Cha

diff

rot

and

Jin-Hyuk Bae, DD

This research was supported by BasicMinistry of Education, Science and T

aChief, Dental Department, The ArmbResident, Department of ProsthodocAssistant Professor, Department ofdProfessor, Department of ProsthodeProfessor, Department of Prosthodo

The Journal of Prosthetic

nges in the cutting efficiency of

erent types of dental diamond

ary instrument with repeated cuts

disinfection

S, MS,a Jaeyoung Yi, DDS, MS,b

Sungtae Kim, DDS, PhD,c June-Sung Shim, DDS, PhD,d andKeun-Woo Lee, DDS, PhDe

College of Dentistry, Yonsei University, Seoul, South Korea; School ofDentistry, Seoul National University, Seoul, South Korea; The ArmedForces Daegu Hospital, Daegu, South Korea

Statement of problem. Cutting efficiency is one of the most important factors to consider when a specific dental diamondrotary instrument is selected. However, the selection of a dental diamond rotary instrument is based on clinical experiencerather than any scientific evidence.

Purpose. The purpose of this study was to identify how the cutting efficiency of different types of dental diamond rotaryinstrument changed with repeated cuts and disinfection.

Material and methods. Four types of diamond rotary instrument from 2 dental manufacturers (Shofu, Jin Dental) wereinvestigated with a high-speed air-turbine handpiece. The groups were as follows: S cham group (n¼10): chamfer design fromShofu; J cham group (n¼10): chamfer design from Jin Dental; S thin group (n¼10): thin tapered design from Shofu; J thingroup (n¼10): thin tapered design from Jin Dental. Changes in the cutting efficiency of diamond rotary instruments on glassceramic blocks were measured after repeated cuts. Changes in cutting efficiency also were measured for 30 diamond rotaryinstruments, the same type as those used in group J cham after disinfection with ethylene oxide gas, immersion in solution, orautoclaving. One-way ANOVA, 2-way ANOVA, and repeated-measures ANOVA were used to identify differences in cuttingefficiency, in total cutting efficiency, and change trend in cutting efficiency (a¼.05). The Tukey honestly significant differencemethod was used for the post hoc tests. The principal metal components of the diamond rotary instruments were detectedwith x-ray spectrometry.

Results. The mean (standard deviation [SD]) total cutting efficiency after 10 cuts in the 4 groups was in the following order:J cham group (0.210 �0.064 g/min) > S cham group (0.170 �0.064 g/min) > J thin group (0.130 �0.042 g/min) > S thingroup (0.010 �0.040 g/min) (P<.05).The decrease in the cutting efficiency was greatest after the first cut. The cuttingefficiency was not influenced by repeated disinfection.

Conclusion. The cutting efficiencies of diamond rotary instruments with different designs and particle sizes showed a decreasingtrend after repeated cuts but did not show any change after various disinfecting procedures. (J Prosthet Dent 2014;111:64-70)

Clinical Implications

Irrespective of particle size or disinfection procedure, cutting efficiencygradually decreases after repeated use. The greatest decrease was foundafter the first use.

Science Research Program through the National Research Foundation of Korea (NRF) funded by theechnology (2010-0007829).

ed Forces Daegu Hospital, Daegu, Korea.ntics, Department of Prosthodontics, College of Dentistry, Yonsei University.Periodontology, Dental Research Institute, School of Dentistry, Seoul National University.ontics, Department of Prosthodontics, College of Dentistry, Yonsei University.ntics, Department of Prosthodontics, College of Dentistry, Yonsei University.

Dentistry Bae et al

Table I. Diamond rotary instruments in 4 study groups (n¼10)

GroupDiamond Rotary

Instrument Manufacturer

S cham Chamfered, 102R Shofu

J cham Chamfered, CF910T Jin Dental

S thin Thin tapered, 104R Shofu

J thin Thin tapered, CF1007T Jin Dental

January 2014 65

The high-speed air-turbine hand-piece was introduced into dentistry in1957 and remains widely used. Thediamond rotary instrument was firstintroduced in the late 19th century andhas been used widely in the field ofrestorative dentistry1,2 Diamond rotaryinstruments have been used for overalltooth preparation, marginal finishing,enameloplasty, and finishing of thedefinitive restoration. The efficiency ofthese procedures depends on the cut-ting efficiency, which should be care-fully considered when selecting a dentalcutting instrument suitable for eachprocedure. Cutting efficiency has beencommonly defined as the quantity ofsubstrate that can be removed within aspecified time. A longer cutting timeindicates lower cutting efficiency.3 Pre-vious studies compared the differencesin cutting efficiency between diamondrotary instruments and tungsten car-bide burs, and showed a differencebetween the two.4-7

