2. Direct retainers
It is defined as a clasp or attachment applied to an abutment tooth for the purpose of holding a
removable denture in position.
It is that component part of removable partial denture that is used to retain and prevent
dislodgement consisting of a clasp assembly or precision attachment. -GPT
Retention: that quality inherent in the prosthesis which resists the force of gravity, adhesiveness
of foods, and the forces associated with opening of the jaws. -GPT
Direct retention: retention obtained in a removable partial denture by use of clasps or
attachments which resist the removal from the abutment teeth. -GPT
3. Direct
Retainers
Intracoronal
Extracoronal
• Internal Attachment
• External Attachment
• Stud Attachment
• Bar Attachment
• Special Attachment
Manufactured
Custom Made
Occlusally
approaching
Gingivally
approaching
• Bar Clasp
• T Clasp
• Modified T clasp
• Y Clasp
• I Clasp
• Infrabulge Clasp
• I bar
Circumferential
Embrasure
Onlay
Half and half
Grasso’s Clasp
Continuous Clasp
• Simple Circlet
• Reverse Circlet
• Multiple Circlet
• Ring
• Fishhook
• Combination
4. Clasps- Definition and Introduction
Defined as “a part of a removable partial denture which acts as a direct retainer and/or stabilizer for
the denture by partially encircling or contacting an abutment tooth” -GPT
Also defined as “component of partial denture that rests against the vertical enamel surfaces of an
abutment tooth and aids in bracing and retention.”
•In 1916 Prothero proposed the Cone theory to explain the basis for clasp retention
•He described the shape of crown of premolars and molars to be equivalent to two cones sharing a
common base
•The upper cone resembles the occlusal half of the tooth and the lower cone resembles the cervical
half of the tooth
•A clasp tip that ends below the junction of the two cones will resist movement in the upward
direction because it has to deform to be released across the junction of the bases of the two cones
•The degree of resistance to deformation determines the amount of clasp retention.
5. Component Parts of a Clasp
•Retentive arm
•Reciprocal arm
•Shoulder
•Rest
•Minor connector
6. Retentive Arm
•Defined as “a flexible segment of a removable partial denture which engages an undercut on an
abutment and which is designed to retain the denture” -GPT
•It is that part of the clasp comprising of the retentive clasp arm and the retentive terminal
•The retentive clasp arm is not flexible and is located above the height of contour. The retentive
terminal is flexible and lies below the height of contour.
•Height of contour is defined as “a line encircling a tooth designating its greatest circumference at
a selected position” -GPT
•Kennedy named the greatest convexity of the tooth as the height of contour.
•Cummer called it as the guideline that helps in the placement of the components of the clasp.
•DeVan named the surfaces sloping superiorly as the suprabulge area and the surfaces sloping
inferiorly as the infrabulge area.
7. Reciprocal arm
•A clasp arm or other extension used on a removable partial denture to oppose the action of some
other part or parts of the prosthesis” -GPT
•It is located on the side of the tooth opposite to the retentive arm.
•It resists the lateral forces exerted by the retentive arm when it passes through the height of contour
during the placement and removal of the RPD.
•It is always placed in the suprabulge area. It may act as indirect retainer when placed on an abutment
located anterior to the fulcrum line of the partial denture.
•Thus the rigid reciprocal arm can resist the rocking of the denture base
General functions of reciprocal arm are as follows-
•Provides stability and reciprocation against retentive arm
•The denture is stabilized against horizontal movements
•Acts as an indirect retainer to a minor degree
8. Shoulder
•It is the part of the clasp that connects the body to the clasp terminals
•It lies above the height of contour and provides stabilization against horizontal displacement
•It is rigid and lies above the height of contour
Body
• Body of the clasp is designed such that it contacts the guide plane of the abutment
during insertion and removal
• The tissue surface of the body of clasp which is closely related to the guide planes is
known as a proximal plate of the direct retainer.
