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TrabalhoEspanhol
OpenBiomedEngJ.2007;1:
71–76.
RadiofrequencyHeatingoftheCornea:
AnEngineeringReviewofElectrodesandApplicators
EnriqueJBerjano,*1EnriqueNavarro,2VicenteRibera,2JavierGorris,2andJorgeLAlió3,4
1InstituteforResearchandInnovationonBioengineering,TechnicalUniversityofValencia,Valencia,Spain
2NepturyTechnologies,Almassora,Spain
3CorneaandRefractiveSurgeryDepartment,Vissum-InstitutoOftalmológicodeAlicante,Alicante,Spain
4PathologyandSurgeryDepartment,UniversidadMiguelHernández,Elche,Spain
*AddresscorrespondencetothisauthorattheInstituteforResearchandInnovationonBioengineering,TechnicalUniversityofValencia,Valencia,Spain;E-mail:
eberjano@eln.upv.es
Abstract
Thispaperreviewsthedifferentapplicatorsandelectrodesemployedtocreatelocalizedheatinginthecorneabymeansoftheapplicationofradiofrequency(RF)currents.Thermokeratoplasty(TKP)isprobablythebestknownofthesetechniquesandisbasedontheprinciplethatheatingcornealtissue(particularlythecentralpartofthecornealtissue,i.e.thecentralstroma)causescollagentoshrink,andhencechangesthecornealcurvature.Firstly,wepointoutthatTKPtechniquesareacomplexchallengefromtheengineeringpointofview,duetothefactthatitisnecessarytocreateverylocalizedheatinginapreciselocation(centralstroma),withinanarrowtemperaturerange(from58to76ºC).Secondly,wedescribethedifferentapplicatordesigns(i.e.RFelectrodes)proposedandtestedtodate.Thisreviewisplannedfromatechnicalpointofview,i.e.thetechnicaldevelopmentsareclassifiedanddescribedtakingintoconsiderationtechnicalcriteria,suchasenergydeliverymode(monopolarversusbipolar),thermalconditions(dryversuscooledelectrodes),lesionpattern(focalversuscircularlesions),andapplicationplacement(surfaceversusintrastromal).
INTRODUCTION
Localizedheatingofthecorneahasbeenemployedsince1889fordifferenttherapeuticandsurgicalobjectives.Thebestknownofthesetechniquesisprobablythermokeratoplasty(TKP),whichisbasedontheprinciplethatheatingcornealtissue(particularlythecentralpartofthecornealtissue,i.e.thecentralstroma)causescollagentoshrink,andhencechangesthecornealcurvature[1].However,theprocesscanbeusedforothertherapeuticobjectives.TKPisacomplexchallengefromtheengineeringpointofview(seeFig.11).Ononehand,itisnecessarytocreateverylocalizedheatinginapreciselocation(centralstroma),keepingtheendotheliumthermallyprotected(whichislocatednomorethan300μmdistantfromthecentralstroma).Theendotheliumisamono-cellularlayerinthehumaneyewhichisnonregenerable,andhencethetemperatureatthispointshouldalwaysbemaintainedatasafelevel(e.g.lowerthan45ºC).Consequently,theprocedurerequiresanextremelyhighspatialresolution.Thetemperatureattheepitheliumisnotsocritical:
someTKPtechniquescombinesurfacecoolingtocreateatemperatureprofilewithalowtemperatureattheepithelium(seesolidlineinFig.11).Incontrast,othertechniques(likeintrastromalapplicators)heatboththeepitheliumandthecentralstroma(seedashedlineinFig.11).
Fig.
(1)
Left:
Schematicdiagramofthecorneashowingtheanteriorlayer(epithelium),theposteriorlayer(endothelium),separatedbythestroma.Theaimofthethermokeratoplasty(TKP)procedureistocreatesafelocalizedheatinginthecentralstroma.Right:
Optimum(solidline)andsuboptimum(dottedline)thermalprofilesinthecorneaforthemokeratoplasty(TKP).TKPtechniquesareacomplexchallengefromtheengineeringpointofview,duetorequiring:
1)anextremelyhighspatialresolution(i.e.itisnecessarytocreateverylocalizedheatinginapreciselocationofthecentralstroma–redzone–,thermallyprotectingtheendothelium–greenzone–,whichisplacednomorethan300μmdistantfromcentralstroma);and2)ahighresolutionofthetemperaturereachedatthetargetpoint(temperaturesrangingfrom58to76°Carerequiredtoshrinkthecollagen,andtemperaturesover79°Careharmful).Sincetheepitheliumisalayerwithregenerationcapability,itcanstandheatingduringTKP(dottedline).However,intheoptimumTKPtechniquetheepitheliumshouldbekeptcool(solidline)andtheheatfocusedonthecentralstroma.
Itisknownthatcollagenshrinkageoccursattemperaturesrangingfrom58to76ºC.However,higherthermallevels,79ºCorover,definitelyleadtorelaxationofthecollagenandcompletelossofitselasticity,inducingimportantkeratocyteproliferationandacceleratingcollagenturnover(i.e.provokingaregressionofthecorrectedrefractiveerror)[2].Therefore,theprocedurealsorequireshighresolutionofthetemperaturereachedatthetargetpoint(seeFig.11).
