Direct and indirect retainers

Vinay Pavan Kumar .K 2nd year P G student

Dept of ProsthodonticsAECS Maaruti College of Dental Sciences

Direct retainers

Indirect retainers

Definition

Classification

Principles of design

Extracoronal retainers

Types

Definition

Factors affecting

types of indirect retainersRequirements

Direct retainer: It is that component of a

removable partial denture that is used to retain

and prevent dislodgment, consisting of a clasp

assembly or a precision attachment (GPT 8)

Direct retainers

   Intracoronal Extracoronal

Precision Semi-precision   Extracoronal Clasps

Attachment

Intracoronal retainers

In 1906 the principle of the internal attachment was first formulated by Dr. Herman E.S. Chayes

Extracoronal RetainersProthero provided a Conceptual Basis for mechanical

retention

Clasps mainly divided 2 types

Occlusally approaching which approach the undercut

from the occlusal area and gingivally approaching which

enter the undercut crossing the gingival margin.

Clasp Assembly

The part of a removable dental prosthesis that acts as a

direct retainer and/or stabilizer for a prosthesis by

partially encompassing or contacting an abutment tooth.

Components of the clasp assembly include the clasp,

the reciprocal element, the cingulum, incisal or occlusal

rest, and the minor connector.

Parts of clasp assembly

Circumferential Clasp(Retentive Arm)

Reciprocating(Bracing) Arm

DistalOcclusalRest Seat Proximal

Plate

Principles of Clasp Design

1. Encirclement- more than 180 degrees in greatest circumference if

the tooth engaged by the clasp assembly

2. Occlusal rest - to prevent the movement of the clasp arms

cervically.

3. Each retentive terminal should be opposed by a reciprocal

component

4. Clasp retainers on abutment teeth adjacent to distal extension bases should be designed to avoid direct transmission of forces to the abutment

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

6. Reciprocal elements – junction of gingival and middle third

Terminal retentive arm – gingival third

Functional requirements of the clasp

Retention

Support

Stability

Reciprocation

Encirclement

Passivity

Retention

Is obtained by the incorporation of a flexible element of

the clasp into the undercut.

Sufficient undercut to be engaged to obtain desired

retention.

Force from the clasp arm on flexing must be within the

tolerance of the PDL and must be less to prevent

deformation of the clasp arm itself.

Factors affecting retention

◦Tooth factors: Size of the angle of cervical convergence

◦How far the clasp terminal is placed into the angle of cervical convergence

Material used: cast chrome (0.010”) cast gold (0.015”) wrought alloy (0.020”)

Clasp arm flexibility

Length of the clasp

•The longer the clasp arm the more flexible.

• Flexibility is directly proportional to the cube of its length.

•By increasing the length, the horizontal stresses imparted to the abutment during placing and removal is reduced

Cross section: round > half round

Modulus of elasticity: more the modulus - less flexibilityDiameter of clasp: flexure inversely proportional to the

diameter.Alloy: wrought > cast

Support Support is the quality of the clasp assembly to resist

displacement of the prosthesis in the apical direction.

a rest must contact the surface of the abutment tooth at

a properly prepared surface- rest seat

A properly prepared rest will prevent the

tissueward movement of the prosthesis.

maintains the position of the clasp assembly in

relation to the abutment.

Transmits forces along the long axis of the

abutments

Stability

Resistance to horizontal displacement

Reciprocation

Counteracts lateral displacement of an abutment when retentive clasp terminus passes over the height of contour

Encirclement

Prevent movement of abutment away from associated clasp assembly

More than 180 degrees

Passivity

Prevent the transmission of the adverse forces

to the associated abutment

Be passive until a dislodging force is applied

Classification of extra-coronal retainers

Supra bulge clasps (occlusally approaching, circumferential clasps)

Infrabulge clasps (gingivally approaching, projection or bar clasps)

Combination clasps

Circumferential clasps

The cast circumferential clasp design was introduced by

Dr N B Nesbitt in 1916.

Simple, easy to construct- excellent support, bracing,

retentive properties.

Close adaptation to tooth therefore minimises food

entrapment

Disadvantage- covers large amount of tooth surface

Circlet clasp.

Reverse circlet

Multiple circlet clasp

Embrasure clasp.

Reverse action or hair pin clasp

Ring clasp.