Most studies exhibited a reductionin cutting efficiency after repeated use.However, few studies have been per-formed to show the detailed changesin the cutting efficiency of diamondrotary instruments. Siegel and vonFraunhofer3 compared the cutting effi-ciency among 20 diamond rotary in-struments after 10 cuts. They showedthe effect of the handpiece load on thecutting efficiency, which was estimatedaccording to the amounts of substrateremoved from the first to fifth cuts andthe sixth to tenth cuts.8 Siegel and vonFraunhofer4 also compared the cuttingefficiency among 3 diamond rotary in-struments with different grit sizes. Thedifference in cutting efficiency betweentungsten carbide burs and diamondrotary instruments was also comparedafter 3 successive cuts through castingalloys.4,7 Chung et al9 showed that thecutting efficiency of 3 diamond rotaryinstruments decreased significantly af-ter the first cycle (30 seconds). How-ever, no difference was noted in thecutting efficiency after the fifth cycle.Pilcher et al10 compared the cuttingefficiency between diamond rotary in-struments used for a single patient and

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multiple patients after 20 cuts, andshowed that the decrease in cutting ef-ficiency was largest after the first cut.

The service life of a rotary cuttinginstrument is another key factor. Therepeated use of a rotary diamond in-strument reduces its cutting efficiencybecause of abrasion and/or the deposi-tion of substrate remnants.3 A shortservice life means that cutting efficiencydecreases rapidly and frequent instru-ment replacements are necessary. Steril-izing the diamond rotary instrument ismandatory to prevent cross infection.Hooker and Staffanou11 reported thatsome diamond particles disappearedafter sterilization with an ultrasoniccleaner and autoclaving. Harkness andDavies12 reported that corrosion of thenickel matrix can occur when ultrasoniccleaner and disinfectants with a pHhigher than 11 were used for steriliza-tion. Disinfectants accelerated the lossof diamond particles. In contrast, Gur-eckis et al13 reported that the cuttingefficiency appeared not to be affectedafter 10 successive disinfection pro-cedures with cold sterilization with 2%glutaraldehyde, autoclaving, dry heatsterilization, or chemical vapor steriliza-tion. However, the cutting efficiencytended to decrease from the first to thesixth disinfection.

The cutting efficiency, service life,and ability to sterilize a diamond rotaryinstrument without decreasing the cut-ting efficiency are essential factors inselecting a diamond rotary instrument.However, dentists tend to select anduse diamond rotary instruments basedon their own clinical experiences ratherthan on scientific evidence. Accord-ingly, this study examined differences

in the cutting efficiency among dia-mond rotary instruments with differentconfigurations. Changes in cutting ef-ficiency after repeated cuts and disin-fection also were investigated. The nullhypotheses were that no differenceswould be found in the cutting efficiencyafter repeated cuts of the 4 differentdiamond rotary instruments used andthat no differences would be found inthe cutting efficiency with the variousdisinfection methods.

MATERIAL AND METHODS

This study investigated 4 types ofdiamond rotary instruments. These in-struments were manufactured by 2companies and are available in theKorean domestic market (Table I). Amachinable ceramic block (Macor;Corning) was used as the cutting sub-strate. Chlorhexidine gluconate (5%)solution (Sungkwang, Bucheon) wasused as the cold disinfectant.

Experiment in cutting efficiency:handpiece

A high-speed air-turbine rotaryhandpiece (Hi Torque 535B; YoshidaDental) was used. This handpiece wasmounted onto a cylinder attached toan acrylic resin board by a frictionlessbearing (Fig. 1). A constant force of1.65 N was applied at a distance of5 cm from the pivot. The force on thediamond rotary instrument was ap-proximately 0.92 N. The handpiecewas operated at 320000 rpm undera coolant water spray of 25 mL perminute according to the manufacturer’srecommendations.

d

Pivot Weight (W) Instrument tip

(p)

D

1 Schematic diagram of handpiece loading and determi-nation of force at rotary instrument tip.

Table II. Sterilization methods usedin 3 additional groups (n¼10)

