direct retainer (dr vikas ppt.ppt

7/22/2019 direct retainer (dr vikas ppt.ppt

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DR VIKAS AGGARWAL


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COMPONENTS OF REMOVABLE PARTIAL DENTURE

MAJOR CONNECTOR-the part of a partialremovable dental prosthesis that joins the componentson one side of the arch to those on the opposite side

(GPT 8TH

)


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MINOR CONNECTORthe connecting link between

the major connector or base of a partial removabledental prosthesis and the other units of theprosthesis, such as the clasp assembly, indirectretainers, occlusal rests, or cingulum rests (GPT 8TH)


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DIRECT RETAINER- that component of a partial

removable dental prosthesis used to retain and preventdislodgment, consisting of a clasp assembly orprecision attachment (GPT 8TH)


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INDIRECT RETAINER- the component of a partialremovable dental prosthesis that assists the direct

retainer(s) in preventing displacement of the distalextension denture base by functioning through leveraction on the opposite side of the fulcrum line whenthe denture base moves away from the tissues in pure

rotation around the fulcrum line (GPT 8TH)


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GUIDING PLANE- vertically parallel surfaces onabutment teeth or/and dental implant abutments

oriented so as to contribute to the direction of thepath of placement and removal of a removabledental prosthesis (GPT 8TH)


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6) REST- unit of a partial denture that is placed on anabutment tooth , ideally in a prepared rest seat, so that it

limits movement of denture in a gingival direction andtransmits functional forces to the tooth. They aredesignated by the surface of the tooth prepared to receivethem: Occlusal rest- placed on the occlusal surface of a posterior tooth

Lingual rest- placed on the lingual surface of an anterior tooth, Incisal rest- placed on the incisal edge of an anterior tooth.


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Denture base part of a denture (made of metal or resin)that rests on a residual bone covered by soft tissue , to

which teeth are attached and which effects the transfer of

occlusal forces to supporting oral structures. Tooth replacements-refers to artificial teeth placed in

denture. They can be acrylic, resin metal or porcelain


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Retention is that quality inherent in theremovable partial denture that resists the vertical

forces of dislodgement for e.g., the force ofgravity, the adhesiveness of foods or the forcesassociated with the opening of the jaws

Adirect retainer is that component of aremovable partial denture used to retain andprevent dislodgement, consisting of a claspassembly or precision attachment (GPT 8TH )


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DIRECTRETAINER

EXTRACORONAL

RETENTIVECLASP

ASSEMBLY

SUPRABULDGE INFRABULDGE

ATTACHMENTS

INTRACORONAL

PRECISIONATTACHMENT

SEMIPRECISIONATTACHMENT


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EXTRACORONAL ATTACHMENTS

1900- introduced by Henry. R .Boos

1908- modified by F.Ewing.Roach

located outside the normal clinical

contours of abutment crowns derive their retention from closely

fitting components termedmatrices and patrices.


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The male unit is soldered to the surface of the

abutment crowns, forming a projection to which

the female element, buried within the denture,

can be joined. The male portion projects as an L

shaped bar with a ball joint on the lower extremely

The female section fits over the bar and engages

the sides of the ball connection of the male.

Advantages

They provide excellent resistance to toothdistal and lateral displacing forces.

Stress breaking effect.

They require no buccal retainers or lingual bracing

arms.

esthetics

They do not interfere with the contour of the

abutment crown.

Disadvantages

Difficulty in maintaining plaque free environment

Dalbo attachment


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EXTRACORONAL DIRECT RETAINER/COMPONENTS

Retentive clasp assembly

m/c method of extracoronal retention

first appeared in the dental literature with Dr W. G. A.Bonwill's description in 1899

In1916 Prothero advanced cone theory as the basis forclasp retention. He described the shape of crowns ofpremolar and molar teeth as that of two cones sharing acommon base.


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The line at which the two converging cones meet is

called the height of contour, a term first used byKennedy .

De Van : SUPRABULGE and INFRABULGE

Cummer called it GUIDELINE How clasp helps in retention??

If the clasp terminal is placed cervical to this line, it

has to deform in order to escape from undercut area. Indoing so, it generates resistance called retention.


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Rest : It is the part of theclasp that lies on theocclusal, lingual orincisal surface of a toothand resist tissue ward

movement of the clasp.

Body of the clasp : It isthe part of the clasp that

connects the rest andshoulder of the clasp tothe minor connector.

Basic parts of a clasp assembly :


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Shoulder : It is the part of the clasp

that connects the body to theclasp 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.


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Retentive arm : It is the part of theclasp comprising the shoulder

which is not flexible and islocated above the height of thecontour.

Retentive terminal : It is the terminalend of the retentive clasp arm. Itis the only component of theremovable partial denture thatlies on the tooth surface cervicalto the height of the contour. Itpossesses a certain degree of

flexibility and offers the propertyof direct retention.


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Extracoronal circumferential direct retainer


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


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PRINCIPLES OF CLASP DESIGN

1) Encirclement basic principle ,more than 180 degree in the greatestcircumference of the tooth

can be continuous contact, such as in a circumferential clasp,

or discontinuous contact, such as in the use of a bar clasp.

The clasp should make contact with three areas 1) occlusal rest andbody area 2) the reciprocal terminal area 3) the retentive terminal area.


