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Journal of the Formosan Medical Association (2016) xx, 1e7

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ORIGINAL ARTICLE

Cyclic fatigue behavior of nickeletitaniumdental rotary files in clinical simulated rootcanals

Chih-Wen Chi a, Chun-Chieh Li b, Chun-Pin Lin a,Chow-Shing Shin b,*

a Graduate Institute of Clinical Dentistry, School of Dentistry and National Taiwan University Hospital,College of Medicine, National Taiwan University, Taipei, Taiwanb Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan

Received 25 May 2016; accepted 3 June 2016

KEYWORDScyclic fatigue life;nickeletitaniumendodonticinstrument;

two-step fatigue test

Conflicts of interest: The authors h* Corresponding author. Department

10617, Taiwan.E-mail address: csshin@ntu.edu.tw

Please cite this article in press as: Chcanals, Journal of the Formosan Med

http://dx.doi.org/10.1016/j.jfma.2010929-6646/Copyright ª 2016, FormosaBY-NC-ND license (http://creativecom

Background/Purpose: Dental rotary instruments can be applied in multiple conditions of ca-nals, but unpredictable fatigue fracture may happen. This study evaluated the fatigue livesof two batches of nickeletitanium (NiTi) dental rotary files operating in clinically simulatedroot canals.Methods: Single-step cyclic fatigue tests were carried out to assess the performance of twobatches of NiTi files (ProTaper and ProFile) in nine combinations of simulated canals (cylinderradii 5 mm, 7.5 mm, and 10 mm, and insertion angles 20�, 40�, and 60�). Two-step cyclic fa-tigue tests were carried out in simulated root canals with the same radius by using thefollowing two sets of insertion angles: (20�, 40�), (20�, 60�), (40�, 20�), and (60�, 20�). Fracturesurfaces were observed by scanning electron microscopy.Results: The single-step cyclic fatigue results showed that cyclic fatigue lives of the filesdecreased with increasing insertion angles or decreasing cylinder radius. The ProFile #25 .04file was more fatigue resistant than the ProTaper F2 file. In two-step cyclic fatigue tests,the total fatigue lives were usually more than 100% when the files operated at a lower strainand then at a higher strain. By scanning electron microscopy, a larger area of fatigue striationcorresponded to a longer fatigue life.Conclusion: Cyclic fatigue life can be influenced by the strains and geometries of files. The fa-tigue life was prolonged when the files operated at a lower strain and then at a higher strain.However, the fatigue life was shortened if the loading sequence was reversed.Copyright ª 2016, Formosan Medical Association. Published by Elsevier Taiwan LLC. This is anopen access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

ave no conflicts of interest relevant to this article.of Mechanical Engineering, National Taiwan University, Number 1, Section 4, Roosevelt Road, Taipei

(C.-S. Shin).

i C-W, et al., Cyclic fatigue behavior of nickeletitanium dental rotary files in clinical simulated rootical Association (2016), http://dx.doi.org/10.1016/j.jfma.2016.06.002

6.06.002n Medical Association. Published by Elsevier Taiwan LLC. This is an open access article under the CCmons.org/licenses/by-nc-nd/4.0/).

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Introduction

Nickeletitanium (NiTi) rotary instruments have popularlybeen used in endodontic treatments for more than twodecades. Compared with conventional stainless steel, NiTialloys offer superior flexibility due to the occurrence ofausteniteemartensite transformation. This property con-tributes to useful applications of the alloys in endodonticswhere root canal treatments require the insertion of small-diameter rotary files into slender and highly curved canals.Benefits of NiTi rotary instruments are that these can shapethe canal more efficiently and prevent canal aberrations,such as ledge, perforation, and transportation in the curvedroot canal. Furthermore, NiTi rotary instruments canmaintain the original canal shape and simplify clinicalprocedures, which facilitate endodontic treatments andincrease their success rates.1e4 Nonetheless, the mainproblem of NiTi rotary instruments was the fracture of in-strument in the root canal.5