Back action and reverse back action clasp

Simple Circlet clasp

Tooth support RPD

Undercut remote from

edentulous area

Half round

Disadvantages

- Increase tooth coverage

- compromised esthetics

Variations of circlet clasp

Back action clasp

Reverse back action

Ring clasp

C clasp or hair-pin clasp

Reverse circlet clasp

Undercut located adjacent to edentulous area

Kennedy class I ,II

Disadvantages

- Lack of rest adjacent to edentulous area

- Poor esthetics

Multiple circlet design

2 simple circlet clasp joined at the terminal aspect of

their reciprocal elements

Principle abutment is periodontal compromised and the

forces are distributed between multiple abutment teeth

Embrasure clasp

2 simple circlet joined at bodies

Used on side of the arch where there is no edentulous

space

Can be used only when adequate tooth preparation is

possible

C-clasp design

Fish hook” or “Hairpin” claspSimple circlet clasp with loop back retentive armSufficient crown height

Disadvantages - Insufficient flexibility - Tooth coverage - Esthetics compromised

Combination claspCast metal reciprocal arm and wrought wire retentive

arm abutment adjacent to Kennedy class I and II area Advantage• kinder to the tooth can engage greater undercut

Disadvantage• more prone to breakage than cast• minimal stabilizing

Gingivally approaching clasps /Bar/Roach type

Approach the undercut gingivally and have a push type of retention.

Approach arm

• It is a minor connector that connect the retentive tip to the denture base.

• It crosses the gingival margin at right angle and it is the only flexible minor connector.

• Flexibility of the clasp is controlled by the taper and length of the approach arm

• More esthetic

Retentive terminal

• It should end on the surface of the tooth below the

undercut.

T-claspKennedy class I and IIUndercut locate adjacent edentulous area

Contraindication - Severe soft tissue undercut - Height of contour locate near occlusal surface

Modified T-clasp

No retentive horizontal projectionKennedy class I and IIUndercut locate near adjacent edentulous areaCanine and premolarAdvantage - esthetics

Y-clasp

Equivalent to T-clasp

Approach arm terminates in the cervical third

Mesial and distal projection terminate near occlusal

surface

I bar

Kennedy class I and II

RPI - Mesial rest - Proximal plate - I bar

Flexible clasps

A comparative study on Co-Cr and Acetal resin clasps; Pal .H etal TPDI • January 2014, Vol. 5, No. 1 pg 9- 13

• Acetal resin clasps are esthetic and are available in sixteen different shades.

• To evaluate the effect of cast Co-Cr and acetal resin clasp on the surface of tooth.

• The retentive force of cast Co-Cr clasp showed a decrease from 12.4 N to 8.1 N.

• The retentive force of acetal resin clasp, reduced from 5.2 N to 4.03 N at the completion of experiment.

• Acetal resin clasps do not abrade the surface of tooth and maintain retention

Implants as direct retainers

• Eliminates a visible clasp

• placement of an implant within a modification space to the advantage of retentive needs requires consideration of anterior, mid, or distal placement

• retainers utilizing teeth have always been restricted to tooth locations at either end of a span

Indirect retainerResists rotational displacement of an extension base

from the supporting tissueKennedy class I, II and IV

Factors determining indirectretainer

Occlusal rests must be held in rest seats by direct

retainer

Distance from fulcrum line

Placed on definite rest seat to prevent slippage

Rigidity

Auxiliary functions

Reduce A-P tilting of abutments

Stabilization – auxiliary guide planes

Anterior teeth stabilized

Auxiliary rest – stress distribution

Visual indication for reline

Forms of indirect retainer

Auxiliary occlusal rest

Canine extension from occlusal rest

Continuous bar retainer & lingual plate

Rugae Support

Auxiliary Occlusal Rest

Canine rest

Rugae Support

Major connectors - cingulum bars and linguo plates

References Carr AB, Mc Givney GP, Brown DT, McCracken’s

Removable Partial Prosthodontics, 12th edition , Canada, Elsevier Publishers, 2005 , pp:68-102

Stewart, Phoenix, Cagna, De Freest, Clinical Removable Partial Prosthodontics, 3rd edition, 2001, USA, Quintessence publishers, pp:53-96

Grant AA, Johnson W, An Introduction to Removable Denture Prosthetics, 1st edition, USA, Churchill livingstone, 1983, pp: 96-101

Davenport JC , Basker RM, Heath JR, Ralph JP, Glantz PO, Retention ,Brit Dent J 2000;189(12):646-657

Davenport JC , Basker RM, Heath JR, Ralph JP, Glantz PO, Hammond P, Bracing and reciprocation ,Brit Dent J 2001;190(1):10-14

Davenport JC , Basker RM, Heath JR, Ralph JP, Glantz PO, Hammond P, Clasp design , Brit Dent J 2001;190(2):71-81

Davenport JC , Basker RM, Heath JR, Ralph JP, Glantz PO, Hammond P, Indirect Retention ,Brit DentJ 2001;190(2):128-132