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2) The occlusal rest should be located in prepared

recess on the occlusal surface so all forces aretransmitted along the long axis of the abutment

3) Each retentive terminal should be opposed by areciprocal component capable of resisting any

pressure exerted by retentive arm during removalor insertion

4) Clasp retainers on abutment teeth adjacent to

distal extension bases should be designed so thatthey will avoid direct transmission of tipping androtational forces to the abutment


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5) Unless guiding planes will

positively control the path ofremoval and stabilizeabutments against rotationalmovements, retentive clasps

should be bilaterally opposed,i. e., buccal retention on oneside of the arch should beopposed by buccal retention

on the other, or lingual on oneside opposed by lingual on theother


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6) To resist dislodgment path of escapementfor retentive clasp terminal must be otherthan parallel to the path of removal ofprosthesis

7)The amount ofretention should always bethe minimum necessary to resist reasonable

dislodging forces. 8) Reciprocal elements of the clasp assembly

should be located at the junction of thegingival and middle thirds of the crowns of

abutment teeth. The terminal end of theretentive arm is optimally placed in thegingival third of the crown. These locationspermit better resistance to horizontal and

torqueing forces.


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All clasps must be designed so that they satisfythe following six basic requirements:

1.Retention2.Support

3.Stability

4.Reciprocation5.Encirclement

6.Passivity


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1) RETENTION

Retention is provided mainly by the retentive arm but also by the effectivedesign and adequate construction ofother parts

The retention must be very definitely limited and minimal .

The retention should be balanced

the path of escapement of a retentive terminal must not be parallel to

path of appliance removal


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Guiding planes control the path of

placement and removal; they can alsoprovide additional retention for the partialdenture by limiting the possibilities thatexist for its dislodgment. The more vertical

walls (guiding planes) that are preparedparallel, the fewer the possibilities that existfor dislodgment

Therefore without guiding planes, claspretention will either be detrimental orpractically non-existent. If clasp retention isonly frictional because of an activerelationship of the clasp to the teeth, thenorthodontic movement or damage toperiodontal tissue, or both, will result.Instead a clasp should bear a passiverelationship to the teeth except when a

dislodging force is applied


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Factors affecting retention

Size and the distance into the angle of cervicalconvergence

To be retentive a tooth must have

an angle of convergence cervical

to the height of contour.

The location and depth of a tooth

undercut available for retention are

relative to the path of placement

and removal of the partial denture. Greater the degree of gingival

convergence, the greater will be

the retention of the clasp, provided

all other factors are equal


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When the angle of convergencebetween two abutments differsuniformity of retention can beobtained by placing the clasparms into the same degree ofundercut (i.e. both .02"). Aguiding principle of partialdenture design is that retentionshould be uniform in magnitudeand bilaterally opposed amongstabutments.

The tool used to identify the proper position for each clasp terminus is

called an undercut gauge. Undercut gauges are available in 0.010-,

0.020-, and 0.030- inch configurations


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Degree of undercut

When all other clasp factors

remain constant, positioning

the retentive clasp terminus

at a greater horizontal

undercut will result in

increased retentive force.Therefore, a retentive clasp

terminus positioned in a

0.020-inch horizontal

undercut (B) will provide

greater retentive force thanwill a clasp positioned in a

0.010-inch undercut (A).


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FLEXIBILITY OF CLASP ARMLENGHT OF CLASP ARM

Flexibility

LENGTH (POWER OF 3)

The retentive circumferential clasp armshould be tapered uniformly from its pointof origin through the full length of theclasp arm.

For the same length, the bar clasp arm isless f lexible than a circumferential clasparm because of its half round cross section

which lies in several planes and prevents

total flexibility. In practice however, the gingivally

approaching clasp is longer and moreflexible, with less retentiveness thanocclusally approaching clasp


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DIAMETER OF

CLASP ARM FLEXIBILITY

1/(DIAMETER)3 The greater the average

diameter of a clasp armthe less flexible it willbe.

A clasp should be half as

thick at the tip than atthe origin.


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CROSS-SECTIONAL FORM

Round form is used in distal extension cases where as half

round is used in tooth supported partial dentures


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Material used for clasp arm : Two types of materials areused : Type IV gold alloys and cobalt-chrome alloys whichhave different modulus of elasticity.

The modulus of elasticity of cobalt-chrome alloys is greaterthan that of casting gold. Therefore a clasp of the samecross-section is stiffer in cobalt-chrome than in cast gold.

This can be overcome by using longer clasps of thinnersection.

The retentive terminal has to be flexible and therefore havelow modulus of elasticity. The reciprocal elements have to be

stiff and unyielding and have high modulus of elasticity.


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Structure of the alloy: The alloy may be cast or wrought

in nature. Wrought wires have greater flexibility than a caststructure due to its grain structure being fibrous.

The tensile strength of a wrought structure is at least 25%greater than that of the cast alloy from which it was made.

Wrought forms can be used in smaller diameters toenhance the flexibility. They offer minimum friction andcan have a stress breaking effect.

Disadvantage of wrought alloy is recrystallization can lead

to fracture


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2.SUPPORT Property that resist displacement of clasp in

gingival direction.

