s for reducing cutting temperature in ultrasonic cutting of bone.pdf
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轴类零件工艺分析及抛光装置设计,零件,工艺,分析,抛光,装置,设计
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Methods for reducing cutting temperaturein ultrasonic cutting of boneAndrea Cardoni, Alan MacBeath, Margaret Lucas*Department of Mechanical Engineering, University of Glasgow, Glasgow G12 8QQ, UKAvailable online 30 June 2006AbstractUltrasonic cutting is widely used in food processing applications to produce a clean and accurate cut. However, it is yet to be adoptedas an instrument of choice in orthopaedic applications, mainly due to the high temperatures that can be generated at the cut site and theconsequent requirement to use additional cooling. For example, if cutting temperatures above 5560 ?C are reached, particularly for sus-tained periods, bone necrosis can occur, compromising post-operative recovery.A recent study by the authors has shown that the thermal response in natural materials, such as wood and bone, is affected by theabsorption of ultrasonic energy and conduction of heat from the cut site. In this paper the dependency of cutting parameters, such asblade tip vibration velocity, applied load, tuned frequency and coupling contact conditions, on the thermal response are reported andresults show that it is possible to maintain cutting temperatures within safety limits by controlling the cutting parameters. A novel cuttingblade design is proposed that reduces frictional heat generated at the cut site. Through a series of experimental investigations using freshbovine femur it is demonstrated that the cutting temperature, and hence thermal damage, can be reduced by selecting appropriate cuttingparameters and blade profile.? 2006 Elsevier B.V. All rights reserved.Keywords: Ultrasonic bone cutting; Temperature; Experimental modal analysis; Modal coupling1. IntroductionBone cutting instruments, such as burs, saws and chisels,offer limited precision and manoeuvrability to surgeons 1and often result in tissue burning, formation of debris anddamage of adjacent tissue. An alternative bone cuttingdevice is an ultrasonic blade, Fig. 1(a), which is tuned toa longitudinal vibration mode at a frequency in the lowultrasonic range (20100 kHz). The reported benefits ofultrasonic cutting of hard tissue include elimination ofswarf, reduced reaction forces and a more accurate cut.Ultrasonic osteotomy is not a novel concept, withdevices dating back to 1957 2. However, limitations intool and transducer design and the lack of suitable methodsfor fine-tuning power control, considerably restricted theearly development of the technology. In the last fifteenyears, following improvements in transducer design andthe development of more sophisticated electromechanicalpower control, interest has been renewed in ultrasonic sur-gical devices 3.The current challenge for ultrasonic bone cutting residesin the development of tuned systems capable of deliveringsufficient acoustic power to cut hard tissue without exceed-ing the temperature of bone necrosis. To overcome theproblem of tissue burning, ultrasonic cutting devices usu-ally need to incorporate cooling systems, which deliverwater (or saline solution) to the cut site 3,4, but this offersadditional problems of cross-contamination. This studyinvestigates opportunities for controlling the cutting tem-perature, by studying the effects of cutting parametersand cutting blade geometry on cutting temperature, withthe aim of designing an ultrasonic cutting device capableof deep cuts in bone without the need for a cooling system.0041-624X/$ - see front matter ? 