The influence of number of autoclave treatmentcycles (N ) on rotational speed and total indicatedrun-out of commercially available air-turbinehandpieces from five manufacturers was investi-gated at N =0, 50, 100, 150, 200, 250 and 300cycles, and the significance in the test results wasassessed by Dunnett’’s multiple comparison test.Some air-turbine handpieces showed the signifi-cant differences in rotational speed at N =300cycles, however, the decreases of the rotationalspeeds were only 1 to 3.5 percent. Some air-turbinehandpieces showed the significant differences intotal indicated run-out, however, the respectivevalues were smaller than that at N =0 cycle.Accordingly, it can be considered that the ballbearing in the air-turbine handpieces is not affect-ed significantly by autoclave. To further evaluaterotational performance, this study focused on therotational vibration of the ball bearing compo-nents of the air-turbine, as measured by FastFourier Transform (FFT) analysis; the power spec-tra of frequency of the ball’’s revolution, frequencyof the cage’’s rotation and frequency of the ball’’srotation were comparatively investigated at N =0,150 and 300 cycles, and the influence of autoclavewas evaluated qualitatively. No abnormalities in theball bearings were recognized.

Key words: Autoclave sterilization, Rotationalspeed, Total indicated run-out, Air-tur-bine handpieces.

Introduction

Social awareness of infectious disease such ashepatitis B, hepatitis C and acquired immune deficien-cy syndrome (AIDS) is increasing. In dental fields, therehas long been concern that patients may contractsuch infectious diseases via contact with poorly steril-ized instrument or appliances, particularly becausepatients commonly bleed during dental treatment.Furthermore, dentists, dental hygienists and assis-tants are also at risk of contracting such diseases viacontact with patient blood and saliva. Therefore, suffi-cient measures to protect against infection must betaken1,2.

In light of this social background, manufacturers ofair-turbine handpieces have actively improved theirproducts to facilitate autoclave sterilization. Dentistsalso understand the necessity of sterilizing air-turbinehandpieces after each patient. However, there issome concern among dentists that if air-turbine hand-pieces are sterilized frequently under severe environ-mental conditions, such as those present in an auto-clave, the rotational performance is adversely affected.In order to alleviate such misgivings, we performed arecent literature search regarding the durability of air-turbine handpieces with sterilization, but were able tofind only three reports3-5. Two reports3,4 did not focus onthe rotational speed or eccentricity of the test mandrel;the testing criteria focused on the number of cycles untiluse of the air-turbine handpiece was impossible.Accordingly, no correlations between changes in rota-tional performance and sterilization treatment could bedetermined. The third report5 focused on the rotationalspeed, the stall torque, the bearing resistance, the

Original Article

Influence of Number of Dental Autoclave Treatment Cycles on RotationalPerformance of Commercially Available Air-Turbine Handpieces

Masahiro Nagai and Kazuo Takakuda

Department of Biodesign, Division of Biosystems, Institute of Biomaterials and Bioengineering, Tokyo Medicaland Dental University

J Med Dent Sci 2006; 53: 93–101

Corresponding Author: Masahiro Nagai2-3-10 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-0062, JapanReceived December 6, 2005; Accepted March 17, 2006

noise and the light output of fiber-optic of the air-turbinehandpieces during the course of daily use in generaldental practice. However, the report5 did not focus onthe eccentricity of a rotational tool.

In the present study, we investigated the influence ofthe number of autoclave cycles on the rotational per-formance (rotational speed and eccentricity) of the mostcommonly used commercially available air-turbinehandpieces in Japan. Furthermore, we diagnosed therotational conditions of ball bearing in the air-turbinehandpieces by using the Fast Fourier Transform (FFT)analyzer.

MATERIALS AND METHODS

Air-turbine handpiecesThe air-turbine handpieces used in this study were

commercially available products. They were made byfour Japanese manufacturers and one German manu-facturer, as shown in Table 1, with two products comingfrom each manufacturer. The air-turbine handpiecesmade by Osada and Yoshida used steel ball bearings,while the air-turbine handpieces made by Morita,Nakanishi and Kavo used ceramic ball bearings. Inaddition, the air-turbine handpieces made by Kavo wereequipped with an automatic air pressure regulator.Furthermore, all the air-turbine handpieces included adevice in the head that prevented oral fluids and otherforeign substances from entering.