Support in a clasp is generally provided by the rest.Thus while chewing food the rest prevents tissue

ward movement of the clasp assembly, plus directsthe force along the long axis of the tooth, thusreduces periodontal tissue damage.

Secondary support is obtained by the rigidcomponents i.e. body and shoulder of the clasp

which are placed above the greatest diameter of thetooth.


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3. STABILITY

Resistance to horizontal displacement of a prosthesis.

All clasp component except retentive terminal providestability

Greatest contribution to the stability is offered byreciprocal element, shoulder and vertically orientedminor connector.

The components of the cast circumferential clasp offerbetter stabilization than either the bar clasp or thewrought wire clasp, because of greater amount ofrigidity of the clasp material.


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4. RECIPROCATION

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

Ideally, reciprocal elements of the clasp

assembly should be located at the junction ofthe gingival and middle thirds of the crownsof abutment teeth. The terminal end of theretentive arm is optimally placed in thegingival third of the crown. These locations

will permit the abutment teeth to better resisthorizontal and torqueing forces.

Reciprocal arm is not tapered like retentivearm.

It is tapered in one dimension(OG)


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During insertion of the prosthesis, the reciprocal elementshould contact the abutment slightly before the retentivearm contacts the abutment. Contact of the reciprocal

element should be maintained while the retentive terminuspasses over the height of contour and into the prescribedundercut.


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If the retentive terminus contacts the abutment before the reciprocal element

contacts the abutment damaging non-axial forces may be applied to the

abutment. For this reason, the abutment surface that will be contacted by

the reciprocal element should be parallel to the removable partial denture'spath of insertion and removal.


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5.ENCIRCLEMENT

The clasp must be designed

to encircle more than 180 degrees.

Encirclement may be:

Continous contact (circumferential) Broken contact (bar)

In bar, clasp contact at 3 diff. tooth areas:

Occlusal rest, retentive terminal, reciprocal terminal


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6. PASSIVITY The retentive clasp arm should be passive (no active

force) until a dislodging force is applied.

The retentive arm should be activated only whendislodging forces are applied to the removable partial

denture


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Classification of extra-coronal retainers:1.Supra bulge clasps (occlusally approaching,

circumferential or encircling clasps).The retentive arm approaches the undercut area from

the suprabulge direction.

2.Infrabulge clasps (gingivally approaching, projection orbar clasps)

The retentive arm approaches the undercut from theinfrabulge direction ,e.g. Bar clasp arm (I-Bar etc.)

Circumferential Clasp


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Circumferential Clasp

Introduced by Dr N. B. Nesbitt in 1916

Easiest to design and construct. Also it is easy to repair.

The most logical clasp to use with all tooth-supported partialdentures because of its retentive and stabilizing ability

Design Rules for use

Retentive clasp arm occlusal and Retentive terminal gingival to height of contour

Terminal toward occlusal surface, never towards thegingiva.

Retentive tip terminate at mesial and distal line angle andnever in the centre of facial or lingual surface.


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DISADVANTAGES:

increased risk fordecalcification

covers large area

width of occlusal table is

increased More metal is visible

ADVANTAGES:

Good support ,bracing and

retentive properties

displays close adaptation to theabutment and therefore

minimizes the entrapment of

food and debris Easy to make and repair


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Types of circumferential clasps

Simple Circlet clasp.

Reverse approach circlet.

Multiple circlet

Embrasure clasp Ring clasp.

Back action clasp.

Reverse action / hair pin clasp.

Half-and-half clasp.

Combination clasp.

Onlay clasp.

Extended clasp


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Simple Circlet clasp

Disadvantages

Because of half round cross-

section geometry adjustment

of clasp arm is difficult.

increase the occlusal table

Increased calcification and

compromised esthetics

Cannot be used in distal

extension cases

Advantages

Easy to construct, repair.

It provides better support, stability,

reciprocity, encirclement, and passivity

Approaches the undercut

on abutment from the

edentulous area.

Engage the undercut

remote from edentulous

space

Most widely used least complex in

design

Clasp of choice for

tooth supported

RPD


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Reverse-action clasp

Features Retentive undercut adjacent to

edentulous space.

m/c DB under undercut

Used when bar clasp cannot be

used due to soft/hard tissue

undercut

Advantages

In distal extension partial dentures, asdenture base is depressed under

function, retentive clasp tip rotate

gingivally to enter greater amount of

undercut and reduce torsional stresses

transmitted to abutment.

Disadvantages

As the shoulder of the clasp extendsover the abutments mesial marginal

ridge, it may be difficult to provide

adequate clearance without removing

significant tooth structure

The gingival mucosa may not be well

protected because of the mesial

occlusal rest resulting in food traps

between proximal plate and surface of

abutment.

It may be esthetically compromised as

it has a mesial approach.


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Multiple circlet clasp.

Indication:

When additional retention is needed.

Tooth borne partial denture Multiple clasping required, when the

partial denture replaces an entire half of

the dental arch.

Used in the form of splinting tooth.

Disadvantage

Two embrasure approaches arenecessary rather than a single

common embrasure for both clasps

Features The multiple clasp is simply two

opposing circumferential claspsjoined at the terminal end of thetwo reciprocal arms.

Upon emerging through occlusalembrasure retentive arm engagesopposite line angle

.

