Dental Clasp designs 1 /certified fixed orthodontic courses by Indian dental academy

CLASP DESIGNS.

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A

removable partial denture must
have

Support
 Stabilization
 Retention



RETENTION
FOR RPD
 Is

derived mechanically by
placing the retaining elements on
the abutment tooth.


Direct retainer
 Is

that component of removable
partial denture used to retain and
prevent dislodgement , consisting
of clasp assembly or precision
attachment.
-GPT.



Two types of direct retainers :
1) Intra coronal retainer (Attachments).
2) Extra coronal retainer (clasps).


Intra coronal retainer


Extra coronal retainer.


Two configurations are
1) Manufactured retainers (Dalbo)
2) Custom-made retainers:
a) Occlusally approaching (Circumferential or
Akers clasp)
b) Gingivally approaching (Bar or Roach
clasp)



Principle of extra coronal direct
retainer.
Resistance of the metal to the
deformation.


Clasp




Is the component of the clasp assembly
that engages a portion of the tooth surface
and either enters an undercut for retention
or remains entirely above the height of
contour to act as a reciprocating element.
Generally, it is used to stabilize and retain
a removable prosthesis.
-GPT.


Clasp assembly.


The part of a removable partial denture
that acts as a direct retainer and or
stabilizer for a prosthesis by partially
encompassing or contacting an abutment
tooth.
-GPT.


Component
parts.
Rest

Function

Location

Support

Occlusal, Lingual, Incisal

Minor
connector

Stabilization

Clasp arms

Stabilization

Proximal surfaces extending
from the prepared marginal
ridge to the junction of the
middle and gingival third of
abutment crown.
Apical portion of middle third of
crown.

Reciprocation
Retention

Apical portion of middle third of
crown
Gingival third of crown in




The critical areas of an abutment that is
responsible for retention, stabilization,
reciprocation and guide planes can only
be identified with the use of dental
surveyors.




General considerations:
A clasp should be located at the undercut
area in relation to the determined path of
insertion and removal of the prosthesis.


Prothero’s cone theory.
According to him , the
tooth is considered as a pair
of cones sharing a common
base
A clasp arm or tip that
ends on a cervical cone
would resist movement in the
occlusal direction because to
release from the tooth it
would be forced to undergo
deformation.






The line which the two converging cones
meet is called the height of contour, the
term first coined by Kennedy.
Devan referred to the surface of a tooth
that is occlusal to the height of contour as
supra bulge, and the tooth surface that is
cervical to its height of contour as infra
bulge.

Component parts of a clasp.




The component parts of a clasp assembly
may be rigid or flexible.
The flexible components are designed
below the height of contour


Component parts of a clasp








Retentive clasp arm
Retentive terminal
Reciprocal arm
Occlusal rest
Shoulder
Body
Minor connector

Retentive arm
A flexible segment of a removable partial
denture which engages a under cut on an
abutment which is designed to retain the
denture – GPT
Retentive arm
retentive clasp arm
+
retentive terminal




Retentive arm


Retentive clasp arm - not flexible
- located above the
height of contour

Retentive terminal

- flexible
- located below the
height of contour


Reciprocal arm
A component of a clasp assembly
specifically designed to provide
reciprocation by engaging a reciprocal
guiding plane and
it counteracts the action of the clasp during
the removal and insertion of the partial
denture - GPT


Reciprocal arm




Location -

on the side of the tooth
opposite to the retentive arm
always placed in the supra
bulge area


Reciprocal arm




Function -

resists the lateral forces
exerted by the retentive arm
when it passes through the
height of contour
- acts as an indirect retainer
when placed on an abutment
located anterior to the
fulcrum line

Shoulder






Part of the clasp that
connects the body to the
clasp terminal
Lies above the height of
contour
Provides stabilization
against horizontal
displacement