GroupSterilizationmethod

J cham Auto Autoclaving

J cham Chx Chlorhexidine gluconate

J cham Gas Ethylene oxide gas

66 Volume 111 Issue 1

Experiment in cutting efficiency:diamond rotary instruments

Sample size was determined based onprevious studieswitha similardesign.3Tenspecimens each of 102R and 104R in-struments (Shofu) and 10 specimens eachof CF910T and CF1007T instruments(Jin Dental) were investigated. Accordingto the manufacturers’ descriptions, all ofthese have a medium grit. The cuttingsurface of the 102R and CF910T in-struments were 9-mm long, and the in-struments were chamfered, with adiameter of 1 mm in the thickest area ofthe cutting surface. The cutting surfaces ofthe 104R and CF1007T instruments were10mmin length, and the instrumentsweretapered, with a diameter of 0.7 mm in thethickest area of the cutting surface.

A machinable ceramic block (Macor;Corning) was used as a substrate forcutting. This block is composed of 55%fluorophlogopite and 45% borosilicateglass, and has a hardness of 250 KHNand an elastic modulus of 66.9 GPa;these values are similar to those of toothenamel (hardness, 300 to 340 KHN andelastic modulus, 84 GPa). The ceramicblock was cut into 70 specimens ofsize 20 � 20 � 20 mm (LianyungangHighborn Technology). Forty of thesespecimens were divided into 4 groupsbased on the type of diamond rotary in-strument used (Table I).

Each specimen was positioned sothat the cutting surface of the rotaryinstrument could be applied to thespecimen 9 mm from the tip of therotary instrument. The specimen wascut for 1 minute. Thereafter, the cutting

The Journal of Prosthetic Dentis

debris was washed from the ceramicblock for 60 seconds by using an ultra-sonic cleaner (UC-100; Coltene Whale-dent). After drying, the weight of thespecimen was measured with an elec-tronic scale with a resolution of 0.01 g(SD-2020; Ohaus). The difference be-tween the weight before and after cuttingrepresented the lost weight, and the cut-ting efficiency was calculated by dividingthe lost weight of the ceramic block bytime. The handpiece was then rotated for60 secondswithout cutting after applyinglubricant (Turbine spray, YoshidaDental)for 1 second in accordance with themanufacturer’s description. The aboveprocess was performed 10 times witheach instrument, thereby yielding 400measurements of cutting efficiency.

Experiment in change of cuttingefficiency by repetitive disinfection

Thirty Macor blocks and diamondrotary instruments (CF910T), the sametype as those used in group J cham, wereused. Experimental groups were dividedinto 3 subgroups (J cham Auto, J chamChx, J cham Gas) according to thedisinfection methods applied (Table II).Ten CF910T rotary instruments wereused for each group. After each cut, theinstrument was washed for 60 secondsin an ultrasonic cleaner and then steril-ized by autoclaving, cold sterilization, orethylene oxide gas. The above processwas repeated 10 times after each cut.The cutting efficiency was measured atotal of 300 times.

Autoclaving disinfection was per-formed at 125�C and 0.7 kPa for 15

try

minutes in a pressure steam sterilizer(S-330/33L; Yoshida Dental); immersionin cold sterilizing solution was performedfor 20 minutes in a 5% chlorhexidine glu-conate solution (alphahexidine; Sungk-wang), andgasdisinfectionwasperformedat 160�C for 4 hours in an ethylene oxidegas sterilizer (Steri-Vac 5XL; 3M).

A platinum coating was applied tothe 102R, 104R, CF910T, and CF1007Tinstruments that were not used for cut-ting. Two rotary instruments from eachgroup were used for 10 cuts. They werethen attached to a scanning electronmicroscope (SEM) stud. Images weremade at �45 and �200 magnifications(S-3000N; Hitachi). Component anal-ysis was performed by using energydispersive x-ray spectrometry.

Software (PASW 17.0; SPSS Inc)was used for statistical analysis. One-way ANOVA was used to identify dif-ferences in cutting efficiency with thediamond rotary instruments (S cham, Jcham, S thin, J thin), disinfectionmethod (J cham, J cham Auto, J chamChx, J cham Gas) and repeated-measures 2-way ANOVA were used toidentify the differences in the total cut-ting efficiency with the configurationsof the diamond rotary instrument.Repeated-measures ANOVA was usedfor the change trend in the cutting ef-ficiency with the configuration of thediamond rotary instrument and disin-fection method. The Tukey honestlysignificant difference method was usedfor the post hoc tests.

