Three-dimensional Analysis of Cutting Behavior of

Three-dimensional Analysis of Cutting Behavior ofNickel-Titanium Rotary Instruments by MicrocomputedTomographyYa Shen, DDS, PhD, and Markus Haapasalo, DDS, PhD

Abstract

The cutting behavior of nickel-titanium rotary instru-ments with and without irrigation was evaluated in abovine bone model. Six brands of NiTi rotary instru-ments were constrained into a curved trench. The tipsof the instruments were bent to create a 1-mm longinitial contact with the floor of the trench. After a seriesof 100, 200, 300, 400, and 500 (1,500 total) push-pullstrokes on each rotating instrument, the grooves werescanned by microcomputed tomography. The volume ofremoved material and the maximum depth of the cutgroove were measured. Irrigation increased the cuttingefficiency of the instruments significantly, except forLiberator (Li). There was a significant correlation be-tween the extracted volume and the maximum depth.The volume removal rate was highest with K3 and Li(dry) and with K3 and FlexMaster (FM) (irrigationgroup). The maximum cutting depth was highest withFM and K3 in both dry and irrigation groups. Thecutting behavior of NiTi rotary instruments dependsboth on experimental setup, instrument design, andcutting condition. (J Endod 2008;34:606–610)

Key Words

Cutting behavior, instrument, microcomputed tomog-raphy scan, nickel titanium

Since the introduction of the nickel-titanium (NiTi) rotary endodontic instruments in1991, endodontic therapy has coupled 360° rotary instrumentation with the new

technology of the NiTi alloy. Using NiTi rotary instruments for root canal instrumenta-tion has enabled clinicians to more predictably and efficiently create consistently ta-pered preparations while minimizing procedural mishaps, especially in curved canals(1). Although these instruments have been applied with great success, progress ininstrument design and surface treatment continues to be made even after sixteen years.In addition to shaping ability, flexibility, resistance to breakage and cutting efficiency arethe most desirable mechanical properties for endodontic instruments. Cutting dentin isan essential step during root canal treatment. It contributes greatly to the removal ofinfected dentin and provides an adequate funnel-shaped preparation (2). However,because of their pseudoelastic properties, NiTi instruments must be machined ratherthan twisted, which may lead to surface defects within the cutting surfaces and affecttheir cutting efficiency (3).

Currently, there are approximately 30 different brands and designs of NiTi rotaryendodontic instruments on the market. Although the advertising literature is rich inclaims of superiority of various NiTi rotary instrument designs, few of these claims canbe supported by well-designed objective studies in endodontic literature. No standardsexist for either the cutting or machining effectiveness of NiTi rotary instruments, norhave clear requirements been established for resistance to wear. Thus, the goals of thisinvestigation were to (1) examine cutting behavior using a standardized approach and(2) determine the effect of irrigation on the above parameters of NiTi rotary instru-ments. The null hypothesis was that NiTi rotary instruments of different types have asimilar cutting behavior when used dry and with sodium hypochlorite (NaOCl) irriga-tion.

Materials and MethodsDentin-Substitute Specimens

Bovine femur bone was selected as the testing material (4). The microhardness ofthe specimens was 50 HV (Vickers hardness), which compares well with dentin (5).The bones were cleaned with 5.25% NaOCl (90 seconds) and processed into 10 �12� 5 mm slabs with a band saw under cooling water. A 0.8-mm diameter trephinedrill was used to cut a camber trench of 8.0� 0.8 mm (length� width). The dimen-sions of the trench were enough to keep the file from slipping and binding. At the top,the chamber trench was 1.5 mm deep. The vertical height from top to bottom of thechamber trench was 7.5 mm, and the chamber trench had a radius of about 14.5 mm.Five parallel trenches at intervals of 1 mm were prepared on a slab. The bottom of theslab was 15 degrees from the horizontal level (Fig. 1). The slabs were ultrasonicallycleaned in tap water for approximately 120 seconds before the experiment.

