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efects in Nickel-Titanium Instruments after Clinical Use.art 2: Fractographic Analysis of Fractured Surface in aohort Study

a Shen, DDS, PhD,* Gary Shun-pan Cheung, BDS, MDS, MSc, PhD,† Bin Peng, DDS, PhD,‡nd Markus Haapasalo, DDS, PhD*

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bstracthe purpose of this study was to investigate theode of failure of three brands of nickel-titanium

NiTi) instruments (ProTaper [Dentsply Maillefer, Bal-aigues, Switzerland], ProTaper for Hand Use [Dentsply

aillefer], and K3 [SybronEndo, Orange, CA]) that sep-rated during clinical use. A total of 79 fractured in-truments were collected from three endodontic clinicsver 16 months. The fracture surface of each fragmentas examined by scanning electron microscope. Mostf the rotary instruments (78% of K3 and 66% ofroTaper) failed because of fatigue fracture, whereas1% of NiTi hand instruments failed as a result of shearp � 0.01). The fracture mode of shaping files in rotaryroTaper was different between two different clinicsp � 0.05). All surfaces with fatigue fracture (n � 47)evealed the presence of either one or two crack ori-ins. The vast majority (86%) of K3 fatigue failure hadwo crack origins that could be found not only at theutting edge but also at various places along the fluteompared with only 28% of ProTaper showing multiplerack origins (p � 0.01). The latter showed one crackrigin in 81% of the fatigued shaping files but only 37%or finishing files (p � 0.01). The results of this studyndicated that the failure mode of NiTi instruments iselated to preparation technique. (J Endod 2009;35:33–136)

ey Wordslinical, failure, fatigue, nickel-titanium, rotary instru-ent, shear

From the *Division of Endodontics, Department of Oraliological & Medical Sciences, Faculty of Dentistry, Universityf British Columbia, Vancouver, Canada; †Area of Endodontics,aculty of Dentistry, The University of Hong Kong, Hong KongAR, China; and ‡Key Laboratory of Oral Biomedical Engineer-ng of Ministry of Education, School and Hospital of Stoma-ology, Wuhan University, Wuhan, China.

Supported in part by Post-Doctoral Fellowship Awards,aculty of Dentistry, University of British Columbia, Canada.

Address requests to reprints to Dr Markus Haapasalo,ivision of Endodontics, Oral Biological and Medical Sciences,BC Faculty of Dentistry, 2199 Wesbrook Mall, Vancouver,C, Canada V6T 1Z3. E-mail address: markush@interchange.bc.ca.099-2399/$0 - see front matter

Copyright © 2008 American Association of Endodontists.

foi:10.1016/j.joen.2008.10.013

OE — Volume 35, Number 1, January 2009

onsurgical root canal treatment has benefited from the introduction of nickel-titanium (NiTi) rotary instruments (1). However, there is an increasing concern

bout instrument fracture, as evidenced by the amount of reports on this problem2–9). A recent survey of endodontic specialists and general dental practitioners in thenited Kingdom has indicated that the great majority (92.6%) of endodontists used NiTiotary systems. The majority of users (93.6% of endodontists and 79.5% of generalractitioners) had experienced instrument fracture (10). Increasing numbers of NiTiotary instruments of various designs are now marketed; new instruments and materialseem to appear faster than clinicians could learn about the preceding versions. This hasreated an educational challenge for practitioners, universities, and manufacturers,equiring a greater degree of cooperation among these groups than ever before. Clini-ians should use only those instruments and materials shown to be safe and effective inndependent studies.

Although a number of studies have attempted to examine the reasons for fracturef NiTi rotary instruments after clinical or simulated use (2, 6, 11, 12), very few haveerformed a systematic examination of the fracture surface to identify the mode orechanism of fracture. A large-scale clinical study can provide detailed and reliable

nformation about the mechanism of fracture on different NiTi instruments and con-ribute to better understanding of how and why the fracture occurs. Such knowledge ismportant for the prevention of instrument fracture. Therefore, the aim of this study waso investigate the mode of fracture in NiTi instruments discarded from different clinicso identify the fracture mechanism.