Autoclave sterilization conditions The sterilization conditions for small medical auto-

claves are specified in JIS T7324-20056. In addition, thesterilization conditions for air-turbine handpieces, asspecified in “Dental handpiece - high-speed air-turbinehandpieces” (JIS T59067), call for a sterilization tem-perature of 132±2°C and a sterilization pressure of 0.2MPa for 5 min. Rotational performance must not dete-riorate after autoclaving, even if sterilization treatment isrepeated for 250 cycles. In this study, the autoclaveused was the Super Clave HF260 (Hillson Dec Co.) andthe sterilization temperature and pressure were set at132±2°C and 0.2 MPa, respectively, as specified in JIST59067. However, sterilization was performed for 15 minand the number of cycles was 300. These conditionswere severer than the JIS regulations7.

Testing procedureFor the repeated sterilization test, one cycle (N=1

cycle) was as follows. First, the air-turbine handpieceswere lubricated with turbine spray using nozzles pro-vided by the manufacturers for 2 s, were immediatelyrotated without applied load for 30 s after inserting atest mandrel (Í1.6 × 19 mm), and were sterilized for15 min in the autoclave after removing the test mandrel.When setting the air-turbine handpieces into the auto-clave chamber, all air-turbine handpieces were tilted atabout 20°, with the heads placed upward, in stainlesssteel container. The container was then set in the auto-clave chamber. In addition, the positions of the air-tur-bine handpieces in the container were changed after

M. NAGAI and K. TAKAKUDA J Med Dent Sci94

Table 1. Commercially available air-turbine handpieces used in this test.

every cycle in order to minimize the influence of air-tur-bine handpiece position. After each cycle, the contain-er in which the air-turbine handpieces were placed wasimmediately removed from the autoclave chamberand was allowed to cool to room temperature.Lubrication was repeated as described above, and thiswas repeated for 300 cycles. None of the air-turbinehandpieces were subjected to load during testing.This is because the present study was conducted inorder to investigate the influence of autoclave on theball bearing parts of the air-turbine handpiece.

Evaluation methodRotational performance of the air-turbine hand-

pieces was evaluated based on rotational speed,eccentricity of test mandrel in rotation and powerspectral analysis of the ball bearings supporting thebody of the rotating air turbine, without applied load.Rotational speed and eccentricity were measuredevery 50 cycles, and power spectral analysis was per-formed every 150 cycles using sound level meter (NA-40, Rion Co.) and FFT analyzer (SA-74, Rion Co.).Rotational performance tests were carried out as fol-lows: 1) Air-turbine handpieces were rotated withoutapplied load; however, at cycle N=0, when the rotationalspeed of each air-turbine handpiece reached themanufacturer’s recommended rotational speed (R* inTable 2) by adjusting the compressor air valve, the sup-ply air pressure, po, was measured using a digital pres-sure gauge connected at the coupling joint of the air-turbine handpiece. This value (po) was used when sup-plying the air pressure used to rotate the respectiveair-turbine handpieces in the rotational performance

tests. In Table 2 the measured values of R* and po foreach air-turbine handpiece are shown.

2) When arriving at the predetermined number ofautoclave cycles, the air pressure of each air-turbinehandpiece was adjusted to po via the compressor airvalve (Table 2).

3) After the rotational speed of each air-turbinehandpiece became stable, as the rotational perfor-mance tests, the rotational speed, RN, and the total indi-cated run-out, δN, were measured simultaneouslyaccording to the testing methods depicted in Fig. 1. Thetests were performed continually from 30 to 50 s andwas performed three times. Power spectrum wasmeasured based on the sound generated from the air-turbine head.