Cutting Behavior Test Device

The instruments were mounted on an electric motor with a 1:16 reduction contra-angle handpiece (ATR Tecnika Vision, Dentsply Maillefer, Tulsa, OK). The contra-anglewas attached to the descending crosshead of an Instron Testing Machine (MechanicalTester 8841; Instron Corp., Canton, MA). The slab was glued onto the Instron. Thecrosshead testing was performed at 3 Hz (36 mm/s). The instrument was operated aspush-pull on the trench, at a constant speed of 350 rpm. The instrument was perpen-

From the Department of Oral Biological & Medical Sci-ences, Division of Endodontics, Faculty of Dentistry, Universityof British Columbia, Vancouver, Canada.

Address requests for reprints to Dr Markus Haapasalo,Division of Endodontics, Oral Biological & Medical Sciences,UBC Faculty of Dentistry, 2199 Wesbrook Mall, Vancouver,BC, Canada V6T 1Z3. E-mail address: [email protected]/$0 - see front matter

Copyright © 2008 by the American Association ofEndodontists.doi:10.1016/j.joen.2008.02.025

Basic Research—Technology

606 Shen and Haapasalo JOE — Volume 34, Number 5, May 2008

dicular to the ground, starting from the top of the trench. The tip of theinstrument was bent 1 mm in contact with the floor of the trench from 1mm below the trench top (Fig. 1). The Instron chart recorded the forcegenerated, which included compression and tension during the use ofeach instrument. The compression force was analyzed as a function oftime generated by test instruments during cutting at a constant rate ofdescent.

Six brands of #30, 0.06 NiTi 21-mm long instruments were tested:ProFile (Dentsply Maillefer, Ballaigues, Switzerland), FlexMaster(VDW, Munich, Germany), K3 (SybronEndo, Orange, CA), Hero Shaper(Micro-Mega, Bensancon, France), Liberator (Miltex Inc, York, PA),and Alpha (Brasseler, Lemgo, Germany) (Table 1). This specific sizewas used because it ensured a measurable volume, and it is commonlyused clinically. Every instrument was run on five trenches and under-went 100 strokes (33 seconds) in the first trench. Subsequently, thesame instrument underwent a new series of 200, 300, 400, and 500

strokes (for 66, 100, 133, and 166 seconds) in the second, third,fourth, and fifth trench. Each instrument was tested under dry condi-tions and with continuous NaOCl (fresh preparation) irrigation, respec-tively. Five and a quarter percent NaOCl was used at a flow rate of 20mL/min. A total of 10 selected instruments from each brand were tested:5 without irrigation (dry) (PFD, FMD, K3D, HSD, LiD, and AlD) and 5with NaOCl irrigation (PFH, FMH, K3H, HSH, LiH, and AlH).

Evaluation Procedures

A microcomputed tomography system (�CT-80; Scanco Medical,Bassersdorf, Switzerland) was used to scan the bone specimens after100, 200, 300, 400, and 500 strokes. Four hundred slices with a voxelsize of 30� 30� 30�mwere acquired with each scanning procedureand a three-dimensional (3D) surface model was reconstructed. Theseries of images of each bone specimen after the experiments wasimported into the image processing software package Amira 4.0 (Mer-cury Computer Systems, Chelmsford, MA). The image stacks were seg-mented by grayscale, and the bone specimen surface models were re-constructed after the experiments, and the surface model wastransformed into a 3D model with the polygon file format using Amirasoftware on a workstation. Volumes of interest were selected extendingfrom the top to the bottom of the trenches. Extracted volumes weremeasured at 100, 200, 300, 400, and 500 strokes, respectively. Thevolume removal rate per unit time of instruments at 100, 300, 600,1,000, and 1,500 strokes (or at the time 33.3, 100, 200, 333.3, and 500seconds) was calculated. (The volume removal rate per unit time �added extracted volume/time; time � the number of strokes/fre-quency).