Materials and MethodsThree NiTi rotary systems: ProTaper rotary (PT) (Dentsply Maillefer, Ballaigues,

witzerland), ProTaper for Hand Use (PHU) (Dentsply Maillefer), and K3SybronEndo, Orange, CA) have been adopted at three endodontic clinics, each in atomatologic school in China (Beijing, Guangdong and Wuhan). Two clinics (A and B,dentity not revealed) used only the PT instruments, and the other (clinic C) used bothHU and K3 instruments (by different endodontists who consistently used only one of

he three brands). There was a guiding principle in each clinic for their use. Eachnstrument was a limited to maximum number of uses according to the tooth beingreated: 30 root canals in clinic A (PT); 16 canals (for molars) to 30 canals (othereeth) in clinic B (PT), 16 canals (for molars) to 25 canals (other teeth) in clinic Cither by PTU or K3. Instruments would also be discarded after a single use in veryomplex, severely curved, or calcified canals. The collection of clinically used NiTinstruments has been described earlier (13). In short, a total of 79 fractured instru-

ents were collected from these three clinics over a 16-month period from January006 to April 2007. The fractured instruments were ultrasonically cleaned and auto-laved. The length of the fractured segment was examined under a stereomicroscopeMicrodissection; Zeiss, Bernried, Germany) at 30� magnification with a precision of.1 mm. The separated fragment was then mounted so that its long axis was normal to

he microscope stage for fractographic examination under scanning electron micro-cope (SEM) (Stereoscan 260, Cambridge Instruments, Cambridge, UK). The mode of

racture was classified as “fatigue” or “shear” according to Cheung et al. (14).

Defects in N.T. Instruments after Clinical Use. Part 2 133

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As for fatigue failure specimens, the crack initiation site(s) wasdentified by noting the chevron pattern, also called “herringbone

arks,” on the fracture surface and by referring to the high-power viewshen necessary. The fractographic appearance of a fatigued metallicaterial always progresses from the crack origin to a zone of fatigue

triations (ie, microscopic, incremental marks left by a growing fatiguerack) and, finally, a region of dimple rupture (15). The number ofrack origin(s) for each specimen was recorded. Data were analyzed bysing a Fisher exact or Student t test, where appropriate, in softwareSPSS for Windows 11.0; SPSS, Chicago, IL).

ResultsOf the 1,682 NiTi instruments collected, 79 (5%) were fractured,

ither because of fatigue (n � 47; 59%) or shear failure (n � 28; 35%)f the material. Fractographically, almost all of the fractured PHU

nstruments (10/11; 91%) had concentric abrasion marks encir-ling a central region with microscopical dimples, indicative ofhear fracture of the material (Fig. 1A); a less amount (31%, n � 18 outf 59) of the PT engine files showed these marks (Fisher exact test, p �.01) (Table 1). Microscopic fatigue striations were found in some

igure 1. (A) Fracture surface of a separated PHU finishing F1 file showing shearf fatigue striations. (B) Fracture surface of a separated PT F1 file showingagnification view of B showing clusters of fatigue striations (arrows). (D) Frac

rigins (arrows): (E) one at the cutting edge and (F) the other at the flute. A hig

rrow) and at the flute (F, arrow), respectively.

34 Shen et al.

8% (7 out of 9) of K3 instrument, whereas only 9% of hand-operatedHU instruments showed such sign of material fatigue; the differenceas statistically significant (Fisher exact test, p � 0.01). Most of the

otary instruments (PT and K3) failed because of fatigue fractureFisher exact test, p � 0.05). For PT instruments, there was significantifference in fracture mode on shaping files between two clinics, And B; there were more fatigue failure for clinic B than for clinic AFisher exact test, p � 0.05); however, no significant difference wasbserved in the fracture mode of finishing files between clinic A and BFisher exact test, p � 0.05). The broken fragment caused by shearailure (3.0 � 1.1 mm) was significantly shorter than that for fatigueailure (3.6 � 1.2 mm) collectively (t test, p � 0.05).

