The extracoronal or clasp direct retainer is used more frequently than the internal attachments and consists of two basic types:
A clasp arm or tip that ends on a cervical cone will resist movement in a occlusal direction because to release from the tooth it would be forced to undergo deformation.
These factors are critical to the health and longevity of abutments.
Preservation of such teeth as abutments would necessitate the use of bar clasps with increased length of clasp arm of round cross-section, of type IV casting gold alloy or wrought gold wires.
Wrought wire clasp omnidirection movement so maximum dissipation of forces. Decresed tooth contact. Can placed in deeper undercut so esthetic in nature. Difficult lab procedure and increase sussptible to damage. Used in mesiogingival undercut and large tissue undercut prevent giving bar clasp. Adjustable but can be distorted if mishandleled by patient.
Included the features of both bar and c clasp. Simultanoeus contact of teeth seen
Point of force applicarion iw lower than the odlusal rest.
Prosthesis design should be kept as simple as the clinical situation permits. Unnecessarily complicated designs may be avoided by producing improved abutment contours. In many instances, this may be accomplished by carefully recontouring the surfaces of an abutment. In other instances, it may require the placement of one or more fixed restorations. Although the latter approach will result in added expense for the patient, the simplification in design can significantly improve the comfort, function, and longevity of the prosthesis.
Direct retainers in removable partial dentures
Dr Shebin Abraham
Dept. of Prosthodontics
Basic part of clasp assembly.
Analysis of tooth contour.
Basic Principles of clasp design.
Cast circumferential clasp.
Other types of clasp.
Types of retention
1. By clasps
2. Acting through
polished surface of the
3. Tissue coverage.
“That component of a partial removable dental
prosthesis used to retain and prevent dislodgment,
consisting of a clasp assembly or precision
Direct retention :
“Retention obtained in a partial removable dental
prosthesis by the use of clasps or attachments that
resist removal from the abutment teeth” –GPT-8
Classification Of Direct Retainers.
Clinical removable prosthodontics:- STEWART’S 3rd edition
Intra coronal Extra coronal
Proposed by Dr Herman E S Chayes in 1906.
Cast or attached within the contours of natural teeth(abutment).
Keyway and key…..Opposing vertical walls provides retention.
Retention is achieved by frictional resistance.
1. Precision attachment manufactured by high precision technique and
2. Semi precision attachment less intimate contact between matrix and
matrix component. Unlike precision attachment they consist of
tapering walls and are casted from wax or plastic pattern.
First proposed by Henry H Boos 1900 later modified by
Ewing F Roach in 1908.
Located outside the teeth.
Retention from mechanical resistance.
Permit vertical movement during vertical loading.
Minimize potentially damaging forces to abutment Stress
breaking or stress directing effects.
Occlusally approaching /
Suprabulge / Ney Type I
clasp / Circumferential:
• Approaches the tooth
undercut from an occlusal
• It is attached to metal
framework above the
height of contour.
Gingivally approaching /
Infrabulge/ Bar/ Roach / Ney
Type II Clasp:
• The retentive arm originates
from the metal base or
traverses soft tissue and
• Approaches the tooth
undercut area from a
The basic parts of a clasp
assembly include the following:
It is the part of the clasp that lies on the occlusal, lingual or incisal surface
of a tooth and resist (tissue ward) movement of the clasp by ensuring
that the retentive terminals of the clasp remain fixed in the desired or
planned depth of undercut.
Body of the clasp
It is the part of the clasp that connects the rest and shoulder of the clasp
to the minor connector.
It must be rigid.
Above the height of contour.
It is the part of the clasp that connects the body to the clasp
The shoulder must lie above the height of contour and provide
some stabilization against horizontal displacement of the
A rigid clasp arm placed above the height of contour on the
side of the tooth, opposing the retentive clasp arm.
It is the part of the clasp comprising the shoulder which is not flexible and is located
above the height of the contour
It is the terminal end of the retentive clasp arm. It is the only component of the
removable partial denture that lies on the tooth surface cervical to the height of the
contour. It possesses a certain degree of flexibility and offers the property of direct
It is a component of the bar clasp.
