Retention/cosmetic dentistry courses

Clasp retention
- the effects of variables
INDIAN DENTAL ACADEMY
Leader in continuing Dental Education
www.indiandentalacademy.com

Component parts of RPD
Major connectors
Minor connectors
Rests
Direct retainers
Indirect retainers
Denture bases

Forces acting on the partial denture
Removable partial denture is subjected to a
composite of forces arising from three
principal fulcrums.
Movement about an axis through the most
posterior abutments (Principal fulcrum line).
Movement about the longitudinal axis (in the
sagittal planes the distal extension moves in
rotary direction about the residual ridge)
Movement about an vertical axis located near
the center of the dental arch.

Movement about an axis through the most posterior abutments .
(Principal fulcrum line)
(in sagittal plane)

Movement about an axis through the most
posterior abutments .(Prnicipal fulcrum line)
Force activated by Resultant
force
Counteracted by
Sticky foods Vertical lift Retention
Tongue and
muscle forces
Vertical lift Adequate denture
base coverage
Gravity (maxillary) Vertical lift Indirect retainers ,
Gravity
(mandibular)
Occlusal load Movement
towards the
residual
ridge
Rests , Adequate
denture base
coverage, choice of
connector

Movement about the longitudinal axis (in the frontal plane
as the distal extension moves in rotary direction about the
residual ridge)
Force
activated by
Resultant
force
Counteracted by
Occlusal force
on one side of
the arch
causes lifting
forces on the
contra lateral
side of the
arch
Twisting
and tilting
Rigid connectors
Direct retainer
design
Denture base
coverage
Denture tooth
placement and
Contour of the
denture base.

Movement about an vertical axis located near the
center of the dental arch
(in horizontal plane)
Force
activated by
Resulta
nt force
Counteracted
by
Masticatory
stress
Twisting
and
spreadi
ng of
RPD
Adequate bracing:
Rigid
connectors
Connector
choice
Denture base
coverage
Occlusal balance
contour of the
denture base.

That quality inherent in the prostheses
acting to resist the forces of
dislodgement along the path of
placement.
Retention –Definition (gpt-7)

Sources of retention in RPD
Physiologic retention
Mechanical retention
Frictional retention

Physiologic retention
Proportional to the tissue covered by
denture base.
Important in extension base partial
dentures.
Maximum coverage is needed so that
the primary retention line is so far
anterior that indirect retention is
ineffective.

Maximum coverage is needed so that the
primary retention line is so far anterior
that indirect retention is ineffective.

Mechanical retention
Provided by direct retainers
DIRECT RETAINERS
Intra coronal Extra coronal
Precision
attachments
Semi precision
attachments
Retentive
clasp
assemblies
Attachments
Suprabulge Infra bulge

Retentive clasp assemblies
Retention clasp assembly represent most
common method of extra coronal direct
retention.
First appeared in dental literature with W.
G. A. BONWILL description in 1899.
Proper use of dental surveyor is only
reliable method of effectively analyzing
teeth for their contribution to the retention
of partial denture.

Classification of the clasps
 Based on design :
Circumferential type (Akers, supra
bulge) clasp,
Bar type clasp (Vertical
projection, Roach, infra bulge).
 Based on construction :
1. Cast clasp,
2. Wrought wire clasp,
3. Combination clasp

Parts of a clasp
Clasp assembly consists of :
Rest,
Reciprocal arm
Retentive arm,
, minor connectors

The component of the clasp that provides
vertical support or prosthesis is called
rest and the portion of the abutment tooth
prepared to receive rest is called rest
seat.
Properly prepared rest seats and rests
serve to :
Resist displacement of prosthesis
towards supporting tissues.
Transmit functional forces parallel
to the long axis of abutments.