For every instrument with fatigue failure, the crack origins andreas showing microscopic fatigue striations and dimple rupture coulde identified on all fracture surfaces. Almost all PT instruments showedrack initiation at the cutting edge (Fig. 1B). For the K3 instruments,rack initiation appeared somewhat erratic; origins of fatigue crackingould be found not only at the cutting edge but also at various placeslong the flute (Fig. 1D–F). The number of crack initiation sites wasignificantly different between PT and K3 (Fisher exact test, p � 0.01);

e of the material; note the presence of circular abrasion marks and the absencee failure with two crack origins at the cutting edge (arrows). (C) A higherrface of a separated K3 size 30 .06 taper showing fatigue failure with two crackagnification view of D showing the crack origin situated at the cutting edge (E,

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6% of K3 but only 28% of the PT instrument had two crack originsTable 2). There was also a significant difference in the number of cracknitiation sites between shaping and finishing files on PT instrumentFisher exact test, p � 0.01).

DiscussionImprovements in instrument design, alloy, and instrumentation

echniques have taken place in the last decade. However, the separationf NiTi instruments remains a problem and may occur suddenly andnexpectedly (16, 17). Although many studies have addressed this issueased on the collection of used instruments that came from one dentalractice (2, 8, 9, 12), little was known about instruments used in aariety of practices. Large clinical samples with detailed information areore valuable than small samples. Only one study of 7,159 NiTi rotary

nstruments collected from 14 endodontic clinics in four countries haseen available; they reported an instrument fracture rate of 5% to 1.5%ere classified as “torsional” and 3.5% as “flexural” (fatigue) failures6). This classification was based on the “longitudinal” or, more accu-ately, lateral view of the instrument at low power (2). However, al-hough such examination in the lateral view allows the detection oflastic deformation in the separated instrument, it fails to indicate thectual mechanism involved in the fracture process. Fractographic ex-mination aims to identify features on the fracture surface that wouldndicate the origin and (direction of) propagation of the crack(s) lead-ng to material failure (15). Samples collected from several groups ofxperienced endodontic practitioners would give an indication of theracture mechanism of different NiTi instruments that are subjected toeuse (after proper cleaning and sterilization procedure) in a multitudef clinical cases.

Root canal instruments are subjected to various stresses duringlinical use. Both the instrumentation technique and instrument designan influence the magnitude of stress concentration and likelihood ofnstrument fracture (18 –21). Recent studies have indicated that theomplexity of canal anatomy and its initial diameter are important fac-ors contributing to fracture of NiTi rotary instruments (22), emphasiz-ng the need for coronal pre-enlargement before the use of NiTi instru-

ents at working length (23, 24). It is well known that different practicexists between operators and clinics as to how rotary instruments maye used. In this study, Gates-Glidden drills (instead of a PT-Sx instru-ent) was used in clinic B for coronal flaring after a “glide path” was

ecured with manual K-files (up to size 15). This may explain why theracture mode of S1 files was different from those of clinic A. TheroTaper instruments consist of two shaping and three finishing instru-ents. The design of the shaping and finishing file is different. The

iameter and taper at the apical few millimeters of the finishing file is

ABLE 1. Fracture Modes of NiTi Instrument as Determined by Fractographic E

PT-A†§�

S-A¶ F-A# Subtotal S-B¶

Fatigue failure 9 4 13 (52) 22Shear failure 12 0 12 (48) 4Total 21 4 25 26

� shaping file; F � finishing file; H � hand file; A � clinic A; B � clinic B.

Two specimen was lost during processing in PT-B and K3 group.

Significant difference in fatigue and shear failure between rotary and hand PT instruments (p � 0.0

Significant difference in fatigue and shear failure between PHU instruments and K3 (p � 0.01).

No significant difference in fatigue and shear failure between PT instruments and K3 (p � 0.05).

Significant difference in fatigue and shear failure between clinic A and B of PT instruments (p � 0.0

Significant difference in fatigue and shear failure between clinic A and B of Shaping files (p � 0.05

No significant difference in fatigue and shear failure between clinic A and B of Finishing files (p � 0

reater than that of a shaping file, making the finishing files stronger f

OE — Volume 35, Number 1, January 2009

ith respect to monotonic load, especially in situations in which the firstmm of the instrument was clamped for testing of ultimate strength

25). On the other hand, instruments of a smaller dimension wereenerally more resistant to cyclic fatigue than larger ones (26). Thatay explain the greater amount of shear failure observed in shaping

rotary, 34%; hand, 100%) rather than finishing files (rotary, 20%;and, 75%), which is in agreement with a previous study of the samerands of instrument (27).

