Direct retainers in prosthodontics /certified fixed orthodontic courses by Indian dental academy

DIRECT RETAINERS

INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

DIRECT RETAINERS
 Introduction

The removable partial denture must have
sufficient retention to resist reasonable dislodging
forces.
 Primary Retention: for the removable partial denture

is accomplished mechanically by
elements on the abutment teeth.

placing

retaining

 Secondary Retention: is provided by the intimate

relationship of minor connector contact with the guiding
planes,
denture
bases,
and
major
connectors
(maxillary) with underlying tissues.



Definition: A direct retainer is any unit of removable
dental prosthesis that engages an abutment tooth in
such a manner as to resist displacement of the
prosthesis away from basal seat tissues.



The two basic types of direct retainers:
Intracoronal retainer
Direct Retainers

(precision attachment)
Extracoronal

(clasp )


Retainer

 Intracoronal

Retainers: which is cast or attached
totally within the restored natural contours of an
abutment tooth. This type of retainer is composed of
a prefabricated machined key and keyway, with
opposing vertical parallel walls that serve to limit
movement and resist removal of the partial denture
through frictional resistance.

 Extracoronal

Retainers : which uses mechanical
resistance to displacement by components placed on
or attached to the external surfaces of an abutment
tooth

 Extracoronal D.R.

Circumferential clasp




Components of clasp Assembly
1. Rest – Provides support for prosthesis
2. Body – Connects rest and clasp arms to

minor connector
3. Reciprocal arm – must be rigid above the

height of contour
4. Retentive arm – Provides direct retention

below the height of the contour
5. Shoulder
6. Approach arm
7. Terminal

a. The occlusal
rest
b. The body
c. The shoulders
d. The terminals
e. The clasp arms
f. The strut
g. the approach
arm
h. the terminal




Clasps can be classified on the basis of
construction
1. Cast clasp
2. Wrought wire clasp
3. Combination clasp



Clasps classified on a basis of design
1. Circumferential
clasp
(Akers
–
suprabulge)
2. Bar clasp (vertical projection – Roach
infrabulge)


Circumferential
clasp:
It
approaches
the
undercut from occlusal direction – “pull” type of
retention


Bar clasp: Retentive terminal approaches the
undercut from gingival direction – below the
surveyline – “push” type of retention.


a. Circumferential
clasp
b. Bar clasp


ATTRIBUTES OF PROPERLY
DESIGNED CLASP
 The clasp should possess the attributes

of
 Retention
 Stabilization
 Support
 Encirclement
 Reciprocity
 Passivity

RETENTION
 Retention is the property which enables the clasp to

resist dislodgement from the tooth in an occlusal
direction. A dislodging force may be activated by
speech, muscle action, mastication, deglutition,
sticky foods, or gravity.


Retentive arm of the clasp


 The length of the clasp arm. The greater the length,

the greater the resiliency.
 The diameter of the retentive arm. The smaller the

diameter, greater the resiliency.
 The cross sectional form. A round clasp arm is more

resilient than one that is half round or oval in shape.
 The taper. Proper taper can increase flexibility as

much as fourfold. The retentive arm of the clasp
should taper evenly and uniformly from its origin at
the body of the clasp to its terminal extremity.

 The kind of alloy. The chromium Cobalt alloys have a

higher modulus of elasticity, hence are not
resilient as gold alloys of the same diameter.

as

 The type of alloy. An alloy is wrought form is more

resilient than the same alloy of identical diameter in
cast form, because of its internal structure.
 Heat treatment of the alloy. Proper heat treatment,

gold in particular, will increase the resilience
substantially, whereas improper heat treatment may
render it brittle and with no resilience whatever.

Flex
in
horizontal
plane

Flex in
all
planes

Cast Clasp

Wrought clasp


Uniform
taper
in
thickness
and
necessary

both
width

Clasp terminal should
half as thick as origin

Flexibility of a clasp arm increases
as length of clasp increases

be


STABILIZATION
 Stabilization, or bracing is the resistance which

the clasp contributes to displacement of the
prosthesis in horizontal plane . All of the clasp
components, with the exception of the retentive
terminal, contribute this property in varying
degrees.


SUPPORT
 Support

is the property of the clasp which
enables it to resist displacement in a gingival
direction. The occlusal rest is the prime support
unit of the clasp, although the body and the
shoulder of the clasp, positioned as they are
above the greatest diameter of the tooth, also
contribute a substantial amount of support.


ENCIRCLEMENT
 The clasp should be so designed that it encircles at

least 180 degrees of the crown of the tooth, to
preclude movement of the tooth out of the clasp arms
as strmsses are applied.


Each clasp assembly
must
encircles
more than 180 0

A clasp that fails to
produce encirclement
will act as orthodontic
appliance


RECIPROCITY
• Reciprocation may defined as “ the means by

which one part of the appliance is made to
counter the effect created by another part”.
• Reciprocation

is needed most when the
retentive terminal flexes over the bulge of the
crown during insertion and removal of the
prosthesis.


Reciprocal arm of the clasp is rigid .


PASSIVITY
 When the clasp is in place on the tooth, it should be

passive. Passivity is an important requirement of a
properly designed clasp. A clasp so designed will
permit a slight degree of movement of the base
without transmitting any appreciable amount of the
stress to the abutment tooth.


A clasp must be completely seated to be passive


SIMPLE CIRCLET CLASP
 This is the most widely used clasp
 It is the clasp of choice on tooth supported

removal partial dentures
 This

clasp approaches the undercut on the
abutment tooth from the edentulous areas and
engages the undercut from the edentulous
space.