9. Rest
•A rigid stabilizing extension of a removable partial denture which contacts a remaining tooth or
teeth to dissipate vertical or horizontal forces -GPT
•It is the part of the clasp that lies on the occlusal or lingual or incisal edge or surfaces of the
tooth
•It resists tissueward movement of the clasp by acting like a vertical stop
•Rest also helps to maintain the retentive arm of the clasp in position.
10. Minor Connector
•It is defined as “the connecting link between a major connector or base of removable partial
denture and other units of the prosthesis, such as clasps, indirect retainers and occlusal rests”
•Here, it joins the clasp with the remaining part of the metal framework
•In gingivally approaching clasp its also known as the approach arm.
11. Principles of Clasp Design
The basic principle of clasp design is encirclement i.e to obtain more than 180
degrees of continuous contact for Aker’s clasp and a minimum of 3-point contact
for Roach clasp.
Other principles include-
•Occlusal rest should be designed to prevent tissue ward displacement of the
denture
•Each retentive terminal should be opposed by a reciprocal component
•Balanced retention should be present i.e if a buccal retentive clasp is present on
one side, the same should be present in the opposite side and vice versa
•Path of escapement should never coincide with the path of removal
12. Principles of Clasp Design
•Only the minimum necessary amount of retention should be used
•Primary abutment clasp of a distal extension denture base should never exert
tipping forces on the abutment
•Its preferable to place the reciprocal elements at the height of contour and
retentive elements below the height of contour
13. Functional Requirements of a clasp
Functional requirements of a clasp include-
•Retention
•Stability
•Support
•Reciprocation
•Encirclement
•Passivity
•Aesthetics
14. Retention
•Retention is that quality inherent in the prosthesis which resists the forces of gravity, adhesiveness of foods, and
the forces associated with the opening of jaws -GPT
•Retention is the most important function of the clasp hence it’s the most important functional requirement.
•Retention is provided by the retentive arm of the clasp. The tip of the retentive arm (retentive terminal) should lie
in the undercut to the selected path of insertion.
•The undercut used for retention is known as the retentive undercut or preferred undercut.
The retentive arm should be fabricated according to the following design considerations-
•The retentive arm of the clasp provides retention. The terminal third of it is flexible and it engages the undercut.
Middle third has minimum flexibility. Proximal third is rigid and is located above the height of contour.
•The location and degree of tooth undercut available for retention is relative to the path of insertion of the partial
denture.
•A clasp has rigid and flexible components. The rigid components should be placed in the non retentive areas of
the tooth for a given path of insertion.
•The clasp design for each abutment must be separately considered.
15. Retention
The retentive terminal is forced to deform when a vertical dislodging force is applied
The retentive terminal exhibits a certain amount of resistance to deformation which is proportional to flexibility
of clasp arm.
It is the resistance to deformation that generates retention. The flexibility of clasp varies with the type of alloy
being used-
•The retentive undercut will be present only in relation to a given path of insertion. It is absent in conditions where
the direction of dislodgment of clasp arm is similar to the direction along which the clasp was inserted. Hence a
single path of insertion that does not coincide with the path of displacement must be maintained.
•Retentive undercuts should be located with the help of a surveyor. The following factors should be considered
while determining the path of insertion-
1. Tissue undercuts
2. Location of vertical minor connectors
3. Origin of bar clasp
4. Denture bases
16. Retention
•Guiding planes are defined as “two or more vertically parallel surfaces of abutment teeth so oriented as to direct
the path of placement and removal of partial dentures” -GPT
•Guiding planes should be as parallel as possible to the path of insertion of the denture. If they are not, trauma to
the teeth and supporting structures may occur during insertion and removal of denture.
•This leads to periodontal breakdown of abutment teeth and strain to the parts of denture
•In the absence of guiding planes, the retention from the clasp will be meagre or practically non existent.