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.ultras.2006.06.046*Corresponding author. Fax: +44 (0) 141 330 4343.E-mail address: m.lucasmech.gla.ac.uk (M. Lucas)./locate/ultrasUltrasonics 44 (2006) e37e422. Thermal response characteristics during ultrasonic cuttingPrevious work by the authors has shown that the ther-mal response, measured during ultrasonic cutting of a vari-ety of materials, exhibited two temperature peaks 5,6.Fig. 1(b) shows a typical thermal response measured inbovine femur specimens. Qualitatively similar responseswere measured in artificial bone and several grades ofwood.The first sharp temperature peak in the measuredresponse occurs due to the absorption of the ultrasonicenergy generated by the blade vibration in the materialsample during cut initiation. The temperature peak magni-tude increases with increased static load applied to theblade, because increasing the static load improves couplingbetween the blade and material. The temporal response isindependent of the location of the measurement sensor inthe specimen 4.The second peak in temperature in the measuredresponse occurs due to heat conduction generated by fric-tion between the blade and specimen as the blade pene-trates the material, resulting in a gradual increase thendecay in temperature during the measurement period. Inthis case, the temporal response depends on the locationof the sensor in the specimen. For the same cutting depth,the peak conduction temperature measured in the responsedecreases with increasing applied static load, mainlybecause the cut occurs more quickly.3. Influence of blade design on temperatureFour titanium alloy ultrasonic cutting blades tuned toresonate longitudinally at two distinct frequencies weredesigned and manufactured for the present study (Fig. 2).The dimensions of the blades were determined using finiteelementanalysis(FEA).Twoblades were tuned at19.5 kHz and two at 35 kHz. Although different lengthswere required to tune the blades, the vibration amplitudegain was kept constant and the cutting profile was consis-tent for each blade design.Previous cutting experiments revealed that the sampletemperature associated with ultrasonic energy absorptioncould reach elevated peaks, well above the temperature nor-mally quoted for bone necrosis. However, it is known thatthe necrosis temperature is not a constant and depends onthe duration for which bone experiences the elevated tem-perature. In fact, bone can withstand higher temperatureswithout thermal damage if the duration is very short 7.Fig. 2. Sketch of 35 kHz (short) and 19.5 kHz (long) ultrasonic cutting blades with (a), (b) constant cutting edge section (profile 1) and (c), (d) withindented cutting edge section (profile 2).Fig. 1. (a) Schematic of an ultrasonic cutting system, and (b) temperature response measured in bovine bone during ultrasonic cutting at different staticloads.e38A. Cardoni et al. / Ultrasonics 44 (2006) e37e42The experiments also showed that frictional heat gener-ated during cutting exposed bone samples to prolongedintervals of high temperature. This study aims to investigatethe effect of blade profile on this cutting temperature duringultrasonic cutting of bone and, in particular, the impact ofblade geometries with different areas of contact betweenthebladeandthebone.Therefore,twodifferent cuttingedgeprofiles, one with a constant section and a sharp tip (bladeprofile 1), andthe other with anindentedprofileterminatingin an identical tip (blade profile 2), were incorporated in thetuned blades for each selected frequency (Fig. 2).3.1. Experimental rigThe experimental rig designed to cut bovine femur bonesamples is shown in Fig. 3(a). Samples were cut andclamped onto a metal plate attached to a horizontal guide.The transducer and blade assembly was mounted on a sli-der, which was free to travel along the guide. In order toinvestigate the effects of the applied static force, a systemof a pulley, cable and weights was connected to one sideof the slider. The depth of each cut was monitored usinga dial gauge.Temperature measurements were conducted using sixthermocouples distributed in two rows of three, placed onopposite sides of the cutting line, as depicted in Fig. 3(b).The three thermocouples in each row were placed at dis-tances of 5, 10 and 15 mm from the top surface of the spec-imen. Probes 13 and probes 46 were positioned 1 and2 mm from the line of cut, respectively.3.2. Effects of cutting edge profile, tuned frequency andvibration amplitude on specimen temperatureFirstly, cutting experiments used the pair of bladestuned to 35 kHz. The ultrasonic amplitude was set to23 lm for both blade configurations (profiles 1 and 2),and tests were performed at a series of applied static loadsin the range 2075 N. For each cutting experiment, thetemperature of the sample