Embrasure clasp/ modified crib clasps/ Bonwill


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p/ p /clasp

Features

Two simple circlet clasp

joined at the body. Used on the side where

no edentulous space

Occlusal rest preparation on

both teeth.

Upon emerging through

occlusal embrasureretentive arm engages

opposite line angle

Disadvantages

Sufficient space must be provided

between the abutment teeth in theirocclusal third to make room for the

common body of the clasp. This

involves more of tooth reduction,

which increases caries susceptibility,

and risk of encroachment of the pulp.

Advantages

The double occlusal rests prevent

interproximal wedging by the

prosthesis, which could causeseparation of the abutment tooth

and result in food impaction and

clasp displacement.

In addition to providing support,

occlusal rests also serve to shunt

food away from the contact area.

Ri l


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Ring claspFeatures

It is that form of a circumferential claspthat encircles nearly the entire tooth

from its point of origin. It is usually used

when a proximal undercut cannot be

approached by any other means

Mandibular molars tend to drift

mesiolingually and maxillary molarsmesiobuccally

Retentive undercut on mesiolingual line

angle of mandibular molar and

mesiobuccal line angle of maxillary

molars

The undercut is on the same side as therest seat (i.e. adjacent to edentulous

span)

The lower bracing arm should be at

least 1 mm from the free gingival margin

and relieved to prevent impingement of

the gingival tissues

Should always be used with a

supporting strut on the non-

retentive side with an auxiliary

occlusal rest on the opposite side Omission of the supporting strut will

allow the clasp arm to open and

close with minimum or no

reciprocation.

Ad t


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Advantages

Provides adequate encirclement of

more than 180 degrees.

excellent retention with adequate

flexibility due to increased length of

clasp arm.

restores the occlusal surface by its

anatomy and thereby uprights the tooth

decreasing unfavourable stress to the

abutment.

provides vertical support andprevents tissueward movement.

The auxiliary distal rest prevent

further mesial drift of the tooth.

Indication

It is used mainly when the proximal

undercut cannot be approached directly

from the occlusal rest area, and / or

tissue undercuts prevent a gingivalapproach of the clasp

Disadvantages:a. Covers a large area requiring

meticulous hygiene

b. Very difficult to adjust

Contra indication Limited vestibular depth

contraindicated when the

bracing arm must cross the soft

tissue undercut.


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Back action clasp

This clasp is a modification of the ring

clasp with no apparent advantages.

It lacks a guide plane and the occlusal

rest does not have a rigid support.

Here minor connector is connected to

end of clasp arm and occlusal rest is

left unsupported. Hence it is a

biologically and mechanically unsound

design.


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FISH HOOK/HAIRPIN/ C CLASP

FEATURES Simple circlet clasp with retentive

arm after crossing the facial surface,

loops back in a hairpin to engage

the undercut below its point of

origin.

Upper part is rigid considered asminor connector and lower part is

flexible.

Crown must have sufficient

occlusogingival height.

The bend that connects the upper

and lower parts of the arm shouldbe rounded to prevent strain

accumulation and fracture of the

arm at the bend


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Indication:

Distal extension partial denture.

Mesially inclined posterior teeth.

If proximal undercut must be used

on a posterior abutment and when

the tissue undercut, tilted teeth or

high tissue attachment prevent theuse of bar clasp arm ( although ring

clasp can be placed but lingual

undercut may prevent the

placement of supporting strut

without tongue interference

Indicated when reverse circlet clasp

cannot used because of lack of occ.

space

Contra Indication:

Tight occlusal contact, increase

posterior overbite and short

crown.

The clasp covers considerable tooth

surface and may trap debris..

It has limited flexibility and is

unesthetic for use on an anterior

abutment

Disadvantages Inadequate flexibility

Difficult to fabricate

Accumulation of food and debris

Esthetically unacceptable

Half and half clasp


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Half and half clasp It consists of a circumferential retentive

arm arising from one direction and areciprocal arm arising from another.

The second arm must arise from asecond minor connector, this arm isused with or without an auxiliaryocclusal rest.

Reciprocation is achieved with a shortbar or with an auxiliary occlusal rest.

The principle of half and-half clasp isused only for unilateral partial denturedesign.


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ONLAY CLASP

Is extended occlusal rest with buccaland lingual clasp arm.

The clasp may originate from anypoint on the onlay that will not createocclusal interferences.

Therefore, this type of clasp is usedwhen the occlusal surface of theabutment tooth is below the occlusalplane. The onlay can be used torestore the lost vertical dimension.

Indicated in caries resistant mouth

Extended clasp arm


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Extended clasp armAdvantages:

It has splinting andstabilization action.

Distribution of lateral loads

over two teeth.

Disadvantages:

More tooth structure

covered, easily distorts and

breakage of the arm.

Features

similar to the

circumferential arm but it

covers two teeth.

It remains above the survey

line of the 1st tooth; crosses

the undercut of the

adjacent tooth..

Indications:

Tooth supported RPD.

tooth next to edentulous space that

has no buccal and lingual undercut.

Contra Indication:

Distal extension dentures

because functional forces will

cause rotation around the rest

and upward movement of

clasp tip.

COMBINATION CLASP


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COMBINATION CLASP

In 1965, Dr O. C. Applegate

introduced a modified wrought-wire clasp assembly known asthe "combination clasp

The combination clasp consistsof a wrought-wire retentiveclasp arm and a cast reciprocalclasp arm. .