Body







Part of the clasp that connects the rests and
shoulders of the clasp of the minor connectors.
It is rigid and lies above the height of contour
It contacts the guide plane of the abutment during
the insertion and removal
Part of the body that contacts the guiding plane proximal plate


rest






A rigid extension of a removable
partial denture which contacts a
remaining tooth or teeth to
dissipate vertical or horizontal
forces
part of the clasp assembly that
lies on the occlusal or lingual or
incisal edge or surfaces of the
teeth
Resist tissue ward movement of
the clasp by acting like a vertical
stop

Minor connector




It joins the clasp with the remaining part of
the framework
In gingivally approaching clasp –
Approach arm


BASIC PRINCIPLES OF CLASP
DESIGN






Any clasp assembly must satisfy the basic
principle of clasp design, is
ENCIRCLEMENT
which is that more than 180 degrees of
the greatest circumference of the crown of
the tooth
passing from diverging axial surfaces to
converging axial surfaces







In the form of continuous contact when
circumferential clasp arms are used.
When bar clasp arms are used, at least
three areas of tooth contact must be
embracing more than one half of tooth
circumference
These are the occlusal rest the retentive
terminal area, and the reciprocal terminal
area.

Other principles to be considered in
the design of a clasp are as
follows:






1. The occlusal rest must be designed so that
movement of the clasp arms cervically is
prevented.
2. Each retentive terminal should be opposed by
a reciprocal arm or element capable of resisting
any orthodontic pressures exerted by the
retentive arm.
Reciprocal and stabilizing elements must be
rigidly connected bilaterally









3. Retentive clasps should be bilaterally
opposed,
that is, buccal retention on one side of the arch
should be opposed by buccal retention on the
other, or lingual on one side opposed by lingual
on the other.
In Class II situations the third abutment may
have either buccal or lingual retention.
In Class III situations, retention may be either
bilaterally or diametrically opposed

. A, Retentive clasps should be bilaterally opposed.
B, In Class II situations the retention on the third abutment may be on the buccal or the
lingual
. C, In Class III situations retention may be either (a) bilateral or (b) diametrically
opposed.







4. The path of escapement of each retentive
clasp terminal should never coincide with the
path of removal
5. Amount of retention should be the minimum,
necessary to resist reasonable dislodging
forces.
6. Clasp retainers on abutment teeth adjacent to
distal extension bases should be designed so
that, it should never exert tipping forces on the
abutment teeth.





7. Reciprocal elements of the clasp
assembly should be located at the junction
of the gingival and middle thirds of the
crowns of abutment teeth.
The terminal end of the retentive arm is
optimally placed in the gingival third of the
crown


Functional requirements of a clasp.







Retention
Stability
Support
Reciprocation
Encirclement
Passivity.


Retention.






The function of retentive clasp arm is to
provide retention.
The retentive clasp is divided into three
parts.
The terminal third – engages the undercut
area.
Middle third
- engages a minimal
amount of undercut.



The proximal third - is positioned above
or shoulder
the height of contour.


Three dimensions of a retentive
undercut.


The following factors determine the
amount retention a clasp is capable of
generating.





1) Size of angle of cervical convergence.
2) How far into the angle of cervical
convergence the clasp terminal is placed.
3) Flexibility of the clasp arm, which is the
product of a) Its length which is directly proportional
to the flexibility.





b) Its diameter is inversely proportional
to the flexibility of the clasp arm.
c) Its cross-sectional form or shape, that
is, whether it is round, half-round, or some other
form.
round clasp
- flex in all spatial planes.
half-round clasp-flex only in a single plane.


d) The material of which the clasp is made,
that is whether it is made of a gold alloy,
or chrome alloy.
Chrome alloy have a higher modulus of
elasticity and is less flexible.


A smaller cross sectional form of the clasp
and less depth of the retentive undercut
is engaged when chrome alloy is used and
vice versa while using gold alloy.


Cast vs. wrought alloy.


Support.




