Direct retainer. /certified fixed orthodontic courses by Indian dental academy

DIRECT RETAINERS

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

Contents
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•
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•
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•
•
•
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Introduction
Classification
Types of clasps
Requirements of clasp
Comparison of Circumferential and
Bar clasps
Other clasp system
Recent advances
Conclusion
Bibliography www.indiandentalacademy.com

Introduction
• Retention of removable partial denture is a unique
concern when compared with other prosthesis, when
dealing with the crown of fixed partial denture the
use of preparational geometry & a luting agent
combines to retain a prosthesis, but
• In case of removable partial denture, mechanical
retention is achieved by means of direct retainer by
frictional means, by engaging a depression in the
abutment tooth or by engaging a tooth undercut
lying cervically to its height of contour.

“That component of a partial removable
dental prosthesis used to retain and prevent
dislodgment, consisting of a clasp assembly
or precision attachment”
-GPT.


Classification of Direct Retainers
It is broadly classified into
1 Intracoronal direct retainers
a.Precision attachments
b.Semiprecision attachments

2 Extracoronal direct retainers
a.Retentive clasp assemblies
• Suprabulge
• infrabulge

b.Attachments


INTRA-CORONAL RETAINERS / INTERNAL
ATTACHMENTS / PRECISION ATTACHMENTS:
• It is an interlocking device, one component of
which is integrated into a removable prosthesis to
stabilize and/or retain it.
• This principle was first formulated by Dr.Herman
E.S. Chayes in1906.
• It resides within the normal contours of an
abutment & functions to retain & stabilize a RPD.


Advantages
1. Elimination of visible rest and support
component
2. Better vertical support
3. Horizontal stabilization
4. Better stimulation of underlying tissues because
of intermittent vertical massage


Disadvantages
1. Require prepared abutments and castings
2. They eventually wear with progressive loss of
frictional resistance to denture removal.
3. Difficult to repair and replace
4. Complicated clinical and laboratory procedures
5. Difficult to place completely within the
circumference of tooth
6. Expensive

Limitations
1. Size of the pulp depends on age
2. Short or abraded teeth
3. Not to be used in extensive tissue supported
distal extension cases unless some form of stress
breaker is used.


EXTRA-CORONAL DIRECT RETAINERS /
CLASPS:
• It consists of components that reside entirely
outside the normal clinical contours of the
abutment.
• They serve to retain & stabilize RPD.
• They are divided into 2 distinct categories
– Extra coronal attachments
– Retentive clasp assembly


EXTRA CORONAL ATTACHMENTS
• It was first introduced by Henry.r.Boos in the
early 1900’s, modified by F. Ewing Roach
• It derives retention from the close fitting
components such as matrices & patrices
• It permits vertical movement of the prosthesis
during occlusal loading.


RETENTIVE CLASP ASSEMBLY
• It’s the most common method for Extra coronal
direct retention.
• It was first appeared in the dental literature with
Dr W.G.A. Bonwill’s description.
• It operates on the principle of resistance of metal
to deformation.


•

In 1916 – Prothero’s
Advanced Cone Theory was
introduced which is the basis of
clasp retention. Shape of the
crowns of PM and molar teeth
can be considered as two cones
sharing a common base.the line
formed at the junction of these
cones represents the greatest
diameter of the tooth.

•
•
•
•

Kennedy called it as Height of contour
Cummer as Guideline
De Van as Supra & infra bulge regions
Height of contour changes depending on position
of teeth. Retentive undercut exists only in relation
to given path of placement or removal.


TYPES OF CLASPS
1) Circumferential/ Supra bulge/ Akers clasp:
– Has two clasp arms that encircle the abutment
tooth. Clasp approaches undercut from above
the height of contour
2) Bar Clasp/ Infra bulge clasp:
– Has an approach arm ending up in two clasp
arms one placed above and the other placed
below the height of contour. Clasp approaches
undercut from below the height of contour.

Component parts of a clasp
•
•
•
•
•
•
•

Rest
Body
Shoulder
Retentive clasp arm
Reciprocal arm
Minor connector
Approach arm

Rest:
 Part of the clasp assembly that
lies on the occlusal or lingual or
incisal edge or surfaces of the
teeth
 Resist’s the tissue ward
movement of the clasp.
 Transmit functional forces along
the long axis of the tooth.