After preparation, the polygon file format surface model was im-ported into software program (Geomagic Qualify, Raindrop Geomagic,Research Triangle Park, NC, USA) for surface editing. The cut grooveswere filled and erased. The surface models were generated before cut-ting. Finally, matched images of the groove before and after cutting wereexamined to evaluate the maximum depth of the cut.

One-way analyses of variance with Scheffe’s post hoc tests wereused to compare means among various groups. Comparisons were thenmade by general linear model (GLM) tests among different types ofinstruments used dry and with NaOCl irrigation on the five grooves.Pearson correlation coefficients were calculated between the maximumdepth and the extracted volume. For all tests, the alpha-type error wasset at 0.05. All statistical analyses were performed with the statisticalpackage SPSS version 11.0 (SPSS for Windows; SPSS Inc, Chicago, IL).

ResultsThe maximum compression forces for the instruments during five

subsequent runs are shown in Table 2. The compression force gener-ated in dry use was higher than under irrigation. The compression forcewas highest with HS and lowest with PF instruments both with andwithout irrigation.

The cutting efficiency of the instruments was higher under irriga-tion than when used dry (Fig. 2). Without irrigation, K3 removed the

TABLE 1. Six Brands of NiTi Instruments (#30, 0.06 taper, 21 mm) Tested

Group Brand Name (manufacturer) Batch Numbers

PF ProFile (Dentsply Maillefer,Ballaigues, Switzerland)

5254370

K3 K3 (SybronEndo, Orange, CA, USA) 03J215JHS HERO Shaper (Micro-Mega, Besancon,

France)051006

FM FlexMaster (VDW, Munich, Germany) 0404310285Li Liberator (Miltex, Inc., New York, PA) 397P0604AI Alpha (Brasseler, Lemgo, Germany) 099487

Figure 1. Test equipment for measuring the cutting efficiency of NiTi rotaryinstruments.


JOE — Volume 34, Number 5, May 2008 3D Analysis of NiTi Rotary Instruments by �CT 607

maximum volume of material per unit time followed by Li, FM, HS, Al,and PF. With NaOCl irrigation, K3 again removed the highest volumefollowed by FM, HS, Li, Al, and PF (Fig. 2). The volume removal rate perunit time was significantly higher when irrigation was used for all in-struments (post hoc test, p 0.05), except for Li instruments, whichshowed no significant differences. Compared with dry instrumentation,the initial volume removal rate for five of the instruments under NaOClirrigation increased between 30% and 172% depending on the instru-ment. For the Li instrument, the increase was only 14%. The volumeremoval rate for all instruments decreased rapidly during continued usefollowing a similar pattern, with the exception of the Al instrument,which showed a lower reduction in volume removal rate under NaOClirrigation during the entire study.

The highest cutting depth was achieved by FM instruments fol-lowed by K3, HS, Li, PF, and Al when used without irrigation (Fig. 3 andTable 3). Under NaOCl irrigation, the order was almost similar: FM, K3,Al, HS, Li, and PF. For all instruments, the cutting depth was higherunder irrigation (post hoc test, p 0.05), although Li instrumentsshowed no statistically significant differences. The increase of depthbetween dry and wet (NaOCl) cutting was from 20% (Li) to 162% (Al)when measured at the first 100 strokes.

Within each instrument, there was a significant difference on thevolume removal rate per unit time between dry and NaOCl conditionsduring the five subsequent runs (GLM test, F� 54.50, p 0.001). Also,there was a significant difference in the maximum cutting depth (GLMtest, F � 346.41, p 0.001). High coefficients of correlation weredetected between the maximum cutting depth and the extracted volume

in FMD, FMH, PFD, PFH, K3D, K3H,HSD,HSH, LiD, LiH, AlD, andAlH (r2�0.96, 0.95, 0.92, 0.83, 0.99, 0.86, 0.99, 0.93, 0.95, 0.96, 0.98, and 0.92,respectively; p 0.001).