Factors affecting retention-
•Depth of the undercut: Deeper the undercut, greater is the retention. A retentive undercut has
three dimensions, namely-
1. buccolingual width of the undercut
2. distance between the survey line and the tip of the clasp arm
3. mesiodistal length of clasp arm below the height of contour
17. Retention
Factors affecting retention-
•Taper of the clasp arm: The clasp arm should taper uniformly from its origin to its tip.
2.The dimension at the tip should be half of that in the origin.
•Cross sectional form: A half round clasp arm is flexible only in one plane, whereas a round clasp
is flexible in all planes, however they are avoided due to difficulty in
fabrication.
2. A cast retentive arm is used mainly in tooth supported partial dentures
because they need to flex only during placement and removal.
3. In a Kennedy’s class 1 situation, the clasp has to flex during functional
movements too. A round clasp is preferred in these conditions
•Approach of the clasp arm: The gingivally approaching clasp arm has a better rentention as it pushes against the height
of contour during dislodgment
19. Stability
It is defined as,"The quality of a denture to be firm, steady, or
constant, to resist displacement by functional stresses, and not to be
subject to change of position when forces are applied"— GPT.
• All components of the clasp except the retentive arm provide
stability.
• Cast circumferential clasps offer greater stability because it has a
rigid shoulder.
• Wrought clasps have a flexible shoulder and bar clasps do not have
a shoulder; hence, they offer lesser stability
20. Support
It is defined as, 'To hold up or serve as a foundation or prop for—
GPT.
• It is the resistance to the movement of the denture in a gingival
direction (along the path of insertion).
Whereas retention is the resistance to the movement of the denture
against the path of insertion)
• It is provided by occlusal, lingual and incisal rests.
21. Reciprocation
•It is defined as, "The means by which one part of a prosthesis is made to counter the effect created by
another part"—GPT
•It is provided by a rigid reciprocal arm.
•It resists the stresses generated by the retentive arm. It also stabilizes the denture against horizontal
movement. In other words, it helps to hold the tooth when the retentive arm is active. If the reciprocal arm
is absent there will no resistance available for the action of the retentive arm
•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.
22. Encirclement
•"It is the property of the clasp assembly to encompass more
than 180° of the abutment tooth either by continuous or broken
contact to prevent dislodgement during function".
•Each clasp must encircle more the 180° of the abutment tooth.
•Encirclement 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
rest, Reciprocal arm
•This embracement prevents the clasp assembly from moving
out of the confines of the tooth during function.
23. Passivity
•It is defined as "The quality or condition of inactivity or rest assumed by the
teeth, tissues, and denture when a removable partial denture is in place but not
under masticatory pressure"—GPT.
•The retentive function should act only when dislodging forces are present.
•If the clasp is not seated properly, the retentive forces act continuously on the
tooth leading to pain and tenderness.
24. Types of Clasps
They are-
•Cast circumferential clasp: "A clasp that encircles a tooth by
more than 180°, 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 infrabulge
(gingival convergence) area”
•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 gingiva-occlusal direction“
•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"
25. Cast Circumferential Clasp
•They are popularly known as Aker's clasps. These clasps embrace more than half of the abutment
tooth. They may show a continuous or a limited three-point contact with the tooth.
• This architecture helps the clasp to hold the abutment firmly enough to prevent the rotation of the
denture.
•Advantages • Easiest clasp to make and repair.
• Less food retention
• Derives excellent support, bracing and retention.
•Disadvantages • It covers a large tooth surface area. It also alters the Buccolingual width of the crown.
This affects the normal food flow pattern leading to food accumulation. This causes
decalcification of the tooth structure.
• Difficult to adjust with pliers because of it's half-round 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.
26. Simple Circlet Clasp
• Most versatile and widely used.
•Best for tooth supported Partial dentures.
•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 (i.e.
buccolingually but not occluso-gingivally).
•They cannot be used for distal extension cases as
they engage a mesiobuccal undercut.