During use, NiTi instruments are subjected to alter-nating tensile and compressive stresses for rotating underbending. As a result, a fatigue fracture is a dominant fail-ure mode of rotary endodontic instruments.6,7 A number ofinvestigations on fatigue properties of NiTi alloys andcomponents have been carried out by the endodontic8e20

and material engineering communities.21e24 The formerstudies focused on the failure of actual rotary instrumentsunder a loading condition that closely mimics clinical ap-plications. Most studies have focused on the effects ofrotating speed,8 high-temperature sterilization,9,10 corro-sive environment of sodium hypochlorite irrigant,11 clinicaluses,12e15 and different instrument designs.16 The latterstudies concentrated on understanding the microstructuralmechanisms behind fatigue failures and the factors thataffected fatigue lives. It has generally been concluded thatthe major factor that determines fatigue life is theamplitude of the applied strain. Fatigue tests were origi-nally performed with bulk materials such as bars andplates.25 However, recently wires are being used for thetests21e24 presumably because NiTi components in theforms of wire and tubing are used widely in medical ap-plications. Nevertheless, the connection between end-odontic application and engineering mechanism wasinsufficient.

For the endodontic community, fatigue lives are usuallycorrelated with curvature radius and/or angle of bendinginside a root canal. However, different workers quantifythe degree of bending in different terms. For example,some employ the Schneider’s method to quantify theseverity of bending,8 while others use a radius of curvaturetogether with an angle of curvature.17,18 Young and VanVliet26 tried to bridge this gap by testing NiTi wires, andconcluded that NiTi wire diameter and strain amplitude aretwo of the stronger parameters for cyclic fatigue. There-fore, cyclic fatigue resistance should be evaluated with NiTiinstruments having different geometries and applyingdifferent strains in the simulated root canals.

Root canal systems of teeth are complicated and varied.Each tooth might have more than one root canal, and eachcanal has different geometries and curvatures. In a clinic,repeated applications of the same NiTi rotary instrument indifferent canals are common. Therefore, the same NiTi

Please cite this article in press as: Chi C-W, et al., Cyclic fatigue behavcanals, Journal of the Formosan Medical Association (2016), http://dx

rotary instrument may often work in more than one con-dition. There were several studies on the properties of NiTiinstruments after clinical application, including their sur-face decay, torsional fracture, cyclic fatigue, and defor-mation.27,28 The period of cyclic fatigue is decreased afterprolonged usage.12e14 However, all studies so far focusedon a one-step condition. There has been no study on theeffect of cyclic fatigue life in two different curved canals ina row, although this may be closer to the actual clinicalcondition.

In this study, we compared the resistance of cyclic fa-tigue on two types of files, ProTaper (PT) F2 and ProFile(PF), when operating in simulated root canals. Further-more, we evaluated the cyclic fatigue lives of two batchesof NiTi instruments using two-step cyclic fatigue tests.

Methods

Two batches of NiTi instruments, PF #25 .04 (Maillefer,Dentsply, Ballaigues, Switzerland) and PT F2 (Maillefer,Dentsply), were chosen in this study. Cyclic fatigue test wascarried out by operating the rotary instrument with ahandpiece (TCM Endo; Nouvag, Goldach, Switzerland) in asimulated root canal consisted of a circular steel cylinderand an exterior steel block with matching curvature, asshown in Figure 1A. Nine combinations of cylinder radii(R Z 5 mm, 7.5 mm, and 10 mm) and extents of insertion(A Z 20�, 40�, and 60�) were tested and displayed(Figure 1B). Rotating speed was fixed at 300 rpm. All thetests were performed by one experienced operator, and thespecimens were video recorded with a webcam during thecourse of experiment. This allowed an accurate estimationof the number of cycles performed. Three files were testedin each of the simulated root canals (n Z 3) till failure. Thenumber of cycles to fracture, Nf, was noted.

In this part of experiment, two different canal geome-tries were employed. The instruments were first run forhalf of the number of cycles to failure in the first canalconfiguration (nf1/Nf1 Z 0.5). The same instrument wasthen run in the second canal configuration till failure andthe remaining fatigue lives were recorded (nf2). For each ofthe three cylinder radii, the following combinations ofinsertion angles were employed in the two-step tests:(A60�, A20�), (A40�, A20�), (A20�, A40�), and (A20�, A60�).