Is the property of a clasp that resists
displacement of the clasp in a gingival
direction.
The support units of a clasp are occlusal,
lingual, or incisal rests.


Stability.




All clasp components except the retentive
clasp terminals contribute to this property
in varying degrees.
Cast circumferential –offers greatest
clasp
amount of stability.


Reciprocation.


Each retentive clasp terminal must be
opposed by a reciprocal clasp arm or
another element of the partial denture
capable of resisting horizontal forces
exerted on the tooth by the retentive arm.


Reciprocation.






The reciprocal arm of the clasp is
positioned on the opposite side of the
tooth from the retentive arm.
It also stabilizes the denture against
horizontal movement.
It should be placed preferably at the
junction of the gingival and middle thirds of
the abutment tooth (always positioned
above the height of contour.)

Reciprocation.







It should contact the abutment
tooth along with or before the
retentive arm during insertion or
removal.
Other parts which offer
reciprocation are:
Lingual plate major connector.
An additional occlusal rest
placed on the opposite side of the
tooth along with the minor
connector.

Encirclement.


It is the property of
the clasp assembly to
encompass more
than 180 degree of
the abutment tooth
either by continuous
or broken contact to
prevent dislodgement
during function.









It can be either a continuous contact as in
circumferential clasp or a broken contact
as in bar clasp with at least 3 different
areas of contact.
The three points of contact are:
retentive terminal
occlusal contact
reciprocal arm

Passivity.








A clasp in place should be completely
passive.
The retentive function is activated only
when dislodging force are applied to
the partial denture.
A clasp must be completely seated on
a tooth to be completely passive.
If a clasp designed to reach a retentive
undercut of 0.010 inch cannot reach
that depth, it will exert a constant force
on tooth. Over time this can produce
pain or tooth movement





The types of clasps are:.
Circumferential or Aker ' s clasps.
Vertical projection or Bar or Roach
clasps .


Cast circumferential clasp


A clasp that encircles a tooth by more
than 180 degrees, including opposite
angles, and which usually has total
contact with the tooth (throughout the
extent of the clasp), with at least one
terminal being in the infra bulge (gingival
convergence) area"
- GPT

Vertical projection clasp / Bar
clasp / Roach clasp:




A clasp having arms which are bar type
extensions from major connectors or from
within the denture base;
the arms pass adjacent to the soft tissues
and approach the point or area of contact
on the tooth in a gingivo- occlusal
direction" - GPT.


Continuous clasp




A metal bar usually resting on the lingual
surface of teeth to aid in their stabilization
and to act as an indirect retainer"
- GPT.


Cast Circumferential Clasp








They are popularly known as Aker’s
clasps.
These clasps embrace more than half of
the abutment tooth.
This architecture helps the clasp to hold
the abutment firmly enough to prevent the
rotation of the denture.
They approach the undercut from an
occlusal direction.








Advantages:
Easiest clasp to make and repair.
Less food retention
Best when applied in a tooth supported
partial denture.
Derives excellent support, bracing and
retention









Disadvantages :
It covers a large tooth surface area.
It also alters the Bucco lingual width of the crown
This affects the normal food flow pattern leading
to food accumulation.
This causes decalcification of the tooth
structure.
Damage to soft tissue will occur due to lack of
physiological stimulation.
Difficult to adjust with pliers because of it's halfround configuration.





If these clasps are placed high (more occlusally)
on the tooth, the width of the food table
increases leading to generation of greater
occlusal forces.
All cast circumferential clasps should never be
used to engage the mesio buccal undercut of an
abutment adjacent to the distal edentulous
space (Fig. 18.167). Hence they cannot be used
for cases with an undercut away from the
edentulous space.

Types of Cast Circumferential
Clasps







1. Simple circlet clasp.
Most versatile and widely used.
Best for tooth supported partial denture.
It approaches the undercut from the
edentulous space.
It engages the undercut, located away
from the edentulous space.