It is a minor connector that projects from the
framework, runs along the mucosa and turns to cross
the gingival margin of the abutment tooth; to
approach the undercut from a gingival direction.
Analysis of tooth contour:-
Before moving on to principles of clasp design its vitally
important to consider how tooth contour & RPD components
must be related to allow a stable prosthetic function.
What is path of insertion??? path of removal ??and height of
Point of maximum convexity or the term height of contour
Dr Edward Kennedy in 1985.
This critical area of an abutment that provide for retention &
stabilization can only be identified with the use of dental cast
Proposed “cone theory” of clinical
crown in 1916.
Provided conceptual basis of mechanical
Contours of clinical crown resembles two
cones sharing a common base.
The line formed at the junction of this
base represents the greatest diameter of
This greatest diameter is called height of
contour or point of maximum convexity.
Devan  referred to the
surface occlusal to the height of
contour as suprabulge, & the
surface inclining cervically as
o When the surveyor blade contacts a tooth on the cast at its
greatest convexity, a triangle is formed.
o The apex of this triangle is at the point of contact of surveyor
blade with the tooth and base is towards the gingival tissues.
o This apical angle is called angle of cervical convergence. The
importance of this angle lies in its relationship to the amount
According to Stewart basic principles are:-
“Retention is the inherent quality of the clasp
assembly that resists forces acting to dislodge
components away from the tooth structure.”
No single component of a clasp assembly is solely responsible
for prosthesis retention.
Rather, it is effective design and accurate construction that
make the removable partial denture retentive.
The amount of retention designed into a removable partial
denture should always be the minimum necessary to resist
reasonable dislodging forces.
A rigid clasp flexing over the height of contour may transfer
harmful stresses to an abutment during insertion, removal, and
functional movement of the prosthesis.
An only a minimum area of contact should be seen.
Amount of retention
Factors that effect retention can be divided into -
1. Tooth factors
Size of angle of cervical convergence
How far clasp is placed in angle of cervical convergence.
2. Prosthesis factors
Clasp cross-sectional form[ shape]
Material used for making clasp[ alloy]
Length of clasp arm-
Longer the clasp arm the more flexible it will be.
Circumferential clasps more retentive than bar clasp for a given clasp length.
The clasp arm should taper from the point of origin to provide its flexibility.
Diameter of the clasp:
The greater the diameter of a clasp arm the less flexible it
will be.(only in uniform taper)
If its taper is absolutely uniform ,the avg diameter will be at
a point midway between its origin & its terminal end.
But if taper is not uniform a point of flexure –therefore a
point of weakness will exist.
Flexibility may exist in any form, but is limited to only one direction in the case
of the half-round form
The only universally flexible form is the round form.
Clasp arm should only flex away from tooth so half round is used.
Round shaped clasp arm used only in distal extension denture bases so that it
can flex in all directions during functional movement and minimize stresses.
Material used for construction:
Gold alloy greater flexibility than chrome alloys ,
Disadvantage of cast gold alloys bulk of the prosthesis
Chrome alloys have a higher modulus of elasticity & therefore
So in less undercut areas CoCr alloy can be used but in cases
of deep undercut wrought metal can be used.
Relative uniformity of
1. Retention on all principal abutments should be as equal as
2. Retentive clasp arms should be located so that they lie in the
same approx.. Degree of undercut on each abutment tooth.
3. Retentive clasp positioning should also be same on the
contralateral side of arch.(atleast for one teeth).
“Support is the quality of a clasp assembly that resists
displacement of a prosthesis in an apical direction.”
Provided by occlusal rest.
1. A properly prepared rest seat and corresponding rest serve to resist
displacement of the prosthesis toward the supporting teeth and soft
tissues, thereby ensuring that the clasp assembly maintains its intended
relation to the abutment, and
2. Transmit functional forces parallel to the long axes of the abutments.
“Stability is the quality of a clasp assembly that resists
displacement of a prosthesis in a horizontal direction.”
