2. principles of designing rpd with special emphsis on support and perio

GOOD MORNING

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 considerations4) Forces influencing the magnitude of stress5) Biomechanical consideration of individual

component

6) Philosophies of designs

7) Essentials of designing

8) Procedure of designing

9) Conclusion

10)References

• INTRODUCTION

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

INDICATION FOR REMOVABLE PARTIAL 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

INDICATION FOR REMOVABLE PARTIAL 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 11 th 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 IClass IIClass 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 planeStewart’s clinical Removable Posthodontics 4th edition

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 designMcCracken’s Removable Partial Prosthodontics 11 th 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. CLASP AS A FACTOR IN STRESS

• More flexible the retentive arm of claspless 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 RESTS1) Occlusal rest2) Extended occlusal rest3) Incisal ,lingaul or cingulum rest4) Rest on restoration5) 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 forcesMcCracken’s Removable Partial Prosthodontics 11 th 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 appliedMcCracken’s Removable Partial Prosthodontics 11 th edition

CLASP DESIGNS:

• Combination clasp

• Reverse circlet clasp

• Bar clasp

• T Clasp

• Modified T Clasp

• Y clasp

• I clasp

• RPI system

INDIRECT RETAINER

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

• CONCLUSION

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

THANK YOU

2. principles of designing rpd with special emphsis on support and perio

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