Retentive arm is only portion of the RPD
that contacts the surface of an abutment
apical to the height of the contour.
Depending on their approach to the
undercut region of the abutment they are
termed
Supra bulge clasp arm and
Infra bulge clasp arm

Supra bulge clasp arm
originates from a minor
connector occlusal or incisal
to the height of the contour.
The portion of the clasp arm
arising from the minor
connector is known as
shoulder which compromises
the proximal third in a clasp
arm.
From the shoulder the clasp
arm follows gently curved
path on the surface of the
abutment and the clasp arm
passes over the height of
contour and its tip contacts
undercut and this portion of
the clasp is terminus.

The infra bulge clasp arm consists of two distinct
segments approach arm and terminus.
Approach arm is a minor connector that originate
from frame work travels horizontally along the
surface of the mucosa and then turns vertically to
cross the free gingival margin at 90°.
The terminus arises from the vertical portion of the
approach arm and engages undercut and surface of
the abutment.

The component of the clasp assembly from
that braces an abutment during prosthesis
insertion and removal is called the reciprocal
element. The reciprocal element may be a
cast clasp, lingual plate or a combination of
mesial and distal minor connectors.
Reciprocal elements must contact the
abutment tooth at or occlusal to the height of
the contact.

Incorrect Correct

Mechanism of action of reciprocal
arm
As the retentive arm passes over the height of the
contour the arm displace a slight amount of flexure.
This flexure places lateral stress in the abutment. If
the stress is not counter acted it may cause lateral
displacement of the abutment and significant
damage to the supporting periodontium. To prevent
these effects, a rigid reciprocal element must be
used.
During insertion of the prosthesis a reciprocal
element should contact the abutment slightly before
the retentive arm contacts the abutment and this
contact should be maintained while the retentive
terminus pass over the height of the contour and
onto the undercut.

The abutment surface that will be contacted by
the reciprocal element should be parallel to the
path of the insertion of the removable partial
denture.
Minor connectors join the elements of the clasp
assembly to the other parts of RPD and they must
be rigid depending on the philosophy of the
design.
The minor connector serve as guiding plate to
direct insertion and removal of RPD
Reciprocal element to counteract non axial forces.
Approach arm for intra bulge clasp.

Cast circumferential clasp
Was introduced by Dr .N.B. NESBITT in (1916)
Design of choice in tooth supported removable partial
denture.
Advantages:
Simple and easy to construct
Close adaptation to the abutment – minimizes the
entrapment of the food and debris.
Disadvantages:
As the large amount of the tooth surface is covered by the
cast assembly ; if proper oral hygiene is not maintained,
under lying enamel is at under increased risk of
calcification.
Interfere with food flow and bolus during elimination
during the masticatory process. As a result , the abutment
and associated periodontium may be damaged.

Design rules
Clasp should originate from
portion of frame work that lies
above height of the contour
Retentive arm should extend
apically and circumferentially in
gently arcing manner. Terminal
third of the retentive clasp should
pass over the height of the contour
and enter the infrabulge portion of
the abutment. Retentive terminus
should be directed occlusally ,
never towards the gingiva.
Clasp arm should terminate at the
mesial or distal line angle never at
the midfacial or midlingual
surfaces.

Retentive arm should be placed as far as
apically on the abutment as practical.
Reciprocal arm should be located at or
slightly above the height of the contour on
the opposite surface of the abutment teeth.
 In an extension base partial denture clasp
should not engage mesiofacial surface of
the abutment adjacent to posterior
edentulous space.
Distofacial surface of the abutment
surface of the abutment adjacent to the
anterior edentulous spaces

Simple circlet design
Clasp of choice to the
tooth supported
removable partial
dentures.
Clasp originates from
the proximal surface
of the abutment
adjacent to the
edentulous space with
the clasp arms away
from the edentulous
space.

Advantages
Easy to construct
Simple to repair.
Disadvantages
Increased tooth coverage may promote
decalcification and compromise esthetics .
Interfere with the elimination of the food
from the occlusal table.
It can be adjusted in only in one direction
(bl).