Clasp can be adjusted only in one
direction.
They cannot be used for distal extension
cases as they engage a mesio buccal
undercut.








2. Reverse circlet clasp or reverse
approach clasp:
This clasp is used when the retentive
undercut on the abutment tooth is located
adjacent to the edentulous space.
Used in distal extension denture base to
control the stresses acting on the terminal
abutment teeth.










Disadvantages.
If sufficient occlusal clearance is not
present, the thickness of the clasp has to be
reduced. This will affect the strength of the
clasp.
An occlusal rest away from the edentulous
space does not protect the marginal gingiva
adjacent to the abutment tooth.
Poor esthetics as the clasp runs from the
mesial to the distal end of the facial surface
Wedging may occur between the abutment
and its adjacent tooth if the occlusal rest is
not well prepared.





3.Multiple circlet
clasp.
It is a combination of
two simple circlet
clasps joined at the
terminal end of the
reciprocal arms.







It is used for sharing the retention with
additional teeth on the same teeth on the
same side of the arch when the principle
abutment tooth has poor periodontal
support.
It is a mode of splinting weakened teeth.
It’s disadvantages are similar to that of
simple and reverse circlet clasps.







4. Embrasure clasp or
modified crib clasp.
It is a combination of
two simple circlet clasps
joined at the body.
It is used on the side
of the arch where there
is no edentulous space.








The clasp must cross the marginal ridges of two
teeth, emerge to cross the facial surfaces of
both teeth, and engage undercuts on the
opposing line angles of these teeth.
Occlusal rest seat preparation must be made on
both teeth
Tooth structure must be removed from the
buccal inclines of both teeth to provide space for
adequate thickness of the metal.





Indications: It is used in kennedy’s class 2
and class 3 cases without any
modifications.
Occasionally, a very small edentulous
space can be closed by a modified
embrasure clasp called a pontic clasp






5. Ring clasp
Indicated in tipped molars
In Distal edentulous conditions with
distolingual undercut - the retentive arm
is extended all around the tooth from the
disto buccal end to terminate in the
distolingual undercut across the mesial
side of the tooth



As clasp is long – additional support is
provided by auxiliary bracing arm









Disadvantages
Alteration in food flow pattern
Increased tooth surface coverage
Difficult to adjust or repair
Contra indications
Not considered in mandibular molar where
attachment of the buccinator is close to
tooth that the auxiliary arm encroaches on
it






6. fishhook or hairpin or reverse action
clasp
Type of simple circlet clasp.
retentive arm – crosses the facial
surface of the tooth from its point of origin,
loops back in a hairpin turn to engage a
proximal undercut below its point of origin






Upper arm - rigid
Lower arm - flexible
Sufficient space should be present
between the two arms – to avoid food
accumulation & for proper
finishing and polishing










Indication
Crown with sufficient occluso gingival
height
The undercut is adjacent to edentulous
area
Presence of a soft tissue undercut
Reverse circlet can’t be used – lack of
occlusal space





Disadvantages
Poor esthetics
Tends to trap and accumulate food debris







Onlay clasp ;
Extension of a metal crown or onlay with
buccal and lingual clasp arms.
Used in occlusal surfaces of submerged
abutment teeth – thus restoring the
occlusal plane with an onlay.






If onlay clasp is made of chrome alloy –
the opposing tooth should be protected
with a gold crown to avoid attrition of
enamel.
Used in caries resistant mouth.







8. combination clasp
A cast circumferential clasp can’t be used
– when an undercut is adjacent to the
edentulous space since it produce
destructive rotatory forces on the distal
abutment
A flexible wrought wire retentive arm is
used to replace the rigid cast alloy
retentive arm.





It has greater flexibility it can be placed in
a deeper undercut – without any hazard to
the abutment tooth
It is used in maxillary canines and
premolars due to its superior esthetics.