Inordertocreatecornealheating,differentkindsofenergysourceshavebeentested,suchassimplethermalconductionfrompre-heatedprobes,knownasthermokeratophores[3,4],microwaves[5-7],laser[2]andultrasound[8-10],andradiofrequency(RF)currents.However,ourinterestisfocusedontechniquesforcornealheatingbymeansofRFcurrents(≈500kHz),thereforeinthisreviewwedescribethedifferentdesignsofapplicators(i.e.RFelectrodes)proposedandtestedtodate.Thisreviewisplannedfromatechnicalpointofview,i.e.thetechnicaldevelopmentsareclassifiedanddescribedtakingintoconsiderationtechnicalcriteria,suchasenergydeliverymode(monopolarversusbipolar),thermalconditions(dryversuscooledelectrodes),lesionpattern(focalversuscircularlesions),andapplicationplacement(surfaceversusintrastromal).Consequently,thedevelopmentsarenotnecessarilyrelatedtoeitherclinicalproceduresortrademarks.
APPLICATORSWITHSURFACECOOLING
ThefirstapplicatordesignedforapplyingRFcurrentsinthehumancorneawasdescribedbyDossandAlbillar[11]in1980.SinceitwasdevelopedatLosAlamosScientificLaboratory(NM,USA),thedevicewasnamedtheLosAlamosKeratoplastyprobe[1].However,DossandAlbillarcalleditthecirculatingsalineelectrode(CSE),since,whiletheactiveelectrodedeliveredRFcurrents(1.6MHz)aflowofisotonicsaline(at37ºC)wasinfusedoverthecorneasurface[11,12](seeFig.2B2B).
Fig.
(2)
RFelectrodesandapplicatorsforcreatingalesionatacertainpointinthecornea.A:
Apairofelectrodescausealesionbymeansofabipolarapplication.B:
Thecirculatingsalineelectrode(CSE)worksbydeliveringRFcurrentsfromtheactiveelectrodetoadispersiveelectrodewhileaflowofisotonicsalineisinfusedoverthecorneasurface.C:
AsmallsurfaceelectrodeisplacedonthecorneaandRFcurrentsareappliedbetweenthisandadispersiveelectrode.Thelesionishenceconfinedtothecorneasurface.D:
Anintrastromalelectrodeisaneedle-shapedelectrodewhichpenetratesfromthesurfacetothecentralstroma.Thelesionhasmoredepththaninthecaseofasurfaceelectrode(C).
ThebasicideaoftheCSEwastoimprovethetemperatureprofileobtainedbythethermokeratophoreduringthermokeratoplasty(TKP).Thethermokeratophorewasametallicprobepreheatedtoaspecifictemperaturerange(90-130ºC)andplacedonthecorneasurface,i.e.thethermallesionwascreatedbythermalconductionfromthethermokeratophoretowardsthecornealstroma.Consequently,themaximumtemperaturewasreachedatthecorneasurface(≈75ºC),whilethecentralstromaremainedat≈45ºC[11].Incontrast,usingtheCSE,themaximumtemperatureinthecorneawasreachedatthecentralstroma(≈70ºC),bothepitheliumandendotheliumremainingatatemperaturelowerthan50ºC.Theapplicatorhadamonopolarsystem,i.e.theRFcurrentsweredeliveredbetweentheCSE(activeelectrode)andalargedispersiveelectrodeplacedawayfromtheCSE.Thistechniquewasfirsttestedonexcisedpigs’eyes[11],lateronaninvivomodelwithdogs’eyes(sacrificedwithinminutesofthetreatment)[13],andfinallysomepreliminaryclinicaltrialwereconductedonpatientstotreatkeratoconus[14-16].AlthoughtheCSEwasveryeffectiverightaway(thecorneaswereflattened),itwasalsoacommonexperiencethattheflatteningofthesediseasedcorneastendedtodiminishsignificantlywithinafewweeks.Unfortunately,theprotocoldidnotallowperformingre-treatmentstodeterminewhetherlongertermstabilitycouldbeachieved(Doss,personalcommunication,Oct.11,1995).Itisalsoimportanttopointoutthatthistechniquehasneverbeenemployedonrelatively“normal”humancorneassuchasthosefoundinastigmatismandhyperopicconditions(incontrast,inkeratoconusthecorneahasabnormalmorphologyandisalsoverythin).
TheCSEtechniquewasalsoproposedusingabipolarsystem(i.e.withoutusingadispersiveelectrode).Severalarrangementswithavariablenumberofelectrodeswereproposed[17]butnevertested.ThemostnovelideaofCSEwastocoolthecorneasurfaceduringheating.Infact,thesameideawaslateremployedinmicrowavethermalkeratoplasty[5-7],andmorerecently,asystemforsurfacecoolingofthecorneaduringTKPhasbeenpatented,especiallyforelectrical-inducedtechniquessuchasRFormicrowaveTKP[18].
APPLICATORSWITHOUTSURFACECOOLING(DRYAPPLICATORS)
PriortothedevelopmentoftheCSE,someexperimentalstudieswereconductedbyusingatwo-electrodeapplicatorfordeliveringRFcurrents(2MHz)inabipolarsystem(seeFig.2A2A).Theapplicator,knownasthelocalizedcurrentfield(LCF)device,wasalsodevelopedatLosAlamosScientificLaboratory[19].Itwasemployedintheveterinaryfieldforhyperthermictherapyforneoplasiaordegenerativecornealdiseases[