Reverse circlet clasp
Used when the
available undercut is
located at the facial
or lingual line angle
adjacent to the
edentulous space.
Retainer of choice
when the infrabulge
clasp cannot be used

In distal extension applications ,it helps to
control stresses transmitted to the abutment
teeth during functional movement of the
prosthesis
Not a clasp of choice in canine and
premolar areas.
Main problem associated with the clasp is
the strength, and food impaction between
the proximal plate and surface of the
abutment.

Embrasure clasp design
It is the two simple circlet
clasps joined at the bodies.
Used in the side of the arch
where there is no
edentulous space.
Clasp originates from the
minor connector that
traverse the marginal ridge
between teeth
Retentive arms engage
undercuts on opposing line
angles.

Ring clasp design
Indicated on a tipped
mandibular molar
Engages the mesiolingual
undercut of the mesially tipped
molar.
Clasp originates at the mesial
rest and traverses the facial and
distal surfaces of the tooth
,remaining occlusal to the
height of the contour
And at the lingual surface the
clasp arm engages the undercut
at the mesiolingual line angle.
Additional support is provided
by the bracing arm at the facial
surface of the tooth.

Bracing arm
Originates at the minor connector near used to
retain the acrylic denture base.
Provide reciprocation
Runs horizontally across the mucosa apical to the
abutment teeth and then turns occlusally to cross
the free gingival margin and join the clasp
assembly in the midfacial aspect.
An additional rest is placed on the disto occlusal
surface if desired.
Not considered when
There is soft tissue undercut.
Vestibular depth is minimal.

C clasp design
Referred to as fish hook
or hair pin clasp.
It is a simple circlet
clasp where the retentive
arm loops back to
engage an undercut
apical to the point of the
origin.
Occlusal portion of the
clasp should be rigid and
display consistent
dimensions.
Retentive portion of the
clasp should be tapered
and flexible. www.indiandentalacademy.com

Abutment should
Have sufficient vertical height.
Adequate space between the occlusal and retentive
portion of the clasps.
Occlusal aspect of the retentive arm should not
interfere with the opposing teeth in maximum
intercuspation.
Disadvantages
Generally yields inadequate flexibility,
abutment subjected to non axial forces.

Multiple circlet clasp
It involves two circlet
clasps joined at terminal
aspects of the reciprocal
elements.
Indicated when the
primary abutment tooth is
periodontally
compromised and
stresses originating from
prosthesis can be
transmitted to the mutiple
abutment teeth.

Onlay clasp
It consists of a rest the
covers the entire occlusal
surface and serves as
origin to buccal and
lingual arms.
Indicated when the
occlusal surface of the
abutment is apical to the
occlusal plane .
Onlay rest serves as the
vertical stop.
Done in caries resistant
patients. www.indiandentalacademy.com

Half and half clasp
Consists of
circumferential retentive
arm arising from one
direction and a reciprocal
arm arising from other.
second arm arises from
the second minor
connector and it may be
used with or without the
rest.
Reciprocation can be
accomplished by minor
connector avoiding the
much tooth coverage

Combination clasp
O.C. Applegate introduced modified wrought wire clasp
assembly known as the combination clasp.
It consists of occlusal rest, a cast metal reciprocal arm, and
a wrought wire retentive arm.
It is used when
maximum flexibility is desirable such as on abutment
tooth adjacent to the distal extension bases
When a bar type retainer is contraindicated on the
abutment.
Esthetic advantage over cast clasps.
.

The most common use of this clasp is on
abutment tooth adjacent to the distal
extension base where only mesial undercut is
present and
use of the bar clasp is contraindicated
due to presence of large tissue undercut
When the undercut is on the side away
from the extension base the tapered wrought
wire retentive arm offers great flexibility than
does the cast clasp arm and therefore better
dissipates the forces

Advantages
Flexibility
Adjustability
Esthetic advantage over the cast clasp arms
Minimum tooth surface covered because of its
line contact with the tooth
Fatigue failures are less common than the cast
retentive arm
Disadvantages
Extra steps in fabrication of the retentive arm
Distorted by the careless handling of the
patients
As it is bent with the hand the adaptation may
not be so accurate and less stabilization in the
suprabulge portion.

