2. There are two types of direct retainers
Intra-coronal.1
Within the contour of the crown e.g. precision attachment
Extra-coronal.2
Outside the contour of the crown of the tooth e.g. clasp
5. Basic Requirements
All clasps must be designed so that they satisfy the
:following six basic requirements
Retention.1
Support.2
Stability.3
Reciprocation.4
Encirclement.5
Passivity.6
6. Retention.1
Retention is provided by the retentive arm
which prevents the partial denture from
displacement away from the tissues
toward the occlusal.While the partial
denture is seated the retentive tip must be
passive. However, it should touch the
. tooth in the undercut area
7. Support.2
Support in a clasp is generally provided by
the rest. Thus, while chewing food the rest
prevents tissueward movement of the
clasp assembly, plus directs the force
along the long axis of the tooth, thus
. reduces periodontal tissue damage
8. Bracing.3
.Bracing is the resistance to horizontal forces
This provided by the rest, minor connector and
bracing arm. Actually, the bracing arm mainly
provides reciprocation which is resistance to
. force applied by retentive tip
9. Encirclement.4
There must be more than 180 degrees
crown coverage otherwise, the clasp can
.slip-off the abutment tooth
Passivity.5
The retentive clasp arm should be passive
(no active force) until a dislodging force is
.applied
10.
11. Factors that determine the amount of retention
provided by a particular clasp arm
.The size of the angle of the cervical convergence.1
How far into the cervical convergence the clasp terminal is .2
.placed
.Flexibility of the clasp arm.3
:Flexibility of the clasp arm
.The length of the clasp arm.1
.The diameter of the clasp arm.2
.The cross-sectional form.3
.The type of metal used.4
12. :Other factors about clasp design
In a clasp only the tip of the retentive arm should be flexible, the other .1
components are rigid
Selection of clasp will depend on the survey line (height of contour), .2
retention area available, type of partial denture i.e. whether it is tooth-
.supported or distal extention removable partial denture
Flexibility of clasp arm can be increased by curving and/or lengthening the .3
.arm
The thickness of the retentive clasp arm tip should be ½ the thickness at its .4
attachment to the body of the clasp arm
The greater the diameter of the clasp arm, the lesser will be the flexibility .5
.and vice versa
.Wrought metal is more flexible than cast metal.6
If buccal retention is used on the left side of the arch, buccal retention must .7
be used on the right side of the arch
13. :Classification of extra-coronal retainers
Supra bulge clasps (occlusally approaching,.1
).circumferential or encircling clasps
The retentive arm approaches the undercut area from the
.suprabulge direction
Infrabulge clasps (gingivally approaching, projection or.2
)bar clasps
The retentive arm approaches the undercut from the
.)infrabulge direction,e.g.Bar clasp arm (I-Bar etc
Combination clasps.3
a. ½ clasp is circlet and ½ clasp is Bar type or
b. Combination of two types of metals-Chrome-cobalt an
wrought wire
14. Types of circumferential clasps
.Circlet clasp.1
.Half & half clasp.2
.Fish hook or hairpin clasp.3
.Reverse circlet clasp.4
.Embrasure clasp.5
.Ring clasp.6
Back action and reverse back action .7
.clasp
15. .Circlet clasp.1
Is the most common clasp used for -
. removable partial dentures
.The least complex in design-
It has a rigid reciprocal arm, a rest, a -
proximal plate approximating the
edentulous area, and a flexible retentive
.arm ending in an undercut of 0.01 inch
It may be used on canines, premolars, and -
. molars
16. :Variations of circlet clasp are
a) Back action clasp: Provides poor bracing due to
its length. It is used on premolars (use .02
) inch undercut
b) Reverse back action: Minor connector is on
. buccal, otherwise similar to back action
c) Ring clasp: Variation of back action clasp
usually used on upper buccally tilted and
. lower lingually tilted molars
d) Other variations of simple circlet clasp “ C ” clasp
. or hair-pin clasp
17. .The half and half clasp: 2
The half and half clasp is a modification of
the circlet clasp with the reciprocal arm
coming from one direction and the
retentive arm from the other. Two rests
are used for this clasp. It is used on
molars and premolars. The retentive tip is
.placed into a 0.01 inch undercut
18.
19. The fish hook or hair-pin clasp. 3
The fishhook or hairpin clasp is another
modification of the circlet clasp. It is mostly
used on teeth with long crowns. It is rarely
used because so much of the teeth is
covered by the retentive arm. The other
difficulty is lack of flexibility of the retentive
.tip because of the bulk of the clasp
20.
21. :The reverse circlet clasp. 4
The reverse circlet clasp is used when the
retentive undercut is located on the
surface of the abutment tooth adjacent to
.the edentulous space
It is indicated in class I & class II distal
extension partial dentures where deep
tissue undercut precludes the use of
.infrabulge clasp
22. .Embrasure clasp. 5
The embrasure clasp is used when there is
no modification spaces in the portion of
the arch that needs retention. The clasp is
in fact two circlet clasps back-to-back. It
may be used on two molars, a molar and
premolar or two premolars. Preparation of
the double rest and channel going from
the lingual to the buccal of the teeth needs
to be deep enough for strength and not
. compromised by the opposing occlusion
23.
24. .The ring clasp. 6
The ring clasp is used on molars. Some
designers advocate its use for tipped
mandibular molars where there is only a
mesial undercut on the tooth. It has mesial
and distal rests and the reciprocal arm is
. continuous connecting the two rests
