen by visuatinspection with a No. tO K-typefilc. Atl initial apicalpreparations were made at working tength usingcircumferential reaming motions untit the fiie fittoosety.

For groups A through D. preparations were finishedby hand, with a stepback technique.''' Apical prepar-ations were with No. 15 through No. 25 used atworking tength with a push-putt peripheral filingmotion. The coronat third was then flared withGatcs-Gtidden bur Nos. 2 and 3. Preparations werecompteted with stepback of 0.5-nim increments throughNo. 60.

For group E (ProFite Rotary), instruments wereused according to the manufacturer's instructions.'^Instruments were inserted into an MM tOE 6:1reduction contra-angle used in an MM 324 low-speed,high-torque motor (Micromega. Medidenta). Therecommended 350 rpm was used throughout instru-mentation. A No. 4 ProFile tapered instrument wasrotated two thirds of the way down the canal with lightapical pressure. A No. 5 file was then used toapproximately the same depth. Next, a No. 3 file wasrotated to a three-quarter depth of the canal. Then, theapical area was sequentially enlarged with successiveProFile .04 tapers from No. 2 through No. 7. File Nos.5, 6. and 7 were used with stepback of 0.5-mniincrements.

Each file in each group was used for 1 minute ineach canal in each root. This resulted in 2 minutes ofuse per file per root, up to a maximum of 22 minutes,or until the instrument showed wear or breakage.

For all groups, irrigation was copious and frequent,with 2 mL of 2.6% sodium hypochlorite that wasdelivered from a 27-gauge needle after use of each burand file size.

Scanning electron microscopic examination

Files could be studied repeatedly in the scanningelectron microscope without affecting their physicalproperties. " ' ' Thus, the same file was examined new(an internal control ), and then after each usage period.Each file was observed at intervals from time 0 (new.unused controls) to a maximum of 22 minutes, or untilthe instrument became visibly altered (appeared unu-sabte) or separated.

For each viewing, the flies were prepared as follows:They were cleaned in an ultrasonic bath tor 5 minutes toremove debris: mounted on copper tape on stubs (nosputter coating was necessary); and then viewed on anAmray 1820 SEM. Ttie fottowing areas were examined

398 Quintessence International Volume 28, Number 6/1997

Zuolo/Walton

Fig 1 Group A. New stainless steei reamer No. 7 (control],Fiutes, tips, and taper are uniform, matching manufac-turer's adverfised designs.

Fig 2 Groups C and D New Ni-T hand instruments No 3fiie (top) and No. 5 reamer (bottom) (controls). The shapeand cutting edges are as expected.

at different magnifications: the apical one half, the tip,and 10 mm from the tip.

Evahiaiion

Photomicrographs were made to study instrumentdefects created during canal preparation. All photo-micrographs were coded, mixed, and blindly analyzedby two trained evaluators for the following:

1, New instruments. Uniformity ofthe shaft, fiute.and tip. Also examined were the condition and shapeof cutting edges and the presence or absence of defects.

2. Used instruments. The same files were examinedat various usage periods for defects such as nicks,gouges, and untwisting or for fracture with loss ofinstrument segments. The cutting edges of flutes wereexamined for rounding or disruption.

When three or more specimens of used instrumentsshowed signs of excessive wear, or file breakageoccurred, the time of usage was recorded. This wasconsidered the maximum useful time. Observationsand numbers and comparisons between instrumenttypes were reported as descriptive.

Fig 3 Group E, New Ni-Ti rofary No, 5 (control). TTieinstrument shows good quaiity, with a ianded fiute andbuiiet-nosed tip.

was observed with each file size. The instrumentsstudied were generally consistent with the manufac-turer's advenising diagrams (Figs 1 to 3).

Results

New instriimenls

For aU the riew instruments of each group, the tips,fiutes, and shafts were generally very uniform with anabsence of tears, gouges, or other notabie defects. Aregular pattern of grinding and/or twisting ofthe flutes

Used iiislrumenls

Stainless steel

Both the K-Flex files and the ProFile reamers showeda similar pattern of usage at the first two observationtimes ( 1 and 2 minutes). There was little evidence ofwear; used files were similar to controls. After 4minutes, initial rounding of edges was visible in the

Quintessence International Volume 2a, Number 6/1997 399

Zuolo/Walton

Fig 4 Groups A and B Stainless steel hand instruments ¡6minutes' usei The No 4 ProFile reamer (topi showsrounding and loss of sharpness of cutting edges. The No25 K-Flex (bollom) is partially untwisted Both are con-sidered unusable

Fig 5 Group C Nickel-titanium hand instruments (22minutes' use). The No 15 Oopi and No. 25 (boitomjUltra-Flex show slight blunting of flute edges, but littleevidence of other signs of wear Both are consideredusable.

