專業(yè)英語 Unit 33教案學(xué)習(xí)資料

UnitThirtyThreeTypesofbridgeTheappliancesusedtoreplacemissingteethweredefinedinChapter7.Someofthetermsusedinbridgeworkarealsousedinrelationtopartialdentures.?Anabutmentisatoothtowhichabridge(ofpartialdenture)isattached.?Aretainerisacrownorotherrastorationthatiscementedtotheabutment.Thetermsretainerandabutmentshouldnotbeconfusedorusedinterchangeably.?Aponticisanartificialtoothaspartofabridge.?Aspanisthespacebetweennaturalteeththatistobefilledbythebridge.?Thesaddleistheareaoftheedentulousridgeoverwhichtheponticwilllie.?Apierisanabutmenttoothstandingbetweenandsupportingtwopontics.Eachponticbeingattachedtoafurtherabutmenttooth.?Aunit,whenappliedtobridgework,meanseitheraretainerorapontic.Abridgewithtworetainersandoneponticwouldthereforebeathree-unitbridge.?Aconnector(orjoint)connectsapontictoaretainer,ortworetainerstoeachother.Connectorsmayeitherbefixedorallowsomemovementbetweenthecomponentsthattheyjoin.Basicdesign,combinationsandvariationsTherearefourbasicdesignsofbridge,thedifferencebeingthetypeofsupportprovidedateachendofthepontic.Thesamenameisgiventothedesignhowevermanyponticsinthespanandabutmentteethsplintedatoneendofthespan(seeFigure85).Thefourbasicdesignsarethesamewhetherthebridgeisaconventionaloraminimal-preparationtype.Itispossibletocombinetwoormoreofthefourbasicdesignsandtocombineconventionalandminimal-preparationretainersinthesamebridge.Ofthefourbasicdesigns,thefirstthreemaybeeitherconventionalorminimal-preparationtype.Itwouldbeunusualtohaveaminimal-preparationversionofthespringcantileverbridge.ThefourbasicdesignsFixed-fixedbridgeAfixed-fixedbridgehasarigidconnectoratbothendsofthepontic.Theabutmentteetharethereforerigidlysplintedtogetherandforaconventionalbridgemustbepreparedparalleltoeachothersothatthebridge,whichisaminimumofthreeunits,canbecementedinoneplace.Theretainersshouldhaveapproximatelythesameretentionaseachothertoreducetheriskthatforcesappliedtothebridgewilldislodgeoneretainerfromitsabutment,leavingthebridgesuspendedfromtheotherabutment.Tominimizethisriskitisalsoimportantfortheentireoccludingsurfaceofalltheabutmentteethforaconventionalbridgetobeco4eredbytheretainers.Theopposingteethcannotthencontactthesurfaceofanabutmenttooth,depressitinitssocketandbreakthecementlute.Ifthisshouldhappen,theretainerwillnotappearloose,asitwillstillbeheldinplacebytherestofthebridge.However,oralfluidswillenterthespacebetweentheretainerandtheabutmentpreparationandcarieswillrapidlydevelop(seeFigure86).Thisruledoesnotapplytominimal-preparationbridgesinwhichthebondbetweentheretainerandtheabutmenttoothismuchstronger.However,itissometimesnotstrongenoughanddebondingsometimesoccursasaresultofamechanismsimilartothatshowninFigure86.Thisprobablypartlyaccountsforthehigherincidenceofretentionfailurewithminimal-preparationbridgesthanwithconventionalbridge.Atonetimeitwasthoughtthatthesupportfortheabutmentteethateachendofafixed-fixedbridgeshouldbesimilar.Inotherwords,therootsurfaceareaoftheabutmentsshouldbeapproximatelythesame.Todaythisisnotconsiderednecessary.Afixed-moveablebridgehasarigidconnector,usuallyatthedistalendoftheponticandamoveableconnectorthatallowssomeverticalmovementofthemesialabutmenttooth.Themoveableconnectorshouldresistbothseparationoftheponticfromtheretainerandlateralmovementofthepontic(seeFigure87).Figure85Fourbasicbridgedesignsofconventionalbridges.aFixed-fixeddesignBothupperandlowerbridgewillbefixed-fixed,thelowerretainedbyfullcrownsonthecaninetoothandcentralincisor.Theupperbridgewillberetainedbythecanineteethonly.Fixed-moveablebridgeOccasionallythefixedandmoveableconnectorsarereversedbutthishasanumberofdisadvantages.Theretainerwiththemoveableconnector(theminorretainer)issmallerandlessvisibleandsoisbetterinthemoreanteriorabutmenttooth.Mesialdrifttendstounseatdistalmoveableconnectors,butisresistedbymesialones.Themoveableconnectorcanbeseparatedbeforethebridgeiscementedandsothetwopartsofthebridgecanbecementedseparately.Theabutmentteethdonotthereforehavetobepreparedparalleltoeachotherandtheretentionfortheminorretainerdoesnotneedtobeasextensiveasforthemajorretainer.Neitherdoesitneedfullocclusalprotection.