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

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

Retentive Clasp
“A flexible segment of a removable partial denture which
engages a under cut on an abutment which is designed
to retain the denture” – GPT




Terminal 3rd / retentive terminal/ Terminus
Middle 3rd
Proximal 3rd


Terminal 3rd / retentive terminal/ Terminus
– It is flexible and should point towards occlusal
surface
– Only component to lie cervical to height of
contour to provide direct retention.

Middle 3rd
– Provides limited flexibility
Placed above the height of contour

Proximal 3rd
– Rigid
– Placed above www.indiandentalacademy.com
the height of contour

Reciprocal Arm:
• It is rigid and placed above the
height of contour.
• It is not tapered like retentive
clasp
• It is designed to contact the tooth
before the retainer clasp does
and to remain in contact while
the retentive terminal passes the
height of contour.

Purpose:• Resist tipping force generated by
the retentive terminal as it passes
over the height of contour when
partial denture is inserted or
removed.
• Helps in stabilization and
reciprocation against retentive
arm
• May act as indirect retainer if
placed on the opposite side of
fulcrum

Minor Connector:
• Part of the clasp that
joins the body to the
remainder of the
framework


Approach arm:
• It projects from the framework,
runs along the mucosa turns to
cross-gingival margin of
abutment tooth.
• Body and retentive terminal
attached to it.
• It is slightly flexible.


Requirements of the clasp
1.
2.
3.
4.
5.
6.

Retention
Support
Stability
Reciprocation
Encirclement
Passivity


Retention
“It is the quality of the clasp
assembly that resists forces
acting to dislodge components
away from the supporting
tissues”.
•

Provided by the retentive
clasp,


Amount of retention depends
1. Flexibility of clasp arm.
2. The depth of the retentive terminal
extending into the undercut.
3. The amount of clasp arm that extends
below height of contour.

Flexibility depends on:
•

Length of clasp arm
(Double the length 5 times is the
flexibility)

•

Diameter of clasp arm is
inversely proportional to the
flexibility

•

Cross-sectional form of the
clasp
–
–

Round -increases flexibility
and ability to flex in all planes
Half round - decreases
flexibility and flexes only in
single plane www.indiandentalacademy.com



Curvature of the clasp arm

More the curvature makes the clasp more rigid.
Material
•

•
•
•

Cr-Co alloys have higher Modulus of elasticity than gold
alloys so reduced flexibility. To obtain equal retention a
greater depth of undercut is required for a wrought wire
clasp.
Materials used and the undercut to be engaged
Cast chrome
- 0.010”
Cast gold
- 0.015”
Wrought alloy
- 0.020”

2, The depth of the retentive terminal extending
into the undercut.
•

Its described in 2 distinct dimensions
– A medio-lateral or horizontal dimension.
– A occluso-apical or vertical dimension.


The position of the terminus is
determined by the dental surveyor, by
using the undercut gauge to visualize the
amount of undercut.
Angle formed between the analyzing rod
& the tooth surface apical to the height of
contour is angle of gingival convergence
If the angle of gingival convergence is
great, either the tooth is recontoured or
specific clasp is given.

Support
“It is the property of the clasp that
resist displacement of the clasp in
the gingival direction”.
• it is provided by occlusal, lingual
and incisal rests
• It should transmit forces parallel
to the long axis of the abutment.


Stability
“It is the resistance to horizontal
displacement of the
prosthesis”.
• It is provided by all
components except retentive
terminal
• Cast Circumferential clasp
provides the greatest stability.

Reciprocation:
“It is the quality of the clasp assembly
that counteracts lateral displacement of
an abutment when the retentive clasp
terminal passes over the height of
contour”
•

It must be rigid

• It should be placed above the height of
contour. Preferably at the junction of the
middle & gingival 3rd.
•

If the height of contour lies in the occlusal 3rd
of the tooth either the tooth has to be reshaped
or a lingual plate is more effective.

Encirclement:
“It is the characteristic of the clasp
assembly that prevents movement
of the abutment away from the
associated clasp assembly”.
• Clasp must encircle 1800 or half of the
circumference of the tooth
• It may be continuous
(circumferential) or broken (bar clasp).
If broken it must contact at least 3
different areas of tooth.


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.