It helps the denture be steady constant firm and resist
displacement due to function stresses and also prevent
change in position of the denture.
It is provided by :-
1. Reciprocal element.
2. The shoulder(s) of a cast circumferential retentive clasp.
3. Vertically oriented minor connectors.
“Reciprocation is the quality of a clasp assembly that counteracts lateral
displacement of an abutment when the retentive clasp terminus passes
over the height of contour.”
As the retentive arm passes over the height of contour it flexes creating
lateral forces damage to the tooth.
The reciprocal element may be a
1. Retentive arm of clasp
2. Lingual plating,
3. Combination of mesial and distal minor connectors.
Points to be remembered while
1. To optimize reciprocation, the axial surface of an abutment
should be prepared parallel to the path of insertion and removal.
2. It should be placed above the height of contour.
3. To provide true reciprocation, the reciprocal clasp arm must be
in contact during the entire period of retentive clasp
“Encirclement is the characteristic of a clasp assembly that prevents
movement of an abutment away from the associated clasp assembly”.
Clasp assembly 180 degrees contact.
The engagement can be in the form of continuous contact, such
as circumferential clasp, or discontinuous contact, such as Bar
Both provide tooth contact in at least 3 areas encircling the
“Passivity is the quality of a clasp assembly that
prevents the transmission of adverse forces to the
associated abutment when the prosthesis is
When fully seated it should be passive.
Should be activated only when dislodging forces are applied.
1. Retentive clasp element should always be placed on facial surface of
tooth.(mainly in premolars)
2. Only one retentive element should be used per tooth opposed by
a reciprocal element.
3. Clasp retainers on the abutment adjacent to distal extension bases
should be designed so they will avoid direct transmission of tipping &
rotational forces to the abutment.
4. The path of escapement for each retentive clasp terminal must be other
than parallel to the path of removal for the prosthesis to require clasp
engagement with resistance to deformation.
CRITERIA FOR CLASP SELECTION
Requirements of retention and stability depending
on the number, configuration of edentulous areas.
Nature of support
Root size and form
Presence of excessive tissue undercut
Oral hygiene and patient awareness
First proposed by Dr N B Nebbit. Later
modified by Dr Polk E Aker.
Simple and easy to construct
Large amount of tooth surface that is
covered by the clasp assembly leads to
1. The clasp should arise from the main body of the clasp
assembly above the height of contour. The retentive arm
should extend cervically and circumferentially in a gently
2. All the components of the C clasp should be present above
the height of contour except the retentive tip.
3. The retentive terminus should always be directed towards
the occlusal surface never towards the gingiva.
4. It should always terminate at the mesial or distal line angle
never at midfacial or midlingual surface.
5. The retentive arm should be positioned as far apically on the
abutment as is practical.(not impinging the gingiva)
6. Special considerations in case of distal extension case:-
A cast circumferential clasp should not be used to engage (a) the
mesiofacial surface of an abutment adjacent to a posterior edentulous
the distofacial surface of an abutment adjacent to an extensive anterior
Simple circlet clasp.
It is one of the most commonly used
clasps retentive and stabilizing
The basic design consist of
1. buccal retentive arm and
2. a lingual reciprocal arm originating
from a common body.
3. Occlusal rest.
With retentive arm projecting away
from the edentulous space.
1. Fulfils the design requirements of support, stability,
reciprocation, encirclement, and passivity.
2. Its uncomplicated design features make it easy to construct and
relatively simple to repair.
1. If used in distal extension base due to fulcrum rotation of the
clasp it can cause damage to teeth.
2. Can increase the circumference of teeth and lead to food
accumulation and decalcification.
Reverse circlet clasp
Used when undercut is located at the facial distoangle adjacent to an
It consists of a mesial occlusal rest, a horizontal reciprocal arm, and a
retentive arm engaging the distobuccal undercut adjacent to the
Decreases the harmful stresses to teeth.
Week clasp if sufficient preparation is not done.
Poor aesthetics' if used in premolars and cuspids.
