bardsley 2011 journal of tics

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The Effect of Three Rotational Speed Settings on Torque andApical Force with Vortex Rotary Instruments In Vitro

Sean Bardsley, DDS,* Christine I. Peters, DMD,†

and Ove A. Peters, DMD, MS, PhD †

Abstract

Introduction: Both the number of rotations in curvedcanals and torque are related to fracture resistance of nickel-titanium rotaries via the respective mechanismsof brittle and flexural failure. Increased rotational speed(rotations per minute [RPM]) may lead to higher cuttingability and could overcompensate for increased fatigue.The impact of three RPM settings on peak torque (Nmm)and apically directed force (N) during root canal prepa-ration were investigated in vitro. Methods: S-shapedcanals in plastic blocks (n = 12/group) were instru-

mented with Vortex rotaries (Dentsply Tulsa Dental,Tulsa, OK) sizes #15 to 30 with a .04 taper. Rotarieswere used in a manufacturer-recommended sequence:#30, 25, and 20 in a crown-down approach progres-sively deeper into the canal, #15 to the working length,and apical enlargement with sizes 20 and 25 to WL. Atotal of 216 preparation procedures were performedusing a custom testing platform. RPM was set at 200,400, or 600; automated axial feed mirrored clinicalhandling, resulting in two in-and-out movements, eachto preset insertion depths. Torque and apical forcewere continuously recorded and peak values statisticallycontrasted using analysis of variances. Results: No file

fractures were observed in any of the three experimentalgroups. Peak torques and forces varied by instrumentsize and were highest at 200 RPM for all sizes; torqueand force were reduced by 32% and 48%, respectively,at 400 RPM (P < .001). Increasing RPM to 600 did notresult in further reductions. The number of discerniblepeaks for torque (threshold: 0.3 Nmm) and force(threshold: 0.2 N) significantly decreased from 200RPM to 400 RPM and did not decrease further with600 RPM. Conclusions: Under the present experi-mental conditions, rotational speed had a significantimpact on preparation with Vortex rotaries, with instru-ments at 400 RPM generating less torque and forcecompared with 200 RPM. (J Endod 2011;37:860–864)

Key WordsForce, nickel-titanium, rotations per minute, torque, vortex

The use of engine-driven nickel-titanium (NiTi) root canal instruments has many advantages over hand instrumentation, including less canal transportation, less

blockage, and more dentin-conserving canal shapes (1). In order to minimize the inci-dence of instrument fractures, two parameters are of importance: apically directedforce andtorque (forcerequired to rotate theinstrument duringcontactwith root canal walls) (2, 3).

Instrument separation may occur as torsional (ductile) and fatigue (brittle) frac-

ture (4) or caused by a combination of the two effects (5). Although a multitude of rotary techniques are advertised on the dental market, one of the best documentedinstruments is ProFile (Dentsply Tulsa Dental, Tulsa, OK), which was first introducedin 1994 as ProFile Series 29 rotary instruments (6, 7).

Recently, the design of ProFile instrument was updated, and the system is now availableunder thename of ProFile Vortex rotaries (DentsplyTulsaDental).Vortex filesare manufactured from modified NiTi raw material, also known as M-wire. M-wire wasintroduced in 2008 and is produced by applying a series of heat treatments to wireblanks. Preliminary evidence suggested that using M-wire increased the fatigue lifespanof rotary instruments while maintaining the same torsional properties as traditionally ground instruments (8). However, the increased fatigue life of instruments manufac-tured from M-wire is equivocal (9, 10). Vortex rotaries have a triangular cross-section without radial lands and a specific helical angle. The manufacturer suggests

that this geometry promotes a more efficient cutting behavior and less ‘‘threading-in’’effect.

Another factor that influences cutting efficacy is speed (rotations per minute[RPM]). Recommendations given in textbooks for rotary speed vary from instrument to instrument (11); the manufacturer suggests that Vortex files be operated at up to500 RPM (12). Various reasons exist to keep speed with NiTi rotaries low (eg, below 300 RPM) including longer time to fatigue failure (5) and less incidence of taper lock (13). However,a recentarticle advocateshigher RPM with Vortexinstruments based onthe assumption that higher cutting efficiency would more than compensate for thesedisadvantages (14). The authors concluded that the heat-treated NiTi alloy, M-wire, with its extended fatigue resistance, used to manufacture Vortex rotaries would beparticularly suited for higher RPM.

