DIRECT RETAINERS

Retention is that quality inherent in the

removable partial denture that resists the

vertical forces of dislodgement for e.g., the force

of gravity, the adhesiveness of foods or the forces

associated with the opening of the jaws.

A retainer is defined as any type of clasp, attachment, device etc used for the fixation, stabilization or retention of a prosthesis.Direct retention is the retention obtained in a removable partial denture by the use of attachments or direct retainers to resist the displacement or removal of the partial denture from the abutment teeth in a direction opposite to that of their insertion.

Direct retainer is any unit of removable partial

denture that engages an abutment tooth in

such a manner as to resist displacement of

the prosthesis away from basal seat tissues

by 1. Functional means by engaging a tooth

undercut lying cervically to the height of the contour.

2. Mechanical means

DEFINITION

According to the Glossary of Prosthodontic Terms, a direct retainer is that component of a removable partial denture used to retain and prevent dislodgement and consisting of a clasp assembly or precision attachment.

Factors affecting the retention in Removable Partial Dentures:

I. Primary retention : This is mechanical in action.

II. Secondary Retention: Achieved by intimate relationship of the denture base and the rigid major connector to the underlying soft tissues.

a. Adhesion b. Cohesion c. Atmosphere pressure d. Molding of tissuese. Effect of gravity

Classification of direct retainers

Direct Retainers are of two types:

Extra Coronal

Intra Coronal

Its parts are :

•Reciprocal arm (blue).

•Retentive arm (red).

•Shoulder and rest (yellow).

•Minor connector (green).

Requirements of a clasp design:

1. Retention 2. Support 3. Stability 4. Reciprocation 5. Encirclement 6. Passivity

Retention:The path of escapement of each retentive

clasp terminal should be other than parallel to the path of removal of the prosthesis.

The amount of retention always should be the minimum necessary to resist reasonable dislodging forces.

Only the terminal third of an occlusally approaching clasp should engage the undercut

The retentive clasp is divided into 3 parts; each with its arm functional requirement. The terminal third is flexible and engages the undercut.

The middle third has a limit degree of flexibility and may engage a minimal amount of undercut.

Proximal third or shoulder is rigid and must be positioned above the height of contour.

Support:Property of the clasp that resist the

displacement of the clasp in gingival direction.Primary support units of a clasp are

occlusal, lingual or incisal rest.The occlusal rest must be designed so that

cervical movement of the clasp arm is prevented.

Stabilization or Bracing: It is the resistance which the clasp contributes to

displacement of the prosthesis in a horizontal plane.

All of the clasp components with the exception of the retentive terminal contribute this property in varying degrees.

Bracing elements united by rigid major connector are capable of distributing horizontal forces throughout the partially edentulous arch.

The components of the cast circumferential clasp offer better stabilization than either the bar clasp or the wrought wire clasp because of greater amount of rigidity of the clasp material.

Reciprocation

Reciprocation may be defined as “the means by which one part of the prosthesis is made to counter the effect created by another part”.

For effective reciprocation clasps should be planned and designed so that the two arms of the clasps are in balanced.

Encirclement:More than 1800 of greatest circumference of

the tooth must be included passing from diverging axial surface to converging axial surface.

This may be in the form of continuous contact when circumferential clasp arms are used.

Bar clasps are used, at least 3 areas of tooth contact must be embracing more than ½ the tooth circumference. These are occlusal rest area, the retentive terminal area and reciprocal terminal area.

Passivity:

A clasp in place should be completely

passive and the retentive function is activated

only when dislodging forces are applied to the

partial denture.

Location of components:The reciprocal element of the clasp

assembly should be located at the junction of the gingival and middle 3rd of the crowns of an abutment teeth. The terminal end of the retentive arm is placed in the gingival 3rd of the crown.

There are basically 2 categories of clasps:

1. Circumferential or Akers clasps & 2. Vertical projection, or bar or Roach clasps

Circumferential clasps/ Occlusally approaching/Aker Clasp

• Circumferential clasp• Embrasure clasp.• Ring clasp.• Back action clasp.• Reverse action / hair pin clasp.• Multiple clasps.• Combination clasp.• Onlay clasp.

Bar/Roach clasps / Gingivally approaching clasps.

T-clasp

Modified T-clasp

Y-clasp

I-clasp

RPI concept.

CIRCUMFERENTIAL CLASP

Reverse circlet design

multiple claspMultiple clasp is two opposing simple circlet clasp joined at

the terminal end of the two reciprocal arms.

EMBRASSURE CLASP Bonwill clasp Rib clasp Back to back clasp

This clasp is essentially two single circlet clasps joined at the body.

RING CLASP

Hair pin or reverse action clasp or C-clasp design

INFRABULGE CLASPT-clasp, modified T clasp, Y clasp and I bar

DisadvantageFood accumulationFlexibility Horizontal stability

DESIGN RULES

Approach arm- must not impinge soft tissuecross perpendicular to free gingival margin Terminal clasp position - Apically on the abutment

T-CLASP DESIGNKennedy class I and IIUndercut located adjacent to edentulous area

O.01’’Contraindication- Severe soft tissue undercut- High of contour locate near occlusal surface

A = Approach armB= Vertical projection of approach armC= Location where B cross perpendicular to free gingivalD=Point of first tooth contactE=Terminus of the retentive claspF= Encirclement portion

MODIFIED T-CLASP

No retentive horizontal projectionKennedy class I and IIUndercut locate near adjacent edentulous

areaCanine and premolarAdvantage- Improve esthetic compare with conventional

T clasp

Y-CLASP DESIGN:

•Equivalent to T-clasp•Mesial and distal projection terminate near occlusal surface

I-CLASP OR I BARContact area•2-3 mm. in height•1.5-2 mm. in width

Mesial rest

Kennedy class I and II

RPI- Mesial rest- Proximal plate- I bar