Tikrit University College of Dentistry Lec-8- prosthetic 3rdclass/Dr.Reem

RetainersThereare2typesofretainers:

1-directretainer2-indirectretainer

Directretainer

AnyunitofRPDthatengagesanabutmenttoothtoresistdisplacementAnyunitofRPDthatengagesanabutmenttoothtoresistdisplacementoftheprosthesisawayfrombasalseatoftheprosthesisawayfrombasalseattissue.tissue. GreatlyinfluencedbythestabilityandsupportoftheprosthesisprovidedbymajorandGreatlyinfluencedbythestabilityandsupportoftheprosthesisprovidedbymajorandminorconnectors,rests,andtissuebasesminorconnectors,rests,andtissuebases.

Thereare2typesofdirectretainer:

1-extracoronal2-intracoronal

EXTRACORONALDIRECTTRETAINER

A- Retentiveclaspassemblies:-representthemostcommonmethodforextracoronaldirectretention.Eachclaspassembliesconsistfromretentivearm(buccalsurface),reciprocalarm(lingualsurface),rest,anditsminorconnector.

claspscandividedintotwotypesaccordingtothedirectionofapproachofretentivearmtotheundercutareainto:-1- Suprabulgeclasps(occlusallyapproaching,circumferentialorencirclingclasps):Theretentivearmapproachestheundercutareafromthesuprabulgedirection.2-Infrabulgeclasps(gingivallyapproachingorbarclasp)Theretentivearmapproachestheundercutfromtheinfrabulgedirectione.g:barclasp(Ibarclasp)

Tikrit University College of Dentistry Lec-8- prosthetic 3rdclass/Dr.Reem

suprabulgeinfrabulgeTounderstandthemechanicsofsucharetainer,thepractitionermustunderstandtwoimportantconcepts-(thepathofinsertionandremovalfortheprosthesis,andheightofcontourforeachabutment).Toproperlydesignaremovablepartialdenture,thepractitionermustconsiderthepathalongwhichtheprosthesiswillbeinsertedandremovedfromitsfullyseatedposition.Uponinsertion,theclasparmswillcontacttheaxialsurfacesoftheabutments.Theclasparmpassoverthegreatestconvexitiesoftheabutments.Oncethepointofgreatestconvexityisreached,allowtheeachretentiveclasptoreturntoitspassivestate.Properuseofadentalsurveyoristheonlyreliablemethodofeffectivelyanalyzingteethfortheirpotentialcontributiontoretentionofremovablepartialdenture.

SUPRABULGECLASPAsuprabulgeclasparmoriginatesfromaminorconnectorocclusalorincisortotheheightofcontour.Theportionofclasparmthatarisesfrom

Tikrit University College of Dentistry Lec-8- prosthetic 3rdclass/Dr.Reem

theminorconnectorisknownastheshoulder.Fromtheshoulder,theclasparmfollowsagentlycurvingpathwayonthesurfaceofabutment.Theclasparmpassesovertheheightofcontour,anditstipcontactsapreciselymeasuredundercut.Thisportionoftheclaspisknownasterminus.

Theshouldersegmentisrigid,therigidityrequirethatshoulderbepositionedatorocclusaltotheheightofcontour.Themidsectionoftheclasparmextendsfromtheshoulder.Becauseofthecrosssectionalareaisslightlysmallerthanshoulderanditissomewhatflexible.Theclaspterminus(retentivetip)asmallercrosssectionalareaandthereforepermitagreateramountofflexure.Theclaspterminusistheonlypartofaremovablepartialdenturetocontacttheabutmenttoothapicaltotheheightofcontourwhentheprosthesisisfullyseated.

Tikrit University College of Dentistry Lec-8- prosthetic 3rdclass/Dr.Reem

Thecomponentofaclaspassembliesthatbracesanabutmentduringprosthesisinsertionandremovaliscalledareciprocalelement.Topreventlateraldisplacementoftheabutmentandsignificantdamagetothesupportingperiodontiumreciprocalarmshouldbepresent.Thereciprocalarmmustcontacttheabutmenttoothatorocclusaltotheheightofcontour.REQUIREMENTOFACLASPASSEMBLY1- Retention2-support3-stability4-reciprocation

5-encirclement6-passivity.1- RETENTION:-Isthequalityoftheclaspassemblythatresistsforcesactingtodislodgecomponentsawayfromthesupportingtissues.Retentionisprovidedbytheretentivearmwhichpreventsthepartialdenturefromdisplacementawayfromthetissuestowardtheocclusal.Whilethepartialdentureisseatedtheretentivetipmustbepassive.However,itshouldtouchthetoothintheundercutarea.2-STABILITY:-Isthequalityofaclaspassemblythatresistsdisplacementoftheprosthesisinahorizontaldirection.Thisprovidedbytherest,minorconnectorandbracingarm.Actually,thebracingarmmainlyprovidesreciprocationwhichisresistancetoforceappliedbyretentivetip.3-SUPPORT:-Isthequalityofaclaspassemblythatresistsdisplacementofaprosthesisinanapicaldirection.Supportinaclaspisgenerallyprovidedbytherest.Thus,whilechewingfoodtherestpreventstissuewardmovementoftheclaspassembly,plus