Fatigue and shear failure are cited as the main reasons for instru-ent fracture. Fatigue fracture is caused by crack initiation at the sur-

ace and transgranular crack growth, which are likely to occur when thenstrument is rotating within a curvature (14). Shear failure is the resultf an applied shear moment exceeding the elastic limit of the material.his may occur when the tip of the instrument is forced into a narrowanal lumen, generating high torsional stress (21, 24). The results ofhe present study indicate that shear failure predominated in the handnstruments (PTU), whereas a majority of the rotary instruments of theame (PT) or another (K3) brand failed because of fatigue. This mighte explained by the fact that different magnitude and direction oftresses were generated by the two different methods of use (28). The3 file was introduced as a file of the “third generation” with an asym-etrical cross-section, radial land reliefs, and a slightly positive rake

ngle. Of the three asymmetrical radial lands, two are broad and re-essed, whereas the third is a narrow, full land. Accordingly, Schäfer etl. (28) found that K3 instruments were significantly stiffer than all otheriTi rotary instruments of the same size and taper. Mechanical behavior

s related to cross-sectional design, the applied torque, and the momentf inertia of the instrument area. As the area of the inner core of theross-section increased, the instrument was more torque resistant19). Therefore, it was not surprising that K3 instruments failed mainlyecause of fatigue in clinical use.

During the manufacture of NiTi instruments, small machiningcratches and grooves are invariably left on the surface. These surface

ation

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-B# Subtotal S-H F-H Subtotal

4 26 (76) 0 1 1 (9) 7 (78)2 6 (18) 7 3 10 (91) 06 34* 7 4 11 9*

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No. of CrackOrigins

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1 25 3 1 12 6 5 0 6

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Significant difference in the number of fatigue crack origins between PT shaping and finishing files of

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Significant difference in the number of fatigue crack origins between PT and K3 instruments of fatigue

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Defects in N.T. Instruments after Clinical Use. Part 2 135

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cratches can serve as notches that would concentrate the stress (4, 9,7, 27, 29), limiting the instrument’s fatigue life. The crack origins weresually found at the cutting edge region in most instruments. This is toe expected because when a circular beam is bent, its outermost fibersre subjected to the greatest stress and strain. Indeed, the first fatiguerack often is initiated at the surface (when there is no major internallaw or stress-concentration feature elsewhere) (15). There seemed toe some effect of the cross-sectional shape on crack initiation with someifference between brands in the number of crack origins. For the onerand of instrument (K3) with an irregular cross-sectional shape, cracknitiation occurred at a corner along the flute in some specimens (Fig.D). Similar patterns of cracking have also been reported by Cheungnd Darvell in a laboratory fatigue study (30).

The number of crack origin has never previously been examineduantitatively on discarded NiTi instruments after clinical use. A singlerigin usually indicates a failure with low stress, whereas the presencef multiple origins may be the result of high stress concentrations. Theighly concentrated stresses at the cutting edge of ProTaper instrumentight cause the machining defects to “open” and become microcracks

31), especially more two crack origins instruments occurred on fin-shing files. The potential shortcomings of the original ProTaper instru-

ent in the “Finisher” series were high bending moment and stiffness.he ProTaper Universal file with a reduced bending moment and in-reased flexibility was launched recently as an improvement to the orig-nal design. A mathematical simulation found that stress concentrationas situated at the trailing edge adjacent to rather than at the cuttingdge on the ProTaper Universal file (20). In theory, this could help tolleviate the combined effect of having both the greatest stress concen-ration and the site of rubbing contact with the root canal wall at theame location.

Under the conditions of this study, the fracture mode of rotary andand-operated NiTi instruments appeared to be different, with shearailure being more prevalent in the latter. The K3 instruments failed

ainly because of fatigue fracture, most of which showed the presencef more than one crack origin. The preparation technique has a sig-ificant influence on the fracture behavior of ProTaper instruments.

AcknowledgmentsThe authors thank Mr Andre Wong for the technical assistance

ith the SEM.

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