Indicated on abutment adj. todistal extension space, when theundercut is on mesiobuccalsurface.


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The wrought-wire component is circular in cross

section, thereby permitting flexure in all

directions.

This omnidirectional flexure allows the clasp to

flex in all planes and can minimize the transfer

of potentially harmful forces to the abutment

b) In addition to advantages of

flexibility, adjustability, and

appearance, wrought-wire

retentive arm makes only line

contact with abutment tooth,

rather than broader contact ofcast clasp


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Uses

Kennedy Class I or Class II posterior edentulous area

when the usable undercut is located at the mesiofacialline angle of the most posterior abutment

It is used on abutment tooth adjacent to a distalextension base where where a large tissue undercut

contraindicates a bar type retainer.

ADVANTAGES


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ADVANTAGES Not only the flexibility is maximal, but it is effective

in any direction.Greater flexibility of wrought wire actas stress equalizer, prevent undesirable forces.

Can be placed in deeper undercut or in gingival third ofclinical crown so more esthetic appearance. So often usedon maxillary canines and premolars.

Round wrought wire makes only a line contact, can beused in caries prone mouth.

readily adjustable to more retentive position. better esthetics (due to its round form and smaller

diameter - 18 gauge)

can be placed in 0.02" undercut due to its flexibility

(allows lower placement for better esthetics)


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Disadvantages More prone to breakage or damage.

Need extra lab procedures Easily distorted by careless handling by patient during

removal, who tend to remove it by lifting retentiveportion of wrought wire clasp.

Retentive arm not possess bracing or stabilizing qualitydue to its increased flexibility.

There are four laboratory approaches to construct


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There are four laboratory approaches to constructwrought wire clasp arm (JPD 1973)

1. Cast to.

2. Soldered to the minor connector.

3. Soldered to the base retentive mesh area.

4. Attached only in the resin of the denture base.This is most commonly used in the repair of brokenclasp components.

Wire components which have been incorporated

into the casting are 42 per cent less flexible thanwire components soldered to the base retentivemesh or unsoldered and buried in resin.


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Clasp that allow functional movement

Clasp assemblies that accommodate functional prosthesismovement are designed to address the concern of a Class Ilever.

The concern is that the distal extension acts as a long "effortarm" across the distal rest "fulcrum" to cause the clasp tip"resistance arm" to engage the tooth undercut.

This results in a harmful tipping or torquing of the tooth andis greater with stiff clasps and more denture base movement.

Two strategies are adopted to either change the fulcrumlocation and subsequently the "resistance arm" engagingeffect (mesial rest concept clasp assemblies), or to minimizethe effect of the lever by use of a flexible arm (wrought-wireretentive arm)

Gingivally approaching clasps/infra bulge/push clasp/roach


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clasp

Clasp is termed by Dr. F. Ewing

Roach in 1930 Approaches the retentive undercut

from a gingival direction.

Push type of retention, which isbetter than pull type retention of

roach clasp. The bar clasp is classified by shape

of the retentive terminal T, modifiedT, I, Y forms, all of which originatefrom the denture base frame work

and approaches the undercut fromgingival direction

It is easier to seat but difficut toremove

Ad t


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Advantages

Minimal tooth contact and minimal distortion ofnormal tooth contours leading to improved tissuestimulation, oral hygiene, caries and periodontalproblems.

Improved esthetics if the approach portion of the armis not visible as it crosses the gingiva.

Increased retention because of tripping action.

Decreased torqueing forces applied to terminalabutments in distal extension RPD.

Large undercut can be engaged.

Di d t


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Disadvantages

Cannot be used in the presence of soft tissueundercuts, shallow vestibule and high frenumattachments.

Bracing action provided by bar clasp is considerable

less than that provided by cast circumferential clasps. Food entrapment.

Difficult to fabricate and adjust.


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a) horizontal projection portion of the

approach arm (A),

b) vertical projection aspect of the

approach arm (B),

c) location where the approach arm

crosses perpendicular to the free

gingival margin (C),

d) point of first tooth contact at or

occlusal to the height of abutment

contour (D),

e) terminus of the retentive clasp

contacting the abutment apical to

the height of contour(E),

f) encirclement portion of the clasp

contacting the abutment occlusal to

the height of contour (F).

Design rules


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Design rules

The approach arm must not impinge on the soft tissues adjacent to

the abutment The approach arm should cross perpendicular to the free gingival

margin.

The approach arm should never be designed to "bridge" an area ofsoft tissue undercut since this will produce an increased risk of food

entrapment and may result in irritation of the soft tissue To optimize flexibility, the approach arm should be uniformly

tapered from its origin to the clasp terminus.

The clasp terminus should be positioned a far apically on the abutmentas is practical. Proper placement of the clasp terminus yields a decrease

in leverage-induced stresses resulting from movement of theprosthesis.

The minor connector that attaches the occlusal rest to the frameworkshould be rigid and should contribute to the overall bracing andstabilization characteristics of the prosthesis.


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Approach arm must not bridge area of soft issue undercut can lead to food entrapment

and soft tissue irritation


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Upon loading of the extension base, the distal rest serves as a centre of

rotation. The tip of the retentive clasp moves apically and mesially. This

minimizes potentially harmful torqueing forces while transmitting a relatively

small, mesially directed force to the abutment. The mesially directed force is

well tolerated as a result of sound contact with the adjacent natural tooth.

