DIRECT RETAINERS

Mechanical retention of a removable partial denture is provided by the

direct retainers or clasps, and the denture base. The direct retainers
provide the partial denture with the ability to resist movement away from
the basal seating tissues. The denture base can provide some retention
by adhesion, cohesion, atmospheric pressure, polished surface
contours, and gravity.
There are basically two types of direct retainers:
1. Precision or semi-precision attachments. These are usually
premanufactured and either machined or cast in the laboratory. They
can be either placed intracoronal or extracoronal in conjunction with the
fabrication of a single crown or fixed bridge.
There is a male-female union between the abutment tooth and the RPD
framework. The primary advantage of using attachments as direct
retainers is esthetics as the retentive clasp arm is eliminated from the
design. A disadvantage of using these attachments is they can be
costly, they are difficult to use, and they require more maintenance than
a conventional clasp design.
2. Conventional extracoronal cast clasp, where the retention is usually
provided by a flexible arm that flexes over the area of greatest contour
into an area of lesser contour. This is usually called a clasp.

Occlusally approaching clasp Gingivally approaching clasp

A-Shoulder or point of origin.
B-middle or transitional portion.
C-terminal or retentive portion.

Clasps must meet the requirements of:

a-Support
b-Retention
c-Cross-tooth reciprocation
d-Encirclement (encompass more than 180 o of the abutment tooth)
e-Be passive when the removable partial denture is seated
Most clasps consist of one or more of the following components:
Rest
Retentive arm
Reciprocal arm
Minor connector
Most clasps contain a single retentive arm. The action of the retentive
clasp when flexing over the height of contour is counteracted by the
reciprocating clasp.
The reciprocal clasp or plate, contacts the portion of the abutment tooth
parallel to the path of placement and removal, either before or at the
same time the retentive clasp contacts the tooth, and remains in
contacts as the retentive clasp arm flexes over the height of contour.
The reciprocal component is located 180 around the tooth from the
direction of force of the retentive clasp arm. The reciprocal component
can also provide for bracing and occasionally indirect retention.
All clasps contain one or more minor connectors. This element joins the
clasp arms and the rest (occlusal, cingulum, etc.) to the major
connector.
Types of clasps:
There are two types of retentive clasps based on the direction from
which they approach the undercut on the tooth:
1. Circumferential (suprabulge or pull clasp) - These clasps arise from a
minor connector or lingual plate and crosses cervically over the height of
contour of the tooth into the retentive undercut area.

2. Bar (infrabulge or push clasp) these clasps originate from the denture
base retention minor connector and cross the soft tissue of the facial of
the alveolar ridge before contacting the tooth in the desired undercut
area. The portion of the bar clasp that extends from the denture base is
termed the arm. The retentive portion of the clasp arm approached the
undercut on the tooth from the cervical direction. Due to the friction of
the " tripping " or "pushing" action of the bar clasps, they generally will
provide more retention with the same amount of undercut and cervical
convergence than a circumferential clasp.

There are multiple factors that are involved in determining the amount of
retention that will be provided by a clasp and are:
a-The angle of cervical convergence.
b-How far into the angle of cervical convergence the retentive tip of the
clasp is placed.
c-The type of material from which the clasp is made (different materials
have varying degrees of flexibility.
d-The shape and contour of the retentive clasp.

The retentive clasp arms should be tapering. The terminal one-third of

the retentive clasp is the portion that flexes, and the rest is rigid. The
reciprocal clasp should possess little if any taper. The larger the
diameter of a retentive clasp arm the less flexible it will be. Most cast
clasps are half-round and flex away from the tooth, in one direction, with
edgewise flexing minimal or non-existent. Wrought wire is also used for
retentive clasps and are flexible, that is, able to flex in all directions. This
is thought to be a very desirable property in not placing too much stress
on the abutment tooth, and is desirable in stress distribution during
function. This type of clasp is also more easily adjusted during function.