(occlusalforcesappliedtothetoothsurfacenotcoveredbytheretainerwilldepressthetoothinitssocketandtherewillbemovementatThemoveablejointratherthanrupturingofthecementlute(seeFigure86).bFixed-moveabledesignwithDoinlayinthelowerseenpremolarandfullcrownonthemolartooth.Thisbridgehasbeenpresentfortwentyyears,infeetsolongthattheocclusalsurfaceofthecrownhaswornthrough(seeChapter13).ThemoveableJointcanbeseenbetweentheponticandtheminorretainer.Itwouldnotnormallybeasobviousasthis.cCantileverdesignBothlateralincisorsareponticssupportedbycrownsonthecanineteeth.Bothbridgesareall-porcelainandtherightonehasbeenpresentforfourteenyears.Theleftonefracturedfatersevenyearsandwasreplacedwithanotherallporcelainbridge,whichisstillpresent.dSpringcantileverdesignwithfirstmolartoothasabutment.Thereisamidlinediastemaandadiastemabetweenthelateralincisorandcanineonthesideofthemissingcentralincisor.Anyotherbridgedesignwouldhaveinvolvedclosingoneorbothofthesespecs.Afixed-moveableminimal-preparationbridgecannothavethemoveablejointwithinthecontouroftheoriginalabutmenttooth,asthisisnotpreparedsufficientlyforthemoveableconnector.Thismeansthatthemoveableconnectorisintheponticofthebridgeandisusuallyathroughandthroughtubeandpinarrangement.Thismeansthatthetwopartsofthebridgeareconnectedbutifeitherdebondsthenitcanberemovedwithoutdisturbingtheotherpartofthebridgeandrecemented.However,thisdesignofmoveablejointiscumbersomeandtheundersurface,whichapproximatestheridge,isdifficulttoclean.Itisthereforenotcommonlyused.Figure86Aconventionalfixed-fixedbridgeshouldhavealltheoccludingsurfacesoftheabutmentteethprotectedbytheretainers.otherwiseanocclusalforcedirectedattheunprotectedareawilldepresstheabutmenttoothinitssocketwhilethebridgeandtheotherabutmenttoothholdtheretainer.Thiswillbreakdownthecementlute,causingleakage.Theretainerisheldinplacebythebridgeandsosecondarycariesdevelopsrapidly(seeFigure146.page232)Figure87aAniveablecibbectirmseorated.bAcryllcburn-outpatternsforpatternsformoveableconnectors.TheblueIsverytapered,theredmoreparallel-sided.CantileverbridgeAcantileverbridgeprovidessupportfortheponticatoneendonly.Theponticmaybeattachedtoasingleretainerortotwoormoreretainerssplintedtogether,buthasnoconnectionattheotherendofthepontic.Theabutmenttoothorteethforacantileverbridgemaybeeithermesialordistaltothespanbutforsmallbridgesareusuallydistal.SpringcantileverbridgeSpringcantileverbridgesarerestrictedtothereplacementofupperincisorteeth.Onlyoneponticcanbesupportedbyaspringcantileverbridge.Thisisattachedtotheendofalongmetalarmrunninghighintothepalateandthensweepingdowntoarigidconnectoronthepalatalsideofasingleretainerorapairofsplintedretainers.Thearmismadelongandfairlythinsothatitisspringy,butnotsothinthatitwilldeformpermanentlywithnormalocclusalsurfaces(i.e.,exceedtheelasticlimit).Forcesappliedtotheponticareabsorbedbythespringinessofthearmandbydisplacementofthesofttissuesofthepalatesothatexcessiveleverageforcesdonotdisturbtheabutmentteeth.Theabutmentsareusuallythetwopremolarteethsplintedtogether,orasinglepremolarormolartooth.CombinationdesignsThefourbasicdesignscanbecombinedinavarietyofways.Inparticular,thefixed-fixedandcantileverdesignsareoftencombined.Inlargerbridgesadditionalcantileverponticsmaybesuspendedfromtheendofalargefixed-fixedsection(seeFigure88).Similarly,itispossibletocombinefixed-fixedandfixed-moveabledesigns.Itislesscommonforspringcantileverbridgestobecombinedwithothertypesofbridge,butoccasionallyaspringcantileverarmisattachedtoaposteriorfixed-fixedbridge.Itispossibletocombineabridgewitharemovablebuccalflangethatreplaceslostalveolartissue.HybriddesignsTherearethreedifferenthybriddesigns:?Fixed-fixedwithoneconventionalandoneminimal-preparationretainer?Fixed-moveablewithaminimal-preparationretainercarryingthemoveableconnector?Fixed-movablewiththeconventionalretainercarryingthemoveableconnector.Thefirsttwodesignsshouldnotbeused.Ineithercaseiftheminimal-preparationretainerbecomesdebondedthenitwillnotbepossibletorecementitwithoutremovingtheconventionalretainerwhomaywellinvolvedestroyingthebridge.Thethirddesignisacceptableandmaywellbetheoneofchoicegivencircumstancesinwhichoneoftheabutmentteeth(usuallythemesialone)alreadyhasarestorationwhichcouldbereplacedbymeansofaninlayorotherconventionalretainer.Theotherabutmenttoothisunrestoredortherestorationdoesnotinvolvethesurfacestobecoveredbyaminimal-preparationretainer.Thesecircumstancesoccursurprisinglycommonlyandsothisdesignofbridgeisbeingusedincreasing.VariationsRemovablebridgesAllthedesignsdescribedsofararepermanentlycementedinthepatient'smouth.Withlargebridgestherearedisadvantagesinpermanentcementationinthatthemaintenanceandfurtherendodonticorperiodontaltreatmentofabutmentteethisdifficult,andifsomethinggoeswrongwithonepartofthebridgeorwithoneoftheabutmentteeth,usuallythewholebridgehastobesacrificed.Forthisreasonlargerbridges,includingfullarchbridges,aresometimesmadesothatthedentistwithoutbeingdestroyedcanremovethem.Cementingindividualcastgoldcopingstoeachoftheabutmentteethdoesthis.Someofthecopingshavethreadedsleevessolderedintothem,andthebridgeisthencementedtothesecopingswithweakcement.Itisheldinplacewithlinescrewsthatpasthroughholesin.theretainersofthebridgeintothethreadedsleeves.Figure88Alargesplint/bridgewithcantileveredpontics.aTheworkingdies.Figure88bThemetalframeworkshowingtwocantileveredponticsontherightofthepicture.Inothercasesthebridgeisremovablebythepatientandhasnocementorscrews.Theadvantageofthisisthatcleaningaroundtheabutmentteethandundertheponticsismucheasier.Thebridgehastowithstandhandlingbythepatientandsoitisusuallymadeinabasemetalwithacrylicfacings.Thebasemetalismorerigidthanapreciousmetalframeworkandtheacrylicfacingsarelessliabletochipifthebridgeisdropped.Theycanalsobereplacedwithouttheriskofdistortingtheframework.AdvantagesanddisadvantageofthefourbasicdesignsAcomparisonofconventionalfixed-fixed,fixed-moveableandcantileverbridgesisshownonpage240-241Springcantileverbridgesarediscussedseparately.SpringcantileverbridgeThesearedealtwithseparatelybecausetheyareusedonlytoreplaceupperincisorteeth,usuallywhentherearespacesbetweentheincisorteethandtheadjacentpotentialabutmentteetharesound.Theyshouldnotbeusedtoreplacelowerincisorsastheshapeoftheridgeandtheresilienceofthesofttissuesarenotsuitedtoabsorbingpartoftheocclusalloading.Theadvantagesofthedesignare:theponticnotbeingattachedtoadjacentteeth,spacescanbepreservedbothsidesofit,andthesoundnaturalanteriorteethdonotneedtobepreparedasabutments.Disadvantages:somepatientsfindthepermanentbarrunningacrossthepalateintolerable.Althoughmostgetusedtothissurprisinglywell,itisverydifficulttopredictthosewhowillnotbecomeaccustomedtoit.Ithasonoccasionbeennecessarytoremoveaperfectlysoundspringcantileverbridgefornootherreasonthanthatthepatientcouldnottoleratethefeelingofthebar.