direct retainers dr.jyo

DIRECT DIRECT RETAINERSRETAINERS

Dr. Jyothsna S. ReddyII Yr P.GRagas Dental College

CONTENTSCONTENTS

IntroductionIntroduction Classification of direct retainers Extracoronal direct retainersExtracoronal direct retainers The basic parts of a clasp assembly The basic parts of a clasp assembly Principles of clasp design Criteria for clasp selection Factors affecting retention of a Factors affecting retention of a

claspclasp

CONTENTSCONTENTS

Types of clasp retainersTypes of clasp retainers

Intracoronal retainersIntracoronal retainers

Classification of intracoronal intracoronal

retainersretainers

Selection Selection of intracoronal retainersintracoronal retainers

Summary & conclusionsSummary & conclusions

BibliographyBibliography

RetentionRetention is that quality inherent in the is that quality inherent in the

removable partial denture that resists the removable partial denture that resists the

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

force of gravity, the adhesiveness of foods force of gravity, the adhesiveness of foods

or the forces associated with the opening of or the forces associated with the opening of

the jaws. the jaws.

TERMINOLOGIES :

A A retainerretainer is defined as any type of clasp, is defined as any type of clasp, attachment, device etc used for the attachment, device etc used for the fixation, stabilization or retention of a fixation, stabilization or retention of a prosthesis.prosthesis.

Direct retentionDirect retention is the retention obtained is the retention obtained in a removable partial denture by the use in a removable partial denture by the use of attachments or direct retainers to resist of attachments or direct retainers to resist the displacement or removal of the partial the displacement or removal of the partial denture from the abutment teeth in a denture from the abutment teeth in a direction opposite to that of their insertion.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 GPT 8, 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.

Mechanical RetentionMechanical Retention Mechanical retention is obtained by placing Mechanical retention is obtained by placing

portions of the partial denture into tooth or soft portions of the partial denture into tooth or soft tissue undercuts. tissue undercuts.

Maximizing the other retentive factors can Maximizing the other retentive factors can minimize the need for mechanical retention.minimize the need for mechanical retention.

Most mechanical retention is derived from the Most mechanical retention is derived from the use of direct retainers (clasp assemblies) use of direct retainers (clasp assemblies) utilizing tooth undercuts. There are two classes utilizing tooth undercuts. There are two classes of mechanical retainers: of mechanical retainers: intracoronalintracoronal and and extracoronalextracoronal

NEED FOR THE RETENTIONNEED FOR THE RETENTION WEIGHT AND GRAVITY OF THE WEIGHT AND GRAVITY OF THE

PROSTHESISPROSTHESIS MUSCULAR MOVEMEMTSMUSCULAR MOVEMEMTS ACCIDENTAL DISLOADING FORCESACCIDENTAL DISLOADING FORCES FOR STABILITYFOR STABILITY STICKY FOODSTICKY FOOD PSYCHOLOGICAL BENEFITPSYCHOLOGICAL BENEFIT

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

1. Clasps which engage undercuts.2. polished surfaces.3. Coverage of mucosa by the denture.

REMOVABLE PARTIAL DENTURE

TISSUESUPPORTED

TOOTH SUPPORTED TOOTH & TISSUE

SUPPORTED

RETENTION IN RPD

Primary Secondary

By mechanical means By denture base

Friction Engaging depressionOn abutment

Engaging cervical undercut

MODES OF MECHANICAL RETENTION

Direct retainer Indirect retainer

Intra coronal Extra coronal Rests Some majorconnectors

Classification of Retainers

Intracoronal

ExtracoronalPrecision

attachment

Prefabricated attachment

Internal attachment

Manufactured

Clasps

Occlusally approaching

Gingivally approaching

Flexible clips & rings

EXTRACORONAL DIRECT EXTRACORONAL DIRECT RETAINERSRETAINERS

Occlusally Occlusally approaching / approaching / Suprabulge Suprabulge /Akers/ Ney Type /Akers/ Ney Type I clasp / I clasp / CircumferentialCircumferential

Gingivally Gingivally

approaching / approaching / Infrabulge/ Bar/ Infrabulge/ Bar/ Roach / Ney Type Roach / Ney Type II ClaspII Clasp

Extracoronal direct Extracoronal direct retainers (Clasps)retainers (Clasps)

most commonly used retainer.most commonly used retainer. Prothero (1916) - Prothero (1916) - “Cone theory” “Cone theory” to to

explain the basis for clasp explain the basis for clasp retention. He described the shape retention. He described the shape of crowns of premolar and molar of crowns of premolar and molar teeth as that of teeth as that of two cones sharing a two cones sharing a common base. common base. One cone One cone approximates the occlusal portion approximates the occlusal portion of the tooth, and the other the of the tooth, and the other the cervical portion. A clasp arm or tip cervical portion. A clasp arm or tip that ends on a cervical cone would that ends on a cervical cone would resist movement in the occlusal resist movement in the occlusal direction because the clasp arm direction because the clasp arm would have to undergo deformation would have to undergo deformation to move in the occlusal direction.to move in the occlusal direction.

Extracoronal direct Extracoronal direct retainers (Clasps)retainers (Clasps)

The line at which the two converging The line at which the two converging cones meet is called the cones meet is called the “height of “height of contour”, contour”, a term first used by a term first used by KennedyKennedy. . CummerCummer called it as the guideline that called it as the guideline that helps in the placement of the components helps in the placement of the components of the claspof the clasp

Height of contour (GPT Height of contour (GPT 8)8)

A line encircling a tooth and A line encircling a tooth and designating its greatest designating its greatest circumference at a selected axial circumference at a selected axial position determined by a dental position determined by a dental surveyor; a line encircling a body surveyor; a line encircling a body designating its greatest designating its greatest circumference in a specified plane.circumference in a specified plane.

