Removable Denture 007

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Removable Partial Dentures

Direct Retainers

Dr.Mohammad Al Sayed

19l4/2008



T here are two types of direct retainersT here are two types of direct retainers

11.Intra.Intra--coronal coronal

Within the contour of the crown e.g. precision attachmentWithin the contour of the crown e.g. precision attachment

2 2 .Extra.Extra--coronal coronal

Outside the contour of the crown of the tooth e.g. claspOutside the contour of the crown of the tooth e.g. clasp



I ntracoronal retainer

consists of a key and keyway



Ex tracoronal circumferential direct retainer



Basic RequirementsBasic Requirements

All clasps must be designed so that they satisfy the All clasps must be designed so that they satisfy thefollowing six basic requirements:following six basic requirements:

11.Retention.Retention22.Support.Support

33.Stability.Stability

44.Reciprocation.Reciprocation

55.Encirclement.Encirclement

66.Passivity.Passivity



11.Retention.Retention

Retention is provided by the retentive armRetention is provided by the retentive arm

which prevents the partial denture fromwhich prevents the partial denture from

displacement away from the tissuesdisplacement away from the tissuestoward the occlusal.While the partialtoward the occlusal.While the partial

denture is seated the retentive tip must bedenture is seated the retentive tip must be

passive. However, it should touch thepassive. However, it should touch thetooth in the undercut area.tooth in the undercut area.



2 2.Support .Support

Support in a clasp is generally provided bySupport in a clasp is generally provided by

the rest. Thus, while chewing food the restthe rest. Thus, while chewing food the restprevents tissueward movement of theprevents tissueward movement of the

clasp assembly, plus directs the forceclasp assembly, plus directs the force

along the long axis of the tooth, thusalong the long axis of the tooth, thus

reduces periodontal tissue damage.reduces periodontal tissue damage.



33.B

racing .B

racing

Bracing is the resistance to horizontal forces.Bracing is the resistance to horizontal forces.

This provided by the rest, minor connector andThis provided by the rest, minor connector andbracing arm. Actually, the bracing arm mainlybracing arm. Actually, the bracing arm mainly

provides reciprocation which is resistance toprovides reciprocation which is resistance to

force applied by retentive tip.force applied by retentive tip.



44.E ncirclement .E ncirclement

There must be more thanThere must be more than 180180 degreesdegrees

crown coverage otherwise, the clasp cancrown coverage otherwise, the clasp can

slipslip--off the abutment tooth.off the abutment tooth.

5 5.Passivity .Passivity

The retentive clasp arm should be passiveThe retentive clasp arm should be passive(no active force) until a dislodging force is(no active force) until a dislodging force is

applied.applied.





F actors that determine the amount of retention

provided by a particular clasp arm

11.The size of the angle of the cervical convergence..The size of the angle of the cervical convergence.

22.How far into the cervical convergence the clasp terminal is.How far into the cervical convergence the clasp terminal isplaced.placed.

33.Flexibility of the clasp arm..Flexibility of the clasp arm.

F le x ibility of the clasp arm:F le x ibility of the clasp arm:

11.The length of the clasp arm..The length of the clasp arm.

22.The diameter of the clasp arm..The diameter of the clasp arm.33.The cross.The cross--sectional form.sectional form.

44.The type of metal used..The type of metal used.



O ther factors about clasp design:

11.In a clasp only the tip of the retentive arm should be flexible, the other .In a clasp only the tip of the retentive arm should be flexible, the other

components are rigidcomponents are rigid

22.Selection of clasp will depend on the survey line (height of contour),.Selection of clasp will depend on the survey line (height of contour),

retention area available, type of partial denture i.e. whether it is toothretention area available, type of partial denture i.e. whether it is tooth--

supported or distal extention removable partial denture.supported or distal extention removable partial denture.

33.Flexibility of clasp arm can be increased by curving and/or lengthening the.Flexibility of clasp arm can be increased by curving and/or lengthening the

arm.arm.

44.The thickness of the retentive clasp arm tip should be ½ the thickness at its.The thickness of the retentive clasp arm tip should be ½ the thickness at its

attachment to the body of the clasp armattachment to the body of the clasp arm

55.The greater the diameter of the clasp arm, the lesser will be the flexibility.The greater the diameter of the clasp arm, the lesser will be the flexibility

and vice versa.and vice versa.

66.Wrought metal is more flexible than cast metal..Wrought metal is more flexible than cast metal.

77.If buccal retention is used on the left side of the arch, buccal retention must.If buccal retention is used on the left side of the arch, buccal retention must

be used on the right side of the archbe used on the right side of the arch



C lassification of e x traC lassification of e x tra- -coronal retainers:coronal retainers:

11.Supra bulge clasps (occlusally approaching,.Supra bulge clasps (occlusally approaching,

circumferential or encircling clasps).circumferential or encircling clasps).

The retentive arm approaches the undercut area from theThe retentive arm approaches the undercut area from the

suprabulge direction.suprabulge direction.

22.Infrabulge clasps (gingivally approaching, projection or .Infrabulge clasps (gingivally approaching, projection or bar clasps)bar clasps)

The retentive arm approaches the undercut from theThe retentive arm approaches the undercut from the

infrabulge direction,e.g.Bar clasp arm (Iinfrabulge direction,e.g.Bar clasp arm (I--Bar etc.)Bar etc.)