Basic rules of clasp design:

a-All clasp assemblies must encompass more than 180o of the tooth.
At least three areas of tooth contact must be embracing more than one-
half of the tooth circumference for this rule to be satisfied.
b-Generally, the retention on all clasped abutments should be nearly
as equal as possible.
c-The rest must prevent cervical movement of the clasp arm.
d-Retention should be minimum to resist reasonable dislodging forces.
e-Generally, retentive clasps should be bilaterally opposed. For
example, buccal retention on one side of the arch should be opposed by
buccal retention on the other side, or lingual opposed by lingual, etc.
f-Reciprocating arms should be located at the junction of the gingival
and middle thirds of the tooth. The retentive arm tip should be placed in
the middle of the gingival third, no closer than 1 mm to the marginal
gingiva.
g-The simplest clasp for the given survey line should be used. If the
survey line can be modified to simplify the clasp design, then it should
be considered.
Description of surveying lines:
Survey lines can be classified as either type I, II or III.
Type I :
A type I survey line is "S" shaped with the portion adjacent to the
edentulous space being low on the tooth (cervical). The portion away
from the edentulous space is higher on the tooth (occlusal). The
deepest undercut is located on the portion of the tooth away from the
edentulous space.
Type II :
A type II survey line is "S" shaped with the portion adjacent to the
edentulous space being high on the tooth (occlusal). The portion away
from the edentulous space is lower on the tooth (cervical). The deepest
undercut is located on the portion of the tooth adjacent to the
edentulous space.
Type III :
The type III survey line is straight or "U" shaped. It is usually higher on
the mesial and distal of the tooth, with the bottom of the "U" being more
cervical placed. The deepest undercut can be anywhere along the
survey line. This survey line will usually provide minimal retention.

Type I Surveying line Type II Surveying line Type III Surveying line

Circumferential clasp:
Survey Line: Type I
Amount of Undercut: Cr/Co or Ni/Cr alloys - premolar .010", molar .010"
-.020"
Indications:
1. Type I survey line
2. Tooth supported RPD
3. Tooth supported modification space

Contraindications:
1. Type II or III survey line
2. Tooth-tissue supported RPD
3. When facial clasp arm is unesthetic
Advantages:
1. Simplest design.
2. Maximum support, bracing, retention, encirclement, and reciprocation
3. Strong and not easily deformed.
Disadvantages:
1. Maximum metal-tooth contact
2. Difficult to properly adjust, usually only in a horizontal plane
3. Can be difficult to repair, as are all cast clasps

cast circumferential clasp

Ring clasp:
Survey Line: Type II
Amount of Undercut: Cr/Co or Ni/Cr alloys - premolar .010", molar .010"
-.020"
Indications:
1. Type II survey line
2. Tipped or tilted primary abutment tooth (usually mandibular molars)
3. Tooth supported RPD
4. Tooth supported modification space
Contraindications:
1. Type I or III survey line
2. Tooth-tissue supported RPD
3. Soft tissue undercut in the area of the support arm
4. Muscle/frenum attachments in the area of the support arm
5. When a more simple clasp will be acceptable.
Advantages:
1. Can use the Type II survey line without a bar clasp
2. Maximal support, bracing, retention, encirclement, and reciprocation
Disadvantages:
1. Complex design
2. On larger molars will require support arm for reciprocating clasp to
prevent clasp from being distorted and over flexing, also for increased
ease of casting
3. Support arm will be a food trap and may impinge on soft tissue
4. Maximal metal-tooth contact
5. Difficult to adjust, usually on in a horizontal plane
6. Very difficult to repair
 Ring clasp

Cast ring clasp with buccal support arm and mesial-lingual retention with a
type II surveyline line. Support arm may be necessary to add strength to clasp
arm to prevent overflexing and distortion.

Reverse action or hairpin clasp:

Survey Line: Type II
Amount of Undercut: Cr/Co or Ni/Cr alloys- premolar .010", molar .010" -
.020"
Indications:
1. Type II survey line
2. Soft or hard tissue undercuts and/or high muscle and frenum
attachments
3. Tooth supported RPD
4. Tooth supported modification space
Contraindications:
1. Type I or III survey line
2. Small teeth where clasp would be too short or too rigid
3. Survey line too occlusally located - the clasp would be too high and
interfere with occlusion
Advantages:
1. Can use Type II survey line without using a bar clasp
2. Not easily distorted due to short length
3. Maximum support, bracing, retention, encirclement and reciprocation
Disadvantages:
1. Maximum metal-tooth contact
2. Complicated design
3. Clasp arm is usually minimally flexible
4. Not easily adjustable
5. May be difficult to place the occlusal portion of the clasp low enough
on the tooth to insure that it won't interfere with occlusal function.

