Principles of Removable Partial Dentures

Principles of Removable Partial
Dentures with special emphasis on
support and periodontal
consideration of remaining teeth
By
Dr ZARIR RUTTONJI

CONTENTS
1) Introduction
2) Principles of desining
3) Stress considerations
4) Forces influencing the magnitude of stress
5) Biomechanical consideration of individual
component
6) Philosophies of designs
7) Essentials of designing
8) Procedure of designing
9) Conclusion
10)References

INDICATION FOR REMOVABLE PARTIAL
DENTURES
Stewart’s Clinical removable partial Prosthodontics 4th edition pg-8
1.Long standing edentulous area
2.No abutment tooth posterior
to the edentulous space
3.Reduced periodontal support
for remaining teeth
4.Need for cross ach stablization

DENTURES continued
Stewart’s Clinical removable partail prosthodontics 4th edition pg-8
5.Excessive bone loss within
the residual ridge
6.Physical or emotional
problem exhibited by the
patient

DENTURES continued
8) Immediate need to
replace extracted teeth
9) Patient desire
10)Unfavourable
maxillomandibular
relation.
Stewart’s Clinical removable partail prosthodontics 4th edition pg-8

PRINCIPLES OF DESIGN
• 1953,Dr.A.H.Schmidt gave the basic principles
of designing
1.Dentist should have thorough knowledge
2.Treatment plan must be based on complete
examination and diagnosis of individual patient.

3.Dentist must correlate the pertinent factors and
determine a proper treatment plan
4.RPD should restore the function without injury to
remaining oral strutures
5.It is a form of treatment and not a cure.

STRESS CONSIDERATION IN A PARTIAL
DENTURE
The stresses can be divided as:
 Vertical
 Horizontal
 Torsional
Displacing stresses
Dislodging stresses

VERTICAL STRESS
Displacing stresses :
These are the least harmful and are borne well if within
physiologic limits

DISLODGING STRESSES :
These are the forces which tend to lift the partial denture
from it’s rest position

HORIZONTAL STRESS
They originate as a component of rhythmic chewing
stroke. These forces are effective in mesio-distal
and buccolingual direction.
These lateral stresses are most damaging.

TORSIONAL STRESS
It is a twisting rotational type of force. It’s a
combination of vertical and horizontal force

FORCES ACTING ON
PARTIAL DENTURE

McCracken’s Removable Partial Prosthodontics 11th edition
Is a rigid bar supported
somewhere along its
length..
The point where the bar is
supported is called the
fulcrum
Three classes of levers
(based on location of
fulcrum, resistance and
direction of effort (force).
Class I
Class II
Class III
LEVER

CLASS I LEVER
Fulcrum lies in the centre,
Resistance is at one
end and force at the other.

CLASS II LEVER
Fulcrum is at one end effort
at the opposite end and
resistance in the centre.

CLASS III LEVER
Fulcrum is at one end,
resistance at opposite
end and effort is in the centre.

INCLINED PLANE
• Forces against an inclined plane may result in
deflection of that which is applying the forces or
may result in movement to the inclined plane,
neither of these is desirable
.
Stewart’s clinical Removable Posthodontics 4th edition

FULCRUM ON HORIZONTAL PLANE:
• Extends through the rest of principle abutments.
• Rotational movement of the denture in the sagittal plane.

DENTURE BASE MOVES AWAY FROM
SUPPORTING TISSUES:
Counteracted by:
Direct retainer
and
Indirect retainer

DENTURE BASE MOVES TOWARDS THE
SUPPORTING TISSUES:
Counteracted by:
1) Occlusal rest.
2) Tissues of
supporting
ridge

FULCRUM ON THE SAGITTAL PLANE
Extends through the occlusal rest on the terminal
abutment and along the crest of the ridge.
Movement is in the frontal plane

Counteracted by:
• Rigidity of major and
minor connector and
their ability to resist
torque.
• Close adaptation of the
denture base along the
lateral slopes and the
buccal slopes of the
palate and ridge.
• Direct retainer design McCracken’s Removable Partial Prosthodontics 11th edition

FULCRUM LOCATED IN MIDLINE JUST
LINGUAL TO THE ANTERIOR TEETH
(FULCRUM IS VERTICAL)
Rotational movement of denture in horizontal plane
or
flat circular movements of the denture

Counteracted by :
• Stabilizing components
(reciprocal arm and
minor connector)
• Rigid major connector
• Close adaptation of
denture base

