DIRECT RETAINERS

BY DR HISHAM MOUSLY

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Part 2

TYPES OF CLASP ASSEMBLIES:

Clasps Designed to Accommodate Functional Movement

1 .RPI, RPA and Bar clasp2 .Combination clasp

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RPI, RPA, AND BAR CLASP

Distal extension acts as a long “effort arm” across the distal rest” fulcrum” results in harmful tipping or torquing of the tooth .

Two strategies may be adopted to change the fulcrum location (mesial rest concept clasp assemblies), or use of a flexible arm (wrought-wire retentive arm).

Mesial rest includes the RPI and RPA mesio-occlusal rest with distal guiding plane. Proximal plate, in conjunction with the minor connector supporting the rest, provides the stabilizing and reciprocal aspects of the clasp assembly. The I-bar gingival third of the buccal or labial surface of 0.01-inch, no more than 2 mm of its tip contacting.

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RPI, RPA, AND BAR CLASP

Three basic approaches RPI: location of the rest, the design of minor connector (proximal plate) and location of the retentive arm.

Entire length of the proximal tooth surface, from the marginal ridge to the junction of the middle and gingival thirds .

Contraindications for a bar-type (exaggerated buccal or lingual tilts,

severe tissue undercut, or a shallow buccal vestibule)

modification should be considered for the RPI system (the RPA clasp). 4

RPI, RPA, AND BAR CLASP

The term bar clasp is generally preferred over the less descriptive term Roach clasp arm .

The bar clasp arm has been classified by the shape of the retentive terminal, T, modified T, I, or Y.

If a tissue undercut prevents the use of a bar clasp arm, a mesially originating ring clasp, a cast, or a wrought-wire clasp or reverse-action clasp may be used.Indications for use of a bar clasp arm

(1 )a small degree of undercut (0.01 inch) in the cervical third.

(2 )tooth-supported partial dentures .(3 )distal extension base situations. and

(4 )in situations in which esthetic considerations. 5

COMBINATION CLASP

To reduce the effect of the Class I lever in distal extension .

The combination clasp consists of a wrought-wire retentive clasp arm and a cast reciprocal clasp arm. Retentive arm is almost circumferential, but may be used in the manner of a bar.

Advantages of the combination clasp include Flexibility (abutment tooth adjacent to a distal extension base or weak abutment when a bar-type direct retainer is contraindicated)and adjustability and the appearance.

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COMBINATION CLASP

The most common use of the combination clasp is on an abutment tooth adjacent tooth adjacent to a distal extension base where only a mesial undercut exists on the abutment or where a large tissue undercut contraindicates a bar-type retainer.

Disadvantages :(1 )extra steps in fabrication .

(2 )distorted by careless handling ;(3 )less accurately adapted therefore

may provide less stabilization.The disadvantages listed previously should not be prevent its use ,

problems are minimized by selecting the best wrought wire.

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TYPES OF CLASP ASSEMBLIES:

Clasps Designed Without Movement Accommodation

1 .Circumferential clasp2 .Ring clasp 3 .Embrasure clasp 4 .Back action clasp 5 .Multiple clasp 6 .Half-and-half clasp 7 .Reverse- action clasp

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CIRCUMFERENTIAL CLASP ( AKER CLASP)

Most logical clasp to use with all tooth-supported partial dentures because of its retentive and stabilizing ability.

Disadvantages:1 .More tooth surface is covered.

2 .Occlusal approach may increase the width of the occlusal surface of the tooth.

3 .Half-round form prevents adjustment to increase or decrease retention.

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CIRCUMFERENTIAL CLASP

The basic form of the circumferential clasp is a buccal and lingual arm used ( one retentive clasp arm, opposed by a nonretentive reciprocal arm).

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RING CLASP

Circumferential type, which encircles nearly all of a tooth from its point of origin used when a proximal undercut cannot be approached by other means.

Supporting strut on the nonretentive side,with or without an auxiliary occlusal rest on the opposite marginal ridge. The advantageof an auxiliary rest is that prevent further movement.

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EMBRASURE CLASP

Unmodified Class II or Class III partial denture, no edentulous spaces are available on the opposite side .

