專(zhuān)業(yè)英語(yǔ) Unit 32教案學(xué)習(xí)資料

UnitThirtyTwoTHECOMPONENTPARTSOFAPARTIALDENTUREPartialdenturesaredefinedasrestorationscapableofbeingremovedfromthemouthbythepatient.Thecomponentsofpartialdenturesmaybelistedas:l.saddles2.occlusalrestsandsimilarcomponents3.directretainers4.indirectretainers5.connectorsTHESABDLESThesaddleisthepartofapartialdenturethatreplaceslostalveolartissueandcarriesartificialteeth.Thefittingsurfaceofasaddlecanbemadeofacrylicresinormetal;bothhaveadvantagesanddisadvantages,butthemajorityismadeinacrylicresin,andcertainlythismaterialshouldbeusedexclusivelyforfree-endsaddles.Acrylicteetharealwaysfixedtothesaddlethroughabondwithresin.Thesaddlemayhaveafittingsurfaceofcastmetalontowhichacrylicresinbasematerialisattachedbyretentiontags.Inthesecasestheunionoftoothtotheresin,beingofaphysic-chemicalnature,isalwayssufficientlystrong.Insomecasesofcloseanteriorocclusionasmallsaddlemaydisplayaweaknessoftheunionbetweenmetallicbaseandinterveningresin.OCCLUSALRESTSANDSIMILARCOMPONENTSAnocclusalrestisametalprojectionattachedtoapartialdenture,extendingoverandbearingontheocclusalorpreparedsurfaceofastandingtooth.Itmustfitthesurfaceaccuratelyandmustbecastandnotmadefromwroughtmetal.Anocclusalrestservesthesepurposes:l.Ittransmitsverticalloadtothetooth.2.Ittransmitssomelateralloadtothetooth.3.Ifdeflectsfood.4.Itmayimproveocclusion.5.Itmayactasanindirectretainer.ONLAYSWhenanocclusalrestisextendedtocoverthegreaterproportionoftheocclusalsurfaceofatoothitiscalledanonlay.Onlaysmaybecastingoldorcobaltchromiumalloy.Butwhenmadeofgoldtheapplianceisunnecessarilyheavyandcostly.Ithasbeensuggestedthatthehardnessofcobaltchromiumalloywouldhaveanexcessiveabrasiveactionupontheenamelofopposingnaturalteeth,butinpracticethisdoesnotseemtooccur.Itispossiblethatinsomecases,theremaybemoremarkedfacetingthanusual,butthedegreeisslight.Onlaysservethefollowingfunctions:l.Theymaysupportapartialdenture.2.Theymayconnectanoverclosedocclusion.3.Theymayimprovetheocclusion.EMBRASUREHOORSThesearemetalattachmentstoapartialdenturewhichareplacedintheembrasurebetweentwocontiguousnaturalteeth.Theycovertheocclusalorincisalsurfacesoftheteethandcontinueovertothelabialorbuccalsurfaces,butneverextendbelowthesurveyline,andconsequentlyplaynopartindirectretention.Allembrasurehooksmayservethefollowingpurposes:l.Supportthedenture.2.Bracethedenture.3.Resistanterior-posteriorstress.4.Splintnaturalteeth.5.Actasanindirectretainer.DIRECTRETAINERSDirectretentioniseffectedbyprecisionattachmentsorbyclasps.ClaspsAclaspisametalcomponentofapartialdenture,whichfitsagainsttheverticalenamelsurfaceofastandingtooth,aidinginthebracingendretentionoftheappliance.Althoughitisnecessarytoretainpartialdenturesthisisnottheonlyfactortobeconsideredindesigningappliancesintendedtoconservetheremainingmouthstructures.Thecontroloflateralloadisofparamountimportanceanditsdistributionbetweenstandingteethandridgesbymeansofclasparmsisoneofthemostsuccessfulmethodsofpreservingthealveolarboneandtheremainingteeth.PRECISIONANTACHMENTSTheseattachmentsarenormallysuppliedtothe'dentistprefabricatedbyamanufacturer.Theyconsistoftwoparts,commonlydesignatedmaleandfemale,thatarematchedtointerlocktogether.Oneofthesepartsisattachedtotheabutmenttoothandtheothertothesaddleofthedenture.Theirfunctionistoprovidepositivedirectretentionforapartialdenture.Inthisrespecttheymayprovemoredeficientthanclasps,buttheclinicalsituationsinwhichtheyareusedrequirecarefulassessment,and,althoughinallcasesthepatient'sstandardoforalhygienemustbegood,thisfactorisofevengreaterimportancetothesuccessforaprecisionattachmentpartialdenture.Preskelhassensiblyclassifiedthetypeofprecisionattachmentsasfollows:l.intracoronal2.extracoronal3.stud4.barINDIRECTRETAINERSAlthoughotherfactorsparticipate,directandindirectretainerslargelyeffectresistancetothedisplacementofpartialdentures.TheprincipleofindirectretentioncanbediscussedinrelationtoaClassIlowerdenture,havingclaspsandrestsonthefirstpremolarsandthesaddleconnectedbyalingualbar.Whenadisplacingforceactsononeofthesaddlesofthisdenture,eitherthroughtheactionofastickybolus01foodorthroughmuscularactionattheperipheryofthesaddlewhenthemouthiswidelyopened,thedenturetendstorotateaboutanaxisinthevicinityoftheclaspedteeth.Consequently,adthesaddlesrotateawayfromtheridge,thelingualbarmovesdownwardstowardsthefloorofthemouth,If,however,resistanceisofferedtothisdownwardmovementofthelingualbar,amorestableapplianceresultsandtheprincipleofindirectretentionisused.Indirectretention,therefore,resistsocclusallydisplacingforces'acting'onthesaddleofadenturebycreatingaresistanceontheoppositesideofthefulcrumaxistothatonwhichthedisplacingforceisapplied.Ifacontinuousclaspisincorporatedinthedesign,whenanocclusallydisplacingforceactsonthesaddles,thelingualbarcannotrotatetowardsthefloorofthemouthsincethecontinuousclasprestsonthecingulaofthestandingteeth.Rotationisthereforepreventedbyareactionisallthenaturalteethuponwhichthecontinuousclasppresses.Alternativelytheindirectretainermayberegardedasshiftingtheaxisofrotationanteriorlyfromthepremolarstothemesio-incisalpoint.CONNECTIONBARSANDPLATESThesaddlesandretainingcomponentsofpartialdenturesareconnectedeitherbybarsorplates.Thedifferencebetweenthemisintheamountoftissuecovered.Thebarcoveringlessthantheplate.Inthelowerjawthedifferencei8clearcut,butintheupperthereisnoexactdividingline,whatisdescribedasabroadbarbyonepractitionermightbecalledaplatebyanother.Bars,however,mustalwaysbeconstructedinmetaltogiveadequatestrengthandrigidityAlingualplateusedinthelowershould,forsimilarreasons,ideally,alwaysbemadeofmetal,butintheupperaplatemaybemadeinacrylicresinormetal.Allconnectingbarsandplatesmustberigidifthelateralloadofmasticationistobesharedbytheridgesandthestandingteethoftheworkingandbalancingsides.Thereforetheymustalwaysbeofsuchdimensionaswillensurethis,takingintoaccountthephysicalpropertiesofthematerialused.Acrylicresinre