Removable Partial Dentures
Direct Retainers
Dr.Mohammad Al Sayed
19l4/2008
There are two types of direct retainers
1.Intra-coronal
Within the contour of the crown e.g. precision attachment
2.Extra-coronal
Outside the contour of the crown of the tooth e.g. clasp
Intracoronal retainer
consists of a key and
keyway
Extracoronal circumferential direct retainer
Basic Requirements
All clasps must be designed so that they satisfy the
following six basic requirements:
1.Retention
2.Support
3.Stability
4.Reciprocation
5.Encirclement
6.Passivity
1.Retention
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.
2.Support
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.
3.Bracing
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.
4.Encirclement
There must be more than 180 degrees
crown coverage otherwise, the clasp can
slip-off the abutment tooth.
5.Passivity
The retentive clasp arm should be passive
(no active force) until a dislodging force is
applied.
Factors that determine the amount of retention
provided by a particular clasp arm
1.The size of the angle of the cervical convergence.
2.How far into the cervical convergence the clasp terminal is
placed.
3.Flexibility of the clasp arm.
Flexibility of the clasp arm:
1.The length of the clasp arm.
2.The diameter of the clasp arm.
3.The cross-sectional form.
4.The type of metal used.
Other factors about clasp design:
1.In a clasp only the tip of the retentive arm should be flexible, the other
components are rigid
2.Selection of clasp will depend on the survey line (height of contour),
retention area available, type of partial denture i.e. whether it is toothsupported or distal extention removable partial denture.
3.Flexibility of clasp arm can be increased by curving and/or lengthening the
arm.
4.The thickness of the retentive clasp arm tip should be ½ the thickness at its
attachment to the body of the clasp arm
5.The greater the diameter of the clasp arm, the lesser will be the flexibility
and vice versa.
6.Wrought metal is more flexible than cast metal.
7.If buccal retention is used on the left side of the arch, buccal retention must
be used on the right side of the arch
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 or
bar clasps)
The retentive arm approaches the undercut from the
infrabulge direction,e.g.Bar clasp arm (I-Bar etc.)
3.Combination clasps
a. ½ clasp is circlet and ½ clasp is Bar type or
b. Combination of two types of metals-Chrome-cobalt an
wrought wire
Types of circumferential clasps
1.Circlet clasp.
2.Half & half clasp.
3.Fish hook or hairpin clasp.
4.Reverse circlet clasp.
5.Embrasure clasp.
6.Ring clasp.
7.Back action and reverse back action clasp.
1.Circlet clasp.
- 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.
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.
2. The half and half clasp:
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.
3. The fish hook or hair-pin clasp
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.
4. The reverse circlet clasp:
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.
5. Embrasure clasp.
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.
6. The ring clasp.
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.