was monitored for 300 seconds,to allow the specimen to cool back to room temperature,independent of the cutting time.In Fig. 4 the responses detected by the six thermocou-ples positioned in the bone specimens, being cut using35 kHz blades with profiles 1 and 2 and with an appliedload of 20 N, are shown. It is clear that significantly lowertemperatures are recorded by all the probes when cuttingwith the blade with the indented profile (profile 2) as shownin Fig. 4(b) and, in particular, a peak temperature 40 ?Clower was detected by probe 1.These improved thermal conditions stem from thereduction in the frictional contact area between the bladeand specimen during cutting. This also results in a fastercut and facilitates the removal of bone debris from thecut site. As a result, debris combustion through frictionalheating, which was previously cited as a key cause of tissueFig. 3. (a) Test rig for ultrasonic cutting experiments and (b) thermocouple locations in specimen.Fig. 4. Temperature responses measured in bone at six thermocouple locations using two 35 kHz blades with (a) blade profile 1 and (b) blade profile 2.A. Cardoni et al. / Ultrasonics 44 (2006) e37e42e39damage, could be reduced 5,6. The effects of applied staticload on temperature, using blade profiles 1 and 2, areshown in Fig. 5 for blade tip amplitudes of 23 lm and40 lm. In the figure, the difference in peak cutting temper-ature recorded between blade profile 1 and blade profile 2,is plotted against the applied static load. The measure-ments consistently recorded a reduction in the peak tem-perature when using blade profile 2. A small number ofdeviations from this trend appear due to slight inaccuraciesin positioning of the probes.Thesameexperimentswereconductedusingthe19.5 kHz blade pair to investigate any frequency depen-dency of cutting temperature. Both blades operated at avibration amplitude of 40 lm, giving the same blade tipvibration velocity as the 35 kHz blades operating at23 lm. In previous studies, vibration velocity has beendemonstrated to be the influencing vibration parameter inultrasonic cutting experiments 6.By comparing Fig. 6 with Fig. 5(a) it is seen that themeasured temperature differences between profile 1 andprofile 2 are much smaller than for the 35 kHz bladesand, therefore, at 19.5 kHz the peak cutting temperatureis not so dependant on the cutting edge profile for the sameblade tip vibration velocity. Again, at 19.5 kHz, the bladewith profile 2 cuts faster than the blade with profile 1.4. Blade redesign for improved vibration performanceAlthough the cutting blades have been tuned in a longi-tudinal mode, it has been shown previously that cuttingperformance is critically dependent on the vibration char-acteristics of the tuned blade. In this case, it was found thatthe tuned 19.5 kHz blade with profile 2, showed signs ofpoorer than expected cutting performance which couldprovide an explanation for the differences between the 35and 19.5 kHz blades in terms of cutting temperature.Therefore, a study of the vibration characteristics of the19.5 kHz blade with profile 2 was carried out.4.1. Linear and nonlinear modal couplingThe vibration characteristics were determined via exper-imental modal analysis (EMA) using a 3D laser Dopplervibrometer (LDV) and LMS modal analysis software.Fig. 7(a) shows a side view of the measured tuned modeshape, which reveals a significant flexural contribution tothe longitudinal mode of the blade. Fig. 8(b) shows theflexural mode that occurs at a resonant frequency veryFig. 5. Difference in peak cutting temperature between blade profile 1 and profile 2, both at 35 kHz, versus static load at blade tip vibration amplitudes of(a) 23 lm and (b) 40 lm.Fig. 6. Difference in peak cutting temperature between blade profile 1 andprofile 2 versus static load measured using the 19.5 kHz blade and tipamplitude of 40 lm.Fig. 7. Tuned longitudinal mode determined by (a) EMA and (b) FEA.e40A. Cardoni et al. / Ultrasonics 44 (2006) e37e42close to the longitudinal mode frequency and is the cause ofthe modal coupling at the tuned frequency.Moreover, when the device was driven at the tuned fre-quency, a large amount of energy leaked into an internalmodal response at half the