Direct retainers i

Content
• Definitions
• Forces on RPD
• Concept of Height of contour, Suprabulge, Infrabulge and Prothero’s
cone theory.
• History.
• Types of Direct Retainer (According to Stewart’s)
• Brief on Intracoronal & Extracoronal Retainers.
• Structure of Clasp Assembly.
• Requirements of A Clasp Assembly.

Learning Objectives
Sr.no Topic Domain Significance
1. Definition of Direct
Retainers
Cognitive Must know
2. Height of Contour,
Prothero’s Cone Theory
Cognitive Must know
3. Classification Cognitive Must know
4. Components Cognitive &
Psychomotor
Must know
5. Requirements Cognitive &
Psychomotor
Must know

Definitions (GPT-9)
• Direct Retainer: That component of a removable partial denture used
to retain and prevent dislodgment, consisting of a clasp assembly or
precision attachment.
• Direct Retention: Retention obtained in a removable partial denture
by the use of clasps or attachments that resist removal from the
abutment teeth.

“To assume that retention is provided solely by the retentive arm or any
other single element of the clasp assembly is to misunderstand the co-
ordinated function of a well designed removable partial denture”
-Kenneth Lowe Stewart

Forces Acting On an RPD
The Three
Classes of
Levers in RPD
The three classes of levers. Classification is based on location of the fulcrum (F), resistance (R), and direction of effort
(force) (E). In dental terms, E can represent the force of occlusion or gravity; F can be a tooth surface such as an occlusal
rest; and R is the resistance provided by a direct retainer or a guide plane surface.

• J. H. Prothero (1916)-Cone Theory
• Dr Edward Kennedy (1928)- Coined the term Height of Contour
• M. M DeVan – Introduced the term Supra Bulge and Infra Bulge

History
• Bonwill WGA(1899), in his paper “New methods of clasping artificial dentures to
human teeth without injury Vs Immovable Bridges” Gave the first proper
description of clasp assemblies.
• The principle of the internal attachment was first formulated by Dr. Herman E.S.
Chayes (1906).
• A J Fortunati (1923) was the first to introduce the surveyor and Ney Company
was the one to manufacture it.
• The Suprabulge clasp is also known as the Akers Clasp, design of the clasp was
first standardized by Dr. Polk Akers(1928).
• The Infrabulge Clasp was first described and used by Dr. F Ewing Roach (1930).

Thomas J. Donahue (1988) stated factors that augmented the function of direct retainers
they are as follows:
1. The duplication of direct retainer function by other prosthesis components
2. Physiologic adjustment of the framework to assure contacts with abutment teeth that are
consistent with the design and intended functions of the components and that transfer stress in
a manner those teeth are designed to accept
3. Intimate adaptation of denture bases to the residual mucosa, with recall visits to monitor
this adaptation
4. Specific loading of the denture bases through selective placement of artificial teeth
5. Splinting of abutment teeth.
Thomas J. Donahue, Factors that augment the role of direct retainers in mandibular distal-extension dentures, JPD dec 1998 vol 60 number 6
Tsau-Mau Chou, Angelo A. Caputo, Dorsey J. Moore, Bin Xiao, Photoelastic analysis and comparison of force-transmission characteristics of intracoronal attachments with clasp distal-extension
removable partial dentures, THE JOURNAL OF PROSTHETIC DENTISTRY, SEPTEMBER 19.39 VOLUME 92 NUMBER 3

• Tsau-Mau Chou et al (1989) in a photoelastic study compared the load-transfer
characteristics of various bilateral distal-extension removable partial denture designs
embodying intracoronal attachments and clasp assemblies and concluded that The RPI
clasp assembly generated the most uniform stresses. In general, intracoronal retainer
frameworks produced higher stresses than did the clasp frameworks.
• H. ITOH et al (2008): studied the relationship between dynamics of abutment and RPD,
the study concluded that rigid retainer and major connector contribute to the success of
RPD and it provides minimum movement of the abutment teeth.
• Sherif A. Sadek et al (2018) :In a study for search of aesthetic material for clasps in the
anterior region the authors used other methods and by utilising different materials, these
included covering the retainers with tooth-coloured acrylic resin, as well as the introduction
of esthetic materials as; Thermoplastic Acetal, Versacryl, and Thermopress. It was
concluded that the non-metallic Acetal resin clasp shows superior physical properties
regarding colour stability.