27. Reverse Circlet
•This clasp is used when the retentive undercut on the
abutment tooth is located adjacent to the edentulous space.
• Consider a distal edentulous condition. Usually the clasp
will arise from the distal surface of the abutment to reach
the mesial undercut. But this clasp is designed in such a way
that the clasp arises from the mesial side and ends on the
distal undercut.
•Usually Bar clasps are preferred for distal extension cases.
•These clasps are used when a bar clasp is contraindicated,
e.g.
- If there is an undercut area in the ridge
- Presence of a soft tissue undercut caused by bucco-version
of the abutment tooth.
.
28. Multiple Circlet
•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 side of the arch
when the principal 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
29. Embrasure Clasp
•It is a combination of two simple circlet clasps joined at tile body,
• It is used on the side of the arch where there is no edentulous space.
The clasp crosses the marginal ridges of two teeth to form the double
occlusal rest.
•The clasp emerges on the facial surface and splits into two retentive
arms. Each retentive arm engages the undercut located on the
opposite side of the tooth.
•Interproximal tooth structure should be removed to provide sufficient
thickness of the metal. The clasp may break if the metal is too thin.
• Indications: It is used in Kennedy's class II and class III cases without
any modifications.
• Occasionally, a very small edentulous space can be closed by a
modified embrasure clasp called pontic clasp
30. Ring Clasp
•Consider a distal edentulous condition with a distolingual undercut
where a reverse circlet clasp cannot be placed (no buccal
undercut).
•In such cases, the retentive arm is extended all around the tooth
from the distobuccal end to terminate in the distolingual undercut
across the mesial side of the tooth.
•It is used in cases with lingually tipped molar abutments.
•Mandibular molars usually tip mesiolingually and the maxillary
molars tip mesiobuccally. Hence, the retentive undercut will be on
the mesiolingual side for the lower molar and mesiobuccal side for
the upper molar.
•As the clasp is long, additional support should be provided by
adding an auxiliary bracing arm from the denture base minor
connector to the center of the ring clasp on the buccal surface
31. Fishhook Clasp/Hairpin Clasp
•It is a type of simple circlet clasp, which after crossing the facial
surface of the tooth loops back to engage the proximal
undercut beneath its point of origin. It is used in conditions
where the undercut is near the edentulous space. 136
•Upper arm is rigid and the lower arm is flexible. The upper arm
should be positioned above the height of contour in such a way
that it does not interfere with occlusion.
•Indications
The undercut is adjacent to edentulous area.
Presence of a soft tissue undercut.
•Disadvantages
It has poor aesthetics.
It tends to trap and accumulate food debris
32. Onlay Clasp
•It is an extension of a metal crown or onlay with buccal and
lingual clasp arms.
•It is used in the occlusal surfaces of submerged abutment
teeth (that are below the occlusal plane) so that the normal
occlusal plane can be restored with an onlay.
•If the onlay clasp is made of chrome alloy, the opposing tooth
should be protected with a gold crown, because the chrome
alloy can produce massive attrition of enamel.
• As this clasp covers large amount of tooth structure, it may
lead to breakdown of enamel surfaces. Hence, it should be
used only in a caries resistant mouth.
33. Combination Clasp
• A cast circumferential clasp cannot be used when an undercut is adjacent to the
edentulous space, as it will produce destructive rotatory forces on the distal
abutment.
• In such cases, a flexible wrought wire retentive arm is used to replace the rigid
cast alloy retentive arm. These clasps are called combination clasps as they
combine the two.
• As it has a greater flexibility it can be placed in a deeper undercut. It is used in
maxillary canines and premolars due to its superior aesthetics.
• Advantages:- The round configuration of the wrought wire gives two advantages
It has a thin line contact, which collects less debris and is easy to maintain.
-It can flex in all planes.
•Disadvantages :-Tedious lab procedures.
-Easily breaks or distorts.
-Poor stability.