De Van (1955) referred to the De Van (1955) referred to the surface of a tooth that is occlusal to surface of a tooth that is occlusal to the height of contour as the height of contour as “suprabulge”“suprabulge”, and the surface , and the surface inclining cervically as inclining cervically as “infrabulge”.“infrabulge”.

The basic parts of a clasp assembly The basic parts of a clasp assembly :: Rest : It is the part of

the clasp that lies on the occlusal, lingual or incisal surface of a tooth and resist tissue ward movement of the clasp.

Body of the clasp : It is the part of the clasp that connects the rest and shoulder of the clasp to the minor connector.

Shoulder : It is the part of the clasp that connects the body to the clasp terminals. It must lie above the height of contour and provide some stabilization against horizontal displacement of the prosthesis.

Reciprocal arm : A rigid clasp arm placed above the height of contour on the side of the tooth, opposing the retentive clasp arm.

Retentive arm : It is the part of the clasp comprising the shoulder which is not flexible and is located above the height of the contour.

Retentive terminal : It is the terminal end of the retentive clasp arm. It is the only component of the removable partial denture that lies on the tooth surface cervical to the height of the contour. It possesses a certain degree of flexibility and offers the property of direct retention.

Minor connector : It is the part of the clasp that joins the body of the clasp to the remainder of the framework and must be rigid.

Approach arm : It is a component of the bar clasp. It is a minor connector that projects from the framework, runs along the mucosa and turns to cross the gingival margin of the abutment tooth to approach the undercut from a gingival direction.

PRINCIPLES OF CLASP DESIGN

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.

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.

Secondary support is obtained by the rigid Secondary support is obtained by the rigid components i.e. body and shoulder of the clasp components i.e. body and shoulder of the clasp which are placed above the greatest diameter of which are placed above the greatest diameter of the tooththe tooth

Reciprocation: Each retentive terminal should be

opposed by a reciprocal arm capable of resisting any orthodontic pressures exerted by the retentive arm. Reciprocal and stabilizing elements must be rigidly connected bilaterally.

This arm is positioned on the opposite side of the tooth from the retentive arm.

In addition to reciprocating stress generated against

the tooth by the retentive clasp, it also play an

important role in stabilizing the denture against

horizontal movement.Reciprocal clasp must be rigid, it is not tapered as the

retentive clasp. Reciprocal arm should be positioned on the surface of a tooth is reasonably parallel to the denture’s path of insertion and removal.

It must be positioned above the height of contour at the junction of the gingival and middle 3rd.

To reciprocate the forms properly, it should contact the tooth at the same time or before the retentive arm does.

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.

A gingivally approaching clasp contacts the tooth surface only at its tip.

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.

Stability / Bracing:All clasp have reciprocal or bracing arm, which provides equal amount of stability.

All clasp terminal except the retentive clasp

terminals contribute to this property in

varying degree.Eg: Cast circumferential clasp great amount of stability, because its shoulder is rigid and it aids in stabilization.The wrought wire clasp has flexible shoulder Bar clasp does not have a shoulder so both provide less stability.

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.

Criteria for Clasp Retention

1. Surveyline location and degree of undercut.

2. Requirement of retention and stability

depending on whether upper or lower arch

and configuration of edentulous area, axis

of rotation and selection of retainers.

SURVEY LINE Blatterfein in 1951 put forth a

simple and comprehensive classification of surveyline with suggestion on clasp selection.

He divided the buccal and lingual tooth surface into two halves using a vertical imaginary line through the long axis of the tooth. These halves were described as the nearzone and farzone depending on its closeness to the edentulous space.

He described four kinds of surveyline:

1. Typical surveyline or medium

2. Atypical A or Diagonal

3. Atypical B or High

4. Atypical C or Low

Typical or medium:Extends from the occlusogingival midpoint in the near zone to the junction between the occlusal two third and cervical one third in the far zone.

Clasps suggested for use where such a survey

line exists include the occlusally approaching and gingivally approaching clasps.

Atypical A or diagonal:This runs diagonally across the tooth surface from a high position in the nearzone to a low position in the farzone.- A reverse action or hairpin clasp is recommended.- Gingivally approaching clasp may also be used.

Atypical B or High Surveyline:This type of surveyline is parallel to the occlusal surface and lies close to it.A wrought occlusally approaching clasp arm may be used.If accompanied by a low surveyline on the opposite side of the tooth, a ring clasp or back action or reverse back action have been recommended.

Atypical C or Low Surveyline:

The low surveyline is parallel to occlusal surface but has just above the level of the gingival margin.

In this type of survey line, the extended clasp arm is recommended. Undercut may be developed by recontouring the tooth.

Ney: Recommended 3 basic surveyline with an appropriate clasp form.

Class I :

Surveyline runs diagonally across the tooth surface from a low position on the side of the rest to a high position on the other proximal side.

A cast occlusally approaching arm or its variants, back action, reverse back action and ring clasps are recommended.

Class II:

Similar to Blatterfein atypical A or Diagonal surveyline. Here gingivally approaching is recommended .

Class III:

It is the same as the Blatterfein Atypical

B or High surveyline. The wrought wire

arm is used.

Factors affecting retention of a Factors affecting retention of a claspclasp

Size of the angle of cervical Size of the angle of cervical convergenceconvergence

When the surveyor blade contacts a tooth on the cast at its greatest convexity, a triangle is formed, the apex of which is at the point of contact of the surveyor blade with the tooth, and the base is the area of the cast representing the gingival tissues. The apical angle is called the angle of cervical convergence.

To be retentive a tooth must have an angle of convergence cervical to the height of contour.