Currently, no data are available on physical parameters (eg, torque and apically

directed force) for Vortex rotaries in simulated clinical use. Moreover, it is as yet unclear what the changes in such physical parameters are when RPM is increased.Therefore, the aim of the present in vitro study was to investigate the impact of threeRPM settings on peak torque and apically directed force during root canal preparation.

Materials and MethodsNew sets of Vortex .04 taper rotary instruments (Dentsply Tulsa Dental) sizes #15,

20, 25, and 30 were used in this study. Each set was used to instrument two simulatedcanals. A total of 36 plastic blocks with S-shaped canals (A0177S; Dentsply Maillefer,Ballaigues, Switzerland) were mounted on scanning electron microscopy (SEM)carriers and distributed into three groups in which canals were shaped with 200,400, and 600 RPM, respectively.

From *Private Practice, San Mateo; and the †Department of Endodontics, Arthur A. Dugoni School of Dentistry, University of the Pacific, San Francisco, California.

Address requests for reprints to Dr Ove Peters, University of the Pacific, Arthur A. Dugoni School of Dentistry, 2155 WebsterStreet, San Francisco, CA 94115. E-mail address: [email protected]/$ - see front matter

Copyright ª 2011 American Association of Endodontists.doi:10.1016/j.joen.2011.01.022

Basic Research — Technology

860 Bardsley et al. JOE — Volume 37, Number 6, June 2011

mailto:[email protected]


http://dx.doi.org/10.1016/j.joen.2011.01.022

http://dx.doi.org/10.1016/j.joen.2011.01.022





Following manufacturer recommendations, simulated root canals were negotiated to their terminus with stainless steel K-filesand the balanced force technique (15). Between instruments, canals were lubricated with liquid soap (16). This was intended to mimicclinical conditions in which irrigants are present as closely aspossible. In particular, the generation of frictional heat was avoided.The working length (WL) was established at 16.5 mm using a size #15K-file. Patency was confirmed with a size #10 K-file to 1 mm past the

canal terminus.Canals were then prepared using a custom torque bench that hasbeen described in detail previously (5, 17), with automatic axial feedsimulating clinical instrument use. To determine correct settings forinsertion depth, pilot trials in a separate batch (n = 6) of the plasticblocks were performed to establish time and speed needed toadvance Vortex rotaries in a crown-down approach into an S-curvedcanal in 2 in-and-out insertions (Fig. 1 A). The first instrument, size#30, was to reach the area just before the curve, and each subsequent rotary would reach progressively deeper into the canal until size #15arrived at the WL. In order to achieve this goal, programming was set so that sizes #30, 25, and 20 reached 11 mm, 13 mm, and 15 mm,respectively. Finally, Vortex sizes #20 and 25 were used for a secondtime in each canal to the WL and working in two in-and-out insertions.

This sequence is in line with the manufacturer’s recommendation forsmall canals.

The initial positioning of each plastic block for centered file inser-tion was accomplished manually, and then all steps of canal shaping were produced automatically. Care was taken to set insertion depthlimits in a manner as to avoid overloading individual instruments andto adequately distribute forces among rotaries in the set. Force wasgeneratedfrom thevertical drive of thetorque bench in order to accom-plish the desired movement.

Three variables were continuously measured during each10-second-long instrumentation run (Fig. 1 A) using the sensors built into the torque bench. Torque was registered between the motor andthe instrument with an in-axis sensor (MTTRA 2 with amplifier

Microtest; Microtec Systems, Villingen, Germany; accuracy 0.1 Nmm),force was measured with a strain gauge (A&D 30; Orientec, Tokyo, Japan; accuracy 0.1 N), and insertion depth was controlled by a linearpotentiometer (Lp-100; Midori, Osaka, Japan; accuracy 0.1 mm). Data were digitized at 20 samples per second (20 Hz, PCI-MIO-16XE;National Instruments, Austin, TX) and fed into a computer (G3 PowerMac; Apple, Cupertino, CA). Peaks in torque and force records weredetected with thresholds of 0.3 Nmm and 0.2 N, respectively, and tabu-lated.

Data for peak torque and force were normally distributed, and,consequently, statistical analysis was performed by parametric proce-dures. Torque and apically directed force were contrasted at variousrotational speeds using one-way analysis of variance and Scheff e post

hoc tests, whereas categoric datawerecomparedusing chi-square tests.The level of significance was set at P < .05.