Reverse action or hairpin clasp

Embrasure clasp:
Survey Line: Type I
Amount of Undercut: Cr/Co or Ni/Cr alloys - premolar .010", molar .010"
- .020"
Indications:
1. Type I survey line
2. Tooth supported RPD
3. Desire to distribute occlusal support, bracing, and retention in
unilateral distal-extension RPD's
4. Prevention of extrusion of a tooth not contacted by an opposing tooth
Contraindication:
1. When a simpler clasp can be used
2. When adequate occlusal clearance cannot be provided
3. When survey lines will not permit this design
Advantages:
1. May distribute support, retention, and bracing to two teeth
2. Not easily distorted
3. Relatively simple design
Disadvantages:
1. Considerable occlusal clearance must be provided
2. Wedging apart of the abutment teeth may occur if the occlusal rests
do not possess positive seats
3. More enamel reduction is required than with a single tooth clasp
4. Maximum metal-tooth contact
5. Difficult to adjust, usually only on a horizontal plane
6. Difficult to repair

Embrasure clasp

Multiple clasp:
The multiple clasp simply consists of two opposing circumferential
clasps joined at the terminal end of the two reciprocal arms.
Indications:
1-It is used when additional retention and stabilization are needed,
usually on tooth-supported partial dentures.
2-It may be used for multiple clasping in instances in which the partial
denture replaces an entire half of the dental arch.
3-It may be used rather than an embrasure clasp when the only
available retentive areas are adjacent to each other.
Disadvantages:
is that two embrasure approaches are necessary rather than a single
common embrasure for both clasps.

Multiple clasp is actually two opposing circumferential clasps joined at the

terminal end of two reciprocal arms.

Half-and-half clasp:
The half-and-half clasp consists of a circumferential retentive arm
arising from one direction and a reciprocal arm arising from another.
The second arm must arise from a second minor connector, and this
arm is used with or without an auxiliary occlusal rest. Reciprocation
arising from a second minor connector usually can be accomplished
with a short bar or with an auxiliary occlusal rest, thereby avoiding so
much tooth coverage. There is little justification for the use of the half-
and-half clasp in bilateral extension base partial dentures. Its design
was originally intended to provide dual retention, a principle that should
be applied only to unilateral partial denture design

Half-and-half clasp consists of one circumferential retentive arm arising from

the distal aspect and a second circumferential arm arising from the mesial
aspect on the opposite side, with or without a secondary occlusal rest.

Combination clasp:
Survey Line: Type I (usually)
Amount of Undercut: Stellite alloy, premolar .010" - .020", molar .020" -
.030". Slightly less retentive undercut is necessary when 18 gauge is
used than when using 19 gaugeG.e nerally 19 gauge is used for smaller
teeth such as premolars and anteriors, and 18 gauge is used for larger
teeth when more retention is desired.
Indications:
1. Type I survey line
2. Distal extension RPD - because of possible stress-releasing
properties of the wire on the abutment
3. Wrought wire can be placed into a greater undercut than a cast clasp
and may be more esthetic
4. When soft or hard tissue undercuts preclude the use of a bar clasp
Contraindications:
1. Type II survey line
2. When maximum retention is necessary
3. When there is not adequate room to solder the wire far enough distal
to the retentive tip, thus changing the physical properties of the wire
(see physical properties of wrought wire in the chapter on metals)
Advantages:
1. Wrought wire is thought to be stress releasing
2. Wire has a line contact on the tooth, therefore, there is less of the
clasp in contact with the tooth, compared to the cast clasps (this is
better for hygiene, with less food retention under the clasp)
3. May be more esthetic than a cast clasp due to more cervical
placement
4. Can be added after fabrication of the framework, which may make
fitting of the framework easier
5. Much easier to adjust than cast clasps
6. Easier to repair than cast clasps
Disadvantages:
1. Easily comes out of adjustment if the patient removes the RPD by
the wrought wire clasp and not as instructed.
2. An added laboratory step is necessary for the fabrication of a
combination clasp, which increases the laboratory fees to you (this is
not a good reason not to use a wrought wire clasp)

Combination clasp (facial wrought wire retentive arm, soldered to base retentive
minor connector and cast lingual reciprocating arm with cast occlusal rest.