Types of bar clasps


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Types of bar clasps

T- barModified T-barY- barI- bar


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T CLASP Retentive terminal and itsopposing encircling fingerproject laterally from the

approach to form T. Both projections should point toward the occlusal surface. The retentive terminal must cross under the height of

contour to engage the retentive undercut.

Used in distal extension base situations, on distobuccalundercut. Dont use it on mesiobuccal undercut.


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It shouldnot be used if approach arm bridge a softtissue undercut.

If T clasp used on tooth where height of contour isclose to occlusal surface then a large surface is

covered b/w approach arm and tooth leads to foodaccumulation.


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MODIFIED T CLASP

it is a T clasp with non

retentive finger of the

crossbar of T terminal is

omitted. It is used on canines and premolars for esthetic reasons.

Potential danger the encirclement of 180 degrees issacrificed for esthetics.

.


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Y CLASP

Basically it has T- barconfiguration , usedwhen height of contour

on the facial surface ofthe abutment tooth ishigh on the mesial anddistal line angles but lowat the center of the facialsurface

I CLASP/ 1 BAR


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I CLASP/ 1 BAR

Derives its name from its shape The clasp arm contacts the

abutment surface over an area thatextends from the measuredundercut to the height of contour

Typically, the contact areabetween the clasp and theabutment is 2.0 to 3.0 mm inheight and 1.5 to 2.0 mm in width.

The approach arm has a half round,cross-sectional geometry and ischaracterized by a gradual anduniform taper throughout its length.


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Often, the I-clasp design is used in conjunction with a

mesial rest preserving the abutment from torqueingforces

This design is commonly used in the treatment ofKennedy Class I and Class II partially edentulous

arches it may occasionally be used on the distobuccal surface

of maxillary canines for esthetic reasons.

Disadvantage:

Encirclement horizontal stabilization may becompromised.


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I-clasp Is An Integral Retentive Component In TwoDistinct Design Philosophies:

The Mesial Rest/I-bar Concept And

The RPI Concept.


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I BAR CONCEPTIt is modified type roach clasp designed by Kratochvil in 1963, 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.


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DESIGN Mesial rest

The mesioocclusal rest with the minorconnector is placed into themesiolingual embrasure, but notcontacting the adjacent tooth.

Rest are extensively prepared as in

premolars it involves marginal andtriangular ridges where as in molar itextends into central fossa.

Due to this occlusal forces are directedvertically thus eliminating harmful

lateral stresses. It shifts the fulcrum line more anteriorly


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Proximal plates

A distal guiding plane is prepared to receive a proximal

plate. Usually guiding plane is 2-5 mm but in this case it is

very long as it extends onto attached gingiva for 2 mm

The buccolingual width of the guiding plane isdetermined by the proximal contour of the tooth.

The proximal plate in conjunction with the mesialocclusal rest and minor connector provides thestabilizing and reciprocal aspects of the clasp assembly.This configuration permits improved stabilization ofthe prosthesis

Long guide plane provides increased horizontal

stability reduces food impaction between the tooth and the

proximal plate

Provides reciprocation during insertion and removal ofthe prosthesis

Distributes occlusal forces throughout the arch


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I bar retainer

Arm of I bar is long

tapering with half roundcross section

Retentive I bar is placed on

buccal surface ofabutment mesial to mesiodistal height of contour

The I bar should extend

about 2 mm above toothtissue junction

Reciprocation here ismesiodistally


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Advantages

1. Food accumulation is minimized because tooth

contours are not significantly altered.

2. The clasp terminus disengages from the tooth whenan occlusal load is applied to the adjacent distalextension base.

3. Because the approach arm does not contact theabutment, lateral forces are minimized.

Disadvantages

1. Less horizontal stability than other types of claspassemblies

2. Less retention

RPI CONCEPT


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RPI CONCEPT

RPI concept was given by KROLL 1973

RPI means mesial rest, proximal plate and I bar.

The principle was stress control with minimal

tooth and gingival coverage. Normally retentive and reciprocal units act

bucco-lingually but here they act in mesio-distaldirection.

He modified I bar clasp concept to meet minimalcoverage criteria

Mesial rest modification


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Mesial rest extends only in triangular fossa even in molarpreparation. It doesnt cover entire marginal ridge.

Proximal plate modification

The prepared guiding plane is 2 to 3 mm highocclusogingivally, and the proximal plate contacts onlythe apical 1 mm of the guiding plane.

Relief is provided at the tooth-tissue junction to allowthe proximal plate to disengage when loaded.

The stated purpose for reducing the proximal plate is toimprove gingival health

I bar modification

Modifications in 1-bar configuration and placement areneeded to compensate for reduced tooth contact by the

proximal plate. The 1-bar terminus is pod shaped to allow additional

tooth contact, and the vertical portion of the clasp armassumes a more mesial position to achieve efficientreciprocation from the smaller proximal plate.

RPI CONCEPT


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RPI CONCEPT


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ADVANTAGES More esthetic due to minimal tooth contact

Good for caries prone patients

Adequate encirclement by engaging more than

180 degree. Encirclement provided by mesial rest and

proximal plate

Under vertical masticatory forces, both the plate

and I bar disengage the abutment.

