Direct and Indirect Retainers

1 Direct and Indirect RetainersDEPARTMENT OF PROSTHODONTICS& IMPLANTOLOGY, SRM KDC & H Direct Retainers Structure of clasp assembly Requirements of clasp assembly Cast circumferential clasp Infrabulge clasp Indirect retainersRetainers:any type of device used for thestabilization or retention of a prosthesis.(GPT 8) Direct retainer:that component of apartial removable dental prosthesis usedto retain and prevent dislodgment,consisting of a clasp assembly or of Direct retainersIntracoronalExtracoronalClasps According to construction Cast Wrought wire Combination According to design Circumferential Bar typeIntracoronal Direct Retainers Introduced by Herman Chayes in 1906 Consists of two components-Matrix (slot)-Patrix (flange)Intracoronal Direct retainersAdvantages Elimination of visibleretention and supportsystem Better vertical support Better stimulation of underlying soft tissuesDisadvantages Require prepared abutment and castings Complicated clinical and lab procedures Eventually wear Difficult to repair and replace Least effective on short tooth Extracoronal Direct retainersRetentive clasp assembliesMechanics of retainer can be understood with two concepts path of insertion and removal.

2 And height of contour Prothero s cone theory Share common base referred to as greatest diameter of tooth Edward kennedy termed as height of contour DeVan terms Suprabulge Direct Retainers Infrabulge Direct retainerStructure of a clasp assembly Requirements of a clasp assemblyRetention SupportStability ReciprocationEncirclementPassivityRetent ionRetentionThe flexibility of the retentive clasp arm may be influenced by Length cross-sectional form cross-sectional diameter longitudinal taper clasp curvature, and metallurgical characteristics of the alloy. Retention Clasp flexibility increases as clasp length increasesThe mathematical formula for deflection of a uniform cantilever beam. This formula may be expressed as:D= Ewt34PL3where D deflection, P = applied force, L = length, E = modulus of elasticity, w =beam width, and t = beam diameterLongitudinal taperRetentionCross-sectional formRetentionMetallurgical characteristics of the alloy Retention Location of each retentive clasp terminus relative to the height of contour may be described in two distinct dimensions: (1) a mediolateral or horizontal dimension and (2) an occlusal or vertical dimension.

3 Support Support is the quality of a clasp assembly thatresists displacement of a prosthesis in an apicaldirection. Other elements that contact the abutmentocclusal to the height of contour ( , areciprocal element or shoulder of retentive clasp)also may contribute to the support Stability is the quality of a clasp assembly thatresists displacement of prosthesis in a horizontaldirection. All framework components that arerigid and contact vertically oriented hard andsoft tissues may contribute to the stability Reciprocation is the quality of a clasp assemblythat counteracts lateral displacement of anabutment when the retentive clasp terminuspasses over the height of Location of retentive clasp terminusCast circumferential clasp Introduced by Nesbitt in 1916 Simple and easy to fabricate Tooth supported RPD Advantages Disadvantages Design rules for cast circumferential clasp A cast circumferential clasp should originatefrom a portion of the framework that lies abovethe height of contour.

4 Retentive terminus should be directed occlusally should terminate at the mesial line angle ordistal line angle of the abutment The retentive arm should be positioned as farapically on the abutment as is circlet claspReverse circlet claspMultiple circlet claspEmbrasure clasp Ring claspC-claspOnlay claspWrought-wire circumferential clasp It used as early as 1847, the wrought wirecircumferential clasp In 1965, Dr 0. C. Applegate introduced a modifiedwrought wire clasp assembly known as the combinationclasp . consists of an occlusal rest a cast metal reciprocal arm,and a wrought wire retentive arm. The wrought wirecomponent is circular in cross section. Kennedy Class I or Class II posterior edentulous areawhen the usable undercut is located at the mesiofacialline angle of the most posterior abutment. Increased flexibility hence can be used in the greaterundercut area.

5 Minimal tooth surface contactInfrabulge clasp clasp approaches the undercut region of anabutment from an apical direction. Therefore aninfrabulge clasp exhibits a push type ofretention that is more effective than the pull retention associated with a suprabulge clasp. Flexibility of the infrabulge clasp is controlled bythe taper and length of the approach arm. more esthetic than a suprabulge claspDesign rules for infrabulge clasp The approach arm of an infrabulge clasp mustnot impinge on the soft tissues adjacent to theabutment The approach arm should cross perpendicular tothe free gingival margin The approach arm should never be designed tobridge an area of soft tissue undercut uniform tapering The clasp terminus should be more apicallypositioned on the abutmentTypes of infrabulge calspThese clasps are described by their geometric shapes. There are four main types of infrabulge clasp.

6 T-clasp, the modified T-clasp, the Y- clasp, and the I-clasp or design Kennedy Class I or Class II partially edentulous and undercut is located adjacent to the edentulous areaModified T clasp The modified T-clasp is essentially a T-clasp that lacks the nonretentive, horizontal projection. improved esthetics in most applications used when canines or premolars will serve as design Practically Y clasp is equivalent to a T-clasp recontouring of the abutment surface I bar designIndirect Retainer The effect achieved by one or more indirectretainers of a partial removable dentureprosthesis that reduces the tendency for adenture base to move in an occlusal direction orrotate about the fulcrum line (GPT-8)Principles of Indirect retainerForms Of Indirect Retainers Auxiliary Occlusal Rest Canine Rests Canine Extensions from Occlusal Rests Cingulum Bars (Continuous Bars) and LinguoplateReference Phoenix, Cagna, DeFreest, Stewart s Clinical Removable Partial Prosthodontics, 3rdedition, 2003 Quintessence publications Co,Inc.

7 Pp 53-126 Carr , Mc Givney , brown ; Mc Crackens Removable PartialProsthodontics; 11th edition 2005; Harcourt brace and company Asia Pte Ltd NewDelhi Pp 271-299. Renner and Boucher; Removable Partial dentures; 1987, Quintessence Publicationco., Inc Chicago; Pp 53-117. J. C. Davenport et al, Retention, British dental journal, 2000,volume 189, pages646-657 J. C. Davenport et al, Indirect retention, British dental journal, 2001, volume 190,pages 128-132. Removable partial denture an overview pages 51-74 Thank You