Guiding planes determine the path of placement and removal of a partial denture. Therefore without the use of guide planes, clasp retention will either be detrimental or practically non existent.

The guide plane moves down the proximal surface which is prepared on the distal aspect of the tooth. When the denture is fully seated, the plane contacts the lower parts of that surface.

DEGREE OF UNDERCUTDEGREE OF UNDERCUT

Relative uniformity of retention will depend on the location of the retentive part of the clasp arm which is not in relation to the height of contour but in relation to the angle of cervical convergence.

Retentive clasp arms must be located so that they lie in the same approximate degree of undercut on each abutment tooth, despite the variation in the distance below the height of contour.

The measurement of the degree of undercut by mechanical means is achieved by the help of an undercut gauge attached to a dental surveyor.

The retentive force is dictated by tooth shape and by clasp design. Though clasps 1 and 2 are in an undercut of 0.25mm, 1 offers more retention than 2.

Flexibility of clasp armsFlexibility of clasp armsLength of the clasp arm : The longer the clasp arm, the more flexible it is, all other factors being equal. The length of a clasp arm is measured from the point at which a uniform taper begins.

A Co-Cr clasp arm engaging the same degree of undercut will have different flexibility and resistance to distortion on the molar and the premolar because of the difference in length.

Diameter of clasp armDiameter of clasp arm The diameter of a clasp arm is The diameter of a clasp arm is

inversely proportional to its flexibility. inversely proportional to its flexibility. The average diameter to be considered The average diameter to be considered

is at a point midway between its origin is at a point midway between its origin and its terminal end. and its terminal end.

The thickness of the clasp arm in the The thickness of the clasp arm in the buccolingual direction is to be buccolingual direction is to be considered rather than the width in the considered rather than the width in the occluso-gingival direction.occluso-gingival direction.

Cross-sectional formCross-sectional form Round cross-sectional form enables the clasp to Round cross-sectional form enables the clasp to

be flexible in all directions whereas the half-round be flexible in all directions whereas the half-round form limits the flexibility to only one direction.form limits the flexibility to only one direction.

Cast clasps are half round in form and they flex Cast clasps are half round in form and they flex away from the tooth, but edgewise flexing is away from the tooth, but edgewise flexing is limited. limited.

If the cross-sectional area of clasp is If the cross-sectional area of clasp is

doubled, the stiffness will be doubled, the stiffness will be

increased 4 times and the flexibility increased 4 times and the flexibility

reduced 4 times.reduced 4 times.

Material used for the Material used for the clasp armclasp arm

Chrome alloys - higher modulus of elasticity than Chrome alloys - higher modulus of elasticity than do gold alloys and are therefore less flexible. do gold alloys and are therefore less flexible. Therefore, a small cross-sectional form of the Therefore, a small cross-sectional form of the clasp and less depth of retentive undercut must clasp and less depth of retentive undercut must be used.be used.

Because of the internal structure of wrought Because of the internal structure of wrought wire, it has greater ability to flex than is wire, it has greater ability to flex than is permitted by the crystalline structure of cast permitted by the crystalline structure of cast alloy. To obtain equal retention, therefore, a alloy. To obtain equal retention, therefore, a greater depth of undercut is required for a greater depth of undercut is required for a wrought wire clasp than for a cast clasp.wrought wire clasp than for a cast clasp.

Structure of the alloyStructure of the alloy The alloy may be cast or wrought in nature. The alloy may be cast or wrought in nature.

Wrought wires have greater flexibility than a Wrought wires have greater flexibility than a cast structure due to its grain structure being cast structure due to its grain structure being fibrous. fibrous.

The tensile strength of a wrought structure is The tensile strength of a wrought structure is at least 29% greater than that of the cast alloy at least 29% greater than that of the cast alloy from which it was made. from which it was made.

Wrought forms can be used in smaller Wrought forms can be used in smaller diameters to enhance the flexibility and they diameters to enhance the flexibility and they offer minimum friction and can have a stress offer minimum friction and can have a stress breaking effect.breaking effect.

Indirect RetentionIndirect Retention

The reciprocal arm may act as an The reciprocal arm may act as an indirect retainer when it rests occlusal indirect retainer when it rests occlusal to the height of contour on the abutment to the height of contour on the abutment tooth, lying anterior to the fulcrum line. tooth, lying anterior to the fulcrum line.

Lifting of a distal extension base away Lifting of a distal extension base away from the tissues is resisted by a rigid from the tissues is resisted by a rigid arm, which is not displaced cervically arm, which is not displaced cervically

TYPES OF CLASP RETAINERSTYPES OF CLASP RETAINERS Circumferential clasps/ Circumferential clasps/

Occlusally Occlusally approaching/Aker Claspapproaching/Aker Clasp

• Circumferential claspCircumferential clasp• Embrasure clasp.Embrasure clasp.• Ring clasp.Ring clasp.• Back action clasp.Back action clasp.• Reverse action / hair pin clasp.Reverse action / hair pin clasp.• Multiple clasps.Multiple clasps.• Half-and-half clasp.Half-and-half clasp.• Combination clasp.Combination clasp.• Onlay clasp.Onlay clasp.

Bar/Roach clasps / Bar/Roach clasps / Gingivally approaching Gingivally approaching

clasps.clasps. T-claspT-clasp

Modified T-Modified T-

claspclasp

Y-claspY-clasp

I-claspI-clasp

RPI concept.RPI concept.