RESULTS

The total mean (SD) cutting effi-ciency was reduced after 10 cuts in the4 groups in the following order: group

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10.00

0.10

0.20

Cut

ting

eff

icie

ncy

(g/m

in)

0.30

0.40

2 3 4 5Number of cuts

6 7 8 9 10

Group1 S cham2 J cham3 S thin4 J thin

2 Linear graph of average cutting efficiency of each groupwith increase in number of cuts.

January 2014 67

J cham (0.210 �0.064 g/min) > groupS cham (0.170 �0.064 g/min) > groupJ thin (0.130 �0.042 g/min) > group

3 Scanning electron photomicrographs acuts with diamond rotary instrument. Wh

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S thin (0.010 �0.040 g/min) (P<.05).The cutting efficiency differed signifi-cantly among the 4 groups. The cutting

t 200. A, S cham group. B, J cham group.ite circles indicate defects.

efficiency increased with the number ofcutsonly at the thirdmeasurement for theJ cham group and the S cham group,and at the fourth measurement for theJ thin group. It decreased for all othermeasurements. The cutting efficiencydecreased significantly between the firstand second minute in all the groups, butthe subsequent decrease after 2 minutesappeared to be slower (Fig. 2).

The cutting efficiency did not appearto change significantly with the dis-infection method (Table III). SEM im-ages revealed damage to the diamondrotary instrument after 10 cuts in allthe groups (Fig. 3). The particles werelarger for the Jin Dental instrumentsthan for the Shofu instruments. Theparticle size of Jin Dental instrumentswas larger than 200 mm (as revealed inthe images obtained at �200 magnifi-cation) (Fig. 4). The spectrometryexperiments revealed that the prin-cipal metal component was nickeland chromium for the Jin Dental

C, S thin group. D, J thin group after 10

Table III. Mean total cuttingefficiency of 4 disinfection methods

GroupMean (SD) CuttingEfficiency (g/min)

J cham 0.21 �0.065

J cham Auto 0.20 �0.066

J cham Chx 0.20 �0.064

J cham Gas 0.21 �0.064

4 Scanning electron photomicrograph at �200 of J chamgroup before cut. White arrow indicates particle larger than200 mm.

Spectrum 1

0 2 4 6 8 10 12 14keVFull scale 1692 cts; Cursor: 0.000

Ni Ni

Ni

B

Spectrum 1

0

Cr

CrNi Ni Ni

2 4 6 8 10 12 14keVFull scale 1607 cts; Cursor: 0.000

Cr

A

5 Energy dispersive x-ray spectroscopy analysis. A, S chamgroup. B, J cham group.

68 Volume 111 Issue 1

instruments and nickel for the Shofuinstruments (Fig. 5).

DISCUSSION

The results of this study demon-strated that significant differences didexist in the changing trends of cuttingefficiency among the 4 different dia-mond rotary instruments after repeatedcuts. No significant difference was no-ted in the cutting efficiency amongthe various kinds of disinfection used.Therefore, the null hypotheses were re-jected. This study found that the cut-ting efficiency differed significantlyaccording to the types of diamond ro-tary instrument. The cutting efficiencywas higher for chamfered rotary in-struments than for thin tapered rotaryinstruments. This is attributable to thelarger diameters and hence larger cut-ting area of the chamfered rotary in-struments with the same handpiecespeed; the peripheral speed is higher foran instrument with a larger diameter.14

This speed enhances the cutting effi-ciency. The change in the cuttingefficiency over time differed amongthe rotary instruments. The high pe-ripheral speed of the chamfered rotaryinstruments appeared to result in asteeply decrease in cutting efficiency.

The cutting efficiency of the JinDental instruments was significantlyhigher than that of the Shofu in-struments, regardless of the configura-tion of the diamond rotary instrument.The irregularity of the grit may be dueto the nonuniform size of the diamondparticles. The SEM images (�200magnification) revealed particles with

The Journal of Prosthetic Dentis

sizes that ranged from 100 to 200 mmonthe same instrument, whereas the SEMimages (�45 magnification) showedthat the particles on the Jin Dental dia-mond rotary instrument were largeroverall; particles larger than 200 mmwere observed at �200 magnification.The diamond rotary instruments of Jin

try

Dental were classified as medium gritsize by themanufacturer. However, theseinstruments should be classified ascoarse grit according to InternationalOrganization for Standardization speci-fications. Siegel and von Fraunhofer4

reported that cutting efficiency in-creased with diamond particle size. It

Bae et al

January 2014 69

is assumed that the cutting efficiency ofthe Jin Dental instruments was greaterthan Shofu due to the difference in par-ticles size.