Location of Retentive Terminal:
• Usually mesial or distal line angle preferably the
facial surface.
• Maxillary premolar rarely shows lingual
inclination. So buccal retentive area.
• Molar teeth exhibit undercut on either or both of
facial or lingual surfaces so retention may be used
on buccal or lingual.


CIRCUMFERENTIAL CLASP/
SUPRABULGE CLASP/ AKERS CLASP
•
•

It was first introduced by Dr N. B. Nesbitt in 1916.
DESIGN RULES


1. Clasp should originate from the portion of the frame work
that lies above the height of contour. Only the
terminus should engage the undercut.
2. Terminus should be directed occlusally .
3. It should terminate at mesial/distal line angle never at mid
facial /mid lingual.
4. Retentive arm should be as far apical on the abutment.
5. For a distal extension base, clasp shouldn’t engage
mesiofacial surface of the posterior edentulous space
distofacial surface of the anterior edentulous space.

&

Advantages:•

Most logical clasp of choice for tooth supported
partial denture because of its excellent support,
bracing and retentive qualities.

•

Easy to design, construct and repair

•

Fewer problems of food entrapment compared to
bar clasp

Disadvantages:•

Covers more tooth surface so decalcification /
caries

•

Damage to gingival tissue because of lack of
physiological stimulation of the tissue.

•

Not possible to adjust with pliers because of its
half-round configuration.

•

Alters the normal bucco-lingual contour of
tooth.

•

Can increase www.indiandentalacademy.com table if
the width of food

Simple Circlet Clasp:
• It is most versatile and
widely used clasp.
• Approaches undercut,
which is remote from
edentulous space.
• It is the clasp of choice in
tooth-supported partial
denture.

Advantages
• Easy to construct, repair.
• It provides better support, stability, reciprocation,
encirclement, and passivity
Disadvantages
• Same as that of cast circumferential clasp


Reverse Circlet Clasp:

• Indicated when undercut
is located adjacent to
edentulous space and bar
clasp is contraindicated.


Disadvantages
• It requires sufficient occlusal
clearance, failure to do so
results in a thin clasp that are
susceptible to fracture.
• Occlusal rest is placed away
from the edentulous space so
does not protect marginal
gingiva.
• Poor clasp esthetically.

Multiple Circlet Clasp:
• it involves 2 simple circlet clasps
joined at their terminal end of
their reciprocal elements.
• Used for multiple clasping in
instances in which the partial
denture replaces an entire half of
dental arch.
• Form of splinting weakened
teeth by a RPD.

Embrasure Clasp/ Modified crib clasp:
•

Two simple circlet clasps
joined a their bodies.

•

Used on the side of the arch
where there is no edentulous
space, used only when space
is gained from opposing occlusion.


Disadvantages:• Requires removal of increased tooth structure and
occlusal rest preparation
• Breakage is more common
• Requires abutment protection in most of the cases
by using inlays or crown since vulnerable areas of
tooth are involved.


Ring Clasp
•

Engages the undercut after
encircling almost the entire tooth
from its point of origin. Indicated in
cases of tipped molars, mandibular
teeth tilted mesio-lingually and
maxillary teeth tilted mesio-buccally

•

In case of mandibular molars,
ring clasp begins on mesio-buccal
surface and terminates in infra-bulge
area on mesio-lingual surface, it
requires additional support with an
auxiliary bracing arm.

Ring Clasp

• If required an additional occlusal
rest on disto-occlusal surface is
given which provides additional
support for the prosthesis.


Disadvantage
• It alters the contour of tooth and interferes with
the normal stimulation of surrounding mucosa.
• It is susceptible to distortion.
Contraindications:1) Limited vestibular depth.
2) When bracing arm must cross soft tissue undercut.

C, Fish Hook, Hairpin, Reverse Action:
•

A simple circlet clasp in which
the retentive arm loops back to
engage an undercut apical to the point
of origin.

• Upper part of retainer arm is rigid and
lower is tapered and flexible.
• Indicated when retainer clasp must
engage an undercut adjacent to the
occlusal rest or edentulous space.

Disadvantages
• It should have a sufficient vertical height.
• Adequate space is required between the occlusal &
apical aspect of the retentive arm to provide
finishing of metal & to avoid entrapment of food
debris.
• Esthetically unacceptable if used on premolar &
canine.
• There should be no interference with the opposing
arch in maximum intercuspation.