Amorecommonproblemisthedifficultyofcleaningunderthebar,particularlyatitsconnectorwiththeretainer.Theeffectivewayofcleaningtheareaistopassdentalfloss,superfloss,orotherthreadmaterialovertheponticandalongthebarandtocleanbothunderitandundertheconnector.Thisrequiresconsiderabledexteritybythepatient.Anumberofspringcantileverbridgeshavefailedbecauseofperiodontaldiseaseonthepalatalsideoftheabutmentteeth.Theponticmayalsotraumatizethegingivalpapillaeoneithersideofitbecausethespringinessofthebarallowsthepontictomoveupanddownslightlyinuse.Thedesignofthebridgerequiresretentionoftheabutmentpreparationstobeverygood.Inexperiencedoperatorsshouldnotfallintothetrapofusingspringcantileverdesignsbecausetheylacktheconfidencetopreparesoundanteriorteethandprefertopractisebridgeworkusingposteriorabutmentsonly.Thedesignchosenshouldbethebestoneinthepatient'sinterest,andiftheoperatorisnotsufficientlyexperiencedtocarryitoutheshouldreferthepatienttoacolleagueorseekguidanceindesigningandmakingthebridge.ADVANTAGESFixed-fixed?Robustdesignwithmaximumretentionandstrength?Abutmentteetharesplintedtogether,maybeanadvantage,particularlywhenteethareuncomfortablymobilefollowingbonelossthroughperiodontaldisease?Thedesignisthemostpracticalforlargerbridges,particularlywhentherehasbeenperiodontaldisease?Theconstructionisrelativelystraightforwardinthelaboratory?CanbeusedforlongspansFixed-moveable?Preparationsdonotneedtobeparalleltoeachother,sodivergentabutmentteethcanbeused?Becausepreparationsdonotneedtobeparallel,eachpreparationcanbedesignedtoberetentiveindependentlyoftheotherpreparation(s)?Moreconservativeoftoothtissuebecausepreparationsforminorretainersarelessdestructivethanpreparationsformajorretainers?Allowsminormovementsofteeth.?Partscanbecementedseparately,socementationiseasy.Cantilever?Themostconservativedesignwhenonlyoneabutmenttoothisneeded?Ifoneabutmenttoothisusedthereisnoneedtomakepreparationsparalleltoeachother;iftwoormoreabutmentteeth,soitiseasiertomakethepreparationsparallel.?ConstructioninthelaboratoryisrelativelystraightforwardDISADVANTAGESFixed-fixed?Requirespreparationstobeparallelandthismaymeanmoretoothreductionthannormal,endangeringthepulpandreducingretention;thestrengthofthepreparedtoothmayalsobereduced?Preparationsaredifficulttocarryout,particularlyifseveralwidelyseparatedteethareinvolved;thepreparationisslowandtheparallelismhastobeconstantlychecked,oralternatively(andwrongly)thepreparationsareover-taperedtoensurethattherearenoundercutsandsoretentionislost?Alltheretainersaremajorretainersandrequireextensive,destructivepreparationsoftheabutmentteeth.?Hastobecementedinonepiece,socementationisdifficult.Fixed-moveable?Lengthofspanlimited,particularlywithmobileabutmentteeth?Morecomplicatedtoconstructinthelaboratorythanfixed-fixed?DifficulttomaketemporarybridgesCantilever?Withsmallbridgesthelengthofspanislimitedtooneorperhapstwoponticsbecauseoftheleverageforcesontheabutmentteeth;ifmoreteetharetobereplacedwithacantileverbridgealargenumberofabutmentswidelyspacedroundthearchmustbeused.?Theconstructionofthebridgemustberigidtoavoiddistortion.ChoiceofmaterialsMetalonlyManyposteriorbridgescanbemadeentirelyofcastmetal,whethertheyarefixed-fixed,fixed-moveableorcantilever,Iftheretainersorponticsdonotshowwhenthepatientsmilesandspeaksthenanall-metalbridgeisthebestchoice.Thematerialnecessitatestheleastdestructionoftoothtissueand,dependingonthechoiceofmetal,maybetheleastcostly.Themarginsarealsoeasiertoadapttothepreparations.Metal-ceramicWhenthestrengthofmetalisrequiredtogetherwithatooth-colouredabutmentorpontic,metal-ceramicisthebestmaterial.Thishasnowreplacedanothercrownandponticfacingmaterials