tuned frequency, which is char-acteristic of a principal parametric resonance 8, Fig. 9.The internally excited mode corresponded to a blade tor-sional mode occurring at 9.9 kHz, shown in Fig. 10.4.2. Improving blade tuned responses via profile alterationThe vibration measurements have illustrated that theresponse of the 19.5 kHz blade is characterised by both lin-ear and nonlinear modal interactions. Such energy leakagescould have an influence on the thermal response duringcutting and, hence, a redesign is proposed to eliminatethese effects. The requirements were to uncouple the longi-tudinal mode from the untuned flexural and torsionalmodes without altering the blade length and maintain suf-ficient amplitude gain in the blade profile to allow the bladeto operate at the required tip vibration amplitudes 9,10.Two indents were incorporated to alter the width of theblade, as shown in Fig. 11(b) (blade profile 3). EMA of thenew blade measured a significant shift in the flexural modefrequency, that uncoupled the flexural mode response fromthe tuned longitudinal mode response. Also, the indentedwidth profile significantly affected the modal frequency ofthe torsional mode, achieving a frequency reduction of1.1 kHz, with the result that the nonlinear modal couplingwas also eliminated. The response of the modified bladeexhibited a linear single frequency response for blade tipamplitudes up to 55 lm.5. Effect of blade profile 3 on temperatureFurther cutting temperature measurements were carriedout using the 19.5 kHz blade with blade profile 3 at 40 lmblade tip vibration amplitude. Fig. 12 shows the differencein peak cutting temperature between the 19.5 kHz bladeswith profiles 1 and 3, for increasing static load. Despitethe improvement in cutting speed due to the new designof profile 3, and the elimination of modal interactions inFig. 8. EMA modal data for the (a) coupled longitudinal-bending mode and (b) bending mode.Fig. 9. Frequency response for system driven at 19.5 kHz in tunedlongitudinal mode.Fig. 10. The internal torsional mode determined by (a) EMA and (b) FEA.Fig. 11. Top view of: (a) the original 19.5 kHz blade with indented cutting edge (blade profile 2), and (b) the redesigned blade with additional indentedwidth profile (blade profile 3).A. Cardoni et al. / Ultrasonics 44 (2006) e37e42e41the vibration characteristics as an influencing factor in theexperiments, no significant temperature reductions couldbe achieved using this blade design. At a higher ultrasonicamplitude, of 55 lm, improved temperature reductionswere recorded in the specimens. The results suggest thatat 19.5 kHz, the influence of the cutting edge configurationon temperatures is more significant at higher ultrasonicamplitudes (see Table 1).6. ConclusionsThe effect of blade profile on the cutting temperature hasbeen studied in order to investigate ways of controlling thetemperature in bone during ultrasonic cutting. The resultsshow that a reduction in the contact area between the bladeand specimen reduces sample temperatures during cutting.In particular, at the higher tuned frequency, the indentedcutting edge profile has proved to provide consistent reduc-tions in cutting temperature for the range of static loadsand two blade tip vibration amplitudes tested. However,at the lower tuned frequency, significant temperaturereductions have only been achieved at the higher tip vibra-tion amplitude. The impact of the blade vibration charac-teristics on blade responses was highlighted and it wasshown how blade profile alterations could eliminate linearand nonlinear modal interactions, thus removing these asinfluences in the cutting experiments.References1 J.Y. Giraud, S. Villemin, R. Darmana, J.Ph. Cahuzac, A. Autefage,J.P. Morucci, Bone cutting, INSERM, Centre Hospitalier Hotel-Dieu, Toulouse, France, 1, 1991.2 A.G. Nielson, J.R. Richards, R.B. Walcott, Ultrasonic dental cuttinginstrument, Int. J. Am. Dental Assoc. 50 (1957) 392.3 T. Vercellotti, Piezoelectric bone surgery techniques in implantology:ridge expansion, sinus lift, bone harvesting and implant site prepa-ration, Academy of Osseointegration. in: 19th Annual Meeting (SanFrancisco, USA), 2004.4 A.
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