ResultsNo Vortex rotary fractured at any speed during testing. Figure 1 A

shows a representative original experiment with a size #25 rotary fileused to the designated length at 200 RPM. Torque, force exerted, andinsertion depth were recorded against time. Time courses for torqueand force were similar for all file sizes and show two torque peaksconcomitant with the two insertions of the file (compare upper andlower tracesin Fig.1 A). Positiveforcewas neededto introducethe rota-ries into the canal and negative force was generated when withdrawingthe rotary against frictional resistance.

TorqueMean torque was always higher for peak 2 compared with peak 1

(Fig. 1 A) and tended to increase towards the apical enlargement phasecompared with the crown down (Fig. 1 B ). Overall, a significant decrease by 32% was measured after changing operational speedfrom 200 to 400 RPM ( P < .001), but only a slight further reduction was noted after switching from 400 to 600 RPM (Fig. 1 B and C andFig. 2). However, forseveral sizes,such torque reduction only occurred

after the change from 200 to 400 RPM. A further increase to 600 RPMproduced no consistent change and, in fact, a rise in torque for Vortex instruments used for further apical enlargement (Fig. 1 B ).

Force All instruments were used in an in-and-out motion to simulate

clinical movements. The first application of each rotary file into an arti-ficial canal did not reach thedesignated WL, but the second preparationphase did. Forces were highest for the initial instrumentation in thecrown-down phase compared with apical enlargement (Fig. 1C ).

Apically directed force was then plotted against torque for thethree speed settings used. Overall, peak force during preparationdecreased significantly by 48% when comparing 200 with 400 RPM

( P < .001). However, raising RPM from 400 to 600 caused a variablereaction for force, a slight increase for peak 1 and a slight reduction forpeak 2 (Fig. 2); similarly, forces increased for the apical preparationphase (Fig. 1C ).

Detection Limit of Testing PlatformThe registered torque values for Vortex instruments were typically

below 10 Nmm, but Vortex rotaries frequently generated 2 Nmm andless at 200RPM. Along with an increase in speed,there wasalso a signif-icant increase in incidence of ‘‘subthreshold’’ measurements (ie, valuesof <0.3 Nmm for torqueor less than 0.2 N for force). At a setting of 200RPM, 63 of 72 and 62 of 72 insertions were within the detection limitsfor torque and force, respectively; when instruments rotated at 400revolutions, corresponding numbers were 52 of 72 and 40 of 72.Finally, at 600 RPM a similar number of usages fell below the detectionlimit with 53 of 72 (torque) and 40 of 72 (force). The increase in theincidence of ‘‘subthreshold data’’ was significant only for 200 versus400 RPM ( P < .05 and .001 for torque and force, respectively).

DiscussionIf torque during root canal preparation exceeds the strength of

a given engine-driven instrument at a given cross-section, the instru-ment will fracture inside the root canal, a situation that clinicians liketo avoid (4, 17–19). During each rotation, an instrument in a curvedcanal is bent repeatedly, and so-called cyclic loading occurs with alter-

nating tension and compression zones. Manufacturing flaws along thesurface can act as points of fracture initiation and will lead to crack propagation across the file diameter (20). Clinically, buildup of torsional (twisting) and flexural (bending) stresses occurs simulta-neously. The amount of cyclic fatigue stress depends on instrument diameter as well as on cross-sectional design (5).

For flexural stress, the lifespan of an engine-driven instrument isdependent on the time it was used in wall contact inside a canal andduration until fracture will be shortened by using higher workingspeeds (21, 22). In contrast, torsional stress may be reduced at higher RPM via higher cutting efficacy (23). A drop in resistance(torque) that is caused by a higher operating speed may compensateor override any negative effect this increased speed has on cyclicfatigue (14).


JOE — Volume 37, Number 6, June 2011 Vortex Rotaries at Different RPM 861



Two design features for Vortex rotaries have been mentioned toimprove cutting behavior and fatigue resistance: a variable helical angle(higher towards the tip) and the use of a special NiTi alloy (M-wire). Inthis investigation, we used a torque-testing device that has been vali-dated earlier for ProFile and ProTaper rotaries (5, 17), which tendto generate higher working torque that the Vortex rotaries used inthe present experiment. In fact, torque values were occasionally aslow as 0.3 Nmm, which is approaching the detection limit of thetorque sensor used. Static fracture loads determined at D3 accordingto ISO3630-1/ANSI No 28 were 1 Nmm and higher (data not shown);taken together with torques during canal preparation measured inthe present study, this suggests a safety quotient $3 for Vortex

rotaries (24). In other words,torsional fracture of Vortexfiles operatedunder the present conditions is unlikely and in fact no instrument frac-tured during the course of the study.