Infrabulge clasps
Clasp that approaches the undercut region of the
abutment from n apical direction.
Different types of infrabulge retainers:
T clasp
Modified T clasp
I clasp or I bar
Y clasp

Design rules
Approach arm should not impinge on the soft tissues of
the abutment.
Tissue surface of the approach arm should be smooth
and well polished.
Approach arm should cross perpendicular to the free
gingival margin.
Approach arm should never be designed to bridge a soft
tissue undercut since it will produce food entrapment
Approach arm should be tapered from its origin to the
terminus
Clasp terminus should be placed as far apically on the
abutment as is practical as it decreases the leverage
induced stresses resulting from the movement of the
prostheses.
Minor connector attaching the occlusal rest to the
framework should be rigid and should contribute to the
overall stabilization and bracing characteristics of the
prostheses. www.indiandentalacademy.com

Advantages :
Minimal tooth contact and minimal distortion of
the normal tooth contours.-improved tissue
stimulation and oral hygiene, decreased caries
and periodontal problems.
Improved esthetics
Increased retention because of the tripping
action.
Decreased torquing forces applied onto the
abutment tooth in the extension bases.
Improved adjustability because of the location of
the bend is less critical than for the
circumferential clasps.

Disadvantages
Cannot be used in presence of soft tissue under
cuts ,a shallow vestibule, or high frenal
attachments.
Bracing action provided by bar clasps is less
than circumferential clasps.
Esthetic s may be affected if the patient has a
high smile line enough to expose the approach
arm as it crosses the gingiva.
May not be retentive unless the rigid elements
determine the specific path of insertion of the
prostheses.

T clasp
 Used in Kennedy Cl I and II
situations when an undercut is
present adjacent to the edentulous
tissues.
 From the point the approach arm
turns vertically to cross the free
gingival margin and contact the
abutment two horizontal projections
arise.
 One projection passes the height of
the contour and enters the undercut
and the other remains occlusal to
the height of the contour.
 If the undercut is present on the
mesio facial surface this clasp
cannot be used in Kennedy Cl 1
and II partially edentulous
application.

Mode of action of the T clasp

Modified T clasp design
It is identical to the T
clasp except that it will
act as non retentive
horizontal projection.
Provides improved
esthetics in most of the
applications.
So used when the canines
or premolars will serve
as abutment.

Y Clasp
Y clasp is formed when approach arm
terminates in the cervical third of the
abutment while the mesial and distal
projections are positioned near the occlusal
and incisal surfaces.
Mechanics are similar to T clasp.

I CLASP
The approach arm
originates from the frame
work and projects
horizontally over the soft
tissues adjacent to the
abutment and a gradual
turn allows this arm to
assure vertical orientation
and cross perpendicular to
the free gingival margin.
The clasp arm contacts the
abutment that extends
from undercut to the
height of the contour.

The contact area between the clasp and the
abutment 2 – 3 mm in height and 1.5 -2 mm in
width.
Commonly used in class I and class II partially
edentulous arches.
The clasp terminus in these cases should be placed
at or mesial to the mid facial prominence of the
abutment.

RPI Concept
First introduced by Kratchovil and later
modified by Krol
RPI clasp assembly consists of mesio
occlusal rest with the minor connector in
the mesiolingual embrasure, a distal
guiding plane and the I bar clasp located
at the 0.01 undercut in the gingival third
of the buccal or labial surface.

Survey lines
Blatterfeins classification of buccal and lingual
survey lines:
Medium survey line
High survey line
Low survey line
Diagonal survey line

Medium survey line
On the buccal and lingual surface of the tooth
equidistant from the occlusal and gingival margin in
the near zone and slightly nearer the gingival margin
in the far zone.

Indicates use of an occlusally approaching arm and
also gingivally approaching arm.
Various forms of bar clasps are appropriate, the length
of the bar used depending upon the resilience required
in the arm.
If a larger degree of the undercut is to be engaged
more resilience is required and the length of the bar is
increased.
It is often necessary to engage undercuts more deeply
so that the end of the clasp will be replaced nearer to
the gingival margin.
The more resilient the arm in these cases the less the
brazing effect, this factor too makes it separate
demand on bar length.