Test mandrels (φ1.6×19 mm) having the dimensionsspecified in the International Standard (ISO8) and JIS7

documents were prepared. The test mandrels weremagnetized using a permanent magnet and wereinserted into the spring-type chuck of the air-turbinehandpieces. The necessity of such magnetizationdepends on the type rotational speed sensor used. Formeasurement of rotational speed and total indicatedrun-out, as shown in Fig. 1, after fixing the air-turbinehandpiece with a fastening device in order to maintainthe test mandrel in a horizontal position, the rotationalspeed sensor was set below the test mandrel. Thelaser beam of the laser displacement meter wasfocused on the test mandrel 6 mm from the air-turbinehead, and both tests were carried out after the air-tur-bine handpiece was activated. The laser displace-ment meter measures the distance between specificpoints on the test mandrel. And it was able to follow themovement of the test mandrel in direction of the laserbeam axis, because the laser displacement metershown in Fig. 1 had a maximum response frequency of10 kHz. The “peak-to-peak” output mode was thusselected for the laser displacement meter. This outputmode measures the difference between the maximumand minimum values over the measured distance,and thus it was possible to measure the total indicatedrun-out change of the test mandrel. The data shown inFig. 1 were transferred to a personal computer via thesensor interface and were converted to rotationalspeed and a total indicated run-out‐time diagrams,respectively. For power spectral analysis, the air-turbinehandpiece was hung in balance from the ceiling. Therotational axis of the test mandrel coincided with thedirection of gravity. A microphone of the sound levelmeter to record sound levels was set at a position 0.45m away from the air-turbine head. The power spectra of

95INFULUENCE OF DENTAL AUTOCLAVE ON AIR-TURBINE

Table 2. Each manufacturer’s recommended rotational speed, R*and the supply air pressure, po at N=0 cycle.

predicted frequencies described later and a majorspectrum with comparatively large power value wereobserved scientifically in the screen of 10-kHz and 20-kHz display modes of the FFT analyzer, and then thetwo images were recorded immediately. Furthermore,those images were compared with those at N=0cycle.

FFT analysis In the case of ball bearings in the dental air-turbine

handpieces, the outer ring is fixed and the inner ring isrotated at a predetermined rotational speed. The rota-tional speed of the air-turbine handpiece, fr, the fre-quency of the ball’s revolution, fa, the frequency of thecage’s rotation, fb, and the frequency of the ball’s rota-tion, fc, were calculated from the following equations:

(1)

(2)

(3)

where, d is the ball diameter, D is the pitch circle ofthe ball and α is the contact angle of the ball. If the ballbearing has a dent or a scratch on the outer ring, andwhen number of balls is z, the frequency, fd, that these

balls pass over the dent or scratch is given by the fol-lowing equation:

(4)

In addition, if the ball bearing has a dent or ascratch on the inner ring, because the relative rotationalspeed of the inner ring for the frequency of the ball’srevolution is (fr ‐ fa), where, the number of balls is z,the frequency, fe, that the balls pass over the dimple orscratch is given by the following equation:

(5)

The frequencies, fa, fb, fc, fd and fe of each air-turbinehandpiece in five manufacturers were predicted byusing Eqs.(1) to (5), where, fr converted the R* valuesshown in Table 2 to Hz, d=1 mm, D=4.76 mm, α=0°and z=7. Here, the numerical values for the ball bearingwere provided by the five manufacturers.

Statistical analysisThe significances of the rotational speed (RN) and

the total indicated run-out (δN) at N=50, 100, 150, 200,250 and 300 cycles were assessed by Dunnett’s multi-ple comparison test; free software:R9 (significancelevel: α =0.05) in comparison with RN and δN at N=0cycles for control group, in order to examine influence ofautoclave sterilization of an air-turbine handpiece.

M. NAGAI and K. TAKAKUDA J Med Dent Sci96

Fig. 1. Measuring methods of rotational speed and total indicated run-out. [Rotational speed meter: JADAS-701, Ogura jewel Ind., Laser sen-sor: LC-2440, Keyence Co., Laser displacement meter: LC-2400, Keyence Co. and Sensor interface: PCD-320A, Kyowa E. I. Co.]

RESULTS

Rotational speed and total indicated run-outIn the time-series diagrams, there were almost no

changes in rotational speed and total indicated run-outduring the course of measurement in all air-turbinehandpieces. Table 3 shows the means of three test val-ues of the rotational speed (RN) measured at the pre-determined number of sterilization treatment cycles(N=0, 50, 100, 150, 200, 250, 300 cycles) with all air-turbine handpieces used in this study. The numericalvalues in parenthesis under each RN value indicatestandard deviation. The value of RN for each air-turbinehandpiece fluctuated as sterilization cycles increased.However, the standard deviations for all RN values wereextremely small. This may be because establishingequal air pressure at cycle 0 was difficult due to fluctu-ations in the digital air pressure gauge reading. Whenthe air pressure was fixed, rotational speed was stable,even when measurement was repeated 3 times.Consequently, small standard deviations wereobtained.