,includingacrylicexceptinspecialcircumstances,suchasthepatientremovablebridges.Proprietaryceramicponticfacingshavealsobeensupersededbymetal-ceramicpontics.Arangerofcompositecrownandbridgefacingmaterialsisnowavailablebutitistooearlytosaywhetherthesehaveanyadvantageovermetal-ceramicmaterials.Itseemsunlikelythattheydobecausetheretentionofthefacingmaterialdependsuponmechanicalundercutsonthesurfaceofthemetalinsteadoftheveryreliablephysicochemicalmetal-ceramicbond.CeramiconlyTheallporcelainbridgeislimitedbyitsrelativelypoorstrengthtotwo-unitcantileverbridgesorthree-unitfixed-fixedbridges.Allporcelainbridgesmadefromconventionalfeldspathicporcelaincanhaveaverysatisfactoryappearance(seeFigure85c).However,withimprovementsinmetal-ceramicmaterials.Theseallporcelainbridgesarenowfallingintodisuse.Thenewercastceramicandreinforcedporcelainmaterialsmayproduceanewgenerationofallporcelainbridges.Oneadvantageoftheall-porcelainbridgeistheeffusebox'principle.Allporcelainbridges,ifproperlydesignedandconstructedhavesufficientstrengthtosurvivenormalfunctionalforcesbutwillbreakifsubjectedtoexcessiveforces.Thispotentialforfracturemaysavetherootsoftheabutmentteethfromfracturingifthebridgereceivesablow.Itisnotuncommonforpatientswholoseatoothasaresultofanaccidenttohaveafurtheraccident,eitherbecauseoftheiroccupationorsportorbecausewithaclassⅡDivisionIincisorrelationship,theirupperincisorsarevulnerabletotrauma.Abrokenbridgeisbetterthanbrokenroots.CombinationsofmaterialsManycombinationsarepossible,butthreedeservespecialmention.Thefirsttwoarecommon.?Ametal-ceramicretainerandponticwithamoveableconnectortoagoldinlayorotherminorretainer.?Anall-metalretainer(afullorpartialcrown)towardstheposteriorendofthebridgewithanteriormetal-ceramicunits.Initiallysolderingstandardcastingalloystometal-ceramicalloysaftertheporcelainhadbeenaddedwasdifficultandfailureswerecommon.Withimprovedmaterialsandtechniques,however,thisisnolongerthesameproblem.Thesolderjointismadeinalowfusingsolderaftertheporcelainisadded,andthebridgecannotbereturnedtothefurnaceforfurtheradjustmentstotheporcelainafteritismade.Figure91showstherangeoftemperaturesofthevariouscomponentsinthemetalceramicsystem.?Aframeworkofstandardcastingalloyandseparatelyconstructedporcelaincrownscementedtothem;thistypeofconstructionisnowuncommon,buttheyarestillseeninanumberofpatientsandneedtobemaintained,sometimesbythereplacementoffracturedcrowns.Minimal-preparationbridgesConventionalbridgesinvolveremovingtoothtissue,orapreviousrestoration,andreplacingitwitharetainer.Thismaybedestructiveoftoothtissueandwillcertainlybetimeconsumingandexpensive.Thealternative,minimal-preparationbridgeinvolvesattachingponticsviaametalplatetotheunprepared(orminimallyprepared)lingualsurfacesoradjacentteeth.Theattachmentismadebyacompositeresinmaterial,retainedbytheacidetchtechniquetotheenamel.Obviouslythesebridgescanbeusedonlywhentheabutmentteethhavesufficientintactenamel.Figure91Typicaltemperaturerangesforthemetal-ceramicprocess.Theevaryaccordingtothemetal,porcelainandsolderused,andwiththetypeoffurnace,inparticularitsrateoftemperaturerise.Figure92showsasimpleclassificationofminimal-preparationbridges;variationsofthistechniqueareshowninFigures94,95-b.MINIMALPREPARATIONBRIDGESDIRECTINDIRECTMacroMechanicalMediumMechanicalMicroMechanicalChemicallyRetention-Retention-e.g.VinginiaSaltRetention-e.g.Adhesive-e.g.e.g.RochetteMeshCrystalBondMarylandPanavia-ex(seeFig.94)(seeFig.92)(seeFig.96)(seeFig.97)Figure92Asimpleclassificationofminimal-preparationbridges?Directbridgesmaybeusingthecrownofthepatient’sowntooth.Thiscanoftenbedoneasasimpleandrapidwayofreplacingatoothlostthroughinjury(