Multiple circlet clasp:
A multiple circlet clasp design involves two simple circlet
clasps joined at the terminal aspects of their reciprocal
Used in periodontally weekend teeth to splint them
Disadvantages are same as circlet clasp.
Also known as the Bonwill clasp.
fabrication of unmodified Class II
or Class III partial denture
when there are no edentulous
spaces available on the opposite
side of the arch to aid in clasping.
Double occlusal rests, two retentive clasp arms and the two
reciprocal clasp arms either bilaterally or diagonally opposed.
Prevent interproximal wedging by the prosthesis, which
could cause separation of the abutment tooth and result in
food impaction and clasp displacement.
In addition to providing support, occlusal rests also serve to
shunt food away from the contact area.
Improper clearance can give rise thin section of clasp leading
Circumferential clasp encircles nearly
all of the tooth from its point of origin.
It is usually used when a proximal
undercut cannot be approached by any
In case of tilted molars
A support strut is provided on the non
excellent retention with
adequate flexibility due to
increased length of clasp arm
Decalcification of teeth
Increased occlusal table.
Poor structure of clasp.
Hairpin clasp or reverse action or
fish hook clasp
A simple circlet clasp in which the retentive
arm loops back to engage an undercut apical
to the point of origin.
Used when a distofacial undercut is present
adjacent to the edentulous space.
Retentive arm has two horizontal
1. The occlusal portion minor connector and
must be rigid.
2. The apical portion pass over the height of
contour to engage the desired undercut.
Sufficient clinical crown height.
Space between occlusal and apical arm.
Occlusal arm shouldn’t interfere within the occlusion
This clasp is indicated when the
1. soft tissue contour precludes use of a bar-type clasp and
2. when the reverse circlet cannot be considered because
of a lack of occlusal clearance.
Half and half clasp
It consists of a circumferential retentive arm arising from one direction
and a reciprocal arm arising from another.
used only for unilateral partial denture design.
Given by O C Applegate.
This type of clasp consists of a wrought wire retentive clasp arm and a
cast reciprocal clasp arm.
Wrought retentive arm is circular in cross section.
Used in deep undercut case.
Indicated when the occlusal surface of the abutment
lies noticeably apical to the occlusal plane.
occlusal surfaces of the abutments are covered with
crowns clasp arms arises
It establishes the occlusal plane.
Indicated in caries free individual.
Occlusal surface should be restored with gold acrylic
Popularized by Ewing Roach in 1930
called it the Bar Clasp.
An infrabulge clasp approaches the
undercut region of an abutment from an
Push type retention.
Flexibility of clasp from length and taper.
More aesthetic than c clasp.
Ex :- y clasp, t clasp, I clasp.
1. The approach arm of an infrabulge clasp must not impinge on
the soft tissues adjacent to the abutment.
2. The approach arm should cross perpendicular to the free
gingival margin. It shouldn’t impinge the underlying gingiva.
3. Shouldn’t be used in area of tissue undercut.
4. Uniform length and adequate taper should be given for
5. The clasp terminus tip should be placed as apical as possible
on the abutment teeth.
Name is from the shape of the retentive
Used in class 1 and class 2 situation.
distofacial undercut is seen.
The retentive terminal consist of horizontal
two projection the one on the distal side
engages the undercut and the one on the
mesial side is above the height of contour.
T clasp is contraindicated when the height of
contour is at the occlusal one thirds.
In modified t clasp the non retentive arm is
Y clasp is similar to t clasp with the approach
arm ending cervical to the retentive arm.
I clasp lack the horizontal retentive arms but
only a horizontal retentive tip.
Only the retentive tip contacts the abutment
surface only at the undercut region.
The amount of contact is about 2 to 3mm in
height and 1.5 mm in width.
Other types of clasp philosophies.
VRHR clasp or Grasso clasp
RPI stands for Rest Proximal plate I bar.
Introduced by Kratochvil in 1963 it consisted of three
different parts connected to the metal framework.
