Biomechanics of Removable partial

denture

INTRODUCTION

Biomechanics basically deals with application of Biomechanics basically deals with application of mechanical principles to biological tissues. In the oral mechanical principles to biological tissues. In the oral cavity one would find a number of sources of stress cavity one would find a number of sources of stress

generation, the human body is built in such a manner generation, the human body is built in such a manner that it learns to adapt to any stressful situation. that it learns to adapt to any stressful situation.

However when we try to create an artificial However when we try to create an artificial replacement of that natural component which is lost, replacement of that natural component which is lost,

we are at a loss in making it fully functional and we are at a loss in making it fully functional and adaptable.adaptable.

Biomechanics (GPT 7):

1: the application of mechanical laws to living

structures, specifically the locomotor systems of the

body. 2: the study of biology from the functional viewpoint. 3: An application of the principles of engineering

design as implemented in living organisms.

Mechanical principles applicable in Removable Mechanical principles applicable in Removable ProsthodonticsProsthodontics

Lever principle

Inclined plane

Snowshoe principle

Lever:Lever: A simple machine consisting of a rigid A simple machine consisting of a rigid

bar pivoted on a fixed point and used to bar pivoted on a fixed point and used to

transmit force, as in raising or moving a transmit force, as in raising or moving a

weight at one end by pushing down on the weight at one end by pushing down on the

otherother..

LeverClass I

Class II

Class III

Inclined plane

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.

Snowshoe principle This principle is based on distribution of forces to as large an area as This principle is based on distribution of forces to as large an area as

possible. Like in a snow shoe which is designed to distribute forces on possible. Like in a snow shoe which is designed to distribute forces on the entire base area of the shoe, a partial denture should cover the entire base area of the shoe, a partial denture should cover maximum area possible within the physiologic limits so as to maximum area possible within the physiologic limits so as to distribute the forces over a larger area.distribute the forces over a larger area.

Stress consideration in a partial denture

The stresses can be divided as:

Vertical

1.Displacing stresses

2. Dislodging stresses

Horizontal

Torsional

Displacing stress

Those forces which are the result of stresses along the long Those forces which are the result of stresses along the long axis of the teeth in a crown to apex direction and the axis of the teeth in a crown to apex direction and the relatively vertical stresses on the ridge mucosa. These are relatively vertical stresses on the ridge mucosa. These are the least harmful and are born well if within physiologic the least harmful and are born well if within physiologic limits.limits.

Dislodging stress

These are the forces which tend to lift the partial denture from its These are the forces which tend to lift the partial denture from its rest position. Reciprocal dislodging action occurs when wide rest position. Reciprocal dislodging action occurs when wide edentulous spaces are interrupted by few teeth thus inviting an edentulous spaces are interrupted by few teeth thus inviting an antero-posterior or lateral tilt of prosthesis.antero-posterior or lateral tilt of prosthesis.

Horizontal stress

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

Torsional stress

It is a twisting rotational type of force. It’s a combination of It is a twisting rotational type of force. It’s a combination of vertical and horizontal force. Torsion is noted most frequently vertical and horizontal force. Torsion is noted most frequently where a long segment acts upon the first abutment it engages. where a long segment acts upon the first abutment it engages. Where the ridge mucosa has higher resiliency torque is higher. Where the ridge mucosa has higher resiliency torque is higher. Torque applies rotation about a fixed point.Torque applies rotation about a fixed point.

Biomechanical consideration of individual component

“Perpetual preservation of

what is remaining is more important than meticulous replacement of

what is lost” De

Van

Major connector

Connects parts of prosthesis located on one side

of the arch with those on the opposite side

Should be rigid:

Distribution of force

Effectiveness of other components

If flexible forces concentrated on individual teeth or

ridge damaging.

Direct retainer

Rest

Relation between rest and abutment should be such that forces should be directed apically down along the long axis of the tooth Stress absorbed by fibers of PDL without getting damaged.

Indirect retainer

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.

Contributes to support and stability of the partial denture counteracts horizontal forces applied to the denture. Long span mandibular lingual bar major connector (even if tooth supported), IR provides additional support and rigidity for lingual bar. Prevents impingement of the lingual bar on the mucosa during function.

Factors influencing magnitude of stress

Length of span

Quality of ridge

Type of mucosa

Clasp type

Clasp design

Amount of tooth contact

Occlusion

Control stress by design consideration

1. Retention as a means of stress control

A) forces of adhesion and cohesion

B) atmospheric pressure

C) Frictional contact

D) Neuromuscular control

Strategic clasp positioning

a) Quadrilateral configuration

b) Tripod configuration

c) Bilateral configuration

Clasp design

a) circumferential clasp

b) bar clasp

c) combination clasp

4 Indirect retention

5 Functional basing

6 occlusion

a) harmonious occlusion

b) size of food table

c) occlusal pattern of posterior teeth

7 Denture base

a) Size & configuration

b) accuracy of adaptation of base to

the tissues

Removable partial dentures by design are intended to be removed from and replaced into the mouth. Because of this, they are not rigidly connected to the teeth or tissues, which means that they are subjected to movement in response to functional loads. These movement can cause generation of stresses which if not controlled , can damage the remaining teeth and supporting bone.Efforts must be always directed towards minimizing the harmful forces on the remaining teeth by proper designing of the RPD.

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