Other clasp designs Other clasp designs RPA clasp.RPA clasp. VRHR clasp (vertical reciprocal VRHR clasp (vertical reciprocal

horizontal retentive)horizontal retentive) Clasps utilizing proximal undercuts Clasps utilizing proximal undercuts

-Mesiodistal clasp -Mesiodistal clasp

-Devan clasp. -Devan clasp. Movable arm clasp.Movable arm clasp. Cingulum claspCingulum clasp

CIRCUMFERENTIAL CLASP

The clasp is usually the most logical to use with all tooth borne partial denture. Because of its better retentive and stabilizing ability.

Basic design of the clasp is a buccal and lingual arm originating from a common body.

Circumferential clasp on a molar

Advantages:

1) The clasp fulfills the requirement of support,

stability reciprocation, encirclement &

passivity

better than any other type of clasp.

2) It is easy to construct.

3) It is simple to repair.

Disadvantages:

1. It tends to increase the circumference of the crown.

2. It is not acceptable in the anterior region.

3. It covers more tooth surface then the bar clasp and prone to caries.

4. Retentive undercuts on some teeth are difficult to reach with retentive terminal of the clasp

EMBRASSURE CLASP Bonwill clasp Rib clasp Back to back clasp

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

Embrasure clasp on a maxillary premolar and molar

Kennedy class II, III, IV cases where no edentulous space on opposite side of the arch.

Sufficient space must be provided between the abutment teeth in their occlusal third to make room for the body of embrasure clasp.

Contact area should not be eliminated completely.

Abutment tooth should be protected with crowns or

inlays if necessary. This depends upon the age of

the patient caries index and oral hygiene.

This clasp should be used with double occlusal rest.

Proximal shoulder be established. To avoid

interproximal wedging by the prosthesis.

This clasp should have two retentive clasp arms

and two reciprocal arms either bilaterally or

diagonally opposed.

Contra indication:

1. Short and bulboss crown.

2. Not preferred in teeth with more undercuts.

Disadvantages:

Needs adequate cleaness in occlusal

surface.

Breakage of inadequate preparation and

clearance

Wedging action.

RING CLASP Indication:1. In tilted molar 2. Single standing abutment

Unsupported mandibular molar tend to drift and tip in a mesiolingual direction.Maxillary molar tip is a mesio buccal direction. So available retentive undercut will be located on the mesiolingual line angles of a mandibular molar and the mesiobuccal line angle of maxillary molar.

This ring clasp permits engagement of this undercut by encircling almost the entire tooth from its point of origin.

Mandibular molar, clasp encircles the tooth beginning on the mesiobuccal surface and terminating in an infra bulge area on the mesiolingual surface.

Because of the length of the clasp, it must be designed with additional support, usually in the form of an auxillary bracing arm.

This can provide reciprocation and some amount of stability for the denture.

The entire clasp except the retentive terminal shall be placed above the height of the contour.

An additional occlusal rest can be placed may provide additional support and prevent mesial migration of tooth.

Contra indication:

1. In mandibular molar, where the

attachment of buccinator muscle is so

close to the tooth, that the auxillary

bracing arm encroaches on it.

2. The bracing arm must cross the soft

tissue undercut.

Advantages:1. Excellent bracing

2. Decreased leverage

3. Less stress to abutment teeth.

Disadvantages:1. Needs long crown and enough occlusal

clearance.

2. Difficult to repair.

REVERSE ACTION CLASP OR HAIR PIN

This clasp is essentially a simple circlet clasp in which the retentive arm after crossing the facial surface of the tooth from its point of origin loops back in a hairpin turn to engage a proximal undercut below its point of origin.

The upper part of the retentive arm must be considered to be minor connector and should be rigid.

The lower part of the clasp arm should be tapered. It is the only flexible part of the clasp arm.

The crown of the abutment tooth must have sufficient occlusogingival height to accommodate this double width of the clasp arm.

The upper and lower arms of the retentive clasp must also be shaped in such a way that food debris will not be retained between them.

There must be enough space between the arms so that the metal may be adequately finished and polished.

Indication:1. Distal extension partial denture.2. Mesially inclined posterior.3. Undercut addition to the edentulous area.4. If proximal undercut must be used on a

posterior abutment and when the tissue undercut or high tissue attachment prevent the use of bar clasp arm.

5. If lingual undercut is present which prevent the placement of a supporting minor connecter without tongue interference hairpin clasp is indicated.

Contra Indication:1. Tight occlusal contact, increase posterior

overbite and short crown.2. Clasp cover considerable tooth surface and

may trap debris. Advantages:

Easier to construct Adjustibility

Disadvantages: Food trapment Esthetics Caries

Multiple clasp is two opposing simple circlet clasp joined at the terminal end of the two reciprocal arms.

MULTIPLE CLASP

MULTIPLE CLASPIndication:1. When additional retention is needed.2. Tooth borne partial denture3. Multiple clasping required, when the

partial denture replaces an entire half of the dental arch.

4. When the principal abutment tooth it periodontal support can be used in the form of splinting tooth.

Advantages:

1. Less metal display

2. Less tooth coverage

3. Leaves room for the mesial portion of the denture base to a larger extent than it would be otherwise.

4. It braces the abutment on the mesial even if the tooth is tipped distally.

5. Marginal gingiva can be left uncovered of the abutment teeth for better tissue health.

Half-and-half claspHalf-and-half clasp

This clasp consists of a circumferential retention arm arising from one direction and a reciprocal arm arising from another minor connector.

This principle of Half and half clasp should be

applied only to a unilateral denture design.

The buccal arm provides for bracing only.

The lingual arm utilizes an undercut adjacent

to the edentulous space for retention.

Indication:

• Lingually inclined premolars where lingual

undercut are close to the edentulous space.

Contra indication:

• Buccally inclined premolars –

If it is used for distal extension RPD a distal

rest should be placed. Mesial rest also can

be used in conjunction with the distal rest.