DiscussionFew studies in the past have reported on the cutting behavior of

NiTi rotary instruments. Plexiglas (6) and extracted teeth have beenused for the evaluations (7), measuring the weight loss of each sampleafter cutting. However, Plexiglas may not be suitable for an objectivestudy of the cutting efficiency of NiTi instruments (8). Lugassy (9) hasshown that according to the orientation of its structure, bovine bonebetter simulates dentin as an experimental model. The availability ofmultiple samples from one location allows various experimental instru-ments to be used on near identical samples. The use of bovine bone,therefore, eliminates the problem of the variability in hardness fromteeth taken from multiple sources. Some researchers used bovine boneto simulate dentin during instrumentation (5, 10, 11). It is not easy toaccurately measure the amount of dentin removed by some NiTi rotaryinstruments via weight measurements because of their relatively lowcutting efficiency. Three-dimensional surface profilometry and�CT arecommon methods applied to reconstruction of 3D surface contour in-formation (11, 12). The�CT can provide a 3D image over the entire cutarea at a high resolution. During this study and pilot experiments, it wasnoticed that the maximum depth of cut by some instruments was lessthan 100 �m over 100 strokes. Therefore, to obtain sufficient materialremoval for reliable measurements, the first run by each instrumentbefore measurement was 100 strokes (33 seconds).

TABLE 2. The Maximum Compression Force (N) of the Six Brands of NiTi Instruments during Five Subsequent Runs

HS* FM K3 Li* Al* PF*

Dry 4.08! 0.38 3.08! 0.20 2.94 ! 0.05 2.26! 0.11 1.92 ! 0.14 1.30! 0.05NaOCl 3.30! 0.06 2.46! 0.06 2.24 ! 0.07 1.90! 0.09 1.22 ! 0.02 0.64! 0.05

HS, HERO Shaper; FM, FlexMaster; Li, Liberator; AI, Alpha; PF, ProFile

*There was a significant difference between groups in dry or NaOCl conditions (post hoc test, p 0.05).

Figure 2. The volume removal rate per unit time ("V/t, mm3/s) of the sixbrands of NiTi instruments during five subsequent runs.

Figure 3. Different grooves were created by the different instruments after 500strokes with NaOCl irrigation: Li (left) and FM (right).



The canal instrumentation is partly characterized by the self-threading effect. The dentin removal process by rotary instruments is acomplex phenomenon that includes abrasive processes. In these pro-cesses, the dentin layer at the top is formed into a chip by a shearingprocess in the primary shear zone. The chip slides up the rake face,undergoing secondary plastic flow because of the forces of friction. Thedentin is removed as small chips produced by a combination of cutting,plowing, and friction mechanisms. The rake angle may be an importantvariable in the mechanism of chip formation (13). Although no studyhas shown that the rake angle of K3 instruments is positive, it is at leastclose to neutral (14). Therefore, it may not be surprising that the vol-ume removal rate per unit time was highest with K3 instruments. How-ever, it is important to notice that, because of its relatively high stiffness,the force by which HS and other stiff files were pressed against the bonesurface was higher than with files which are more flexible, such as PFand Al (Table 2). Obviously, different forces used in instrumentation area limitation in the present study, and this has to be kept in mind whendrawing final conclusions. However, standardizing of the force to beidentical for all six instrument brands would have resulted in greatvariation in the length of the initial contact area (1 mm), which wouldhave been a major confounding factor in the study. In curved canals,which are the prime target and reason to use NiTi rotary files, the degreeof curvature and the stiffness of the file are major factors determiningthe force the individual files are pressed against the canal wall. There-fore, obtaining contact of same length (1 mm) for all files at the begin-ning of the instrumentation in fact helps simulate the true in vivo situa-tion. However, it is obvious that a fully satisfactory experimental designto test the cutting effectiveness of the NiTi instruments still remains achallenge for future developments.