Classification of Direct Retainers
Stewart’s clinical removable partial prosthodontics 4th edition

INTRACORONAL DIRECT RETAINERS
• Principle of the internal attachment was first formulated by Dr. Herman Chayes in
1906
• Functions to retain and stabilize a removable partial denture
Prefabricated
machined key and
keyway
opposing vertical
parallel walls
serve to limit
movement and resist
removal of the partial
denture
frictional resistance

• Precise parallelism of the components is essential well
defined path of placement and removal
McCollum Attachment

Based on method of fabrication and tolerance of fit
between components
Fabricated in metal
using high precision
manufacturing
techniques
Exhibits long,
parallel walls and
exceptional surface
adaptation
Precision
attachments
Usually originates
as wax or plastic
patterns and are
casted in metal later
Less intimate fit
Display gently
tapering walls
Semiprecision
attachments

ADVANTAGES
Elimination of visible retentive
component and of a visible
vertical support through a rest seat
Horizontal
stabilization similar
to that of internal rest
Central Direction of
Forces
McCraken’s Removable Partial Prosthodontics 13th edition

Disadvant
ages :
Require prepared abutments
and castings
Complicated clinical and
laboratory procedures and
difficult to place completely
within the circumference of
abutment tooth
Eventually wear off leading to
loss of frictional resistance
Difficult to repair and replace,
less effective on short teeth
Expensive

EXTRACORONAL RETAINERS
• Consists of components that reside entirely outside the normal clinical
contours of abutment teeth
• Serve to retain and stabilize the removable partial denture

Extracoronal Retainer are available in three
principle form
The clasp-type
retainer
manufactured attachments and include
interlocking components or the use of a
spring-loaded device that engages a tooth
contour to resist occlusal displacement.
manufactured attachment, which uses
flexible clips or rings that engage a rigid
component that is cast or attached to the
external surface of an abutment crown.

Characteristics of Extracoronal Attachments
• Retention is derived from closely fitting
components termed as matrices and patrices
• Permit vertical movement during occlusal
loading
• Minimises the transfer of potentially damaging
forces to the abutment tooth
• Based on stress breaking or stress directing
theories
CEKA Attachment

Extracoronal Retentive Clasp Assemblies
circumferential clasp
arm, which approaches
the retentive undercut
from an occlusal
direction
Bar clasp arm, which
approaches the retentive
undercut from a cervical
direction

Examples of Few Clasp-Type retainer
Bar-type clasp
• Y-bar clasp
• I-bar clasp
• T-bar clasp
• Modified T-bar
clasp
Circumferential clasp
Simple circlet clasp
Reverse circlet clasp
Embrasure clasp
ring clasp
back-action clasp
multiple clasp
Hairpin clasp
Combination
clasp
• Wrought-wire
clasp

Retentive Clasp Assemblies
• To Understand the mechanics of such Retainer an operator must understand 2 concepts
1.Path of Insertion and removal of the Prosthesis
2. Height of contour For each abutment.

• The Clasp Serves its Purpose of retention by engaging the prescribed
undercut and remaining passive until forces act in the prosthesis
• The Clasp assembly is active only during action of dislodging forces
or during insertion.

Relationship between Height Of Contour and
Path of Insertion.
Changing the orientation alters the relationship of surfaces relative to the greatest
circumference and consequently alters suprabulge and infrabulge locations.

Structure Of Clasp Assembly
A- Rest
B- Retentive clasp arm
C- Retentive clasp arm
D- Minor connectors

• Rest: The component of a clasp that provides vertical support for the
prosthesis.
• Retentive arm: it is the only portion of a removable partial denture
that contacts the surface of an abutment apical to the height of contour
. There are two basic forms of retentive arms:
a. Suprabulge
b. Infrabulge

a. Suprabulge Clasp

b. Infrabulge Clasp

• Reciprocal arm: The component of a clasp assembly that braces an
abutment during prosthesis insertion and removal is called a reciprocal
element.
Relationship between clasp assembly and reciprocal arm

• Minor Connectors: these are components which join the clasp assembly to
other components of the RPD.
• Minor connector may serve as:
(1) a guiding plate to direct insertion and removal of the removable partial
denture.
(2) a reciprocal element to counteract non-axial forces produced by a retentive
clasp.
(3) an approach arm for an infrabulge clasp.

Requirement Of Clasp Design
Retention
Retention is the quality of the clasp assembly that
resists force acting to dislodge components away
from the supporting tissue

1. The retentive arm must be designed so that
only the clasp terminus engages the
prescribed undercut.
2. The accompanying rest must provide
support so the clasp terminus is maintained
in an optimal location.
3. The minor connector must be sufficiently
rigid to ensure proper stability and
function of parts of the clasp assembly.

4. The reciprocal element must
contact the abutment slightly before
the retentive element contacts the
tooth, and it must maintain contact
until the prosthesis is fully seated to
protect the abutment from potentially
destructive lateral forces.

5. Components must provide sufficient
encirclement to prevent movement of the
abutment away from the associated clasp
assembly, otherwise retention will be lost.
6. Indirect retainers must resist forces
acting to dislodge the prosthesis from its
fully seated position (these forces may
result from the actions of sticky foods,
gravity, etc).