Cross sections of fracture surfaces were observed byscanning electron microscopy (ABT-60; Topcon, Tokyo,Japan). The fatigue lives of each file were analyzed sta-tistically by analysis of variance with a 95% confidence in-terval using the SAS 6.12 software for Windows (SASInstitute Inc., Cary, NC, USA).

Results

The single-step cyclic fatigue results are shown in Table 1.In both PT F2 and PF #25 .04 groups, the cyclic fatigue livesdecreased with increasing insertion angles and/ordecreasing cylinder radii. For the same canal configuration,PF #25 .04 was consistently more cyclic-fatigue resistantthan PT F2.

Figures 2 and 3 illustrate the results of cyclic fatiguelives in the PT F2 and PF #25 .04 file groups. The lower

ior of nickeletitanium dental rotary files in clinical simulated root.doi.org/10.1016/j.jfma.2016.06.002

Figure 1 Artificial canal set-up for cyclic fatigue testing of rotary instruments showing the radius of curvature R and angle ofcurvature q. (A) Demonstration of artificial canal. (B) Nine artificial canals formed with three radii and exterior steel blocks withdifferent angular extent.

Table 1 Fatigue lives in single-step cyclic fatigue test ofProTaper F2 and ProFile #25 .04.

Canal geometry Fatigue lives

Cylinderradius

Insertionangle (�)

PT F2 PF 25 .04

60 582 � 109 4833 � 630R10 mm 40 2611 � 249 8617 � 868

20 16,965 � 5335 23,022 � 814860 673 � 155 1860 � 1126

R7.5 mm 40 1453 � 284 5285 � 60920 6753 � 471 17,045 � 4080

R5 mm 60 612 � 29 1498 � 49140 618 � 6 2560 � 19520 1993 � 673 3740 � 950

There was significant difference between the two types of filein the same simulated root canal.

Figure 2 Fatigue lives in two-step cy

Cyclic fatigue life of dental rotary instruments 3

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Please cite this article in press as: Chi C-W, et al., Cyclic fatigue behavcanals, Journal of the Formosan Medical Association (2016), http://dx

portion of each bar represented the first step of a cyclic lifetest, which was operated to 50% of fatigue life in that canalconfiguration. The upper portion of the data bar for a canalconfiguration revealed the remaining fatigue lives in thesecond step of the cyclic fatigue test as a percentage of thesingle-step fatigue life in that canal configuration.

Under the condition of R10 mm, when the file receivedhalf of its fatigue life in the simulated canal with insertionangle 20� and then rotated until fracture in simulated ca-nals with an angle of 60� or 40�, it was found that the totalfatigue life (Snf/Nf) was close to 125% for the PT F2 file andalmost 150% for the PF #25 .04 file. Under the condition ofR7.5 mm, the total fatigue life was nearly 127% for the PTF2 file and w140% for the PF #25 .04 file. In the canal with aradius of 5 mm, the total fatigue life was 140% for the PT F2file and 120% for the PF #25 .04 file.

By contrast, when the file was operated in simulatedcanals with an angle of 60� or 40� and then rotated until

clic fatigue test with ProTaper F2.

ior of nickeletitanium dental rotary files in clinical simulated root.doi.org/10.1016/j.jfma.2016.06.002

Figure 3 Fatigue lives in two-step cyclic fatigue test with ProFile #25 .04.

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fracture with an angle of 20�, the total fatigue lives wereclose to 75%, 56%, and 65% for R10 mm, R7.5 mm, and R5mm, respectively. There were no significant differencesbetween the two batches of files.

Fractographic results of the single-step cyclic fatiguetest are exhibited in Figure 4. Fatigue patterns weredivided into three parts: crack initiation, propagation, andductile fracture (Figure 4A). All the fatigue fracturesshowed a single crack initiation. At higher magnification,fatigue striations and ductile dimples were evident(Figure 4B). There were smaller areas of fatigue striationfor the PT F2 file in the simulated canal with R10 mm, A40�

(Figure 4C). In contrast, the striation area was prominentand more than half of the cross section in the simulatedcanal with R10 mm, A20� (Figure 4D).