There is no consensus as to the definition of cutting efficiency onendodontic instruments (6, 11, 15-17). Theoretically, there are severalparameters to evaluate the cutting tool. Besides the material removalrate per unit time, the major parameters in measuring the behavior ofthe cutting tool include the cutting speed, feed rate, the cutting depth,and the cutting environment (9). In addition to the two separated vari-ables “depth of cut” (18) and “extracted volume” (11) that have beenused previously to describe the cutting efficiency of endodontic file, theintegrated 3-dimensional evaluation of cutting condition was intro-duced to more detailed understanding of cutting behavior. Recently,more suitable sophisticated measurement software has become avail-able on prototype basis to allow measurement of basic geometric pa-rameters such as volume as well as additional descriptors such as themaximum cutting depth. However, the precision of the calibration pro-

cedure may be biased by imperfections during by true measurementerrors. Under the conditions of the current study, there was a strongcorrelation between the extracted volume and the maximum depth.Obviously, the relation between extracted volume and the maximumdepth depends on the interaction of a number of factors, such as cross-sectional configuration of shaft, sharpness of flutes, flute design, tipdesign, and forces. The results here indicated that the maximum cuttingdepth was achieved by FM. According to the beam theory, the stiffness ofa structure is determined by the moment of inertia of the cross-section.Generally speaking, a large cross-sectional area carries a large momentof inertia, which makes a structure stiff and difficult to bend (19). Thiswas confirmed by the present findings that, as the area of the inner coreof the cross-section increased (data not shown), the instrument tendedto encounter more compression force.

Cutting fluids provide lubrication between the tool, chip, andworkpiece at low cutting speeds. Lubrication causes the friction coeffi-cient to change between the chip and the tool (13). The resultant forcesare reduced with the decrease in frictional force. In addition, chemicaladditives may act on the root canal dentin to facilitate instrumentation.The use of NaOCl to irrigate root canals is currently the gold standard toachieve tissue dissolution and disinfection (20). Sodium hypochlorite ispresent in root canals before inserting any rotary instrument to providedisinfection as well as lubrication (21). Some studies showed thatNaOCl attacked the organic dentin matrix (22) and reduced root dentinmicrohardness (23). Yguel-Henry et al. (11) evaluated the effects oflubrication on cutting efficiency of Hedstrom and K-files and deter-mined that tap water and 2.5% NaOCl solutions increased the cuttingefficiency compared to dry tests. As expected, the current study showedthat irrigation does in fact increase the cutting efficiency of NiTi rotaryinstruments. However, it has been shown that high concentrations ofNaOCl profoundly affect mechanical dentin properties within the timeframe of endodontic treatment, whereas low concentrations (1%) donot (24).More research is needed to identify different concentratious ofNaOCl and different irrigants that interact with the cutting efficiency ofinstruments.

The changes on the torque and force were different on the differenttypes of instruments when the same irrigation liquid was applied to them(21). The relief or clearance angle provides potential access to thecutting zone for lubrication (13). Li instruments are nonhelical files,which reduce the possibility of debris clogging the flutes. Hence, it wasclearly shown in this study that under the dry conditions, the volumeremoval rate per unit time was higher with Li instruments than with

TABLE 3. The Maximum Cutting Depth (mm) on the Six Brands of NiTi Instruments During the Series of 100, 200, 300, 400, and 500 Strokes