Table 4 shows the mean total indicated run-out val-

ues (δN) calculated by the same procedure as for rota-tional speed. The numerical values in parenthesisunder each δN value indicate standard deviation. Thevalues for δN were around 2 µm and were extremelysmall. Incidentally, as specified in ISO8, eccentricityshould not exceed a total indicated run-out of 0.03 mm.However, the air-turbine handpieces used in thisstudy gave significantly smaller values than the per-missible values specified in ISO. However, the valuesfluctuated somewhat as sterilization cycles increased,regardless of the type of air-turbine handpiece.Furthermore, the standard deviations of all δN valueswere very large, as shown in Table 4. This demon-strates that the δN data, as measured 3 times for eachtest, varied widely. Such scattering may have been dueto problems in the dynamic measurement system,including the resolution (0.2 µm) of the laser displace-ment meter, laser sensor installation, fixing of the air-turbine handpiece and stiffness of the table thatinstruments were placed on.

The mean values shown in Tables 3 and 4 were plot-ted as RN‐N and δN‐N diagrams in Fig. 2. The sym-bols for each product in Fig. 2 correspond to thoseshown in Table 1. On the whole, the rotational speed of

97INFULUENCE OF DENTAL AUTOCLAVE ON AIR-TURBINE

Table 3. Mean values of rotational speed measured at predetermined num-ber of sterilization treatment cycles.

each air-turbine handpiece seems not to be affected byincreasing N values, except those of Nakanishi. Thedetails are determined by Dunnett’s multiple compari-son test to be described next. However, both the rota-tional speeds of N1 and N2 showed a tendency to grad-ually decrease by increasing N values.

The results assessed by the Dunnett’s multiplecomparison test were shown in Tables 5 and 6. In caseof rotational speed, as shown in Table 5, some ofresults of O1-1 and Y2 decreased significantly. In O1-1which is similar to O1-2, although O1-2 showed no sig-nificant difference in all cycles of sterilization, the sig-nificant difference was observed at N=150, 200 and300 cycles. Similarly, Y1 showed no significant differ-ence at all cycles of sterilization. However, the signifi-cant difference was observed at N=100, 200, 250 and300 cycles in Y2. Although the Y1 and the Y2 were ofdifferent types, only the ball bearing part was same.Meanwhile, in case of RN values in N1 and N2, the sig-nificant differences were observed at all cycles ofsterilization.

As shown in Table 6, in case of Y2 and K1-1, the sig-nificant differences were observed in δN values atN=200 and 250 cycles, and in δN values at N=150, 200,

M. NAGAI and K. TAKAKUDA J Med Dent Sci98

Table 4. Mean values of total indicated run-out measured at predeterminednumber of sterilization treatment cycles.

Fig. 2. RN and δN - N diagrams in each air-turbine handpiece, wherethe symbols for each product correspond to those shown in Table 1.

250 and 300 cycles, respectively. However, as shown inTable 4, the δN values in Y2 or K1-1 were smaller thanthose N=0 cycle. N1 and N2 showed no significant dif-ferences in δN values, although both the rotationalspeeds showed a tendency to gradually decrease anddiffered significantly. This is noticeable result and sug-gests that it is not a problem of ball bearing.

FFT analysisFigure 3 shows representative power spectra of air-

turbine handpiece. The frequency domain of noise inthe measurement room was equal to or less than 2kHz. The power spectral diagrams were recordedrespectively when the air-turbine handpiece: M1-1was tested at N=0, 150 and 300 cycles. The arrows inFig. 3 indicate each predicted frequency calculated by

Eqs.(1) to (4): fr=6.2 kHz, fa= fb=2.4 kHz, fc=14.0 kHzand fd=17.1 kHz. As shown in Fig. 3, M1-1 after steril-ized did not exhibit characteristic difference in thepower spectra in comparison with those at N=0 cycle,because similar power spectra were observed at dif-ferent stages of the sterilization treatment and none ofthe power spectra varied in both sides on frequency-axis when the air-turbine handpiece after sterilized wastested. Additionally, an abnormal power spectrum wasnot observed besides the predicted frequencies. Suchsimilarities were observed for all air-turbine hand-pieces tested.