33.Combination clasps.Combination clasps

a. ½ clasp is circlet and ½ clasp is Bar type or a. ½ clasp is circlet and ½ clasp is Bar type or

b. Combination of two types of metalsb. Combination of two types of metals--ChromeChrome--cobalt ancobalt an

wrought wirewrought wire



Types of circumferential claspsTypes of circumferential clasps

11.Circlet clasp..Circlet clasp.

22.Half & half clasp..Half & half clasp.

33.Fish hook or hairpin clasp..Fish hook or hairpin clasp.44.Reverse circlet clasp..Reverse circlet clasp.

55.Embrasure clasp..Embrasure clasp.

66.Ring clasp..Ring clasp.77.Back action and reverse back action clasp..Back action and reverse back action clasp.



Circlet clasp.Circlet clasp...11

-- Is the most common clasp used for Is the most common clasp used for removable partial dentures.removable partial dentures.

-- The least complex in design.The least complex in design.

-- It has a rigid reciprocal arm, a rest, aIt has a rigid reciprocal arm, a rest, aproximal plate approximating theproximal plate approximating theedentulous area, and a flexible retentiveedentulous area, and a flexible retentive

arm ending in an undercut of arm ending in an undercut of 00..0101 inch.inch.-- It may be used on canines, premolars, andIt may be used on canines, premolars, andmolars.molars.



Variations of circlet clasp are:Variations of circlet clasp are:

a) Back action clasp: Provides poor bracing due toa) Back action clasp: Provides poor bracing due to

its length. It is used on premolars (use .its length. It is used on premolars (use .0202

inch undercut)inch undercut)

b) Reverse back action: Minor connector is onb) Reverse back action: Minor connector is on

buccal, otherwise similar to back action.buccal, otherwise similar to back action.

c) Ring clasp: Variation of back action claspc) Ring clasp: Variation of back action clasp

usually used on upper buccally tilted andusually used on upper buccally tilted andlower lingually tilted molars.lower lingually tilted molars.

d) Other variations of simple circlet clasp ³ C ´ claspd) Other variations of simple circlet clasp ³ C ´ clasp

or hair or hair--pin clasp.pin clasp.



The half and half clasp:The half and half clasp:..22

The half and half clasp is a modification of The half and half clasp is a modification of

the circlet clasp with the reciprocal armthe circlet clasp with the reciprocal arm

coming from one direction and thecoming from one direction and the

retentive arm from the other. Two rests areretentive arm from the other. Two rests are

used for this clasp. It is used on molarsused for this clasp. It is used on molars

and premolars. The retentive tip is placedand premolars. The retentive tip is placed

into ainto a 00..0101 inch undercut.inch undercut.





pin clasppin clasp--The fish hook or hair The fish hook or hair ..33

The fishhook or hairpin clasp is another The fishhook or hairpin clasp is another

modification of the circlet clasp. It is mostlymodification of the circlet clasp. It is mostly

used on teeth with long crowns. It is rarelyused on teeth with long crowns. It is rarely

used because so much of the teeth isused because so much of the teeth is

covered by the retentive arm. The other covered by the retentive arm. The other

difficulty is lack of flexibility of the retentivedifficulty is lack of flexibility of the retentive

tip because of the bulk of the clasp.tip because of the bulk of the clasp.





The reverse circlet clasp:The reverse circlet clasp:..44

The reverse circlet clasp is used when theThe reverse circlet clasp is used when the

retentive undercut is located on theretentive undercut is located on the

surface of the abutment tooth adjacent tosurface of the abutment tooth adjacent to

the edentulous space.the edentulous space.

It is indicated in class I & class II distalIt is indicated in class I & class II distal

extension partial dentures where deepextension partial dentures where deep

tissue undercut precludes the use of tissue undercut precludes the use of infrabulge clasp.infrabulge clasp.



Embrasure clasp.Embrasure clasp...55

The embrasure clasp is used when there isThe embrasure clasp is used when there isno modification spaces in the portion of no modification spaces in the portion of the arch that needs retention. The clasp isthe arch that needs retention. The clasp is

in fact two circlet clasps backin fact two circlet clasps back--toto--back. Itback. Itmay be used on two molars, a molar andmay be used on two molars, a molar andpremolar or two premolars. Preparation of premolar or two premolars. Preparation of the double rest and channel going fromthe double rest and channel going from

the lingual to the buccal of the teeth needsthe lingual to the buccal of the teeth needsto be deep enough for strength and notto be deep enough for strength and notcompromised by the opposing occlusion.compromised by the opposing occlusion.





The ring clasp.The ring clasp...66

The ring clasp is used on molars. SomeThe ring clasp is used on molars. Some

designers advocate its use for tippeddesigners advocate its use for tipped

mandibular molars where there is only amandibular molars where there is only a

mesial undercut on the tooth. It has mesialmesial undercut on the tooth. It has mesial

and distal rests and the reciprocal arm isand distal rests and the reciprocal arm is

continuous connecting the two rests.continuous connecting the two rests.





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