FileStrokes

100 200 300 400 500

*FMD 0.19 ! 0.06 0.23 ! 0.02 0.26! 0.10 0.32 ! 0.05 0.42! 0.10FMH 0.26 ! 0.01 0.35 ! 0.01 0.37! 0.01 0.41 ! 0.01 0.49! 0.02*K3D 0.17 ! 0.01 0.19 ! 0.01 0.19! 0.01 0.21 ! 0.01 0.23! 0.03K3H 0.25! 0.01 0.26 ! 0.01 0.26! 0.01 0.27 ! 0.02 0.31! 0.02*HSD 0.13! 0.01 0.14 ! 0.01 0.16! 0.01 0.17 ! 0.02 0.19 ! 0.02HSH 0.23! 0.01 0.24 ! 0.01 0.25! 0.01 0.25 ! 0.01 0.29 ! 0.01*PFD 0.11 ! 0.01 0.11 ! 0.01 0.11! 0.01 0.12 ! 0.03 0.13 ! 0.03PFH 0.14! 0.01 0.15 ! 0.01 0.15! 0.01 0.15 ! 0.01 0.16 ! 0.01LiD 0.10! 0.01 0.13 ! 0.01 0.13! 0.01 0.15 ! 0.01 0.17 ! 0.01LiH 0.13! 0.01 0.14 ! 0.01 0.15! 0.01 0.19 ! 0.01 0.24! 0.03*AID 0.08 ! 0.01 0.09 ! 0.01 0.09! 0.01 0.10 ! 0.01 0.10! 0.01AIH 0.21 ! 0.01 0.30 ! 0.01 0.31! 0.02 0.32 ! 0.01 0.32! 0.01

FMD, FlexMaster (dry); FMH, FlexMaster (with hypochlorite); K3D, K3 (dry); K3H, K3 (with hypochlorite); HSD, HERO Shaper (dry); HSH, HERO Shaper (with hypochlorite); PFD, ProFile (dry); PFH, ProFile (with

hypochlorite); LiD, Liberator (dry); LiH, Liberator (with hypochlorite); AID, Alpha (dry); AIH, Alpha (with hypochlorite)

*There was a significant difference in the same type of instrument showing the maximum cutting depth between dry and NaOCl groups during the five subsequent runs (post hoc test, p 0.05).


JOE — Volume 34, Number 5, May 2008 3D Analysis of NiTi Rotary Instruments by �CT 609

other instruments, but the rate with Li under dry conditions was notsignificantly different from that with irrigation.

Although not reported here, the instrument surface wear, as acorrelation with the cutting behavior, was evaluated at the end of thetotal 1,500 strokes by scanning electron microscope. All instrumentsshowed wear in the form of pitting defects and blunting of the cuttingedges. The fact is that friction is higher when the instrument has a highcutting efficiency. The instruments with more wear had a higher initialefficiency than the instruments with less wear. This was in accordancewith the comment by Kazemi et al. (25).

The combination of surface wear and low microhardness can de-crease the cutting efficiency of NiTi instruments (26). Recently, somestudies have examined the possibility of improving the cutting efficiencyof NiTi instruments, specifically focusing on surface treatment tech-niques. These include the implantation of boron ions (27), a thermalnitridation process (6), physical vapor deposition of titanium nitride par-ticles (26), and cryogenic treatment (28). All of these studies have yieldedpromising results, although the evaluation methods still have limitations.Similarly, the Al instrument surface with a titanium nitride coating hasbeen claimed to guarantee excellent cutting efficiency and to prevent theinstrument from losing its sharpness. The present study clearly showedthat the reduction in cutting efficiency in the process of the cutting wasless with Al than with other instruments during irrigation. This indicatesthat less instrument wear occurred in the Al instruments.

This study presents a standardized method to evaluate the cuttingeffectiveness of NiTi rotary instruments. In the clinic, however, canalcurvatures, instrument sequences, and multiple other factors affect themechanical stress of rotary instruments. Furthermore, the dynamicanalysis of the cutting efficiency is complicated by the physical complex-ity of machine tool systems and cutting processes and the fact that themeasurements are time dependent because the components move rel-ative to each other during the process. Further work is required topredict the static and dynamic behavior of NiTi rotary instruments underdifferent conditions by finite-element analysis. Within the limitations ofthis study, irrigation greatly improved the cutting effectiveness of NiTirotary instruments. The cutting behavior of NiTi rotary instruments de-pend both on experimental setup, instrument design, and cutting con-dition.

AcknowledgmentsThe authors would like to thank Professor Dorin Ruse of the-

Division of Biomaterials, Faculty of Dentistry, University of BritishColumbia for technical assistance with data analysis.

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