Diagonal Survey line
This line lies nearer the
occlusal surface than a
gingival margin in the near
zone of the tooth but in the
far zone the opposite
condition exists and little
undercut is present.
Most commonly found on
buccal surfaces of the
canines and premolars.

If an occlusally approaching arm is
preferred reverse circlet and C
clasp can be used.
When the gingival approaching
arm clasp is preferred L or T bar
clasp can be used.
If there is rather more undercut as
seen in the figure , U shaped bar
clasp can be used.

High survey line
This survey line appears much
nearer to the occlusal than the
gingival surface of the tooth in
both near and far zones.
It may arise as a result of
abnormal tooth form where the
occlusal surface has a
considerably larger perimeter
than the cementoenamel junction
and where only small degree of
convexity of tooth is present.

It results from the inclination of the tooth frequently
found in the lingual surface of the lower teeth and the
buccal surfaces of the upper teeth.
If occlusally approaching arm is required three arm
clasp is to be used but if the arm is to cover only one
surface of the tooth wrought wire is generally
preferred.
When high survey line results from inclined tooth it is
generally found that the opposite surface of the tooth
has little or no undercut.
Then reverse back action and back action clasps are
used.

Gingivally approaching clasp arm find little
application with high survey lines that result
from tilted teeth.
In these circumstances the supporting ridge
generally confirms in its slope to that of the
tooth.
Assuming vertical path of insertion, a greater
tissue undercut exists so only gingivally
approaching clasp cannot be used.

Low survey line
The survey line is traced very low in the buccal
or lingual surfaces of the teeth.
Frequently occurs in:
marked inclinations of the teeth,
High survey line associated on the opposite
surface,
Conically shaped tooth.

Tooth having a low survey line cannot bear a
retentive clasp arm as:
Insufficient Under cut exists to the effective in
retaining the denture,
The clasp arm in such a undercut will be
situated near the gingival margin.
If the tooth support is adequate it is possible to
place the arm within the 1mm of gingiva, but
if the arm is thick non self cleansing area is
liable adjacent to the gingival line and if there
has been gingival recession or gingivectomy
has been done it is necessary to keep the clasp
arm above the cemento enamel junction.

The tooth surface with low
survey line can always face
brazing arm which is of two
kinds:
Reciprocal arm of the
circumferential clasp
If the tooth is tilted wrought wire is
usually placed in the opposite
side.
Additional retention is required
three methods exist for obtaining
it:
Using the near proximal undercut
of the tooth,
Using the extended arm clasp,
Crowning the tooth and developing
suitable contours.

REQUIREMENTS OF CLASP
ASSEMBLY
RETENTION
STABILITY,
RECIPROCATION,
ENCIRCLEMENT,
PASSIVITY,
SUPPORT

Retention
Retention is the quality of the clasp assembly that
resists forces acting to dislodge components away
from the supporting tissues. Each part of the clasp
contribute to some feature of retention;
The retentive arm must be designed so that only the
clasp terminus engages the undercut.
Rest provide support maintaing the clasp terminus in
the optimal location.
Minor connector must be rigid to ensure proper
stability and function of the parts of the clasp
assembly.

Reciprocal element must contact the abutment
before the retentive elementcontacts the tooth .
Components must provide encirclement to
prevent movement of the abutment away from
the assosciated clasp.
Indirect retainers must resist forces acting to
dislodge the denture.
Amount of retention provided by the clasp is
provided by factors like:
Flexibility of the direct retainer
Depth of the undercut
Location of the retentive terminal

Flexibility of the clasp arm
Maximum flexibility of the retentive arm is the
greatest amount of displacement that can occur
without causing permanent deformation of the clasp
arm.
Flexibility of the retentive arm is dependent upon
Length
Diameter
Cross-sectional form
Material which the clasp is made.