DISCUSSION

Rotational speed and total indicated run-outMonaghan and others5 discussed the influence of

99INFULUENCE OF DENTAL AUTOCLAVE ON AIR-TURBINE

Table 5. The results of Dunnett’s multiple com-parison test (α=0.05) of RN.

Fig. 3. Representative power spectra of the air-turbine handpiece,which were measured after predetermined number of sterilizationtreatment cycles, where the respective arrows indicate fr=6.2 kHz, fa=fb=2.4 kHz, fc=14.0 kHz and fd=17.1 kHz.

Table 6. The results of Dunnett’s multiple com-parison test (α=0.05) of δN.

autoclave on ball bearing by means of the bearingresistance. A wear condition of the ball bearing mightbe investigated from such bearing resistance.However, their method is not commonly used. The eval-uation of the ball bearing is considered of value in thecase of air-turbine handpiece that the dentist used indental practice. In case of this study, it was selected toevaluate by means of the total indicated run-out andFFT analysis, because we employed the air-turbinehandpieces without tooth cutting in this test.

The significance regarding rotational speed andtotal indicated run-out is discussed as follows. In O1-1,the RN values at N=150, 200 and 300 cycles, asshown in Table 3, decreased 1 to 3 percent in compar-ison with that at N=0 cycle, although the significant dif-ferences were observed at those N values as shown inTable 5. However, O1-1 showed no significant differ-ences in total indicated run-out as shown in Table 6.Similar to O1-1, the RN values in Y2 decreased 1 to 2percent in comparison with that at N=0 cycle. And theY2 showed the significant differences in the δN values.However, the δN values, as shown in Table 4, weresmaller than that at N=0 cycle. Incidentally, the δN val-ues of K1-1 that the significant differences wereshown in total indicated run-out were small than that atN=0 cycle. If the ball bearing was damaged by auto-clave, the total indicated run-out of test mandrel wouldincrease suddenly even if the damage of the ballbearing was a little. Therefore, such the result may havedepended on the quality of the equipment. In regard tothe measurement of a total indicated run-out, the ballbearing rotates at super-high-speed and sufficientmeasurement technology has not been provided at pre-sent. The development of new measuring equipment isexpected in future.

In case of N1 and N2, the significant differences wereobserved in all RN values, furthermore, both the RN val-ues gradually decreased as shown in Fig. 2.Especially, both the RN values decreased instantly to3.5 and 2.2 percent at N=50 cycles, respectively. Ahypothesis that the ball bearing was affected by auto-clave at the smaller N was rejected, because a cage ofthe ball bearing is made by heat-resistant resin.Accordingly, it was speculated that such the phenome-non depended on other problems aside from ballbearing itself. On the contrary, Monaghan and others5

reported that the rotational speed of several air-turbinehandpieces increased at early use stage. They sug-gested that initial conditions or variation of quality of ballbearing might become a problem as the reason for this.However, the dentist will try enough lubrication and

free-running of the air-turbine handpiece before used inclinical practice for the first time, and then will sterilizethe air-turbine handpieces. Accordingly, their consider-ation is hard to be understood. Hence, it is consideredthat such phenomena would depend on account of oth-ers. And it is hard to make a clear consideration for theearly decrease in this paper. The aim of this paper is toevaluate the affect of an autoclave on the rotational per-formance of ball bearing. So, the ratio of RN at N=300cycles to that at N=50 cycles in N1 and N2 was calcu-lated, respectively. As the result, the decreases were 5percent together. From such the decrease rates, it maybe evaluated that the durability of the air-turbinehandpieces were deteriorated inconsiderably.However, it was suggested that the ball bearing did notbecome dynamically the serious situation at N=300cycles, because N1 and N2 showed no significant dif-ferences in the δN values in all cycles of sterilization. Ifthe ball bearing was influenced by autoclave, the δN val-ues must have shown a tendency to increase. In theactual clinical case, small decrease of rotationalspeed has no influence on the tooth cutting techniquefor a dentist.