Advantages
The round configuration of the wrought
wire – has a thin line contact which
collects less debris and is easy to
maintain.
flex in all planes







Disadvantages
Tedious lab procedures
Easily breaks or distorts
Poor stability


Half and half clasp




It has a retentive arm arising from one direction
and a reciprocal arm arising from another
direction.
Two minor connectors are needed for this
design. The first minor connector attaches the
occlusal rest and the retentive arm to the major
connector. The second minor connector
connects the reciprocal arm which is similar to
the bar clasp with or without an auxiliary rest.





This design produces large tooth
coverage which can be reduced by
converting the reciprocal arm into a short
bar with an auxiliary occlusal rest.
This design is intended to provide dual
retention.











Back-action clasp
It is a modification of the ring clasp
Here the minor connector is connected to the
end of the clasp arm and the occlusal rest is left
unsupported.
Disadvantages :
Lack of support to the occlusal rest reduces its
function.
It has both biological and mechanical unsound
principles.








Grasso’s clasp or VRHR clasp
Developed by Grasso,
This clasp consists of :
A vertical reciprocal arm,
an occlusal rest and
a horizontal retentive arm arising
separately from the major connector.



Advantages :



Minimizes tooth contact without compromise in efficacy.



Does not require the preparation of guide planes.



Suitable for posterior teeth with high survey lines.



The placement of the retentive arm is more aesthetic.





Disadvantages :
Difficult to maintain as the block out zone
between the base of the reciprocal arm
and the tooth tends to collect food debris.


Rest-proximal plate – Akers
retentive arm (RPA) clasp






The RPA clasp is a modified circumferential
clasp
The circumferential arm arises from the proximal
plate adjacent to the edentulous base area.
The retentive component of this direct
retainer - engages a retentive undercut located
on the facial surface of the abutment tooth away
from the edentulous area.







The shoulder of the clasp arm - contacts the
tooth at the height of contour at the junction of
the middle and gingival thirds of the tooth.
This direct retainer satisfies all the requirements
for use in distal-extension RPD situations.
Allows for mesially oriented support, which is
preferable in distal-extension RPDs.





Excellent bracing is derived from the
contact of the minor connector supporting
the rest, the proximal plate contacting the
tooth surface adjacent to the edentulous
base area, and the shoulder part of the
clasp arm.
Acceptable retention is derived from
retentive areas located on surfaces of the
tooth away from the edentulous area.







Requirements for adequate encirclement and
passivity can easily be accommodated by this
direct retainer.
This type of clasp appears best suited for distalextension RPDs - where the undercut is located
away from the edentulous area and
for situations in which it has been determined
that the tooth need not be modified to
accommodate a different type of clasp.

Mesiodistal clasp.






This direct retainer might be considered
for a tooth-borne RPD when maxillary
incisors remain as the anterior abutment
teeth.
Although the design exhibits good
supporting and bracing qualities,
it may be excessively traumatic to the
supporting abutment teeth when used on
distal-extension RPDs.

Mesiodistal clasp.






Retention for this direct retainer is gained
through parallelism and frictional resistance of
the clasp assembly against the natural teeth.
The teeth must be prepared so that their
proximal surfaces are parallel or have a slight
convergence to one another.
This direct retainer, like the other circumferential
designs, poses the same problems of covering
excessive tooth structure and impairing
esthetics.

Mesiodistal clasp.


Bar or vertical projection or roach
clasp.






Approaches the undercut or retentive area
on the tooth from a gingival direction,
resulting in a push type of retention.
Flexibility of the bar type of clasp can be
controlled by the taper and length of the
approach arm.
More esthetic than the circumferential
clasp.






Disadvantages.
1. Greater tendency to collect and hold
food debris.
2. Due to its increased flexibility of the
retentive arm, it does not contribute as
much to bracing and stabilization as most
circumferential clasps do.





Rules to use.
The approach arm must always be
tapered uniformly from its point of
attachment at the frame work to the casp
terminal.


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