Mesial occlusal rest, a distal guide plan, and an I bar
The guide plan contacts the full length of the of the
proximal surface of the tooth.
This design had certain basic disadvantages:
Physiologic relief was required to prevent impingement
of gingival tissues during function.
Since the proximal plate covers a greater surface area
of the tooth, the functional forces are directed in the
horizontal direction, thus the tooth is located more
than the edentulous ridge.
Krol in 1973 made certain modifications in the design
under the “minimal coverage criteria”
Rest preparations are less extensive in the RPI
Rests extend only into the triangular fossa, even in
molar preparations, and canine rest
2-3mm guide plane in which only 1mm contact was
seen from the guide plate.
Arthur Krol JPD 1973;23;408-415
BASIC PRINCIPLES OF RPI
The mesiobuccal rest with the minor connector is placed
into the mesiolingual embrasure, but not contacting the
A distal guiding plane, extending from the marginal ridge
to the junction of the middle and gingival thirds of the
abutment tooth, is prepared to receive a proximal plate
The proximal plate in conjunction with the mesial occlusal
rest and minor connector provides the stabilizing and
reciprocal aspects of the clasp assembly.
The I-bar contributes to the retentive aspect and should be
located in the gingival third of the buccal or labial surface of
the abutment in 0.01 inch undercut.
Three different approaches
to RPI clasp
These approaches are based on the location of
proximal plate ,location of the I bar
The rest-proximal plate-Aker’s clasp was developed and
described by Eliason in 1983.
It consists of a mesial occlusal rest, proximal plate and a
circumferential clasp arm, which arises from the superior
portion of the proximal plate and extends around the
tooth to engage the mesial undercut.
De VAN CLASP
Also called as the mirror view clasp
Two occlusal rest on the lingual side of the teeth.
M.M.DeVan JPD 1955;5,208-14
The vertical reciprocal horizontal retentive arm concept
was developed by Grasso in 1980 and is characterized by:
A distal occlusal rest supported by a minor connector.
A lingual vertical reciprocal component originating from the major
A horizontal retentive arm attached to either the major connector
or the retention latticework for the denture base.
Joseph Grasso JPD 1980,43;618-21
Proposed by J. J. Goodman in 1990, it is an esthetic retentive
concept for distal extension situations.
Rests are placed away from edentulous span. Vertical inter-proximal
reduction of 1 mm between abutment and adjacent
tooth is done.
Optional bucco-lingual retentive groove at mid and gingival
third junction on distal surface of abutment tooth is provided.
Quintessence Int. 1996
Occlusally and gingivally
Relative merits and demerits
The bar clasp approaches the
undercut from below the height
of contour pushes towards the
occlusal surface abutment tooth
• Easier to seat and more
difficult to remove
• Above the height of contour
• Pulls towards the occlusal
surface from the undercut to
It is more flexible because of
which it provides less bracing or
stability against lateral stresses.
• Because of its rigidity it
provides very good stability or
Stress breaking effect
It allows a certain degree of
functional movement of the distal
extension base dissipate the
stresses and lessen the load on
Potential to torque abutment
teeth in distal extension based
partial denture situations.
Contact with tooth
Minimal tooth contact and
less damage to tooth
More tooth contact leading
to food accumulation
Damage to oral tissues
Minimum relief can lead to
tissue damage to mucosa
under the approach arm
Damage to gingiva can take
place during improper
removal of clasp
Very less metal display so
Increased metal display so
The intracoronal retainer is usually regarded as an internal
attachment or precision attachment.
“A retainer consisting of a metal receptacle (matrix) and a
closely fitting part (patrix); the matrix is usually contained within
the normal or expanded contours of the crown on the abutment
tooth and the patrix is attached to a pontic or the removable
partial denture framework.”
To provide movable joint in Removable Bridgework, fixed
To stabilize unilateral saddles.
Titled molars. F.P.D's in severely misaligned abutments.
Use in Over dentures (different forms of retainer are bar,
telescopic, use of auxillary attachments).
Fixed removable implant restorations.