FACTORS INFLUENCING MAGNITUDE
OF STRESSES TRANSMITTED TO
ABUTMENT TEETH
1. Quality of support of ridge
Form of residual ridge
type of mucosal covering
2. Length of span
3. Clasp factor
design
length
material
amount of tooth contact

4. Occlusion
Type of teeth
Harmony of occlusion
5. Areas of the base to which load is applied

1. QUALITY OF SUPPORT OF RIDGE
• Better support by ridge less stress on abutment
teeth
• Large well formed ridges absorb greater stress
less stress on abutment
• Broad ridges with parallel
sides longer flanges on
the denture base stabilize
the denture against lateral
forces.

b. TYPE OF MUCOSA
• Influences magnitude of stresses transmitted to abutment
teeth.
• Healthy mucosa capable
of bearing greater functional
loads than thin atrophic
mucosa
• Soft, flabby, displaceable mucosa
Contribute little to vertical support of denture allows
excessive movement of denture stress transmitted to
abutment teeth

2. LENGTH OF SPAN
• Longer edentulous span
longer denture base
greater force transmitted to
abutment teeth
• Every effort be made to retain a posterior abutment to
avoid class I and class II situation.

3. CLASPAS A FACTOR IN STRESS
• More flexible the retentive arm of claspless
stress to abutment tooth
• But, flexible clasp arm  provides less
stability against horizontal forces increase
stress on residual ridge.
• Decision should be made whether abutment
or ridge requires more protection

In examination phase decide whether ridge or abutment
tooth require more protection
If periodontal
support good
less flexible clasp
like vertical
projection clasp
If periodontal support
weak
use more flexible
clasp like
combination clasp

TYPE ABUTMENT TOOTH SURFACE
• Surface if gold crown offers more frictional
resistance to clasp arm movement than does
enamel surface of tooth.
• Greater stress exerted on tooth restored with
crown than with intact enamel.

AMOUNT OF CLASP SURFACE IN CONTACT
WITH TOOTH
Greater the area of tooth to metal contact between
clasp and tooth
more will be stress exerted on the tooth.

OCCLUSION AS A FACTOR
Disharmonious occlusion
generate horizontal stresses
when magnified by factor of leverage
can transmit destructive forces to both
abutment teeth and residual ridge.

TYPE OF OPPOSING OCCLUSION
• Play important role in determining amount of stress
generated by occlusion
• Natural teeth can exert closing force upto 300
pounds/inch square, whereas, complete denture upto 30
pounds/inch square.
• Therefore RPD constructed against removable prosthesis
is subjected to much less occlusal stress than one
opposed by natural dentition.

• AREA OF DENTURE BASE TO WHICH LOAD
IS APPLIED
• Less movement of base if load applied adjacent to the
abutment tooth than if it is applied to the distal end of
the base.
• movement may be 4 times greater at distal end of base
than next to the clasp.

BIOMECHANICAL
CONSIDERATION OF
INDIVIDUAL COMPONENT

REST
Two basic
types
Primary rest
Secondary rest
DIFERENT TYPES OF RESTS
1) Occlusal rest
2) Extended occlusal rest
3) Incisal ,lingaul or cingulum rest
4) Rest on restoration
5) Rest on cast restorations

REST
FUNCTIONS
• Provide vertical support for the partial denture.
• Maintains components in planned positions.
• Maintains established occlusal relations by preventing
settling of the denture base.
• Prevents impingement of soft tissue.
• Directs and distributes occlusal loads to abutment
teeth.

OCCLUSAL REST

If angle greater than 90 degrees
Forces not along long axis
but will create an inclined
plane effect
Slippage of prosthesis away
from the abutment
Orthodontic like forces McCracken’s Removable Partial Prosthodontics 11th edition

CINGULUM REST VS INCISAL REST
Cingulum rest nearer to center
of rotation
less tipping of tooth
Incisal rest
Longer minor connector required
magnifies the forces being transferred to the
abutment tooth

MESIAL REST or DISTAL REST FOR DEB
RPD???