Onlay Clasp:
• It consists of a rest that covers the
entire occlusal surface & serves as the
origin for the buccal & lingual arms.
• It is indicated when occlusal surface of
abutment tooth is below the occlusal
plane, as a result of tooth, rotated or
tipped. Onlay is used to restore normal
occlusal plane.
• Should be used only in caries resistant
mouth unless tooth is covered by
acrylic or gold crown.

Half and Half Clasp:
• Consists a circumferential retentive
arm arising from one direction & a
reciprocal arm arising from the other.
• The design was originally intended to
provide dual retention, a principle that
should be applied only to unilateral
partial denture design.


Combination Clasp:
• Introduced by Dr O. C Applegate in
1965.
• Consists of an occlusal rest, a cast
reciprocal clasp arm, and a wrought
wire retentive arm.
• Indicated in distal extention cases with
undercut on mesio-buccal surface..


Advantages:– Flexibility
– Better esthetically because it can be placed in
gingival 3rd
– It can be adjusted in all planes
– Because of minimum contact it can be used in
caries prone mouths


Disadvantages:• Extra steps for fabrication
• No bracing or stabilization qualities because of
increased flexibility
• Prone to breakage or damage when the patient
mishandles it.


BAR CLASP/ VERTICAL PROJECTION
CLASP / ROACH CLASP/ INFRABULGE
CLASP
• Introduced in early 1900, but
received attention in 1930 by
DR F. Ewing Roach.
• It approaches the undercut on
the tooth from gingival
direction resulting in Push type
of retention.


Rules for use:•

•
•
•
•
•

The Approach arm must not impinge on the soft
tissue,no relief should be given under the approach arm,
tissue surface of the approach arm should be smooth & well
polished
The Approach arm should cross perpendicular to free
gingival margin.
The Approach arm should never be designed under the soft
tissue undercut.
The approach arm should be uniformly tapered from its
point of origin to the terminus.
The terminus should be positioned as apical as possible.
The minor connector that connects the Rest should be rigid
& stabilize the prosthesis.

Indications:• Retentive undercut adjacent to edentulous area.
• When small degree of (0.01”) undercut exists in
cervical 3rd
• If buccal sulcus is more than 4mm in depth
• Tooth supported cases in the anterior region of
the mouth.

Contraindications:
• Shallow vestibule
• Severe tooth and/or tissue
undercut
• Excessive buccal or lingual tilt
of abutment teeth


Advantages:

• Better retention because increase length of
retentive arm and trip action.
• Better esthetics because of gingival approach


Disadvantages:• Greater tendency to collect and hold food debris
• Reduced bracing and stabilization.


T-Clasp
• It derives its name from the shape
created where the retentive clasp
arm joins the vertical aspect of the
approach arm.
• It is indicated in distal extention
cases with undercut on distobuccal surface and tooth-supported
cases adjacent to edentulous spaces
by using natural undercuts called
Clasping for convenience.

Contraindications
• If soft tissue undercuts exists.
• When the height o contour is located on the
occlusal 3rd.


Modified-T / L Clasp:
• It’s a T clasp that lacks
nonretentive horizontal
projection.
• It is used on canines or
premolar for esthetic
reason.


Y-Clasp
•

A ‘Y’ clasp is formed
when the approach arm
terminates in the cervical 3rd of
the abutment, while the mesial
& distal projections are
positioned near the occlusal/
incisal 3rd.


I-Bar
• Only contact of retentive clasp with
abutment tooth is the tip of the
clasp, which is circular or oval.
• It rarely used on disto-buccal
surface of maxillary canine for
esthetic reason.
• Advantage includes better esthetics,
minimum interference with natural
tooth contours.
• Disadvantage being problems with
stabilization.

RPI – Concept
•

krol in 1973 introduced it.It consists of Mesial
Rest, Proximal plate and I- Bar.

• Mesial rest extends only in the triangular fossa, even
in a molar preparation, circular concave depression
on mesial marginal ridge of the canine.
• The proximal plate contacts only 1mm of guide plane
which is only 2-3mm high occlusogingivally.
• I bar terminus is a pod shaped to allow more tooth
contact.