whichcannotbereimplanted)orwhichhastobeextractedurgently.Sometimesmetalmeshorwireisaddedtothelingualsurfacetoincreasestrength,butthisisnotalwaysnecessary.Ifthenaturalcrownofthetoothisnotavailableorisnotsuitable,anacrylicdenturetoothcanbeusedinthesameway.?Macro-mechanicallyretentivebridges(Rochette,seeFigure94)havelargeundercutperlorationsthroughthecast-metalplate,throughwhichthecompositeflows.Theseholesarecutinthewaxoracrylicpatternwithaburandarethencountersunk.?Medium-mechanicalretentivesystemsallinvolveretentivefeaturescastaspartofthemetalframework(seeFigure95).Theyalladdsignificantlytothecementfilmthicknessinsomeareas,atleast,oftheretainerandtheyallproducelarge,non-undercutlumpsofmetalonthefitsurfacewhichdonotcontributetoretentionbutwhichnecessitatearelativelythickretainer.Thesizeoftheretentivefeaturesisintermediatebetweenmacroandmicromechanicalretentivesystems.Figure94Arochettebridgereplacingonecentrelincisor.Theporcelainisyettobeaddedandthepalatalspurontheponticwillactasahandleuntilthebridgeisfinished,whenitwillberemoved.Figure95Acast-meshbridge.Itisdifficulttoachievegoodadaptationofthemeshovertheentireretainersurfaceandneitheroftheseretainershasretentivefeaturesrightuptotheperiphery.Theaddedthicknessoftheretainercanalsobesee.?Castingthemetalretainerandthenetchingthefitsurfaceproducesmicro-mechanicalretentionbyoneofthreemethods:electrolyticetchinginacidinthelaboratory,electrolyticetchingusingchairsideequipment.Orchemicaletchingwithahydrofluoricacidgeleitherinthelaboratoryoratthechairside.Althoughthesethreesystemsproducedifferentetchpatterns,theyareallveryretentive.Thesizeoftheretentivefeaturesisapproximatelyone-tenththatofthemedium-mechanicalretentivesystemsandtheretentivefeaturesareundercutfromthesurface.Thesmallersizeoftheseetchpitsandtheabsenceofunnecessarynon-retentivefeatures(asinthemedium-mechanicalretentivesystems)allowthinnermetalretainersandathinnercementfilmthickness.Figure96aAMarylandbridge,thedesigneisunsatisfactoryinthattheextensiondistallyontothethirdmolartoothtoincreaseretentionwouldgiverisetnanimpossiblecleaningproblembetweenthesecondandthirdmolar.ThisIsatechniquebridge,notmadeforepatient.Itisshowntoillustrateacommondesignerror.?Chemicallyretentiveresinsarenowavailable.Severalhavebeenmarketedandsome(forexample.Panavia-Ex)haveperformedwellinsomelaboratoryandshort-termclinicaltrials.Theyadherechemicallytorecentlysandblastedmetalsurfacesandareretainedonthetoothbyconventionalacid-etchingoftheenamel.Comparisonofindirectminimal-preparationretentionsystemsAnumberoflaboratorystudiesandclinicaltrialshaveshownthatthemicro-mechanicalandchemicaladhesivesystemsarethemostretentive.However,thechemicaladhesivesystemshaveonlybeenavailableforashorttimeandlonger-termclinicaltrialsarenecessarybeforeitissafetorelytooheavilyonthissystem.Thedisadvantageofthemici0-mechanicalretentionsystemisthatthemetalframeworkshouldnotbetriedinthemouthafterthesurfacehasbeenetched.Thisisbecausetheverydelicateetchpatternmaywellbedamagedorcloggedbydepositsfromsaliva.Thismeansthattheidealistotryintheunetchedframeworkandtheneitherreturnittothelaboratoryforetchingoretchitatthechairside.Thistakestimeandthereforeaddstothecost.Themacro-mechanicalretentivedesign(Rochette)overcomesthisproblembutislessretentiveinmostcasesandbecauseitiscementedwithaconventionalcompositeandthecompositecomesthroughtheperforationstothemouth,itispronetodegradationoveraperiodofyears.However,themainadvantageoftheRochettebridgeisthatitcanberemovedfromthemouthfairlyeasily.Thecompositeisdrilledoutfromtheholesandthebridgecanusuallyberemovedwithouttoomuchforce.ForthisreasontheRochettebridgeisstillusedwhentheabu