Fig 6 Group E Nfckel-titanium rotary instruments (1minutes' use) The Nos. 6, 4, and 2 ProFile (top lo bottom)show no evidence oi wear.

Fig 7 Group E. Nickei-titanium rotary instruments {12minutes' use). TTie No. 3 instrument (top) shows untwistingand breakage. The No 2 instrument (bottom) separatedcleanly with no other visible signs of deterioration. MostNi-Ti rotary instruments were considered unusable at 12minutes or less.

cutting portions, especially in Nos, 15. 20, and 25K-Flex and Nos. 2, 3, and 4 ProFiles. After 6 minutesof use, most showed twisting and irregularities oftheshaft and loss of cutting edges (Fig 4). Alteration oftips was also noted; lai^er K-Flex files (Nos. 30, 35,and 40) and ProFiles (Nos. 5, 6, and 7) demonstratedless wear than did the smaller instruments. A No. 5ProFile separated al 6 minutes.

After 6 minutes, most instruments in these twogroups of stainless steel Hies were deemed unusable.

Nickel-titanium

ProFiie Series 29 reamers and Uhra-Fkx files. Thesehand Ni-Ti files showed similar patterns. All instru-ments (No, 15 through No. 40 Ultra-Rex, No, 2through No. 7 ProFiies) showed very little wearthrough 16 minutes.

After 18 and 20 minutes, some ofthe smallest filesshowed some alterations that appeared in the flutes assmall gouges and irregularities on the cutting edges.

400 Quintessence International Volume 28, Number 6/t997

Zu o I o/Walton

No visible distortion and twisting of the shaft or fluteswas detected. In larger sizes, no wear was evident.

After 22 minittes. the signs of wear were even moreevident in smaller files. A No. 2 ProFile broke atthis time. The larger files showed considerably lessevidence of wear, with no major defects and noseparations.

The experiment was terminated at 22 minutes,although most of these hand Ni-Ti instruments werestill considered usable ( Rg 5 ).

ProFiles .04 Taper Series 29 rotary- instruments.After the first four usage times ( I, 2,4. and 6 minutes),all were like new (similar to controls), as seen in Fig 6.

Atter 8 and 10 minutes of use. initial signs of usagebecame apparent in smaller files (Nos. 2, 3, 4, and 5);there was minor rounding of the cutting edges anduntwisting of fiutes near the tip. File Nos. 6 and 7showed little wear.

After 12 minutes, the rotary instrument Nos. 2. 3.4,and 5 showed marked deterioration and loss of thesharp edges at the cutting portions. Severe distortions(untwisting of the fiutes) were observed. Two files, aNo. 2 and a No. 3, separated during the apical canalpreparation (Fig 7). The No. 6 and No. 7 instrumentsshowed some, but less, evidetice of wear.

Preparation with the Ni-Ti rotary instruments wastherefore discontinued at 12 minutes: most files wereconsidered to be unusable.

Discussion

Of the instruments examined, quality control was verygood. Most instruments appeared as they were suppos-ed tO: this has been shown in other studies.'"'^ Adesign innovation on the rotary instruments, a radiallanded flute and bullet-shaped tip, was observed in allsizes.

According to these findings, the wear of instru-ments could generally be stratified according to size,usage technique, metal type, and times of usage.Smaller files tended to deteriorate and to break earlier,as was shown in another investigation on Canal Masterfiles." Stainless steel wore more rapidly than didNi-Ti. Within the Ni-Ti groups, the hand instrumentsretained quality better than did the rotarj' instruments.As would be expected, longer usage times resulted inincreasing deterioration but with marked differences inrate, depending on other characteristics. So. the leasttendency to wear was found in the large Ni-Ti handinstruments, even through 22 minutes.

Wliy the Ni-Ti hand files were more resistant thanstainless steel to deterioration of cutting edges fromuse is not precisely known. The tnetal has apparentwear resistance and liexibility"'': both properties couldreduce pressure and force on the flute edges andinstrument body.

Although composed of Ni-Ti. the rotary ProFileinstruments occupied an intermediary position in theresults. Their more rapid wear could have been causedby the fact that ihese files were used with a constantspeed in a high-torque handpiece. The torque couid beresponsible for constant instrument deformation, ex-pressed as an untwisting of the fluted portions of thefiles. With a hand instrument, binding or other un-desirable actions are delectable tactilely and corrected;this would not be as true with the less sensitive,high-torque rotary device.