BACK ACTION CLASP

It is a modification of the ring clasp. It has a rest which is connected to a rigid minor

connector. Indication:

For unilateral and bilateral distal extension partial denture.

Combination claspCombination clasp

• This type of clasp consists of a wrought wire retentive clasp arm and a cast reciprocal clasp arm. • The retentive arm is usually occlusally approaching, but it may also be used from a gingivally approaching direction.

Uses: It is used on abutment tooth adjacent to a distal

extension base where only a mesiogingival undercut exists on the abutment or where a large tissue undercut contraindicates a bar type retainer.

The tapered wrought wire retentive arm offers greater flexibility than does the cast clasp arm and therefore better dissipates functional stresses.

AdvantagesAdvantages

Flexibility on account of fibrous grain structure of

the wrought wire retentive arm.

Adjustability : It can be adjusted later to increase

or decrease the retention without danger of

breakage.

Esthetic appearance since it is used in smaller

diameters of round cross-section.

A minimum of tooth surface is covered

because of its line

contact with the tooth, rather than a surface

contact of a

cast clasp arm.

Fatigue failures in service are less likely to

occur with the

tapered wrought wire retentive arm.

DisadvantagesDisadvantages It involves extra steps in fabrication,

particularly when high fusing chromium alloys are used.

It may be distorted by careless handling on the part of the patient.

Since it is bent by hand, it may be less accurately adapted and therefore provide less stabilization above the height of contour.

EXTENDED CLASP ARM

It is similar to the circumferential arm but it

covers two teeth. It remains above the surveyline

of the 1st tooth; crosses the undercut of the

adjacent tooth. It is rarely used direct retainer.

Indications:• Tooth supported RPD.

• Tooth next to edentulous space has no

buccal and lingual

undercut.

• The occlusion in the embrasure area will not

allow passage

of the clasp arm to an undercut on the

second abutment from the edentulous

space.

• The second abutment from edentulous space

has a buccal

undercut available.

Contra Indication:

1. Distal extension dentures because the retentive lies forward of the axis rotation. Functional forces will cause rotation around the rest and upward movement of clasp tip.

Advantages:

1. It has splinting and stabilization action.

2. Distribution of lateral loads over two teeth.

Disadvantages:

1. Tooth structure covered.

2. Easily distorts.

3. Breakage of the arm.

4. If made in gold limited 2 premolar

in Cr. Ch – longer arm can be used.

MESIODISTAL CLASP

Used to clasp canine and central only if little undercut on buccal surface.Disadvantage:

Metal displaces alloy made of gold.

If diastema between the lateral incisor and canine, then the space provides a accommodation for the mesial part of the clasp otherwise space reaction with safe side disc, contact point with L.I. is returned when clasp an position. It is similar to inlay.

ONLAY CLASP1. Extends from an occlusal onlay into an undercut

located mesio distally.2. This clasp is an extended occlusal rest with buccal

and lingual clasp arms.3. This clasp may originate from any point on the

onlay that will not create any occlusal interference.4. If the onlay is prepared with chrome cobalt alloy

and is opposed by natural teeth the occlusal surface should be constructed of acrylic resin or gold, if chrome alloy used, because of its extreme hardness will cause rapid wear of enamel

Indications:

1. When the occlusal surface of the abutment

tooth is below the occlusal plane.

2. Only in caries resistant mouth unless it is

covered by gold crown.

Advantages:

1. Mesially tilted molars to be used to provide retention.

2. 3rd molar occlusion may be improved.

Disadvantages:

1. Difficult to clasp to tooth.

2. Increased contact area, accurate impression and the resultant cast is difficult to achieve.

DEVAN CLASP Uses proximal undercut and has a small

head that bears on tooth entirely below survey line.

Clasp arises and lies closely against at the periphery of the denture base.

Denture base is under extended to provide room for the approaching arm.

It is reciprocated with lingual and palatal minor connecter.

It gives little bracing effect.

Devan claspDevan clasp

Advantages:1. Esthetically acceptable, because of

interproximal location or it is hidden behind the buccal concavity.

2. The distribution of stress during insertion and removal is minimal.

3. Increased retention without tipping action on the abutment.

4. Less chance of accidental deformation because it doesn’t project very far away from the denture base.

GINGIVALLY APPROACHING CLASPS

Infra Bulge Push Clasp Roach Clasp This bar clasp approaches the retentive

undercut in a gingival direction resulting in a push type of retention. This push type of retention is more effective than pull type retention characteristic of circumferential clasp.- tripping action.

This clasp is termed by F.E. Roach in 1930 and hence the name Roach clasp.

The bar clasp is classified by shape of the retentive terminal T, modified T, I, Y forms, all of which originate from the denture base frame work and approaches the undercut from gingival direction.

Advantages:1. Minimal tooth contact and minimal distortion

of normal tooth contours leading to improved tissue stimulation, oral hygiene, caries and periodontal problems.

2. Improved esthetics if the approach portion of the arm is not visible as it crosses the gingiva.

3. Increased retention because of tipping action.4. Decreased torquing forces applied to terminal

abutments in extension RPD.5. Large undercut can be engaged.

Disadvantages:1. Cannot be used in the presence of soft

tissue undercuts shallow vestibule and high frenum attachments.

2. Bracing action provided by bar clasp is considerable less than that provided by cast circumferential clasps.

3. The bar clasp will not totally disengage in certain distal extension cases.

4. Food trapment.5. Difficult to fabricate and adjust.

Indication:

1. Class I and Class II distal extension partial dentures to engage the distobuccal undercut on abutment. It can be employed on canines and sometimes even on molars.

