Direct and Indirect Retainers

DEPARTMENT OF PROSTHODONTICS& IMPLANTOLOGY, 

SRM KDC & H

• Direct retainers1. Intracoronal retainers2. Extracoronal retainers

– Structure of clasp assembly– Requirements of clasp assembly– Cast circumferential clasp – Infrabulge clasp

• Indirect retainers

Retainers: 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 precisionattachment.

Classification of direct retainers

IntracoronalExtracoronal

Clasps• According to construction

– Cast– Wrought wire– Combination

• According to design– Circumferential – Bar type

Intracoronal direct retainers

• Introduced by Herman E.S. Chayes in 1906

• Consists of two components- Matrix (slot)- Patrix (flange)

Intracoronal direct retainers

Advantages • Elimination of visible

retention and supportsystem

• Better vertical support• Better stimulation of

underlying soft tissues

Disadvantages• Require prepared

abutment and castings• Complicated clinical and

lab procedures• Eventually wear• Difficult to repair and

replace• Least effective on short

tooth

Extracoronal direct retainers

Retentive clasp assemblies

Mechanics of retainer can be understood with two concepts path of insertion and removal, 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”

M.M. DeVan terms

• Suprabulge direct retainers

• Infrabulge direct retainer

Structure of a clasp assembly

Requirements of a clasp assembly

Retention SupportStability ReciprocationEncirclementPassivity

Retention

Retention

The 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= Ewt34PL3

where D deflection, P = applied force, L = length, E = modulus of elasticity, w =beam width, and t = beam thickness.

Retention

Cross-sectional diameter Longitudinal taper

Retention

Cross-sectional form

Retention

Metallurgical 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.

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 (e.g., areciprocal element or shoulder of retentive clasp)also may contribute to the support function.

Stability

• 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 ofprosthesis.

Reciprocation

• Reciprocation is the quality of a clasp assemblythat counteracts lateral displacement of anabutment when the retentive clasp terminuspasses over the height of contour.

Encirclement

Passivity

Location of retentive clasp terminus

Cast 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.

• retentive terminus should be directed occlusally• should terminate at the mesial line angle or

distal line angle of the abutment• The retentive arm should be positioned as far

apically on the abutment as is practical.

Simple circlet clasp

Reverse circlet clasp

Multiple circlet clasp

Embrasure clasp

Ring clasp

C-clasp

Onlay clasp

Wrought-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.

• Minimal tooth surface contact

Infrabulge 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 clasp

Design 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 apically

positioned on the abutment

Types of infrabulge calsp

These clasps are described by their geometric shapes. There are four main types of infrabulge clasp.

• T-clasp, • the modified T-clasp, • the Y- clasp, and • the I-clasp or I-bar.

T-clasp design

• Kennedy Class I or Class II partially edentulous and undercut is located adjacent to the edentulous area

Modified 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 abutments.

Y-clasp design

• Practically Y clasp is equivalent to a T-clasp

• recontouring of the abutment surface

I bar design

Indirect 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 retainer

Forms Of Indirect Retainers

• Auxiliary Occlusal Rest• Canine Rests• Canine Extensions from Occlusal Rests• Cingulum Bars (Continuous Bars) and

Linguoplate

Reference

• Phoenix, Cagna, DeFreest, Stewart’s Clinical Removable Partial Prosthodontics, 3rdedition, 2003 Quintessence publications Co,Inc. pp 53-126

• Carr A.B, Mc Givney G.P, brown D.T; 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

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