The incidence of breakage was fairly low overall andwas limited primarily to the Ni-Ti rolary files. Thesewere in the smaller files and occurred when theinstruments were reaching the end of their usefulnessin cutting. This study did not prove that Ni-Ti handfiles would not separate more readily if improperlyused, ie, if the file were bound and locked in the canaland twisted." Kazemi el al found Ni-Ti to break morereadily than stainless steel under this type of testing.We were careful not to bind instruments at any limeduring canal preparation. This further reinforces thatfiles should be discarded after a period of heav>'usage. ' '

This study did not measure whether tiles were losingtheir cutting efficacy with increased usage periods.Thus, there is a need for studies to better clarify thebehavior of Ni-Ti files under other conditions.

References

1. tspüsiLO P, Cunningham C. Comparison of canal preparation wiihnickel titanium and stainless steel instrumEnts. J Endod I995i: i l7_'- i76.

2. Luiten D. Morgan L. Baumgartner C, Marshall JG. Comparison offour instrumentation techniques on apical canal transportation. JEndod I995í2i:26-."ÍO.

3. Himel V Amed K, Wood D. Alhadainy H. Evaluution of nitinoiandstainless steel files jscd by dental students during a laboratoryproficiency exam. Oral Surg Oral Med Oral Pathol 1995:79:233-237.

4. Glossüii C, Halkr R, Dtiven SB. del Rio C. A comparison of rootcanal preparations using NiTi hand. NiTi-engine driven, and K-Flesendodontic instruments. J Endod 1995:21:146-151.

5. Slunc R, Zuoio M, Walton R. Apical transportation: Stainless steelvs NiTi hand vs NiTi rotary labstract RS56I. J Endod 1995:21:216.

Quintessence International Volume Number 6/1997 401

Zuolo/Walton

5. Canalda-Sahli C. Brau-Aquade E, Berastegiii-Jumeno E. A etrni-parison of blending and torsioiial properties of K-files manu fact ui'edwith different nieiüUic alloys. Int Endod J 1996:29:185-189.

7. Siiiiiyii J. Nieliolls J. Stdner J. Comparison of stainless sleel andnickel-titanium inslrumeiits in molar root canal preparation. JEndod Í996;22:I77-IS1.

8. Serene T, Adams J, Saxena A. Niekel Titanium Instruments:Applications in Endodontics. St Louis: Isiiiyaku EuroAmerica,1994:52,59.

9. Kazemi R, Stenman E. Spaogberg L. Maciiiniog ifficiency and wearresistance of nickel-titanium erdodonlic fiies. Orai Surg Orai MedOralPalhol l99fi:fil:596-i)03

10. Scott G. Walton R. Ultrasonic endodontics. Wear of inbtrumeniswith usage J Endod 1986:12:279-283.

11. Ztiolo M, Walton R, Murgel C. Canal Master files: Scanningelectron microscopic evaluation of new instruments and their wearwith clinical usage. J Endod l992;iS:3.^S-339.

Í2. Murgei CAE, Wallon RE. Ritlman B, Peeora JD. A comparison oftechniques for cleaning endodontic files after usage: A quantitativescanning eieclron miLTostopic study. J Endod 1990:15:214-217.

15. Zmener O. Spielberg C. Cleaning of endodontlc instruments beforeuse. Endod Denl Traumatol Í995.I i:10-l4.

Í4. WaiLun R. Rivera E. Cleaning and shaping. In: Walton R.Torabinejad M (eds) Principles and Practice of Endodontics. ed 2.Phiiadelphia: Saunders. I996:chap 13

:lnslructionsforUse.15. Profile 04TaperSeries29RolarylnstruniTdlsa. OK: Tulsa Denial Products. 1194.

16. D'Souza J. Walton R, Maixner D. Cross-endodomic files compared witli inanufa1995:21:599-602.

17. Rowan M, Nicholis J, Steiner J. Torsional properties of stainlesssteel and nickd-titanium flies. J Endod 1996;22:.541-345. D

ctional configuration ofurer^s design. J Endod

Minimally InvasiveRestorations With BondingEdited by Michel Degrange, DCD, DSD, DEOcind lean-Erançois Roulet, Prof Dr Med Dent

Tins BOOK REVIEWS the state of theart iti adhesive materials and tech-niques for restorative dentistry, empha-sizing their role in minimizing the lossof sound tooth structure and improvingesthetics.

Direct and indirect composites andbonded ceramic restorations are coveredin depth, including recent developmentsin ceramic bonding and high-strengthceramics based on alumina. Chapterson resin-bonded fixed partial denturesgive a complete and up-to-date surveyof the subject. Indications, design,effectiveness, reliability, and longevityare discussed, along with avoiding faii-ures and criteria for choosing betv jeenthe resin-bonded bridge and the single-tooth implant.

280 pp; 357 illus 1263 color):ISBN 0-86715-327-X; US $98

To Order

Call Toll Free 1-800-621-0387or Fax 1-630-682-3288

Quintessence Publishing Co, Inc book/402