Plastic blocks with standardized simulated root canals were usedin the present experiment, which is similar to previous studies (17).Plasticblockshavebeenusedformanyyearsnotonlyfortheassessment of shaping capabilities (25) but also for the cutting behavior of NiTirotaries (26); however, cutting of dentin varies from cutting plasticmaterial, and, therefore, caution should be exercised in directly takingthese in vitro data into the clinic.

We tested Vortex rotary files at three different settings, 200, 400, and600 RPM, to assess the influence of these speed settings on torque and

Figure 1. Simulated root canal preparation with Vortex rotaries using an automated torque platform. ( A) Original records with size #25 .04 to 4.5 mm shorter thanthe WL run at 200 RPM. Note the peaks for both torque and force in phase with the two instrument insertions. ( B and C ) Bar diagrams of mean (Æ SEM) peak torque and peak force, respectively, for three different RPM settings ( n = 12).





apical force inside artificial root canals. This selection of RPMsettings wasbased on previous data assessing the effect to rotational speed on rotary breakage (13, 14, 27), including values that are both below and abovethe recommended RPM for Vortex rotaries. We found that raising RPMfrom 200 to 400 may be beneficial for Vortex instruments because it reduced torque and did not lead to fatigue or fractures.

However, raising RPM to 600 did not provide any added benefit.Lopes et al (27) studied the effect of increasing from 300 to 600 RPMon cyclic fatigue of ProTaper instruments. In theirstudy, elevating speedcaused a 29% decrease in the number of cycles to fracture and a 65%decrease in the working time until fracture.

Cheunget al (28) microscopically investigated clinically used Pro-Taper rotaries andfoundthat 25 of 27 instruments fracturedbecause of

flexural (fatigue) fracture. They stated that raising the number of rota-tions increases the strain rate of rotary files and reduces the time foreither stress relaxation or crystalline grid transformation. This may lead to the initiation of superficial microcracks in the NiTi alloy that can quickly propagate to cause complete instrument failure (29).

Apical force was also measured in the present experiment. Thereduction in force followed a similar pattern compared with the reduc-tion in torque, with the exception that initial shaping in the crown-downphase required more force that the final step, which is apical enlarge-ment. This observation is in line with data from Profile .04 instrumentsobtained in a similar experiment (17).

Thelack of benefitfor 600RPM maybe explainedby theparticulardesign of Vortex blades, potentially reaching a cutting optimum at 400

Figure 2. Scattergrams of torque and force at ( A) peak 1 and ( B ) peak 2 at three different RPM settings. Overall means (Æ SEM) are included. Note the number of data points with less than 72 points stemming from subthreshold data for torque, force, or both.


JOE — Volume 37, Number 6, June 2011 Vortex Rotaries at Different RPM 863



RPM in plastic blocks. Another unresolved question is the potential forheat generation and subsequent conformational changes during prep-aration, which may impact cutting behavior. Tobushi et al (30) showedthat higher strain rates lead to structural changes in NiTi alloys withexothermic reaction and, thus, elevated temperatures; Lopes et al (27) had suggested that such temperature raises might lead to fasterfatigue withhigher RPM.However, it is unlikely that clinicalprocedures, with cooling via intracanal irrigants, lead to substantial temperature rai-

ses. There is still a lot of information to be gathered about NiTi R-phase(31) and, for example, exact annealing temperatures associated withthe production of M-wire are not known. However, one can extrapolatefrom annealed conventional NiTi that likely temperature during canal preparation of 150C would be required for the alloy to ‘‘forget’’ all prior deformation (32). In addition to purely a mechanistic approach,clinical effects with higher RPM such as an elevated potential for taperlock should be considered (13); it has been argued that lower speedsmay be safer for NiTi rotaries (33, 34).

In conclusion, under the present experimental conditions, rota-tional speed had a significant impact on preparation with Vortex rota-ries, with instruments at 400 RPM generating less torque and forcecompared with 200 RPM. An additional RPM increase to 600 did not provide any further benefit.

Acknowledgments

The authors deny any conflicts of interest related to this study.

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