Tikrit University College of Dentistry Lec-8- prosthetic 3rdclass/Dr.Reem

directstheforcealongthelongaxisofthetooth,thusreducesperiodontaltissuedamage.4-RECIPROCATION:-Isthequalityofaclaspassemblybythatcounteractslateraldisplacementofanabutmentwhentheretentiveclaspterminuspassesovertheheightofcontour.Lateralforcesmustbeoffsetbycomponentsoftheclaspassembly.5-ENCIRCLEMENT:-Isthecharacteristicofclaspassemblythatpreventsmovementofanabutmentawayfromtheassociatedclaspassembly.Eachclaspassemblymustbedesignedtoprovidedirectcontactoveratleast180degreeofthetooth,scircumference.Thiscontactprovideencirclementandpreventsmovementoftheabutmentawayfromtheclaspassembly.

6-PASSIVITY:-Isthequalityofaclaspassemblythatpreventsthetransmissionofadverseforcestotheassociatedabutmentwhentheprosthesisiscompletelyseated. Theretentiveclasparmshouldbepassive(noactiveforce)untiladislodgingforceisapplied.Flexibilityofclasparmdependon:1- thelengthofclasparm.2- thediameterofclasparm.

Tikrit University College of Dentistry Lec-8- prosthetic 3rdclass/Dr.Reem

3- thecrosssectionalform.4- thetypeofmetalused(wroughtwireismoreflexiblethancastmetal).

Typesofclasps:1.SimpleCircletclasp(circumferential).2.Half&halfclasp.3.Fishhookorhairpinclasp.4.Reversecircletclasp.5.Embrasureclasp.6.Ringclasp.7.Backactionandreversebackactionclasp.1- SIMPLECIRCLETCLASP(circumferential):-

-Isthemostcommonclaspusedforremovablepartialdenture-Theleastcomplexdesign,ithasarigidreciprocalarm,arest,proximalplateapproximatingtheedentulousarea,andaflexibleretentivearm.-itmaybeusedoncanines,premolarsandmolars.

Tikrit University College of Dentistry Lec-8- prosthetic 3rdclass/Dr.Reem

disadvantages:

1-difficultadjustment 2-increasetoothcoverage 3-moremetalcoverage

2- RINGCLASPTheringclaspisusedonmolars.Somedesignersadvocateitsusefortippedmandibularmolarswherethereisonlyamesialundercutonthetooth.Ithasmesialanddistalrestsandthereciprocalarmiscontinuousconnectingthetworests.Asupportingstrutmightbeusedtoprovidearigidreciprocationareafromtheprincipalocclusalresttothestrut.

3- EMBRASURECLASPORBUTTERFLYCLASP:Theembrasureclaspisusedwhenthereisnomodificationspacesintheportionofthearchthatneedsretention.Theclaspisinfacttwocircletclaspsback-to-back.Itmaybeusedontwomolars,amolarandpremolarortwopremolars.Preparationofthedoublerestandchannelgoingfrom

Tikrit University College of Dentistry Lec-8- prosthetic 3rdclass/Dr.Reem

thelingualtothebuccaloftheteethneedstobedeepenoughforstrengthandnotcompromisedbytheopposingocclusion.

EMBERASSURECLASP

4- THEREVERSECIRCLETCLASPThereversecircletclaspisusedwhentheretentiveundercutislocatedonthesurfaceoftheabutmenttoothadjacenttotheedentulousspace.ItisindicatedinclassI&classIIdistalextensionpartialdentureswheredeeptissueundercutprecludestheuseofinfrabulgeclasp.

disadvantages:-

1-itmaybedifficulttoprovideadequateroomforclaspneedremovingasignificantamountoftoothstructure.

Tikrit University College of Dentistry Lec-8- prosthetic 3rdclass/Dr.Reem

2-pooresthetic3-lackofdistalrestmayallowtheprosthesistodamagetheassociatedstructureandovercomebyplacingdistalrest.5- hairpinclasporfishhook:

Thefishhookorhairpinclaspisanothermodificationofthecircletclasp.Itismostlyusedonteethwithlongcrowns.Itisrarelyusedbecausesomuchoftheteethiscoveredbytheretentivearm.Theotherdifficultyislackofflexibilityoftheretentivetipbecauseofthebulkoftheclasp.

6- thehalfandhalfclasp:

Thehalfandhalfclaspisamodificationofthecircletclaspwiththereciprocalarmcomingfromonedirectionandtheretentivearmfromtheother.Tworestsareusedforthisclasp.Itisusedonmolarsandpremolars.