Consistent with the results of Pilcheret al,10 the cutting efficiency markedlydecreased at the second cut in all groups,but the subsequent decrease in the cut-ting efficiency was slower. It is assumedthat the diamond particles were sepa-rated when cutting started. Separation ofthe particles from the matrix left onlysome particles with a large amount ofmetal matrix, thereby decreasing thecutting efficiency.15 In addition, the sep-aration pattern can differ with theproperties of the metal matrix that coversthe diamond particles. Although energydispersive x-ray spectrometry revealedthat the Jin Dental instruments largelyconsisted of nickel and chromium,whereas the Shofu instruments consistedonly of nickel, the initial decreasingtrend in cutting efficiency did not differbetween the Jin Dental and Shofu in-struments, regardless of the configura-tions of the diamond rotary instrument,which suggests that the initial separationtrend of diamond particles from themetal matrix was similar for the rotaryinstruments by 2 different manufacturers(Jin Dental and Shofu). However, it isdifficult to confirm the amount of parti-cle exfoliation from 2-dimensional im-ages. Future studies are required formore detailed measurements of surfaceroughness.

Neither the cutting efficiency nor thetotal cut amount varied with the dis-infection method. None of the 3 in-vestigated disinfection methods hada negative influence on the cuttingefficiency during 10 cuts. A diamond ro-tary instrument may cause cross infectionof lesions that are infected by blood.16

Although the use of disposable in-struments is recommended to minimizethe possibility of cross infection, theircutting efficiency appears to be lowerthan those of standard diamond rotaryinstruments.3,10 Even allowing for therequired disinfection and sterilizationprocedures after each use, standard dia-mond rotary instruments would still bepreferable. The present study found that

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autoclaving had no effect on the cuttingefficiency, which is consistent with thefindings of both Gureckis et al13 andChung et al.9 Ethylene oxide gas sterili-zation also did not influence the cuttingefficiency, which also is consistent withprevious findings. Moreover, 5% chlor-hexidine gluconate solution had noinfluence on the cutting efficiency. Hark-ness and Davies12 reported that the use adisinfectant with a pH that exceeded 11after ultrasonic cleaning may causecorrosion of the nickel matrix, whichwould lead to the loss of diamond par-ticles. Immersion in a cold chemicalsterilant solution instead of ethyleneoxide gas sterilization or autoclaving isnot recommended. However, chlorhexi-dine gluconate was used in the presentstudy because this solution is often usedto remove dentin remnants or bloodclots from the rotary instruments aftertooth preparation and before sterilizationwith ethylene oxide gas or autoclaving. Inthis study, only the CF970T instrumentswere used to examine disinfection andsterilization. Further studies that includevarious diamond rotary instrumentswould enhance the understanding of thechange of cutting efficiency caused by thedisinfection process.

Dentists generally apply a force of0.66 to 2.23 N to the instrument duringtooth preparation.17 Siegel and vonFraunhofer8,18 compared the cuttingefficiency when applying forces of 0.44,0.92, and 1.83 N to the instrumentand found that 0.92 N was the mosteffective force for instruments withmedium-size particles. Because thecutting efficiency will vary with the forceapplied to the instrument, a constantforce of 0.92 N was applied in thepresent study. Even if the use of actualteeth reproduced the best clinical con-dition, their inconsistent thicknessesand hardnesses could result in unreli-able measurements of the cutting effi-ciency. The present study, therefore,used Macor block (nonperforated glassceramic) as a specimen for cutting. Itshardness of 250 KHN and elasticmodulus of 66.9 GPa are similar to thecorresponding values of 300 to 340KHN and 84 GPa for tooth enamel.19

The present study has several limi-tations. Although the same handpiecewas used, revolutions per minute (rpm)and coolant were not monitored. Ercoliet al20 demonstrated no differences inrpm among various diamond rotarycutting instruments with the same airturbine handpiece. However, the lowtorque of an air-turbine handpiececaused load-dependent decreases inrotational rate. The rpm of the hand-piece could change during cut-ting.3,21,22 The heat generation duringtooth preparation may cause pulpdamage.23,24 However, Galindo et al25

and Ercoli et al20 showed that toothpreparation with an adequate waterflow does not generate harmful heat inthe pulp chamber. von Fraunhoferet al26 showed a 215% increase in cut-ting efficiency when the water flow wasincreased from 15 to 44 mL/min.