Sufficient space must be provided between the abutment teeth, yet the contact area should not be eliminated entirely ,

based on the patient’s age, caries index, oral hygiene, tooth contours, used with double occlusal rests, to avoid interproximal wedging by theprosthesis, always be used whenever food impaction ispossible, two retentive clasp arms and two reciprocal clasp arms.

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Back-action Clasp:Modification of the ring clasp;

difficult to justify .

Multiple Clasp: Two opposing circumferentialclasps joined at the terminal end of the two reciprocal arms .When additional retention and stabilization are needed.

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Half-and-half Clasp: Consists of a circumferential retentive arm arising from one direction and a reciprocal arm arising from another .The second arm must arise from a second minor connector ,with or without an auxiliary occlusal rest.

Reverse-action ClaspHairpin; to allow a proximal undercut to be engaged from an occlusal

approach same use of a ring clasp .

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ANALYSIS OF TOOTH CONTOURS FOR RETENTIVE CLASPS

The teeth that are engaged by a removable partial denture must be contoured to support, stabilize, and retain the functioning prosthesis.

Critical areas of an abutment that provide for

retention and stabilization (reciprocation) can onlybe identified with the use of a dental cast surveyor.

Relationship of the vertical arm to the cast

represents the path of placement

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ANALYSIS OF TOOTH CONTOURS FOR RETENTIVE CLASPS

Identifies the location on the clinical crown where the greatest convexity exists. This line, called the height of contour (specific to the surveyor-defined path)

This surveyor-defined path and the subsequent tooth height of contour will indicate the areas available for retention an those available for support, and the existence of tooth and other tissue interference to the path of Placement.

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ANALYSIS OF TOOTH CONTOURS FOR RETENTIVE CLASPS

The importance of angle of cervical convergence lies in its relationship to the amount of retention

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AMOUNT OF RETENTION

Clasp retention is based on the resistance to deformation of the metal. For a clasp to be retentive, it must be placed in an undercut area of the tooth.

Dependent on several factors:

Tooth factors are the size of the angle of cervical convergence (depth of undercut) and how far the clasp terminal is placed into the angle of cervicalconvergence .

Prosthesis factors include the flexibility of the clasp arm ( length, diameter , cross-sectional form or shape, and the material used in making the clasp).

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SIZE OF AND DISTANCE INTO THE ANGLE OF CERVICAL CONVERGENCE

To be retentive, a tooth must have an angle of convergence cervical to the height of contour. This line, which Kennedy called the height of contour, is its greatest convexity.DeVan added the terms suprabulge, denoting the surfaces sloping superiorly, and infrabulge, denoting the surfaces sloping inferiorly.

Any areas cervical to the height of contour may be used for the placement of retentive clasp components, whereas areas occlusal to the height of contour may be used for the placement of nonretentive, stabilizing, or reciprocating components. The location and depth of a tooth undercut available for retention are therefore only relative to the path of placement and removal of the partial denture.

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SIZE OF AND DISTANCE INTO THE ANGLE OF CERVICAL CONVERGENCE

The most suitable path of placement is generally considered to be the path of placement that will require the least amount of mouth preparation necessary to place the components of the partial denture in their ideal position on the tooth surfaces and in relation to the soft tissue.

The path of placement also must take into consideration the presence of tissue undercuts that would interfere with the placement of major connectors, the location of vertical minor connectors, the origin of bar clasp arms, and the denture bases.

Guiding planes control the path of placement and removal, they can also provide additional retention. If some degree of parallelism does not exist during placement and removal, trauma to the teeth and supporting structures.

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LENGTH OF CLASP ARM

The longer the clasp arm the more flexible.Measured from the point at which a uniform taper begins.Bar clasp arm will usually be longer than a circumferential clasp arm.

Diameter of Clasp ArmThe greater the average diameter of a clasp arm the less flexible.Diameter will be at a point midway between its origin and its terminal end.

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CROSS-SECTIONAL FORM OF THE CLASP ARM

The only universally flexible form is the round form, which is practically impossible to obtain by casting and polishing.