2. Where the anterior retention is needed: It is often used on distobuccal surface of maxillary canines and mandibular premolar as the retentive arm can be hidden from the vision. Esthetically this clasp is superior to circumferential clasp and inferior in providing stability because of greater flexibility of the retentive arm.

TYPES OF BAR CLASP

T Clasp Modified T

Clasp Y clasp I clasp

T-Clasp:

• Used in combination with cast circumferential reciprocal

arm.

• The retentive terminal and its opposing encircling finger

projects laterally from the approach arm to form T.

• The retentive terminal must cross under the heoght of

contour to engage the retentive undercut, while the other

finger of the T stays on the suprabulge of the tooth.

• The approach arm should taper gradually

and uniformly

from its origin to the retentive terminal.

• The approach arm contacts the tooth only

at the height

of contour.

Indication:

- Most frequently used is distal extension ridge

where the usable undercuts is on the distobuccal

surface of the terminal abutment tooth.

- It can also be used for tooth supported partial

denture when the retentive undercut is located

on the abutment tooth adjacent to the

edentulous space.

Contra Indication:- Should not be used on a terminal abutment

adjacent to a distal extension base if the usable undercut is located on the side of the tooth away from the edentulous space.

- The T clasp can never be used if the approach arm is in the soft tissue undercut.

- This clasp cannot be used when the height contour is a large space will be created between the approach arm of the clasp and the tooth, which would result in irritation of the lips or cheeks and in trapping food debris.

MODIFIED T-CLASP It is a T clasp with the non retentive (mesial)

finger of the cross bar of the T terminal is

eliminated

Indication:

- Used on canines or premolar for esthetic reasons.

- When used this type of a clasp the encirclement

of the abutment tooth is sacrificed.

Y-CLASP

It is basically a T-clasp, its configuration occurs

when the height of contour on the facial surface of

the abutment tooth is high on mesial and distal line

angles but low on the center of the facial surface.

I-CLASP

Used on distobuccal surface of maxillary canine for

esthetic reason.

Disadvantage:

- The contact of the retentive clasp with the abutment tooth

is the tip of the clasp an area of 2-3 mm. Encirclement

horizontal stabilization may be compromised.

RPI System (Rest, Proximal plate, I-bar)

Kratochvil in 1963 developed the early clasp assembly which consisted of three separate units connected to each other only through the framework. They were the mesial occlusal rest, a distal guide plate and an I-bar retainer. He preferred a full length guide plane that is subsequently relieved in the mouth to prevent torque or binding.

His purposes were:

Elimination of the V-shaped food trap

distal to the tooth.

A highly polished metal contact with the

marginal gingiva, rather than resin .

Intimate metal-to-tooth contact to

minimize food impaction.

This design had certain basic disadvantages:

Physiologic relief was required to prevent

impingement of gingival tissues during

function.

Since the proximal plate covers a greater

surface area of the tooth, the functional

forces are directed in the horizontal

direction.

Krol in 1973 made certain modifications in the design of the proximal plate and named it the RPI bar clasp design.

He had a 2-3mm of contact of the tooth with the guide plane, the section below this point being relieved and he felt that the V shaped space that is left underneath was not as detrimental as the possible restriction of rotation.

This design however, leaves a region occlusal to the proximal plate where contact between the abutment and denture must be made by the replacement tooth.

BASIC PRINCIPLES OF RPI BASIC PRINCIPLES OF RPI CONCEPTCONCEPT

The mesiobuccal rest with the minor connector is

placed into the mesiolingual embrasure, but not

contacting the adjacent tooth.

A distal guiding plane, extending from the

marginal ridge to the junction of the middle and

gingival thirds of the abutment tooth, is prepared

to receive a proximal plate.

The buccolingual width of the guiding

plane is determined by the proximal contour

of the tooth.

The proximal plate in conjunction with the

mesial occlusal rest and minor connector

provides the stabilizing and reciprocal

aspects of the clasp assembly.

The I-bar contributes to the retentive aspect and should be located in the gingival third of the buccal or labial surface of the abutment in 0.01 inch undercut.

The whole arm of the I-bar should be tapered to its terminus, with no more than 2mm of its tip contacting the abutment.

The approach arm must be located at least 4mm from the gingival margin and even more if possible.

OTHER CLASP DESIGNSOTHER CLASP DESIGNS

RPA claspsRPA clasps The rest, proximal plate, Aker’s clasp

was developed and described by Eliason in 1983. It consists of a mesial occlusal rest, proximal plate and a circumferential clasp arm, which arises from the superior portion of the proximal plate and extends around the tooth to engage the mesial undercut.

VRHR Clasp

The vertical reciprocal horizontal retentive arm concept was developed by Grasso in 1980 and is characterized by:

A distal occlusal rest supported by a minor connector.

A lingual vertical reciprocal component originating from the major connector.

A horizontal retentive arm attached to either the major connector or the retention latticework for the denture base.

Cingulum clasp Miller in 1972 designed a clasp to satisfy both

the mechanical and esthetic requirements without the shortcomings of the internal attachment.

The cingulum clasp has 2 lingual clasp arms. The use of this clasp requires that the lingual surface of the abutment tooth be covered with a gold casting.

A guiding plane is incorporated into the distal surface of the crown and the clasp is designed as an integral part of the rigid metal framework.

AdvantagesAdvantages Esthetic A tooth of short clinical crown can be used. The young pulp is not imperiled by close

proximity to metal which shows thermal conduction.

Less expensive.

DisadvantagesDisadvantages The clasp arms are vulnerable to breakage.