I Bar or RPI clasp:

RPI clasp {mesial rest (R), distal proximal plate (P), facial I bar (I)}-
Survey Line: Type I or III
Amount of Undercut: Stellite alloy, premolar and molars .010"
Indications:
1. Type I or III survey line on facial (sometimes lingual) of abutment
tooth
2. Distal extension RPD - thought to be stress releasing on the
abutment tooth
3. Esthetic requirements of the patient
Contraindications:
1. When it is not possible to rest on the mesial of the tooth, due to
fulcrum lines the clasp is unable to release from the undercut with a
distal rest (theoretically)
2. Type II survey line
3. High muscle or frenum attachments
4. Significant hard or soft tissue undercuts
Advantages:
1. Good esthetics
2. Minimal change in natural tooth contour
3. Not easily distorted
4. Increased retention over circumferential clasps with the same amount
of undercut due to "tripping" action
5. Easier to adjust than circumferential clasps
6. Very simple design
Disadvantages:
1. Approach arm may create food trap
2. Bracing and reciprocation is minimal
3. Approach arm can interfere with the placement of prosthetic teeth if
not properly placed in the embrasure of the proposed prosthetic teeth
4. Approach arm may interfere with soft or hard tissues in function such
as an active frenum
5. Encirclement may not be satisfied with improper placement of
retentive tip of the clasp,

(RPI) system

T – Bar clasp
Survey Line: Type II (usually)
Amount of Undercut: Stellite alloy, premolar and molar .010"
Indications:
1. Type II survey line on facial of abutment tooth.
2. Distal extension RPD (non-retentive portion of T - bar needed for
encirclement)
Contraindications:
1. Type I survey line
2. Excessive hard or soft tissue undercuts
3. High muscle or frenum attachments
Advantages:
1. Esthetics
2. Minimal change in natural tooth contour
3. Not easily distorted
4. Increased retention over circumferential clasp with the same amount
of undercut
Disadvantages:
1. More complicated design than I - bar
2. Approach arm may create food trap
3. Approach arminterfere with placement of prosthetic teeth if not placed
in interparochial area of proposed placement of teeth
4. Approach arm may interfere with soft or hard tissue in function
5. Bracing better than -I bar, but not as good as a cast circumferential
clasp
Modified T – Bar clasp -
Survey Line: Type II
Amount of Undercut: Stellite alloy, premolar and molar .010"
Indications:
1. Type II survey line on facial of abutment tooth
2. Distal extension RPD
3. Non-retentive portion of T - bar not needed to provide encirclement
Contraindications:
1. Same as 1 - 3 for T - bar clasp
Advantages:
1. less tooth-metal contact than T - bar clasp
2. Same as 1 - 4 for T - bar clasp
Disadvantages:
1. Same as 1 - 5 for T - bar clasp
Clasp selection:
Successful clasp selection depends upon many factors. The practitioner
should select a direct retainer that will control tipping and torquing forces
on the abutment teeth, provide adequate retention against normal
dislodging forces, and be compatible with the tooth and tissue contours,
and satisfy the patients esthetic and functional requirements. The most
important factor is the location of the retentive areas and placement of
the survey line. The clasp selection will depend upon where the
retentive undercut is located and how much undercut is available. If the
existing undercut area is undesirable, then the contour of the abutment
tooth must be changed. The alteration in the height of contour is
accomplished through the use of fixed restorations or enamel
recontouring. These procedures will allow the clinician to ideally place
the survey line in a more desirable and functional position.
Accurate diagnostic casts are a requirement if an accurate diagnosis is
to be made in regard to clasp selection. The amount of soft tissue
undercut can be determined, if present, to evaluate the possibility of
using a bar clasp. The height of contour must be accurately marked to
evaluate the type of survey line and amount of retentive undercut
available. This accumulation of information will guide the practitioner in
an intelligent and informed selection of the proper clasp design.