The clinical preparation of an actualtooth is a dynamic process in whichthe rotary instrument is kept moving,whereas, in this study, the tooth wasprepared vertically and statically withoutanymultidirectional dynamicmovementof the rotary instrument. Moreover, thehandpiece was rotated around a pivot-ing point and the diamond rotary in-strument had a curved trajectory. Ercoliet al20 and Galindo et al25 controlled themovement of the instrument with com-puter software and monitored rpm andcoolant. Further studies should includethese designs to demonstrate more reli-able data regarding the cutting efficiencyof rotary diamond instruments. Thisstudy assessed the effects of the type ofdiamond rotary instrument and thechange in the cutting efficiency causedby repeated disinfection; the data ob-tained could be used as reference infor-mation when selecting diamond rotaryinstruments. Nonetheless, the negativeeffects exerted by large diamond parti-cles were not verified. In addition,although a change trend in the cuttingefficiency was revealed, this informationwas insufficient in explaining the servicelife of a diamond rotary instrument.Future studies should use differentmethods to facilitate the production ofclear clinical standards.

70 Volume 111 Issue 1

CONCLUSIONS

Within the limitations of this in vitrostudy, the following conclusions weredrawn:

1. Cutting efficiency was signifi-cantly higher for the chamfered dia-mond rotary instrument than for the thintapered diamond rotary instrument. Thedecrease in cutting efficiency with thenumber of cuts was greater for thechamfered diamond rotary instruments.

2. Cutting efficiency decreased as thenumber of cuts increased, regardless ofthe type of diamond rotary instrument,and this decrease was particularly largeafter the first cut.

3. Repeated disinfection by auto-claving, immersion in chlorhexidinegluconate, and ethylene oxide gas sterili-zation did not influence the decreasingtrend of the cutting efficiency of the dia-mond rotary instruments (the same typeas those used in the J cham group).

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4. Siegel SC, von Fraunhofer JA. Cuttingefficiency of three diamond bur grit sizes.J Am Dent Assoc 2000;131:1706-10.

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15. Klimek L, Banaszek K. The mechanisms ofwear in burs with diamond grits. Journal ofSuperhard Materials 2007;29:185-8.

16. Infection control recommendations for thedental office and the dental laboratory. ADACouncil on Scientific Affairs and ADACouncil on Dental Practice. J Am Dent Assoc1996;127:672-80.

17. Elias K, Amis AA, Setchell DJ. The magnitudeof cutting forces at high speed. J ProsthetDent 2003;89:286-91.

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18. Siegel SC, von Fraunhofer JA. Dental cuttingwith diamond burs: heavy-handed or light-touch? J Prosthodont 1999;8:3-9.

19. Fan PL, Stanford JW. Ceramics: their place indentistry. Int Dent J 1987;37:197-200.

20. Ercoli C, Rotella M, Funkenbusch PD,Russell S, Feng C. In vitro comparison of thecutting efficiency and temperature produc-tion of ten different rotary cutting in-struments. Part I: Turbine. J Prosthet Dent2009;101:248-61.

21. Taylor DF, Perkins RR, Kumpula JW. Char-acteristics of some air turbine handpieces.J Am Dent Assoc 1962;64:794-805.

22. Brockhurst PJ, Shams R. Dynamic measure-ment of the torque-speed characteristics ofdental high speed air turbine handpieces.Aust Dent J 1994;39:33-8.

23. Schuchard A. Pulpal response to coolingarmamentaria at ultrahigh speed. J ProsthetDent 1979;41:58-62.

24. Spierings TA, Peters MC, Plasschaert AJ.Thermal trauma to teeth. Endod DentTraumatol 1985;1:123-9.

25. Galindo DF, Ercoli C, Funkenbusch PD,Greene TD, Moss ME, Lee HJ, et al. Toothpreparation: a study on the effect of differentvariables and a comparison between con-ventional and channeled diamond burs.J Prosthodont 2004;13:3-16.

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Corresponding author:Dr Keun-Woo LeeDepartment of ProsthodonticsCollege of DentistryYonsei University50 Yonsei-ro, Seodaemun-guSeoul 120-752SOUTH KOREAE-mail: kwlee@yuhs.ac

Copyright ª 2014 by the Editorial Council forThe Journal of Prosthetic Dentistry.

Bae et al