rpd designing

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Principal Of Partial Denture Design Stresses acting on RPDs are transmitted to the teeth, and to the tissues of the residual ridges. The stresses, which tend to move the PD in different directions are: 1. Masticatory stress( Tissue ward movt). 2. Gravity( Tissue away movt). 3. Sticky food pull the denture occlusaly (Tissue-away movt).

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Page 1: Rpd Designing

Principal Of Partial Denture Design

Stresses acting on RPDs are transmitted to the teeth, and to the tissues of the residual ridges.

The stresses, which tend to move the PD in different directions are:

1. Masticatory stress( Tissue ward movt).2. Gravity( Tissue away movt).3. Sticky food pull the denture occlusaly

(Tissue-away movt).

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4. Muscles and tongue tend to displace denture from its foundation.

5. Intercuspation of the teeth may tend to produce horizontal and rotational stresses unless occlusal is adjusted.

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Displacement to lateral and anteroposterior direction

1 & 2 occlusal contacts 3 oral musculature

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Designing Bracing and StabilityProperly Constructed PD Must Have:

Stability( bracing) resistance to horizontal and lateral displacement. Bracing( providing resistance to lateral

movt.of RPD). Causes of tipping, rocking and

rotation of R.P.D.1. Quality of supporting structure.2. The tissue-ward movement of the free

end base create an axis of rotation around which this appliance is rotated.

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This axis of rotation is called a fulcrum line.According to GPT Fulcrum line is a theoretical line passing

through the point around which a lever functions and at right angles to its path of movement. The determinants for the fulcrum line are usually the cross arch occlusal rests located adjacent to the tissue borne components around which RPD rotates under masticatory forces.

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The line drawn between the retentive tips of a pair of clasps on opposite sides of the arch.

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Multiple fulcrum lines can exist

Where there is more than one clasp axis, as in this Kennedy Class III denture, it is the clasps on the axis closer to the saddle in question which make the major contribution to indirect retention.

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POSSIBLE MOVEMENTS OF PARTIALDENTURE

Possible movements do not occur singularly or independently but tend to be dynamic, and all occur at the same time.

The greatest movement possible is found in the tooth-tissue-supported prosthesis, because of the reliance on the distal extension supporting tissue to share the functional loads with the teeth.

Movement of a distal extension base toward the ridge tissue will be proportionate to the quality of that tissue, the accuracy and extent of the denture base, and the applied total functional load.

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ROTATION AROUND A FULCRUM LINE IN SAGGITAL PLANE One movement is rotation about an axis through the

most posterior abutments. This axis may be through occlusal rests or any other rigid portion of a direct retainer assembly located occlusally or incisally to the height of contour of the primary abutments. This axis, known as the fulcrum line 

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The axis of rotation may shift toward more anteriorly placed components, occlusal or incisal to the height of contour of the abutment, as the base moves away from the supporting tissue when vertical dislodging forces act on the partial denture. These dislodging forces result from the vertical pull of food between opposing tooth surfaces, the effect of moving border tissue, and the forces of gravity against a maxillary partial denture.

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Presuming that the direct retainers are functional and that the supportive anterior components remain seated, rotation—rather than total displacement—should occur.

Vertical tissueward movement of the denture base is resisted by the tissue of the residual ridge in proportion to the supporting quality of that tissue, the accuracy of the fit of the denture base, and the total amount of occlusal load applied.

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Movement of the base in the opposite direction is resisted by the action of the retentive clasp arms on terminal abutments and the action of stabilizing minor connectors in conjunction with seated, vertical support elements of the framework anterior to the terminal abutments acting as indirect retainers.

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Indirect retainers- resist movement of the RPD away from the tissue base. They are placed as far anterior to the fulcrum line as possible (Ideally placed, perpendicular to the fulcrum line).These are1. Rests 2. Minor connectors3. Proximal plates4. Major connector

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Need for Support for the indirect retainer When the saddle is first displaced, mucosal compression beneath the indirect retainer allows the denture to rotate around the clasp axis (fulcrum).

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(1)When possible, the indirect retainer should rest on a surface at right angles to its potential path of movement.

(2) If it rests on an inclined tooth surface, movement of the tooth might occur with resulting loss of support for the indirect retainer.

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This saddle has an occlusal rest and a clasp on the abutment tooth, and the connector is a sublingual bar. Although normally a mesial rest might well be preferred, a distal rest has been used in this example to simplify the explanation which follows. When sticky foods displace the saddle in an occlusal direction the tips of the retentive clasps engaging the undercuts on the abutment teeth provide the only mechanical resistance to the movement. The saddle thus pivots about the clasp tips.

In the maxilla this movement of the saddle away from the ridge may also be caused by gravity.

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If the design is modified by placing a rest on an anterior tooth, this rest (indirect retainer) becomes the fulcrum of movement of the saddle in an occlusal direction causing the clasp to move up the tooth, engage the undercut and thus resist the tendency for the denture to pivot.

F = Fulcrum – indirect retainer, a component which obtains support.R = Resistance – retention generated by the clasp.E = Effort – displacing force, eg a bolus of sticky food.

It can thus be seen that to obtain indirect retention the clasp must always be placed between the saddle and the indirect retainer.The RPD design should strive to reduce the mechanical advantage of the displacing force by placing the clasp axis as close as possible to the saddle and by placing the indirect retainers as far as possible from the saddle.

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Indirect retainers do not prevent displacement towards the ridge. This movement is resisted by the occlusal rest on the abutment tooth and by full extension of the saddle to gain maximum support from the residual ridge. In addition, it may be necessary to compensate for the compressibility of the denture-bearing mucosa by using the altered cast impression technique

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Kennedy I: Indirect retention in this design is provided by incisal rests on LR3 (43) and LL3 (33).

Displacement in an occlusal direction is indicated by an asterisk

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Kennedy II: Indirect retention in this instance is provided primarily by rests on LR4 (44) and LR3 (43) as they are furthest from the clasp axis. The rests on LL5 (35), LR6 (46) and LR7 (47) are close to the clasp axis and therefore contribute little to the indirect retention.

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Kennedy III: In the case of a bounded saddle there is the potential for direct retention from both abutments. When this can be achieved, as for the saddle replacing UR6 (16) and UR5 (15), indirect retention is not required. However, it is not uncommon for only one of the abutments to be suitable for clasping. In this design a clasp on UL3 (23) has been omitted for aesthetic reasons. Under such circumstances indirect retention can be employed, the major contribution being made by the rest on UR7 (17).

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Kennedy IV: In a maxillary denture it is sometimes difficult to achieve much separation of the clasp axis and indirect retainers. In this example, clasps engage the mesiobuccal undercuts on UR6 (16) and UL6 (26) and indirect retention has been achieved by placing the rests on UR7 (17) and UL7 (27) as far posteriorly as possibl

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ROTATION ABOUT A LONGITUDINAL AXIS

A second movement is rotation about a longitudinal axis as the distal extension base moves in a rotary direction about the residual ridge

This movement is resisted primarily by the rigidity of the major and minor connectors and their ability to resist torque.

If the connectors are not rigid or if a stress-breaker exists between the distal extension base and the major connector, this rotation about a longitudinal axis either applies undue stress to the sides of the supporting ridge or causes horizontal shifting of the denture base.

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How to counteract lateral shifting?

1. Bracing the sides of the teeth by means of rigid clasp arms.

2. Use of continuous bar resting on the lingual surfaces of the natural standing teeth.

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Shaded areas resist forces showed by arrows

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Rotation resisted by appropriately placed components