Kratochvil et al (JPD 1963:13;114)
• Suggested using mesial occlusal rest so that the force
delivered to the mesial aspect of the abutment tooth
• That would tend to tip that tooth forward maintaining a
tight contact with the tooth immediately anterior to it
and gaining stabilization and support from the remaining
anterior teeth.
• They suggested that occlusal rest be moved anterior to
better use the residual ridge for support.
.Kratochvil et al influence of occlusal rest position and clasp design on movement of abutment teeth J Prosthet Dent
1963:13;114

John W. Mc Cartney (JPD 1980;43:15)
Did a study on the effect of location of occlusal rest
on abutment tooth movement and concluded that
the mesial rest placement caused less abutment
movement than distal rest placement.
John W.McCartney Effect of location of rest and movement of abutment tooth J Prosthet Dent 1980:43:15

A major connector is the
component of the partial
denture that connects the
parts of the prosthesis
located on one side of the
arch with those on the
opposite side.
 it also provides cross arch
stability to help resist
displacement stresses
MAJOR CONNECTOR

DESIRABLE CHARACTERISTICS OF MAJOR
CONNECTORS
1.Should be rigid
2.Provide vertical support and protect the soft tissue.
3.Provide means of obtaining indirect retention where
indicated
4.Provide an opportunity of positioning denture bases
where needed
5.Maintain patient comfort

MINOR CONNECTOR
1.Primary function is to join
other units of the prosthesis
such as clasp,rests,indirect
retainers and denture bases to
major connector
2.Distribution of stresses

DESIGN CONSIDERATIONS
1.Should have sufficient bulk to be rigid
2. Should be made thick buccolingually and thin
mesiodistally
3.Should be in triangular form in embrassure
4. Should form a right angle with the major connector
5. Sharp angles should be avoided and spaces should not
exist for trapping of food debris.
Miller E.L. : “Text book of Removable Partial Prosthodontics”.

Any unit of
removable dental
prosthesis that engages
an abutment tooth in
such a manner as to
resist displacement of
the prosthesis away
from basal seat tissue.
DIRECT RETAINER

• Prothero Cone theory as basis
of clasp retention
• Described shape of crowns of
premolar and molar teeth as 2
cones sharing a common base .
Line where they meet height
of contour. (represents the
greatest bulge).
• Suprabulge
• Infrabulge

• Height of contour of tooth changes as the vertical
position of tooth changes.
• Surface is retentive if it is cervical to its height of
contour.

• Clasp tip in infrabulge area resist the
movement in occlusal direction because to
release from tooth  it has to undergo
deformation.
• Degree of resistance to deformation
determines the amount of clasp retention.
• Retention varied by:
depth of undercut
Flexibility of clasp arm positioned in undercut
Stewart’s Clinical removable partial Prosthodontics 4th edition

STRATEGIC CLASP POSITIONING AS A
MEANS OF STRESS CONTROL
• Leverages can be controlled to a large extent
by means of clasps, if there are sufficient
abutment teeth and they are strategically
distributed in the dental arch.
• If number and location of potential abutments
is less than ideal harmful effects can be
decreased by strategic placement of clasps.
Stewart’s Clinical removable partial Prosthodontics 4th edition

• Indicated most often in class III arches (with
modification space on opposite side)
QUADRILATERAL CONFIGURATION

TRIPOD CONFIGURATION
Class II situations
Distal abutment on one
side of arch missing
leverage controlled to
some extent by creating
tripod configuration
of clasp placement.

BILATERAL CONFIGURATION
For class I situations
Not considered ideal, but best
option available
Stress must be controlled by
other means.

REQUIREMENTS OF CLASP
DESIGN
:
• Retention:provided by retentive terminal
• Support:mainly by rest
• Stability:all the component except the
retentive terminal
• Reciprocation:reciprocal arm
• Encirclement:must be designed more than
half of the circumference
• Passivity:engages only when dislodging forces
are applied

CLASP DESIGNS:
• Combination clasp
• Reverse circlet clasp

• Bar clasp
• T Clasp
• Modified T Clasp
• Y clasp
• I clasp

Function: 1. to prevent the DEB
from moving away from its seat
because of cheek and tongue
forces, sticky food.
It uses mechanical advantage
of leverage by moving the
fulcrum line farther from the
force

2. Contributes to support and stability of the partial
denture counteracts horizontal forces applied to the
denture.

Effectiveness of indirect retainer depends
upon:
1) Its distance from the fulcrum line
greater the distance between fulcrum line
and IR more effective

2) Effectiveness of direct retainer
Direct retainer must be effective if the
indirect retainer is to function when the
denture rotates
otherwise the partial
denture will be
dislodged.