Relative merits and demerits of occlusally and gingivally approaching clasps


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Retention-The bar clasp is easier to seat on the toothand more difficult to remove than the circumferentialclasp. If all factors i.e. length of clasp arm, depth ofundercut, flexibility of the arm etc are equal, the barclasp is more retentive than the circumferential claspdue to tripping action.

In practice however, the gingivally approaching clasp islonger and more flexible, with less retentiveness thanocclusally approaching clasp

) B i Th i f i l l i i id i h


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2) Bracing: The circumferential clasp is rigid in theupper 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 tostability.

3) Stress breaking effect: The gingivally approachingclasp allows a certain degree of functional movementof the distal extension base which helps to dissipatethe stresses and lessen the load on the abutment.

Occlusally approaching clasps have the potential totorque abutment teeth in distal extension basedpartial denture situations.

4) Tooth contact:

The gingivally approaching clasp contacts minimum tooth structure and


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The gingivally approaching clasp contacts minimum tooth structure andhas a minimum interference with natural tooth contour permittingmaximum natural cleansing action,

whereas the occlusally approaching clasps covers more of toothstructure. This occlusal approach may increase the width of the occlusaltable.

5) Oral health: The gingivally approaching clasp has a likely potential tocreate gingival pathology. The area of food stagnation is at the neck of

the tooth and the cementum in this area is more likely to be affected bycaries than enamel. Trauma to the gingiva can also occur with bar clapsunless sufficiently relieved. Mishandling of the clasps by the patientsduring removal of the prosthesis can result in deformation of the claspand damage to soft tissues.

Decalcification of tooth is more in occlusally approaching clasp due tomore tooth contact.

6) Esthetics: Gingivally approaching clasps are more esthetic thanocclusally approaching clasps except in instances where large amounts ofgingivae are visible on smiling.

RPA CLASP


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The rest, proximal plate, Akers

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 superiorportion of the proximal plate

and extends around the tooth to

engage the mesial undercut.

It is indicated when bar-type

clasp is contraindicated anddesirable undercut is located in

gingival third of tooth away from

extension base area


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RPL CLASP

Ben-Urin 1988 described the rationale for using an L shaped bar

clasp direct retainer for distal extension removable partial dentures.The L shaped bar crosses the gingival margin of the abutment

tooth in the shortest possible line, ascends to the survey line, and

engages the distobuccal undercut.

VRHR CLASP


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VRHR CLASP developed by Grasso in 1980

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 denturebase.

Advantages

Minimum tooth contact

No guide planespreparation required

Used in high survey lines

Esthetically acceptable

Cingulum clasp


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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 theabutment 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 ofthe rigid metal framework.

Advantages


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

proximity to metal which shows thermalconduction.

Less expensive.

Disadvantages The clasp arms are vulnerable to breakage.

Use The cingulum clasp can be used as a retainer on

cuspid teeth when other extracoronal retainers areesthetically unacceptable

Choice of clasp


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Choice of clasp

It depends upon1. Position of tooth undercuts, restorations, occlusion,classification of edentulous arch, tooth type

2. Nature of the bony and soft tissue support. Is there an

unfavourable:

a) bony undercut

b) frenal attachment

c) vestibular depth

3. Esthetics

Kenned I & II (Tooth & Tiss e Borne)


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Tooth supported partial

dentures Clasp of choice: cast

circumferential

if abutment is severely

tilted use (depending onlocation of undercut):

Cast circumferentialclasp with lingual

retention Ring claspwith support

strut

Kennedy I & II (Tooth & Tissue Borne)

For posterior abutments, or any tooth

needing stress release:

Clasp of choice: RPI (mesial rest, distal

proximal plate and I-bar)

If cant use an I-bar in vestibule, because

of

frenumshallow vestibule

deep soft tissue undercut

then use an RPA retainer (mesial rest,

distal proximal plate and wrought wire

clasp)

If cant use a mesial rest because of:rotation

heavy centric contact on mesial

large amalgam restoration on mesial

then use Combination Clasp

INTRACORONAL RETAINERS


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INTRACORONAL RETAINERS

AND ATTACHMENTS The first intracoronal direct retainer was introduced byHerman E. S.

Chayes in 1906.

As its name implies, an intracoronal direct retainer resides within thenormal contours of an abutment and functions to retain and stabilize a

removable partial denture The Glossary of Prosthodontic Terms defines a precision attachment

as a retainer consisting of a metal receptacle (matrix) and aclosely fitting part (patrix); the matrix is usually containedwithin the normal or expanded contours of the crown on the

abutment tooth and the patrix is attached to a pontic or theremovable partial denture framework. The receptacle is alsoknown as female/ keyway. The closely fitting part is also knownas male/key.


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Intracoronal direct retainers may be subdivided intotwo categories based on their method of fabricationand the tolerance of fit between components

A) PRECISION ATTACHMENTS

B SEMIPRECISION ATTACHMENTS Frequently used synonyms of precision attachments

are internal attachment, frictional attachment, slottedattachment and parallel attachment


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MECHANISM OF ACTION When the removable partial denture is

placed in the patient's mouth, the twocomponents interlock in a sliding jointconfiguration. This sliding joint resideswithin the normal clinical contours of anabutment and functions to retain,support, and stabilize the removablepartial denture.

