PRINCIPLES OF DESIGNING REMOVABLE PARTIAL DENTURE

All tooth supported RPD Tooth and tissue supported RPD

All tooth supported RPD

Tooth and tissue supported RPD Class I Class II Class IV

The forces generated in RPD’s can be understood by analyzing the action of two simple machines. Liver and fulcrum Inclined plane

Liver and fulcrum First - class lever Second - class lever Third - class lever

Inclined plane

Forces acting on partial denture This fulcrum controls the movement of denture in sagittal plane. Movement is greatest in magnitude

The second fulcrum line extends from occlusal rest of terminal abutment and along the crest of the ridge. Controls movement in vertical plane. Most damaging

The third fulcrum is located in the vicinity of midline just lingual to the anterior teeth.

Every effort should be made to design the RPD to control or minimize rotational movement related to these three fulcrum lines

Factors influencing magnitude of stresses transmitted to abutment teeth Length of span Quality of support of the ridge Qualities of the clasp Clasp design Length of clasp Material used Abutment tooth surface Occlusal harmony

Thorough knowledge of mechanical & biological factors BASIC PRINCIPLES Thorough knowledge of mechanical & biological factors Complete examination & diagnosis of patients Restoration of form & functions of oral cavity A removable partial denture is a form of treatment & not cure

PHILOSOPHY OF DESIGN Various philosophies have been suggested related to removable partial denture, none has proved to be clinically advantageous over another All of them have produced excellent results if physiological limit of supporting structures is observed Occlusal forces should be distributed evenly and equally over the large supporting area. .

Main factor which influences the design of removable partial denture is type of denture support; whether tooth supported or tissue supported or combination of both. Accordingly, impression procedures will differ for both of them.

THREE BASIC APPROCHES TO DISTRIBUTE OCCLUSAL FORCES 1. STRESS EQUALIZATION 2. PHYSIOLOGIC BASING 3. BROAD STRESS DISTRIBUTION

So it advocates use of Stress Director Or Stress Equalizer 1. STRESS EQUALIZATION According to this concept compressibility & resiliency of tooth periodontal ligament & mucosal tissue is different It causes an unequal distribution of stress on abutment teeth & alveolar ridge resulting in weakening of abutment teeth So it advocates use of Stress Director Or Stress Equalizer

2. PHYSIOLOGIC BASING This theory denies use of stress breaker in removable partial denture It advocates use of functional impression procedure to record the tissue in its functional form It will compress tissue even at rest, so occlusal loading can’t compress tissue further Due to the rebound action of mucosa it will position the artificial teeth above occlusal plane

ADVANTAGES Simplicity of design Good adaptation to underlining mucosa Minimum direct retention required It decreases functional stresses on abutment teeth

DISADVANTAGES Tissues are always in state of compression It increases rate of Resorption of ridge Denture teeth are always slightly above plane of occlusion It causes instability of denture due to less number of retentive components It does not provide stability against lateral forces

3. BROAD STRESS DISTRIBUTION It advocates use of maximum soft tissue area & maximum no of teeth for even distribution of stress It is achieved by use of additional rest, direct retainer & by increasing denture base area

Broad stress distribution

It reduces forces on abutment teeth ADVANTAGES It reduces forces on abutment teeth Health of oral tissues & abutment teeth maintained. Additional rest seats or direct retainers reduces lateral forces on abutment teeth It also acts as a splint

DISADVANTAGES Due to greater amount of soft tissue coverage, it causes discomfort to patient Difficult to maintain oral hygiene properly

ESSENTIALS OF DESIGNING Key factors to be considered while designing of removable partial denture Direct Retainers Rest Indirect Retainers Major Connector Minor Connector Type Of Occlusion Artificial Teeth

Indirect retainer It is employed to neutralize unseating forces Should be located as far anterior of fulcrum line as possible Class I design -- Two indirect retainers Class II design -- one on opposite side

Class I Class II

CLASS III DESIGN 1. Quadrilateral positioning of direct retainer 2. Bracing arm should be rigid

Rests Prepared next to edentulous space Indirect retention Not required Major & minor connectors Same as class I & class II Occlusion Denture base Functional impression not required

Class IV design Preserve labial alveolar bone Direct retention – quadrilateral configuration Major connector –should be rigid with broad palatal coverage Indirect retention – should be as far posterior as possible functional impression – may required

Class IV

A. Circumferential cast clasp DESIGNS OF CLASP A. Circumferential cast clasp

Bar clasp

DESIGNING PROCEDURE Dentist should have thorough knowledge of all components of removable partial denture The design of the removable partial denture should be simple & should follow mechanical & biological standards.