Use The cingulum clasp can be used as a retainer

on cuspid teeth when other extracoronal retainers are esthetically unacceptable

Occlusally and gingivally approaching clasps:

Relative merits and demeritsRetention : The bar clasp approaches the

undercut from below the height of contour and

to resist dislodgement, the clasp pushes

towards the occlusal surface of the abutment

tooth. The circumferential clasp engages the circumferential clasp engages the

retentive undercut from above the height of retentive undercut from above the height of

contour and pulls towards the occlusal surface contour and pulls towards the occlusal surface

from the undercut to resist dislodgement. from the undercut to resist dislodgement.

Bracing : The circumferential clasp is rigid

in the upper two-thirds of the retentive arm

and offers some bracing or stabilization

against lateral stresses. On the other hand,

the bar clasp is flexible throughout its

length and does not contribute to stability.

Stress breaking effect : The gingivally

approaching clasp allows a certain degree of

functional movement of the distal extension

base which helps to dissipate the stresses and

lessen the load on the abutment. Occlusally

approaching clasps have the potential to

torque abutment teeth in distal extension

based partial denture situations.

Contact with tooth structure : The

gingivally approaching clasp contacts minimum

tooth structure and has a minimum interference

with natural tooth contour permitting maximum

natural cleansing action, whereas the occlusally

approaching clasps covers more of tooth

structure. This occlusal approach may increase

the width of the occlusal table.

Damage to oral tissues : The area of

food lodgement is at the neck of the tooth,

with the cementum in this area being more

likely to be affected by caries than enamel.

Trauma to the gingiva can also occur with bar

claps unless sufficiently relieved. Mishandling

of the clasps by the patients during removal

of the prosthesis can result in deformation of

the clasp and damage to soft tissues.

Esthetics : Gingivally approaching clasps

are more esthetic than occlusally

approaching clasps except in instances where

large amounts of gingiva is visible on smiling.

INTRACORONAL RETAINERSINTRACORONAL RETAINERS The intracoronal retainer is usually regarded as The intracoronal retainer is usually regarded as

an internal attachment or precision attachment. an internal attachment or precision attachment. The Glossary of Prosthodontic Terms defines The Glossary of Prosthodontic Terms defines

precision attachmentprecision attachment as:as:““A retainer consisting of a metal receptacle A retainer consisting of a metal receptacle (matrix) and a closely fitting part (patrix); (matrix) and a closely fitting part (patrix); the matrix is usually contained within the the matrix is usually contained within the normal or expanded contours of the crown normal or expanded contours of the crown on the abutment tooth and the patrix is on the abutment tooth and the patrix is attached to a pontic or the removable attached to a pontic or the removable partial denture framework.”partial denture framework.”

As a direct retainer it must

provide :

Support

Retention

Reciprocation

Stabilization

Fixation

The precision attachment is the only type of The precision attachment is the only type of

intracoronal attachment that provides intracoronal attachment that provides Lateral force transmission or bracing from Lateral force transmission or bracing from

the parallel proximal walls of the rest against the parallel proximal walls of the rest against

the rest seat.the rest seat. Occlusal force transmission or support from Occlusal force transmission or support from

the flat gingival floor of the result on the rest the flat gingival floor of the result on the rest

seat.seat. Primary retention from the frictional fit Primary retention from the frictional fit

between the rest and rest seat.between the rest and rest seat.

Advantages of intra coronal retainers:

1. Esthetically acceptable, because not much of

metal display like extracoronal retainers.

2. It is preferred in many of the situation

because of its vertical support through a rest

seat located more favourable to the

horizontal axis of the abutment tooth.

3. Horizontal stabilization to some extent.

Similar to internal rest but extracoronal

stabilization is needed.

4. Stimulation to the underlying tissues

greater when internal attachment are

used because of the intermittent vertical

massage.

Disadvantages of intra coronal retainers:

1. They require preparation of abutment tooth

and casting.

2. Difficult clinical and laboratory procedure.

3. They eventually wear, result in loss of

frictional resistance to denture removal.

4. Difficult to repair and replace.

5. They are effective in longer teeth

and least effective in shorter teeth.

6. Difficult to place completely in the

abutment teeth.

Limitations of intra coronal retainers:

1. Large pulp size which is usually related to

the age of patient.

2. Length of the clinical crown, not used in

short or abraded teeth.

3. Expensive.

4. Distal extension denture bases.

Classification Classification by Good Kind and Baker in

1976 :

1)Intra coronal

a. resilient

b. non resilient

2) Extra coronal

a. resilient b. non resilient

Gerardo Becerra et al in 1987 classified precision attachments as :

1) Intra coronal attachments

a. Frictional

- tapered and parallel walled

boxes and tubes - adjustable

metal plates

- springs

- studs

- locks

b. Magnets

2) Extra coronal attachments

a. Cantilever attachments

- rigid attachments

- movable attachments

b. Bar attachments

Tapered And Parallel Walled Boxes And Tubes

Designed to be used in FPD.Designed to be used in FPD.

Plastic pre fabricated patterns.Plastic pre fabricated patterns.

Provides vertical support and lateral stabilization.Provides vertical support and lateral stabilization.

Simple pin and tube or rectangular block and Simple pin and tube or rectangular block and

boxes.boxes.

E.g. : Mc Collum attachments.E.g. : Mc Collum attachments.

McCOLLUM McCOLLUM ATTACHMENTSATTACHMENTS

Adjustable Metal Plates Similar to block and box variety .Similar to block and box variety . Provided with a narrow slit in the Provided with a narrow slit in the

metal block or male portion of the metal block or male portion of the attachment to increase the friction.attachment to increase the friction.

Provides a simple and effective form of Provides a simple and effective form of direct retention.direct retention.

Atleast 2.5 mm of tooth height is Atleast 2.5 mm of tooth height is required. required.