Advantages:
• The clasp fulfills

the requirement of support,
stability reciprocation, encirclement & passivity
better than any other type of clasp.

• It is easy to construct.
• It is simple to repair.


Disadvantages:
• It tends to increase the circumference of the crown.
• In anterior part of the mouth is not acceptable.
• It covers more tooth surface then the bar clasp and

prone to caries.
• Retentive undercuts on some teeth are difficult to

reach with retentive terminal of the clasp

REVERSE CIRCLET CLASP
Reverse circlet clasp is used when the retentive
undercut is located on the surface of the
abutment tooth adjacent to the edentulous
space.
In distal extension edentulous ridge partial
denture the reverse approach clasp helps to
control the stresses transmitted to the terminal
abutment tooth on the edentulous side.


REVERSE CIRCLET CLASP


MULTIPLE CIRCLET CLASP
It is opposing simple circlet clasps joined at the
terminal end of the 2 reciprocal arms.
It is indicated in periodontally weakened teeth.
This is the form of splinting weakened teeth by
a removable partial denture.


EMBRASURE CLASP
( MODIFIED Crib Clasp)
The embrassure clasp is 2 simple circlet clasps
joined at the body.
It is most frequently used on the side of the
arch where there is no edentulous space.
Occlusal rest preparations must be made on
both teeth and tooth structure must be removed
from the buccal inclines of both teeth to provide
space for adequate thickness of metal.

EMBRASURE CLASP


RING CLASP
Indicated on tipped molars.
Ring clasp engages the undercut by encircling
the entire tooth from its point of origin.
On mandibular molar the clasp encircles the
tooth beginning on the mesiobuccal surface and
eliminating in an infrabulge area on the
mesiolingual surface.
On a maxillary molar the direct of the clasp is
reversed.

RING CLASP


The entire clasp must be placed above the left
of the contours except retentive tip.
It must be designed with additional support in
the form of an auxillary bracing arm.
An additional occlusal rest on the opposite side
of the tooth from the clasp origin provide
additional support and proved further movement
of the mesially inclined tooth.

C clasp Fishhook, or Hairpin Clasp
The C clasp is a simple circlet clasp in which the retentive
arm crosses the facial surface of the tooth from its point of
origin, loops back in hairpin turn to engage a proximal
undercut below its point of origin.
The upper part of retentive arm must be rigid, the lower
apart of the retentive arm should be tapered. It is the only
flexible part of the clasp arm.
The crown of the abutment tooth must have sufficient
occlusogingival height to accommodate this double width
of the clasp arm.
The upper and lower arms of the retentive clasp must be
designed such that food debris will not be retained.

C Clasp, Hairpin Clasp, Fishhook


Indications:
 When

the retentive clasp must engage an
undercut adjacent to the occlusal rest or
edentulous space and a soft tissue undercut
precludes the use of of a bar clasp

 The C Clasp is indicated when the reverse

circlet clasp cannot be used because of lack of
occlusal space.

Disadvantages:
 The

clasp covers a considerable amount of
tooth structure, which may trap food debris.

 It is not good choice for a young patient or one

who is prone to caries,
unacceptable
esthetically,
premolar teeth.


and it is often
particularly
on

ONLAY CLASP


The onlay clasp is an extended occlusal rest with
buccal and lingual clasp arms.

 The clasp may originate from any point on the onlay

that will not create occlusal interferences.
 This clasp is generally indicated when the occlusal

surface of the abutment tooth is below the occlusal
plane, usually as a result of the tooth being tipped or
rotated.
 The onlay is used to restore the normal occlusal

Onlay clasp


COMBINATION CLASP
The combination clasp consists of a Wrought wire
retentive clasp arm and a cast reciprocal clasp arm.
The cast reciprocal arm is normally a circumferential
clasp. But a bar clasp may be used.
The Wrought wire retentive arm is a circumferential
clasp arm.
It is used when maximum flexibility is required.
Esthetics advantage because Wrought structure in
smaller diameter than the cast clasp.

COMBINATION CLASP


Advantages:
 Flexibility.
 Adjustability.
 Appearance of the Wrought wire retentive arm.
 Minimum of tooth surface covered.


Disadvantages:
 It involves extra steps in fabrication.
 It may be distorted by patient.
 It may be less accurately adapted.


Bar or Vertical Project Clasp
Bar clasps approach the undercut from a gingival
direction.
It is more effective than the “pull”
characteristic of circumferential clasps.

retention

Patient in contouring less difficulty inserting and
more difficulty in removing removable partial
denture with bar clasp than with circumferential
clasp.
Flexibility of the bar clasp can be controlled by the
taper and length of the approach arm.
Because of the gingival approach of the bar clasp
it is usually esthetics than a circumferential clasp.

DISADVANTAGES
Greater binding to collect and hold food debris
Because
retentive

of

the

arm

increased
it

does

flexibility

not

stabilization.


of

contribute

the
to

T - BAR CLASP

T Bar clasp is frequently used on distal
extension ridge where the usual undercut
.


I-bar clasp
 Used on distobuccal surface of maxillary

canines for esthetic reasons.
 Retentive tip is in contact with abutment teeth.
 I bar is a component of a partial denture
referred to as RPI concept.


Y- BAR CLASP
 Y-Bar is basically a T-clasp its

configuration
occurs where the it is contour on the facial
surface of the abutment tooth is high on the
medial and distal angles.


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Direct retainers in prosthodontics /certified fixed orthodontic courses by Indian dental academy

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