Parallelism of multiple attachments mustbe carefully considered when designing aremovable partial denture.

The paths of engagement for the

attachments ie, the long axes of theattachments must be parallel to each otherand parallel to path of insertion.

When parallelism is achieved, friction andbinding between components occurs asforces act to dislodge the prosthesis.

ADVANTAGES


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ADVANTAGES

Elimination of a visible retentive component thereby contributing toesthetics.

Better vertical support through a rest seat located more favourably inrelation to the horizontal axis of the abutment tooth.

Compared to conventional occlusal rests, the apical extension of an

intracoronal attachment reduces non-axial loading and diminishesrotational movement of the abutment

It provides horizontal rest and prevents lateral stresses to theperiodontium of the abutment teethwhen inserting or removing theprosthesis.

Cross arch stabilization is improved Broad stress distribution

Applies broken stress philosophythus reduces damaging forcesapplied to abutment

DISADVANTAGES


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DISADVANTAGES The teeth have to be extensively prepared to provide the required space

for the key resulting in encroachment and danger to the health of thepulp tissues and a need for prepared abutments and castings.

Adequate crown length is required to generate the required frictionalresistance.

They require complicated clinical and laboratory procedures.

They eventuallywear with progressive loss of frictional resistance todenture removal.

Difficulty in maintaining hygiene beneath the attachment.

They are difficult to repair and replace.

They are expensive.

It requires the services of a skilled technician.

This form of treatment is not justified in a poorly motivated anddisinterested patient.


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Preisikel 1979 has classified precision attachments into


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Preisikel 1979 has classified precision attachments into

1. Internal attachments ( Chayes attachment, McCollum

attachment, Crismani 699)2. External attachments ( Dalbo attachment)

3. Stud (Dalla Bona, Gerber )

Gerardo Becerra et al in 1987 classified precision attachments

as :jpd 1987 58 322 327


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as :jpd 1987 58 322-327

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


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2) Extra coronal attachments

a. Cantilever attachments

- rigid attachments

- movable attachments

b. Bar attachments


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Tapered And Parallel Walled Boxes And Tubes

Designed to be used in FPD.

Plastic pre fabricated patterns.

Provides vertical support and lateral stabilization.

Simple pin and tube or rectangular block and boxes.

E.g. : Mc Collum attachments.


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McCOLLUM ATTACHMENTS

Adjustable Metal Plates


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Adjustable Metal Plates

Similar to block and box variety .

Provided with a narrow slit in the metal block or maleportion of the attachment to increase the friction.

Provides a simple and effective form of directretention.

Atleast 2.5 mm of tooth height is required. E.g.: Crismani attachment.

Mc Collum attachment.

Stern attachment

Chayes or Rley attachment.


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CHAYESATTACHMENT

CRISMANI

ATTACHMENT

S i


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Springs

Incorporated in the male part to control the friction.

Approximately 4 5 mm of vertical height is required.

E.g.: Schatzmann attachment.


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SCHATZMANN ATTACHMENT

St d


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Studs

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

Direct retention can be obtained by using a stud

which clips into an flexible ring.

Sufficient clearance is required to arrange the

artificial teeth.

E.g.: Ceka attachment

Rotherman attachment


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Ceka attachment


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Magnets Small metal keeper is attached to the

tooth surface, usually into the root

canal and magnet is incorporated into

the resin.

Alloy in the magnet produces a

magnetic force that is strong .

Magnets are brittle and corrode unless

protected in a stainless steel shelf.

Cantilever attachments


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Rigid attachments

They are pin and tube joints that use a slit in thepin or multiple pin tubes and slots to enhance

retentive friction between the parts with the

natural teeth on the either side of the edentulousspace.

These attachments offer excellent stability and

retention in tooth supported partial dentures.

e.g.: Scott attachment

Thompson dowel rest system

Movable attachments.

Th ll th th i t t t d


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These allow the prosthesis to rotate around ahorizontal axis and transmit occlusal forces to the

residual alveolar ridge . E.g.: Dolbo attachments

B h


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Bar attachments

These can be connectedto the cast metal crownsor copings .

Custom made bars can becast with a flat uppersurface to support theprosthesis and parallel

sides that help to stabilizeit.

E.g.: Dolder bar.

Esthetic metal free clasps


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1) Natural Flex clasps: Based on acetyl resin technology

high elastic memory and remarkable

dimensional stability

biocompatible, non-allergenic andmonomer-free

20 times harder than acrylic


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Opti-flex clasps: Is a acetyl resin homopolymer.


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Properties: 20 times stronger than acrylic

High density, hardness,tensile strength compared toother homopolymers

Better wear strength Resistance to staining & water sorption.


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Proflex clear wire clasp:


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ValplastValplast is a flexible denture base resin that is ideal for

partial dentures and unilateral restorations. The resinis a biocompatible nylon thermoplastic .

Cu-Sil


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Cu Sil

Cu-Sil is a patented tissue-bearing appliance

featuring a soft elastomeric gasket which clasps the

neck of each natural tooth, sealing out food and

fluids, cushioning and splinting each natural tooth

from the hard denture base.


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Thank you

direct retainer (dr vikas ppt.ppt

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