1. Large pulp size which is usually related to the age of
2. Length of the clinical crown, not used in short or
abraded teeth.(6mm crown)
4. Distal extension denture bases.
1. Esthetically acceptable, because not much of metal display
like extracoronal retainers.
2. It is preferred in many of the situation because of its
vertical support through a rest seat located more favorable
to the horizontal axis of the abutment tooth.
3. Stimulation to the underlying tissues greater when internal
attachment are used because of the intermittent vertical
4. It permits proper tooth form to be maintained and allows
for control of vertical, mesiodistal and buccolingual
displacement of the prosthesis.
5. It provides for excellent retention of the prosthesis on
account of frictional resistance between opposing
parallel vertical walls that serve to limit movement and
resist removal of the partial denture.
Disadvantages of intra coronal
1. They require preparation of abutment tooth and
2. Difficult clinical and laboratory procedure.
3. They eventually wear, result in loss of frictional
resistance to denture removal.
4. Difficult to repair and replace.
5. They are effective in longer teeth and least effective
in shorter teeth.
6. Difficult to place completely in the abutment teeth.
MATERIAL OF CHOICE
Pt, Iridoplatinum, Gold and Pt, Gold and Pd.
Type III and IV type of gold is to be used for crown
Base metal alloys are also used now a days as low
2 alternate ways to construct crown and rest seat.
Rest seat may be cast against the full coverage restoration
Rest seat may be soldered into place.
Classification by Good Kind and Baker in 1976 :
b. non resilient
Tapered and parallel-walled boxed and tubes.
Adjustable metal plates
How To Choose An Attachment?
It is the length of attachment, not its width that is main criteria in
For each length there are 3 different sizes (width) of precision attachments
(anterior, bicuspid, molar.)
Width is measured from one side of rest to other.
Full length of a precision attachment is 8 mm for full benefit of bracing,
support and retention a minimal of 5mm height is must.
This means that the clinical length of crown must be at least 7 mm so as to
accommodate an attachment of 5mm and in addition a minimum of 2mm
between the gingival floor of attachment and gingival margin.
Otherwise a periodontal problem may be created.
Kennedy’s class I and class II partially edentulous
The most difficult type of treatment plan.
Some practitioners advocate non rigid and resilient attachments and some advocate
resilient attachment in distal extension to minimize rotation and torquing of the
abutment tooth, when the components of an attachment are rigidly connected.
Int J Prosthodont. 1990 Mar-Apr;3(2):169-74
Another philosophy , known as the stable base precision attachment
RPD concept or floating denture base concept recommends
incorporation of rigid internal attachments and a cast metal base
made from mucostatic impression of the residual ridge.
The male portion of the attachment is connected to the denture
base , allowing the complete seating within the abutment.
Kennedy’s class III partially edentulous arch.
• Rigid internal attachments are recommended .
• Provides good retention, support and brazing because of its rigid
• If the posterior abutment prognosis is questionable then a resilient
type of attachments are recommended with anterior abutment.
Kennedy’s class IV partially
The ideal RPD design for such situation
involves the use of a tissue bar placed close to
the edentulous ridge and connected as a fixed
unit to the abutment teeth on either side of
the space using crowns.
Keep the prosthesis design as simple as possible…
Make RPD more comfortable, more efficient and
affordable to the patient.
1. Clinical removable prosthodontics:- STEWART’S 3rd edition
2. Mc cracken removable partial denture prosthodontics – 12th edition.
3. Davenport J.C., Baskar R.M., Heath J.R., Ralph J.P. “A color atlas of RPD”,
Wolfe Medical Publications Ltd., 1988.
4. Krol A.J. “Clasp design for extension base RPD”. J. Prosthet. Dent., 1973;
29 : 408-415.
5. M.M.DeVan JPD 1955;5,208-14
6. Joseph Grasso JPD 1980,43;618-21
7. Quintessence Int. 1996 May;27(5):333-40.
8. Burns DR, Ward JE. Int J Prosthodont. 1990 Mar-Apr;3(2):169-74