Factors On Which Retention Depends Upon
• Size of the angle of cervical
convergence
• How far the clasp terminal is
placed into the angle of
convergence
Tooth
factors
• Flexibility of the clasp arm
{ Clasp length, clasp relative
diameter, clasp cross
sectional form or shape}
• Material used in making clasp
Prosthesis
factors

Angle of cervical convergence
The angle formed between the
analyzing rod and the tooth
surface apical to the height of
contour is called angle of
cervical convergence.
Greater the angle of
Convergence, greater is the
force required to remove the
clasp assembly from the tooth

Horizontal Placement Angle of Convergence

Flexibility of clasp arm
• Maximum flexibility of a clasp arm may be defined as the greatest
amount of displacement that can occur without causing permanent
deformation of the clasp arm
• Influenced by length, cross sectional form and diameter, longitudinal
taper, clasp curvature and metallurgical characteristics of the alloy

D =
4P 𝐿3
𝐸𝑤𝑡3
Deflection Of A Uniform
Cantilever Beam
L= Length
E= Modulus of Elasticity
W= width of the beam
t= thickness of the beam
P= Force Applied

• Clasp flexibility increases as cross-sectional dimensions decrease

Flexible
Rigid

• A circular cross-sectional clasp form imparts omnidirectional flexure,
while a half-round form allows only bidirectional flexure

• The metallurgical properties of an alloy influence clasp flexibility
• Alloys exhibiting higher elastic moduli exhibit greater stiffness, while
alloys displaying lower elastic moduli exhibit greater flexibility

GOLD ALLOY CLAPS ARE
MORE FLEXIBLE THAN
COBALT CHROMIUM

Support &
Stability
Suprabulge
Infrabulge

Reciprocation
• 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.

To Optimize Reciprocation
• The axial surface of an abutment should be prepared parallel to the
path of insertion and removal. Furthermore, the reciprocal element
should contact the abutment at the junction of the gingival and middle
thirds of the crown contours
A. Lingual B. Proximal C. Buccal

Illustrations of
Reciprocation
Cast clasp arms (arrows) serve as the
reciprocal elements for these first
premolar and second molar clasp
assemblies.
Lingual plating (arrow) serves as the
reciprocal element in this first
premolar class assembly.
The combination of mesial and distal minor
connectors (arrows) serves as the reciprocal
element for this first premolar clasp assembly.

Encirclement
• It is the characteristic of a clasp assembly that prevents movement of
an abutment away from the associated clasp assembly. Each clasp
assembly must be designed to provide direct contact over at least 180
degrees of the tooth's circumference.
Continuous
Engagement
Discontinuous
Engagement

Passivity
• Passivity is the quality of a clasp
assembly that prevents the
transmission of adverse forces to
the associated abutment when the
prosthesis is completely seated.

ESTHETICS
• The suprabulge clasp approaches the undercut from an occlusal
direction and is more visible.
• The infrabulge clasp, approaching the undercut from a gingival
direction, has more potential for being hidden in the distobuccal aspect
of a tooth.
• Shaping enamel surfaces and the use of composites can modify the
convexity of a tooth surface and allow placement of clasps into a less
visible position.
• Clasps approaching the undercut from the distal aspect are less visible
than mesially approaching clasps.

Summary
• It is essential to understand that there is an intricacy between the
design of retainers and the morphology of tooth which has to be
utilized to provide retention
• Surveying and interpreting path of insertion is one of the principle step
in deciding the type of retainer.
• All components serve a purpose, sometimes multiple. To do this
effectively multiple considerations are to be met simultaneously.

References
• Stewart’s clinical removable partial prosthodontics 4th edition.
• McCraken’s Removable Partial Prosthodontics 13th edition.
• Thomas J. Donahue, Factors that augment the role of direct retainers in mandibular distal-
extension dentures, JPD dec 1998 vol 60 number 6
• Tsau-mau chou, angelo a. Caputo, dorsey j. Moore, bin xiao, photoelastic analysis and
comparison of force-transmission characteristics of intracoronal attachments with clasp
distal-extension removable partial dentures, the journal of prosthetic dentistry, september
1989 volume 92 number 3.
• Itoh, H. , Baba, K. , Aridome, K. , Okada, D. , Tokuda, A. , Nishiyama, A. , Miura, H. And
Igarashi, Y. (2008), Effect Of Direct Retainer And Major Connector Designs On RPD And
Abutment Tooth Movement Dynamics. Journal Of Oral Rehabilitation, 35: 810-815
• Sherif A.Sadek, Wessam M. Dehis, Hala Hassan Different Materials Used as Denture
Retainers and Their Colour Stability, Macedonian Journal of Medical Sciences. 2018 Nov
25; 6(11):2173-2179.

Direct retainers i

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