The fractographic performance in the two-step cyclicfatigue test was similar to that in the single-step test(Figure 5). There were crack initiation, fatigue striation,and ductile fracture. Regardless of the radius, the areas offatigue striation presented in two-step conditions with(A60�, A20�) and (A40�, A20�) were similar to those in thesingle-step test with A20� (Figure 5A). In the reverse con-dition, the fracture patterns in the two-step conditionswith (A20�, A40�) and (A20�, A60�) were similar to those inthe single-step test with A40� and A60�, respectively(Figure 5B). The above findings applied to both PT F2 and PF#25 .04 files.

Discussion

A lower bending strain is associated with a larger cylinderradius or a smaller angle of insertion. Thus, with the sameradius of curvature, fatigue lives of either design of fileswould be longer when the angle of insertion is smaller. By

Please cite this article in press as: Chi C-W, et al., Cyclic fatigue behavcanals, Journal of the Formosan Medical Association (2016), http://dx

contrast, with the same angle of curvature, fatigue liveswould be shorter when the radius of curvature is smaller.These results corroborated with those observed in previousstudies.14,19 Second, the two designs of NiTi files gavedifferent cyclic fatigue resistance. Under the same exper-imental conditions, cyclic fatigue lives of PF #25 .04 fileswere longer than those of PT F2 ones. The NiTi alloy andsurface treatment for the two file models were the same.The only difference was geometric design, including crosssection and taper. The apical third portion of PF #25 .04 andPT F2 files had various tapers and results due to differentdiameters when the file was inserted into a simulatedcanal. A larger diameter can lead to lower resistance ofcyclic fatigue.20 Therefore, PF #25 .04 was more fatigueresistant than PT F2 when operating in the apical thirdportion of the root canal.

Miner’s rule is commonly employed for accumulativedamage evaluation under multistep loading in fatiguedesign in the engineering community.29 The rule may beexpressed as follows:

n1

Nf1þ n2

Nf2þ :::þ nm

NfmZ1 ð1Þ

where ni is the actual number of cycles spent under loadingi and Nfi is the corresponding fatigue life under thatloading.

In the current tests, a total of two loading steps wereemployed. Prior to the second step of the two-step cyclictest, the files were rotated for half of the fatigue life cor-responding to the canal configuration in the first step. Inother words, n1/Nfi Z 0.5. Regarding cylinder radii or filedesigns, the values of n2/Nf2 are invariably larger than 0.5in the (A20�, A60�) and (A20�, A40�) groups. In other words,the total fatigue lives are longer than that predicted by the

ior of nickeletitanium dental rotary files in clinical simulated root.doi.org/10.1016/j.jfma.2016.06.002

Figure 4 Cross sections in single-step cyclic fatigue test observed with SEM. (A) ProFile #25. 04 in the simulated canal withR10 mm, A40�. (B) Higher magnification of the illustration of Figure 4A; striation and final ductile fracture are shown in the fatiguetest. (C) ProTaper F2 in the simulated canal with R10 mm, A40�. (D) ProTaper F2 in the simulated canal with R10 mm, A20� (the areapointed with the arrow is fatigue striation).

Figure 5 Cross sections of the two-step cyclic fatigue test observed with SEM. (A) ProFile #25 .04 in the two-step fatigue test(R10 mm, A40�, A20�). (B) ProFile #25 .04 in the two-step fatigue test (R10 mm, A20�, A40�) (the area pointed with the arrow isfatigue striation).