Cast retentive clasp arms are more acceptable in

tooth-supported partial dentures in which they are called on to flex only during placement and removal of the prosthesis.

A round clasp is the only circumferential clasp form

that may be safely used to engage a tooth undercut on the side of an abutment tooth away from the distal extension base. 22

MATERIAL USED FOR THE CLASP ARM

Cast gold is that its bulk must be increased to obtain the needed rigidity.

Greater rigidity with less bulk is possible through the use of chromium-cobalt alloys.

Wrought-wire clasp arm has toughness exceeding

that of a cast clasp arm. The tensile strength of a wrought structure is at least 25% greater than that of the cast alloy from which it was made. It may therefore be used in smaller diameters to provide greater flexibility without fatigue.

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RELATIVE UNIFORMITY OF RETENTION

Depend on the location of the retentive part of theclasp arm, not in relation to the height of contour, butin relation to the angle of cervical convergence.

The retention on all principal abutments should

be as equal as possible.

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STABILIZING-RECIPROCAL CAST CLASP ARM

When the direct retainer comes into contact with the tooth, the framework must be stabilized against horizontal movement for the required clasp deformation to occur.

Derived from either cross-arch framework contacts or a

stabilizing or reciprocal clasp in the same clasp assembly.

Must be in contact during the entire period of retentive claspdeformation. Diameter must be greater than the averagediameter of the opposing retentive arm to increase desired rigidity.

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MINOR CONNECTORS

Connects components to the major connector

Direct retainerIndirect retainer

Denture base

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MINOR CONNECTORS

FUNCTIONS OF MINOR CONNECTORS

Unification and rigidityStress distribution

Bracing through contact with guiding planes

Maintain a path of insertion

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TYPES OF MINOR CONNECTORS

Embrasure Minor ConnectorsBetween two adjacent teeth

TYPES OF MINOR CONNECTORS

Embrasure Minor ConnectorsTriangular shaped in cross section

Joins major connector at right angles Relief placed so connector not directly

on soft tissue

GRIDWORK MINOR CONNECTORS

Connect the denture base and teeth to the major connector

GRIDWORK MINOR CONNECTORS

Adjacent edentulous spaces Usually connect major

connector to direct retainersOpen lattice work or mesh

types

GRIDWORK MINOR CONNECTORS

Mesh type Flatter

Potentially more rigidLess retention for acrylic

if openings are small

GRIDWORK MINOR CONNECTORS

Lattice TypePotentially superior retention

Interferes with setting of teeth, if struts are too thick

Both types are acceptable if correctly designed

GRIDWORK RELIEF

Mechanical retention of denture base resin

Allows the acrylic resin to flow under the gridwork

GRIDWORK RELIEF

Relief wax is placed in the edentulous areas

1 mm of relief

JUNCTION WITH MAJOR CONNECTOR

Butt joint with slight undercut in metal

Maximum bulk of the acrylic resin

Prevents thin, weak edges fracturing

MANDIBULAR GRIDWORK DESIGN

Extend 2/3 of the way from abutment tooth to retromolar pad

Never on the ascending portion of the ridge

Stewart's, Fig. 2-55

MAXILLARY GRIDWORK DESIGN

Gridwork2/3 of the length of

from abutment to the hamular notch

Major connector extends fully to the

hamular notch

GRIDWORK DESIGN

Facially just over the crest of the residual ridge

MANDIBULAR TISSUE STOPS

Contact of metal with cast at posterior of distal extension gridwork

Prevents distortion at free end during hydraulic pressure of processing

NO TISSUE STOPS IN MAXILLA

Maxillary major connector acts as a tissue stop (no relief)

PROXIMAL PLATES

Minor connectors originating from the gridwork in an edentulous area

Broad contact with guiding planesMay or may not terminate in an

occlusal rest

PROXIMAL PLATES

PROXIMAL PLATES

Shifted slightly lingually Increases rigidity

Enhances reciprocationImproves esthetics

Often a triangular space below the guiding plane (an undercut)

PROXIMAL PLATES

Rigid, cannot be placed in undercut

Block-out placed in undercuts prior

to waxing and casting the framework