Frank and Nicholls (JPD 1977:38;494)11
-did a study on the effectiveness of indirect retainer and
concluded that use of a mesial rest instead of a distal rest
on the terminal abutment tooth does not decrease
indirect retention.
Thus the choice of indirect retainer location should be
made mostly on the basis of abutment tooth support, a
crown form favoring adequate rest seat preparation, and
the patients esthetic desires.

Frechette et al (JPD 1956;6:195-212)
demonstrated that removal of the indirect retainers
from a RPD results in the application of more
pressure to the direct abutment teeth. The indirect
retainer probably is effective in distributing forces to
teeth other than direct abutments, and in preventing
denture base lifting.

PHILOSOPHIES OF DESIGN
• There are three basic design philosophies:
• Stress Equalization
• Physiologic Basing
• Broad stress distribution

ESSENTIALS OF PARTIAL DENTURE
DESIGN
• Should be systematically developed on the
diagnostic cast based on the following.
1) Where is the prosthesis supported.
2) How the support is connected.
3) How the prosthesis is retained.
4) How the retention and support are connected.
5) How the edentulous base support is connected

WHERE IS THE PROSTHESIS SUPPORTED.
Tooth supported
Tooth - tissue supported:

TOOTH SUPPORTED
1) The most ideal support units are the RESTS.
2) The abutment selected for the support has to be
evaluated for
• Periodontal health.
• Crown- root ratio.
• Crown –root morphology
• Location of the tooth in the arch
• The opposing dentition

TOOTH- TISSUE SUPPORTED
Depends on 6 factors:-
1) Quality of residual ridge
2) The extent to which the ridge will be covered by mucosa
3) The accuracy and type of impression registration
4) The design chracteristics
5) The occlusal load applied

STRESS EQUALIZATION
• The resiliency of the tooth supported by periodontal ligament in
an apical direction is not comparable to the greater resiliency
and displacement of the mucosa covering the dentulous ridge.
• It is the belief of this school of thought that the rigid connection
between the denture base and the direct retainer on the
abutment teeth is damaging

• Thus some form of stress director or stress equalizer is
essential to protect the abutment teeth.
• The most commonly used ones are composed of a hinge
device interposed between the minor connector of the
abutment tooth and the denture base.

PHYSIOLOGIC BASING
 The belief is that the equalization can best and most simply
be accomplished by some form of physiologic basing.
• The physiologic basing is produced either by
• Displacing or depressing the ridge mucosa during the
impression making procedure
• Relining the denture base after it has been constructed.

 Displacing the mucosa during the impression procedure
records it in its functioning and not the anatomic form.
 This denture base formed over displaced tissue, will adapt
more readily to the depressed tissue when occlusal force
acts and will be better able to withstand the force that is
generated

BROAD STRESS DISTRIBUTION
distributing the forces of
occlusion over as many teeth
and as much of the available
soft tissue area as possible.
This is accomplished by the
use of additional rests,
indirect retainers, clasps and
broad coverage denture
bases

STEP BY STEP DESIGNING
Diagnostic cast area of recontouring
Black-mark survey lines desired undercut is measured

Desired rest seat-red desired denture base-blue
Major connector-brown

Retentive elements completed –rt side
Left side occlusal

REFERENCES
1) McCracken’s removable partial Prosthodontics 11th
edition.
2) Stewart’s Removable partial Prosthodontics 4th
edition.
3) David M Watt; A Roy Macgregar :Designing
partial dentures.
4) Miller E.L. : “Text book of Removable Partial
Prosthodontics”. St. Louis, CV Mosby Co.

5) Kratochvil et al influence of occlusal rest position and
clasp design on movement of abutment teeth J Prosthet
Dent 1963:13;114
6) John W.McCartney effect of location of rest and
movement of abutment tooth J Prosthet Dent 1980:43:15
7) David N Firtell reaction of anterior abutment of kennedy
classII removable partial denture to various clasp arm
designs J Prosthet Dent 1985:53;77
8) Richard P Frank direct retainers for distal extension
removable partial dentures J Prosthet dent 1986:56;562

9) Frank and Nicholis an investigation of effectiveness
of indirect retainer J Prosthet dent 1977:38;494
10)Frechette et al the influence of partial denture design
on distribution of force to abutment teeth J Prosthet
Dent 1956:6;195
11)A.H.Schmidt Planning and designing removable
partial denture J Prosthet Dent 1953:3;783

Principles of Removable Partial Dentures

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