Length of the clasp arm
Longer the clasp arm more flexible it will
be (all the other factors being equal.)
Length of the clasp arm is measured from
the point which the uniform taper begins.

Permissible flexibilities of retentive cast
circumferential and bar type clasp arms for
chromium cobalt alloys
Circumferential clasp Bar clasp
Length
(inches)
Flexibility
(inches)
Length
(inches)
Flexibility
(inches)
0-0.3
0.3-0.6
0.6-0.8
0.01
0.02
0.03
0-0.7
0.7-0.9
0.9-1.0
0.01
0.02
0.03

Diameter of the clasp arm
 As diameter of the clasp arm decreases flexibility
increases.
 Flexibility of the half round cast clasp have shown width
to thickness ratio of 2:1 to be optimal.
 Uniformly tapered clasp is more flexible than non
tapered clasp of the same proximal dimensions.
 The cross sectional dimensions of the shoulder of the
clasp should be twice the cross sectional dimensions at
the clasp terminus.

Cross section form of the clasp
A circular cross sectional clasp form
imparts omni directional flexure while
a half round forms allows only bi
directional flexure.
Clasps exhibiting circular cross section
may permit dissipation of the
detrimental forces during functional
movement of the prosthesis as it flexes
in all spatial planes.
Half round clasp flexes in a plane
perpendicular to the flat surface so
stress dissipation is limited.
As a result retentive clasp in circular
form provides advantages.

Material used for clasp arm
Alloys exhibiting greater elastic moduli exhibit
greater stiffness and vis versa.
Cobalt chromium and nickel chromium alloys
have higher elastic moduli than gold alloys.
Clasps made from chromium alloys are rigid, so
gold alloys provide a good flexibility ;but are
quite expensive, so a wrought wire alloy can be
used when greater flexibility is required.

Wrought wire produced by drawing a cast
metal through the die. This process produces
elongation of alloy crystalline micro
structure which imparts greater flexibility
than a conventional crystalline structure.
Flexibility of wrought wire diminished if the
material is allowed to re crystallize which
can occur as a result of cold working and
heating a wrought structure.

Location of clasp terminus
It is related to height of contour and may be
described in two dimensions :
Horizontal,
Vertical.
The position of a clasp terminus with regard to
this dimensions will influence the retentive
forces exerted by the clasp and this process is
accomplished using surveyor.

Height of the contour can be traced on to the
surface of the cast by surveyor and the survey
line separates the supra bulge and infra bulge
portions of the abutments.
The infra bulge portions represent the
mechanical undercuts in the clasp terminus is
positioned into the undercut and the proper
position of the clasp terminus can be identified
by measuring the undercuts using the undercut
gauzes.
They are available in .01,.02,.03 configurations.

Depth of the undercut
The amount of the undercut used in given
situation depends on many factors:
Keeping all other clasp related factors constant,
position of the clasp terminus at the greater
horizontal undercut will result in increased
retentive force.

Angle of gingival convergence
Angle formed between
the analyzing rod and
tooth surface apical to
the height of the contour.
Angle of gingival
convergence becomes
greater , force required
from the removal of the
abutment becomes
greater.
This may provide
advantages or
disadvantages depending
on the clinical situation.

If a severe angle of gingival convergence is
present recon touring the tooth surface to
reduce the angle or placing the clasp terminus
in the smaller undercuts can be done both are
directed at maintaining an optimal retentive
force for chosen clasp assembly

Retention/cosmetic dentistry courses

Empfohlen

Empfohlen

Weitere ähnliche Inhalte

Was ist angesagt?

Was ist angesagt? (20)

Andere mochten auch

Andere mochten auch (8)

Ähnlich wie Retention/cosmetic dentistry courses

Ähnlich wie Retention/cosmetic dentistry courses (20)

Mehr von Indian dental academy

Mehr von Indian dental academy (20)

Kürzlich hochgeladen

Kürzlich hochgeladen (20)

Retention/cosmetic dentistry courses