Cyclic fatigue life of dental rotary instruments 5

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Miner’s rule. In fact, the total fatigue lives were equal tow150% in the simulated canals with R10 mm (A20�, A60�)and R10 mm (A20�, A40�) for PF #25 .04 files, and in thesimulated canal with R5 mm (A20�, A40�) for PT F2 files. Bycontrast, the total fatigue lives were shortened if the in-strument was run first in a high-strain canal and then in alow-strain canal. The values of n2/Nf2 were < 25% in the

Please cite this article in press as: Chi C-W, et al., Cyclic fatigue behavcanals, Journal of the Formosan Medical Association (2016), http://dx

(A60�, A20�) and (A40�, A20�) groups. Moreover, the valueswere reduced to 6% in the R7.5 mm group.

The above findings could be explained as follows: undera small cyclic strain, a fatigue crack was formed at a laterpart of its life. Spending 50% of fatigue life under the smallcyclic strain caused < 50% of accumulated damage, and nocrack was initiated. Conversely, fatigue crack initiation

ior of nickeletitanium dental rotary files in clinical simulated root.doi.org/10.1016/j.jfma.2016.06.002

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occurred early under a large cyclic strain. The crackcontinued to grow even under a smaller strain. In otherwords, the smaller strain would cause more harm if appliedafter a large cyclic strain than when it was applied to thepristine instrument. Comparing the two batches of files, PTF2 demonstrated prolonged lives of 120% and PF #25 .04maintained even longer lives of 140%. This can explain whythe smaller strain formed with the insertion angle A20� canaffect PT F2 more than PF #25 .04.

Prior studies showed that instrument fatigue life maydegrade after clinical use.12e14 We still do not know theexact cause underlying this phenomenon, and sometimes itis attributed to surface defects formed during repeateduse.28 By employing the two-step testing, the current studythrew more light on the possible mechanisms. On the basisof our results, we suggest that after operating in a severelycurved canal, the file should not be used again.

The scanning electron microscopy study showed atypical phenomenon of cyclic fatigue similar to thatdescribed in a previous study.8 There were crack initiation,fatigue propagation, and final ductile fracture in both sin-gle- and two-step fatigue tests. In the single-step fatiguetest, the area of fatigue striation was larger in the lowerstrained condition (A20�) than in the higher strained con-dition (A40� and A60�). A larger area of fatigue striationcorresponded to a longer fatigue life in this study. The finalductile fracture might happen due to excessive load andstrain on the damaged file. The more curved canal applied ahigher strain and a larger load on the file. Therefore, bothfiles had lower fatigue lives in the curved simulated canals(A40� and A60�).

In the two-step fatigue test, the three fatigue fracturepatterns were similar to those in the single-step test. Theareas of fatigue striation presented in the two-step condi-tions of (A60�, A20�) and (A40�, A20�) were larger thanthose in conditions of (A20�, A40�) and (A20�, A60�),respectively. Compared with the single-step fatigue test,the areas of fatigue striation presented in the two-stepconditions of (A60�, A20�) and (A40�, A20�) were similarto those in the single-step test condition of A20�. In thereverse condition, the fracture patterns in two-step testconditions of (A20�, A40�) and (A20�, A60�) were similar tothose in the single-step test conditions of A40� and A60�,respectively. These findings indicate that the developmentof fatigue striation and final ductile fracture was highlycorrelated with the second step of the two-step fatiguetest. However, the first step of fatigue test could cause theinitiation of crack. Actually, crack initiation is the crucialstep influencing the fatigue life. In the less strained con-dition A20�, the damage accumulated slowly and thusprolonged the time for the occurrence of a crack. There-fore, fatigue lives were longer in two-step fatigue testconditions of (A20�, A40�) and (A20�, A60�) than in thesingle-step fatigue test conditions of A40� and A60�,respectively.

The results of the present study demonstrated that thecyclic fatigue life could be influenced by the strain,including the radius and insertion angle of the canal and thediameter of the file. Furthermore, the total fatigue life wasprolonged when the file operated at a lower strain and thenat a higher strain. By contrast, the fatigue life was short-ened if the loading sequence was reversed.

Please cite this article in press as: Chi C-W, et al., Cyclic fatigue behavcanals, Journal of the Formosan Medical Association (2016), http://dx

Acknowledgments

The authors would like to thank The National ScienceCouncil of Taiwan for financial support through the projectNSC96-2628-E-002-223-MY3.

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