E.g.: Crismani attachment.E.g.: Crismani attachment. Mc Collum attachment.Mc Collum attachment. Stern attachment Stern attachment Chayes or Rley attachment.Chayes or Rley attachment.

CHAYES CHAYES ATTACHMENTATTACHMENT

CRISMANI CRISMANI

ATTACHMENATTACHMENTT

SpringsSprings

Incorporated in the male part to control the friction.Incorporated in the male part to control the friction.

Spring activates a plunger rod which protrudes from Spring activates a plunger rod which protrudes from

male part to engage a depression in the female part.male part to engage a depression in the female part.

Approximately 4 – 5 mm of vertical height is Approximately 4 – 5 mm of vertical height is

required.required.

E.g.: Schatzmann attachment.E.g.: Schatzmann attachment.

SCHATZMANN SCHATZMANN ATTACHMENTATTACHMENT

StudsStuds

A metallic stud can be soldered to post and A metallic stud can be soldered to post and core and cemented into an abutment.core and cemented into an abutment.

Direct retention can be obtained by using a Direct retention can be obtained by using a stud which clips into an flexible ring.stud which clips into an flexible ring.

Sufficient clearance is required to arrange Sufficient clearance is required to arrange the artificial teeth.the artificial teeth.

E.g.: Ceka attachmentE.g.: Ceka attachment

Rotherman attachmentRotherman attachment

Ceka Ceka attachmentattachment

LocksLocks

These lock rigidly into the attachments .These lock rigidly into the attachments .

The vertical height required for this attachment is The vertical height required for this attachment is

atleast 6 mm.atleast 6 mm.

Retained with pins or incorporated in post and Retained with pins or incorporated in post and

core .core .

E.g.: T - block attachmentE.g.: T - block attachment

T- BLOCK ATTACHMENTT- BLOCK ATTACHMENT

MagnetsMagnets

Small metal keeper is attached to the tooth Small metal keeper is attached to the tooth

surface, usually into the root canal and surface, usually into the root canal and

magnet is incorporated into the resin.magnet is incorporated into the resin.

Alloy in the magnet produces a magnetic Alloy in the magnet produces a magnetic

force that is strong .force that is strong .

Magnets are brittle and corrode unless Magnets are brittle and corrode unless

protected in a stainless steel shelf.protected in a stainless steel shelf.

Cantilever Cantilever attachmentsattachments Rigid attachmentsRigid attachments They are pin and tube joints that use a slit in the

pin or multiple pin tubes and slots to enhance retentive friction between the parts with the natural teeth on the either side of the edentulous space.

These attachments offer excellent stability and retention in tooth supported partial dentures.

e.g.: Scott attachmentattachment Thompson dowel rest system.

Movable attachments. These allow the prosthesis to rotate around a

horizontal axis and transmit occlusal forces to the residual alveolar ridge .

E.g.: Dolbo attachmentsattachments

Bar attachmentsBar attachments These can be connected These can be connected

to the cast metal to the cast metal crowns or copings .crowns or copings .

Custom made bars can Custom made bars can be cast with a flat be cast with a flat upper surface to upper surface to support the prosthesis support the prosthesis and parallel sides that and parallel sides that help to stabilize it.help to stabilize it.

E.g.: Dolder bar. E.g.: Dolder bar.

Precision attachment Precision attachment selectionselection

Kennedy’s class III partially edentulous arch.Kennedy’s class III partially edentulous arch.

• Rigid internal attachments are recommended .Rigid internal attachments are recommended .

• Provides good retention, support and bracing Provides good retention, support and bracing

because of its rigid interlocking components. because of its rigid interlocking components.

• If the posterior abutment prognosis is If the posterior abutment prognosis is

questionable then a resilient type of attachments questionable then a resilient type of attachments

are recommended with anterior abutment.are recommended with anterior abutment.

Kennedy’s class I and class II Kennedy’s class I and class II partially edentulous archespartially edentulous arches

The most difficult type of treatment plan.The most difficult type of treatment plan.

Some practitioners advocate non rigid and Some practitioners advocate non rigid and

resilient attachments and some advocate resilient attachments and some advocate

resilient attachment in distal extension to resilient attachment in distal extension to

minimize rotation and torquing of the minimize rotation and torquing of the

abutment tooth when the components of an abutment tooth when the components of an

attachment are rigidly connected.attachment are rigidly connected.

Another philosophy , known as the stable Another philosophy , known as the stable

base precison attachment RPD concept or base precison attachment RPD concept or

floating denture base concept recommends floating denture base concept recommends

incorporation of rigid internal attachments incorporation of rigid internal attachments

and a cast metal base made from mucostatic and a cast metal base made from mucostatic

impression of the residual ridge. The male impression of the residual ridge. The male

portion of the attachment is connected to portion of the attachment is connected to

the denture base , allowing the complete the denture base , allowing the complete

seating within the abutment.seating within the abutment.

The hinged or directionally oriented attachment The hinged or directionally oriented attachment

such as the Dolbo attachments are recommended such as the Dolbo attachments are recommended

to provide additional bearing or resistance to to provide additional bearing or resistance to

lateral movement when the residual ridge is lateral movement when the residual ridge is

severely resorbed.severely resorbed.

The Ceka attachments can be used successfully The Ceka attachments can be used successfully

where the ridges are not parallel to one another .where the ridges are not parallel to one another .

Kennedy’s class IV partially Kennedy’s class IV partially edentulous archedentulous arch

The ideal RPD design for such situation involves the The ideal RPD design for such situation involves the

use of a tissue bar placed close to the edentulous use of a tissue bar placed close to the edentulous

ridge and connected as a fixed unit to the abutment ridge and connected as a fixed unit to the abutment

teeth on either side of the space using crowns.teeth on either side of the space using crowns.

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