FFT analysis Among the components of the ball bearing, the

cage is made of heat-resistant resin. The ball bearingmay thus be gradually influenced by the heat of steril-ization, even if heat-resistant resins are used. This mayresult in reduced rotational performance. However,from the former results of total indicated run-out, it wasguessed that the ball bearing would not be influencedby autoclave. Furthermore, as another test for evaluat-ing the ball bearing, a qualitative diagnosis of ball bear-ing was performed while observing scientifically majorpower spectra displaying on the screen of FFT analyz-er, on the basis of each predicted frequency of the com-ponents in the ball bearing.

Each arrow in Fig. 3 indicates the frequencies for fr, fa(=fb), fc and fd as a reference. Thus, each predicted fre-quency, as shown for each result after a predeterminednumber of sterilization treatment cycles (Fig. 3),agreed with almost each peak value from the spectra.However, the spectra for fc and fd were masked by noisebecause the amplitudes of both frequencies wereextremely small. This suggests that an abnormalsound was generated in the frequencies of fc and fd

because the ball bearing showed no signs of damageafter N=300 cycles. Consequently, this suggests thatthe ball bearing did not receive any type of damage byautoclave. Comprehensively, it was demonstrated that

M. NAGAI and K. TAKAKUDA J Med Dent Sci100

all air-turbine handpieces rotated normally afterrepeated sterilization and were not affected by the auto-clave.

From the above-mentioned results, we may specu-late that the rotational performance of air-turbinehandpieces would not be influenced by sterilizationtreatment until N=900 cycles, because the sterilizationperiod was 3 times longer in this study than that rec-ommended by the JIS. One air-turbine handpiece issterilized 2-3 times per day, and five days per week at atypical Japanese dental clinic. This means that the air-turbine handpieces were subjected to the equivalent ofmore than two years of sterilization treatment in the pre-sent study.

CONCLUSION

The influence of number of autoclave treatmentcycles (N) on rotational speed and total indicated run-out of commercially available air-turbine handpiecesfrom five manufacturers was investigated at N=0, 50,100, 150, 200, 250 and 300 cycles, and the significancein the test results was assessed by Dunnett’s multiplecomparison test. Some air-turbine handpiecesshowed the significant differences in rotational speed atN=300 cycles, however, the decreases of the rotation-al speeds were only 1 to 3.5 percent. Therefore, it is notconcluded that those ball bearings received a consid-erable damage by autoclave. Some air-turbine hand-pieces showed the significant differences in total indi-cated run-out, however, the respective values weresmaller than that at N=0 cycle. Therefore, it was sug-gested to be influenced variation of data. Accordingly, itcan be considered that the ball bearing in the air-tur-bine handpieces is not affected significantly by auto-clave.

To further evaluate rotational performance, thisstudy focused on the rotational vibration of the ballbearing components of the air-turbine, as measured byFFT analysis; the power spectra of frequency of theball’s revolution, frequency of the cage’s rotation and

frequency of the ball’s rotation were comparativelyinvestigated at N=0, 150 and 300 cycles, and theinfluence of autoclave was determined. No abnormali-ties in the ball bearings were recognized.

ACKNOWLEDGEMENTS

On the occasion of implementation of this research,authors wish to thank Morita Mfg. Corp., Nakanishi Inc.,Osada Electric Co., Yoshida Dental Mfg. Co., andShirokusu Dental Supply Works for supplying thoserespective air-turbine handpieces shown in Table 1, andMr. Masayuki Kobayashi for his assistance in steriliza-tion treatment cyclic test for air-turbine handpiece.

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Infection Control in Dental Health-Care Setting—2003.Centers for Disease Control and Prevention. Recommen-dations and Reports 2003/52(RR17);1-61.

2 Yamaga M, Kiryu T, Ohfuchi Y et al. Internal contamination ofdental air turbine system—The protective effect of a newlydesign handpiece on bacterial invasion (in Japanese,English abstract). J J Conser Dent 1995;38(2):472-478.

3 Clinical Research Associates Newsletter (in Japanese).Advanced high-speed handpieces—Ten years after the steril-ization procedure was enabled. 2000;4(9):1-2.

4 Clinical Research Associates Newsletter, Additional study．High speed handpieces: A. Use-life sterilized vs. non-sterilizedHPs. The October 2000 Newsletter.

5 Monaghan DM, Wilson NHF, Darvell BW. The performance ofair-turbine handpieces in general dental practice. OperativeDentistry 2005;30(1):16-25.

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