There are two types of direct retainers for removable partial dentures: intra-coronal and extra-coronal. Extra-coronal retainers include various types of clasps that must satisfy requirements for retention, support, stability, encirclement, and passivity. Common clasp types are circlet, half and half, fish hook, reverse circlet, and ring clasps. Clasp design depends on factors like the undercut size and location, as well as the flexibility needed for retention.

1. Removable Partial Dentures
Direct Retainers
By:Ahmed Ali Abbas
theoptimalsmile.wix.com

2. There are two types of direct retainersThere are two types of direct retainers
11..Intra-coronalIntra-coronal
Within the contour of the crown e.g. precision attachmentWithin the contour of the crown e.g. precision attachment
22..Extra-coronalExtra-coronal
Outside the contour of the crown of the tooth e.g. claspOutside the contour of the crown of the tooth e.g. clasp

3. Intracoronal retainer
consists of a key and
keyway

4. Extracoronal circumferential direct retainer

5. Basic RequirementsBasic Requirements
All clasps must be designed so that they satisfy theAll clasps must be designed so that they satisfy the
following six basic requirementsfollowing six basic requirements::
11..RetentionRetention
22..SupportSupport
33..StabilityStability
44..ReciprocationReciprocation
55..EncirclementEncirclement
66..PassivityPassivity

6. 11..RetentionRetention
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 tissues
toward 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 the
tooth in the undercut areatooth in the undercut area..

7. 22..SupportSupport
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 rest
prevents 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 damagereduces periodontal tissue damage..

8. 33..BracingBracing
Bracing is the resistance to horizontal forcesBracing is the resistance to horizontal forces..
This provided by the rest, minor connector andThis provided by the rest, minor connector and
bracing 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 tipforce applied by retentive tip..

9. 44..EncirclementEncirclement
There must be more than 180 degreesThere must be more than 180 degrees
crown coverage otherwise, the clasp cancrown coverage otherwise, the clasp can
slip-off the abutment toothslip-off the abutment tooth..
55..PassivityPassivity
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
appliedapplied..

11. Factors that determine the amount of retention
provided by a particular clasp arm
11..The size of the angle of the cervical convergenceThe size of the angle of the cervical convergence..
22..How far into the cervical convergence the clasp terminal isHow far into the cervical convergence the clasp terminal is
placedplaced..
33..Flexibility of the clasp armFlexibility of the clasp arm..
Flexibility of the clasp armFlexibility of the clasp arm::
11..The length of the clasp armThe length of the clasp arm..
22..The diameter of the clasp armThe diameter of the clasp arm..
33..The cross-sectional formThe cross-sectional form..
44..The type of metal usedThe type of metal used..

12. Other factors about clasp design:
11..In a clasp only the tip of the retentive arm should be flexible, the otherIn 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 tooth-retention area available, type of partial denture i.e. whether it is tooth-
supported or distal extention removable partial denturesupported or distal extention removable partial denture..
33..Flexibility of clasp arm can be increased by curving and/or lengthening theFlexibility of clasp arm can be increased by curving and/or lengthening the
armarm..
44..The thickness of the retentive clasp arm tip should be ½ the thickness at itsThe 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 flexibilityThe greater the diameter of the clasp arm, the lesser will be the flexibility
and vice versaand vice versa..
66..Wrought metal is more flexible than cast metalWrought metal is more flexible than cast metal..
77..If buccal retention is used on the left side of the arch, buccal retention mustIf 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

13. Classification of extra-coronal retainersClassification of extra-coronal retainers::
11..Supra bulge clasps (occlusally approaching,Supra bulge clasps (occlusally approaching,
circumferential or encircling claspscircumferential or encircling clasps).).
The retentive arm approaches the undercut area from theThe retentive arm approaches the undercut area from the
suprabulge directionsuprabulge direction..
22..Infrabulge clasps (gingivally approaching, projection orInfrabulge clasps (gingivally approaching, projection or
bar claspsbar clasps))
The retentive arm approaches the undercut from theThe retentive arm approaches the undercut from the
infrabulge direction,e.g.Bar clasp arm (I-Bar etcinfrabulge direction,e.g.Bar clasp arm (I-Bar etc.).)
33..Combination claspsCombination clasps
a. ½ clasp is circlet and ½ clasp is Bar type ora. ½ clasp is circlet and ½ clasp is Bar type or
b. Combination of two types of metals-Chrome-cobalt anb. Combination of two types of metals-Chrome-cobalt an
wrought wirewrought wire

14. Types of circumferential claspsTypes of circumferential clasps
11..Circlet claspCirclet clasp..
22..Half & half claspHalf & half clasp..
33..Fish hook or hairpin claspFish hook or hairpin clasp..
44..Reverse circlet claspReverse circlet clasp..
55..Embrasure claspEmbrasure clasp..
66..Ring claspRing clasp..
77..Back action and reverse back actionBack action and reverse back action
claspclasp..

15. 11..Circlet claspCirclet clasp..
--Is the most common clasp used forIs the most common clasp used for
removable partial denturesremovable partial dentures..
--The least complex in designThe least complex in design..
--It has a rigid reciprocal arm, a rest, aIt has a rigid reciprocal arm, a rest, a
proximal plate approximating theproximal plate approximating the
edentulous area, and a flexible retentiveedentulous area, and a flexible retentive
arm ending in an undercut of 0.01 incharm ending in an undercut of 0.01 inch..
--It may be used on canines, premolars, andIt may be used on canines, premolars, and
molarsmolars..

16. Variations of circlet clasp areVariations 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 .02its length. It is used on premolars (use .02
inch undercutinch undercut))
b) Reverse back action: Minor connector is onb) Reverse back action: Minor connector is on
buccal, otherwise similar to back actionbuccal, 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 and
lower lingually tilted molarslower lingually tilted molars..
d) Other variations of simple circlet clasp “ C ” claspd) Other variations of simple circlet clasp “ C ” clasp
or hair-pin claspor hair-pin clasp..

17. The half and half clasp:The half and half clasp: 22..
The half and half clasp is a modification ofThe 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 restsretentive arm from the other. Two rests
are used for this clasp. It is used onare used for this clasp. It is used on
molars and premolars. The retentive tip ismolars and premolars. The retentive tip is
placed into a 0.01 inch undercutplaced into a 0.01 inch undercut..

19. 33..The fish hook or hair-pin claspThe fish hook or hair-pin clasp
The fishhook or hairpin clasp is anotherThe 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 othercovered 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 clasptip because of the bulk of the clasp..

21. 44..The reverse circlet claspThe reverse circlet clasp::
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 spacethe 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 oftissue undercut precludes the use of
infrabulge claspinfrabulge clasp..

22. 55..Embrasure claspEmbrasure clasp..
The embrasure clasp is used when there isThe embrasure clasp is used when there is
no modification spaces in the portion ofno 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 back-to-back. Itin fact two circlet clasps back-to-back. It
may be used on two molars, a molar andmay be used on two molars, a molar and
premolar or two premolars. Preparation ofpremolar 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 needs
to be deep enough for strength and notto be deep enough for strength and not
compromised by the opposing occlusioncompromised by the opposing occlusion..

24. 66..The ring claspThe ring clasp..
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 restscontinuous connecting the two rests..