34. Half and Half Clasp
•• It has a retentive arm arising from one direction
and a reciprocal arm arising from another.
• 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 auxilliary
occlusal rest.
• This design is intended to provide dual retention.
35. 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 aspect reduces its function.
- It has both biological and mechanical unsound principles.
36. Grasso’s Clasp
•Developed by Grasso, this clasp consists of a vertical reciprocal arm, an occlusal rest and a horizontal
retentive arm each arising separately from the major connector.
• It is more of a proposed concept.
•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.
- The balance between the retentive and reciprocal components prevents the
whiplash effect of the retentive arm.
•Disadvantages - Difficult to maintain as the block out zone between the base of the reciprocal arm
and the tooth tends to collect food debris.
37. Bar Clasp parts
Approach arm:
•It is a minor connector that connects the retentive tip to the denture base minor connector. It is semi
circular in cross section and should cross the gingival margin at a right angle.
•The approach should closely adapt over the soft tissues and cannot be fabricated over soft tissue
undercuts. This is the only flexible minor connector designed in a RPD.
Retentive terminal:
•It varies for each type of bar clasp. It should end on the surface of the tooth below the undercut.It can
be paired or singular.
•The terminal, which enters the undercut, is called retentive finger and the other terminal is called the
nonretentive finger. It helps to give a push type retentive force.
Advantages of bar clasps - It is easy to insert and difficult to remove.
- It is more aesthetic, as it covers less tooth structure.
Disadvantages of bar clasps - It tends to collect food debris
38. Bar Clasps Types
•T clasp
•Modified T clasp
•Y clasp
•I clasp
•Infrabulge clasp or Mirror view clasp
•I bar
39. T clasp and modified T clasp
T clasp:
•Used in a distal extension denture base with a distobuccal undercut
•Can also be used for tooth supported partial denture with natural
undercuts. Since the clasp is designed to use existing undercuts
without forming new ones, it is referred to as clasping for
convenience.
•It should not be used in a terminal abutment tooth if undercut is
located away from the edentulous area.
•Should not be placed over a soft tissue undercut
Modified T clasp:
•It is similar to T clasp but the non retentive finger of the T terminal
is removed
•It is used in canines and premolars for better aesthetics
40. Y clasp
Y clasp is basically a T clasp modified to suit certain abutments where height of contour is high
at faciomesial and faciodistalline angles but low at the center of the facial surface.
41. I clasp
•Used on distobuccal surface of canines for aesthetics.
• Only the tip of the clasp (2-3 mm) contacts the tooth. Hence, stability and encirclement is
decreased.
42. Infrabulge/Mirror View Clasp
•The approach arm for the retentive terminal arises from the
border of the denture base,either as an extension of a cast
base or attached to the border of a resin base.
• There are two occlusal rests on each abutment tooth.
• The lingual aspect of the abutment may be plated
(supported) or left open.
• It is more flexible because the portion of the metal base
that gives rise to the approach arm has an incomplete cut.
The cut is prepared either by machining or placing a thin
matrix band during casting
•The retentive arm can also be made of wrought wire, which
has higher flexibility. The wire may be soldered to the metal
base or embedded in the resin base.
43. I bar
•It is a modified I type roach clasp introduced by Kratochvil. It has a mesial rest arising from a
major connector, an l-bar retentive arm and a long proximal plate.
• It is designed to reduce tooth contact.
44. Conclusion
• DIRECT RETAINER is defined as a clasp or attachment applied to an abutment
tooth for the purpose of holding a removable denture in position
• CLASP is defined as “a part of a removable partial denture which acts as a
direct retainer and/or stabilizer for the denture by partially encircling or
contacting an abutment tooth”
•COMPONENTS of clasp are Retentive arm, Reciprocal arm, Shoulder, Rest,
Minor connector
•FUNCTIONAL REQUIREMENTS of clasp are Retention, Stability, Support,
Reciprocation, Encirclement, Passivity, Aesthetics