Modifications of RPI System:
RPA
•

Similar to RPI but instead of I
bar, Akers or circumferential clasp
retentive arm is fabricated.


Advantages
• Easier to grasp for removal of prosthesis .
• It can be used in case of large tissue
undercuts, high frenum etc.
• Indicated when bar clasp is contraindicated
and desirable undercuts is located in gingival
3rd of tooth away from distal extention area.

R L S – System by Aviv L. et al.
It consist of
• Mesio-occlusal rest
• Disto-lingual L-bar It is the direct connector,
located adjacent to edentulous ridge.
• Disto-buccal – stabilizer, It disengages from the
tooth as denture base moves tissue wards.

Comparison of Circumferential and Bar
clasps:
Circumferential
clasp

Bar clasp

Retention

Pull type

Push type

Bracing

Greater

Average

Caries
susceptibility

More

Less

Gingival health

Better

Poor

Aesthetics

Poor

Excellent

Tolerance

Well

Poor

Compactness

More

Less

Other Clasp Systems:


Clasps with Splinting Action:

1) Extended arm clasp: made
of Cr-Co alloy It can be
used for splinting maxillary
teeth


Clasps utilizing proximal undercuts:
Saddle lock hidden clasp partial
dentures.
•

It uses only proximal walls of
abutment teeth for retention and
uses infrabulge clasp which
approaches the retention area
from gingival aspect.


Advantages:
– Esthetics
– Increased retention because of trip action
– Can be used when buccal or lingual survey line
unfavourable
– Compact design prevents dislodgement


RECENT ADVANCES:


Esthetic clasp
The various methods used to mask the metallic direct
retainer are as follows.
Macromechanical retention: Retentive beads
and meshwork have been used to retain facing of
either acrylic or composite resin.


Disadvantages
• Bulk that is created by adding the veneer will
enlarge the total size of the clasp thus defeating
the purpose of disguising the clasp,
• Bonding is unreliable,
• GAP formation and microleakage when used in
combination with composites.

• Micromechanical retention: It involves air borne
particle abrasion. This helps to improve retention
between the alloy and the resin.
This new technique involves coating the metal
with silica intermediate layer (SiOx-C) that bonds
to metal and also supplies the -OH group for
silane bonding.
Disadvantages
•

• Bond strengths obtained after the use of
micromechanical systems are insufficient
especially after thermal conditioning.

Tooth coloured occlusal
approaching polymethylene
clasps are alternative to metal
clasps


The Twin-Flex clasp
• This consists of a wire clasp soldered
into a channel that is cast in the
major connector.
• This clasp is flexible, it does not
generate as much as torque when the
distal extension is depressed.
• The ability to adjust this clasp and its
conventional path of insertion
provides an excellent design option
for retention to an adjacent
edentulous segment

Disadvantages
• There is extra thickness of major connector over
the wire clasp,
• an extra laboratory step incurs extra cost,
• Difficulty in repairing the clasp if breakage
occurs


Conclusion:
• How to clasp a tooth depends on survey line, root
size and form and clinical condition of supporting
structures.
• The choice of the clasp design should be based on
biologic as well as mechanical principles.
•

The dentist responsible for the treatment
rendered must be able to justify the clasp design
used for each abutment tooth in keeping with these
principles.

Bibliography
• Mc Cracken’s –Removable partial prosthodontics.
• Stewart’s – Clinical Removable partial prosthodonticsthird edition
• Osborne & Lammie’s – Removable prosthodontics
• The Twin-flex Clasp: An esthetic alternative. J Prosthet
Dent 1997;77:450-2.
• The use of chairside silica for different dental application:
A clinical report. J Prosthet Dent 2002;87:467-72.

• Eliason, C. RPA Clasp Design for Distal
Extension Removable Partial Dentures. J Prosthet
Dent 49:25,1983
• Aviv, I. et al. RLS-The Lingually Retained Clasp
Assembly for Distal Extension Removable Partial
Dentures. Quint Int. 21:221-223,1990.
• Cherkas L. and Jaslow E. Saddle lock hidden clasp
partial dentures. Comp Contin Educ Dent 12:746752, 1991.
• Direct retainers: Esthetic solutions in the smile
zone : JIPS 2005: 5: 1 4-9

• J. C. Davenport Clasp design, BDJ 190, no. 2,
january 27 2001


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