Retainers in RPD/ Labial orthodontics

Retainers in RPD
INDIAN DENTAL ACADEMY
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 Retention can be defined as that
quality inherent in a prosthesis that
resists the vertical forces of
dislodgement.
Or
 It is the resistance to removal from
vertical forces which tend to dislodge
prosthesis away from its foundation.

 Retention for removable partial denture is
achieved primarily by
 Mechanical means (Direct & indirect
retainers)
 Harnessing the patient’s muscular control
acting through polished surfaces of denture
 Using inherent physical forces which arise
from coverage of mucosa by the denture

Movement of partial denture
Distal extension partial
denture can move
along three axis.
 Rotation about an axis
through the most
posterior abutments,

 Rotation about a
longitudinal axis
as the distal
extension base
moves in a
rotary direction
about the
residual ridge.

 Rotation about
an imaginary
vertical axis
near the centre
of the dental
arch.

Retainers
 Direct retainer
 Intracoronal
 Extracoronal
 Indirect Retainer

Intracoronal retainers
 Prefabricated
 Provide retention by frictional resistance

Advantage
 Batter esthetics
 Better vertical support trough a rest seat
located more favorably in relation to
horizontal axis of the abutment tooth
 Horizontal stabilization is similar to that of an
internal rest

Disadvantages
 Require prepared abutments and castings
 Require somewhat complicated clinical and
laboratory procedures
 They eventually wear with resulting loss in retention
 Difficult to repair and replace
 Effective in proportion to their length and are
therefore least effective on short teeth
 Difficult to place completely within the circumference
of an abutment tooth
 More expensive

Extracoronal direct retainer
 Prefabricated- Dalbo
 Custom made- Clasp

Clasp retainer
Component
 Occlusal rest
 Retentive arm
 Reciprocal arm
 Minor connector

Amount of retention
 Size of angle of cervical convergence
 How far clasp arm is place in cervical
convergence
 Flexibility of clasp arm
 It’s length
 It’s relative diameter
 It’s cross-sectional form
 Material of clasp arm

Angle of cervical convergence
 Area cervical to height of contour
 Amount of convergence – relative to
path of placement and removal
 A definitive path of placement is
possible- guiding planes.

How far clasp arm is place in
cervical convergence
 Relative uniformity of retention
 Location of clasp arm in relation to angle of
cervical convergence, not in relation to
height of contour.

Flexibility of clasp arm
 Length α Flexibility
 Diameter 1/ α Flexibility

Flexibility of clasp arm
 Cross sectional form
 Half round form →Unidirectional
 Round form →universal
 Material of clasp: Wrought wire > cast
gold > chrome alloys

Stabilizing- reciprocal cast clasp
arm
 It should be rigid
 It should taper in one dimension only
 It’s average diameter must be greater then
retentive arm

REQUIREMENTS OF
CLASP DESIGN
 Retention
 Stability
 Support
 Reciprocation
 Encirclement
 Passivity

Basic principles of clasp
design
 More than 180° of the greatest
circumference of the crown of the
tooth must be included
 Continuous contact
 Three widely separated area
 Occusal rest must be designed
 Each retentive terminal must
oppose a reciprocal component.

design
 Positive guiding planes
 Retentive clasps should be bilaterally
opposed
 Path of escapement of each
retentive clasp must be other then
parallel to path of removal of the
prosthesis.

design
 Amount of retention should always be minimum
necessary
 Clasp retainer should not transmit off-vertical forces
to the abutment tooth.
 Reciprocal element →at the junction of gingival and
middle thirds
 Terminal end or retentive arm →Gingival third

Type of clasps
 Circumferential clasp
 Ring clasp
 Embrasure clasp
 Multiple clasp
 Half- and –half clasp
 Reverse action clasp
 Bar clasp or RPI clasp

Circumferential clasp
 This clasp consists of occlusal rest & a
buccal and lingual arm originating from
a common body

 Most logical clasp for tooth supported partial
dentures because of excellent support,
bracing and retentive qualities
 Can be used in a wide variety of situations

Disadvantages
 More tooth coverage
 It’s occlusal approach may increase the width of
occlusal surface of the tooth
 More metal may be displayed than bar clasp
 It’s half round form prevents to increase or decrease
retention

Ring clasp
 Encircles nearly all of a tooth from it’s point of origin
 Indications: Tipped molars were a proximal undercut
cannot be approached by any other means

Embrasure clasp
 Essentially two simple circlet clasp
joined at the body

Multiple clasp
 Simply two opposing circumferential
clasps joined at the terminal end of the
two reciprocal arms

Half- and –half clasp
 It consists of a circumferential
retentive arm arising from one
direction and reciprocal arm
arising from another
 The second arm must arise from
a second minor connector and is
with or without occlusal rest

Reverse-action clasp
 When retentive undercut is adjacent to occusal rest
 Bar clasp cannot be used because of soft tissue
undercut
 Ring clasp cannot be used due to lingual undercut

Bar clasp
 The bar clasps approach the
undercut or retentive area on
the tooth from a gingival
direction
 Flexibility of bar clasp → taper
and length of approach arm

Bar clasp
DISADVANTAGES
 Greater tendency to trap food debris
 Does not contribute as much to
bracing and stabilization as most
circumferential clasps do

Bar clasp
 Classified as
 T. Modofied T,I,
or Y

RPI system

RPI system
Three approach for the application of RPI
system

RPI system
 Proximal plate contact 1mmgingival portion of
GP

RPI system
 RPA Clasp

Combination clasp
 It is a circumferential clasp, but bar
clasp may be used
 wrought wire retentive clasp arm and a
cast reciprocal clasp arm
 If high heat chrome alloy is used, the
wrought wire must be soldered to the
completed framework

Combination clasp

How to select a clasp design
 It’s flexibility
 Stabilization for horizontal & rotational
forces
 Sufficient rigidity
 Applicable to malposed or rotated tooth
 Presence or absence of tissue undercut

How to select a clasp design
 Clasp terminal is adjustable or not
 Area to be covered by clasp
 Effect on width of occlusal table
 Can it be replaced.

Indirect Retainers
 Restrict the movement of denture base away from
basal seat, as it tend to rotate around one of the
fulcurum line .
 Rigid unit of partial denture framework, that are
located on definite rest seats on the opposite side of
fulcurum line from the distal extension base .
 Theses component should be placed as far as
possible from fulcurum line for best possible
leverage.

Fulcurum line
 This is an imaginary line passing through teeth and component
part of partial denture, around which denture rotates slightly
when subjected to various forces directed towards or away from
the residual ridges.

Indirect retainer - types
 Auxiliary occlusal rest
 Canine extension from occlusal rest
 Canine rest
 Linguoplate with rest on terminal ends
 Modification areas.
 Rugae support
 The proximal plate

Factor influencing effectiveness
of indirect retainer
 Stable principal occlusal rests
 Distance from the fulcrum line
 Rigidity of the connectors supporting
the indirect retainer
 Effectiveness of the supporting tooth
surface.

CONCLUSION
 We can select any of the clasp
designs but that should be
based on sound biological &
mechanical principles of clasp
designs

THANK YOU
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Retainers in RPD/ Labial orthodontics

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