PRINCIPLES OF RPD DESIGNING

PRESENTED BY:-DR SHEBIN ABRAHAMPOST GRADUATE

DEPT OF PROSTHODONTICS

CONTENTS 1. INTRODUCTION2. BIOMECHANICS OF RPD.3. BIOMECHANICAL CONSIDERATIONS.4. POSSIBLE TYPE OF MOVEMENT TAKING PLACE.5. DIFFERENCES IN DESIGN BETWEEN TOOTH SUPPORTED

AND TISSUE SUPPORTED PROSTHESIS.6. FACTORS CONTRIBUTING TO AMOUNT OF STRESS

TRANSMITTED TO THE ABUTMENT.7. DESIGN CONSIDERATIONS OF PROSTHESIS TO

CONTROL STRESS.8. RPI RPA CONCEPT.9. DESIGN CONSIDERATION FOR OBTURATOR

PROSTHESIS.10.REFERENCE.

INTRODUCTION

BIOMECHANICS OF RPD

WHY IS IT IMPORTANT TO UNDERSTAND THE BIOMECHANICS?????????

• REMOVABLE PARTIAL DENTURE FORCES IN ORAL CAVITY.

• CAUSES MOVEMENT OF VARIOUS COMPONENT OF THE RPD.

• SO ITS IMPORTANT TO UNDERSTAND THE MOVEMENTS TAKING PLACE ON THESE COMPONENTS AND LOGICALLY HELP DESIGN THEM IN ORDER TO CONTROL THE MOVEMENTS TAKING PLACE IN THEM.

• WHAT ARE THE TYPES OF MOVEMENTS TAKING PLACE IN THE ORAL CAVITY????

BIOMECHANICAL CONSIDERATIONS• DESIGNING OF AN RPD IS BASED ON BOTH BIOLOGICAL

AND MECHANICAL CONSIDERATIONS.• MOST OF US DENTIST CONSIDER THE MECHANICAL

ASPECTS BUT IT IS ALSO IMPORTANT TO UNDERSTAND THE BIOLOGICAL ASPECT.

• BIOLOGICAL ASPECTS :-1. WHETHER TOOTH USED FOR SUPPORT CAN BEAR THE LOADS

FALLING ON IT.2. THE TYPE OF UNDERLYING MUCOSA.

• THE RESISTANCE TO LOAD FROM A TOOTH IS BASED ON THE AMOUNT OF FORCE FALLING ON IT, THE DURATION OF FORCE AND DIRECTION OF FORCE APPLIED.

• MECHANICAL ASPECTS:-1. A RPD LEVER, MAINLY DISTAL

EXTENSION.• BASED ON THE TYPE OF LEVER THE

FORCES APPLIED ON THE ABUTMENT TEETH VARIES.

• THE LEVER HAS THE POTENTIALITY TO INCREASE THE FORCES FALLING ON THE TOOTH.

2. CANTILEVER TYPE DESIGN RPD SHOULD BE AVOIDED.

3. TOOTH TENDS TO WITHSTAND VERTICAL FORCES THAN NON VERTICAL FORCES BECAUSE OF THE NUMBER OF PDL FIBRES INVOLVED.

4. AN ABUTMENT TOOTH WILL WITHSTAND NON VERTICAL FORCES IF THE FORCES ARE APPLIED AS CLOSE TO THE HORIZONTAL AXIS OF ROTATION.

POSSIBLE TYPE OF MOVEMENTS TAKING PLACE.

• GREATEST MOVEMENT TOOTH TISSUE SUPPORTED RPD.• MOST COMMON MOVEMENT TAKING PLACE ROTATIONAL

MOVEMENT.• ROTATIONAL MOVEMENT DIFFERENT PLANES, IN A

DYNAMIC COMPLEX WAY.• THIS MOVEMENT IS BASED :-

1. QUALITY OF TISSUE, 2. AMOUNT OF FUNCTIONAL LOAD APPLIED AND 3. ALSO ON THE QUALITY OF THE DENTURE BASE.

• USUALLY THERE ARE THREE TYPES OF ROTATIONAL MOVEMENTS TAKING PLACE:

• FIRST ROTATION • AXIS OF ROTATION

HORIZONTAL PLANE• EXTENDS THROUGH THE REST OF

DISTAL ABUTMENTS.• THIS AXIS IS CALLED FULCRUM

LINE.• ROTATION OF DENTURE IN

SAGITTAL PLANE(TOWARDS OR AWAY FROM RIDGE)

• GREATEST MAGNITUDE OF ALL MOVEMENTS.

• NOT NECESSARILY DAMAGING AS ROTATION IS IN APICAL DIRECTION.

• SECOND FULCRUM LINE SAGITTAL PLANE.

• IT EXTENDS FROM THE DISTAL ABUTMENT OCCLUSAL REST TO THE CREST OF THE RIDGE

• CLASS I SITUATION TWO SUCH LINES, ONE ON EACH SIDE OF THE ARCH.

• ROTATIONAL MOVEMENT OCCURS IN THE FRONTAL PLANE OR FACIOLINGUAL DIRECTION(ROCKING MOVEMENT OF DENTURE)

• LESSER MAGNITUDE• INDUCES HORIZONTAL STRESS ON

THE ABUTMENT.• IT IS MODERATELY DAMAGING.

• THE THIRD FULCRUM MIDLINE JUST LINGUAL TO ANTERIOR TEETH.

• FULCRUM LINE VERTICALLY• ROTATIONAL MOVEMENT IN

THE HORIZONTAL PLANE (IE, THE FLAT, ARCUATE MOVEMENTS OF THE PROSTHESIS).

• THE FORCE RESULTING FROM THIS MOVEMENT IS ALMOST ENTIRELY HORIZONTAL.

• FORCES CAN BE EXTREMELY DAMAGING AND SHOULD RECEIVE SIGNIFICANT ATTENTION DURING THE DESIGN PROCESS.

DIFFERENCE IN DESIGN BETWEEN TOOTH SUPPORTED AND TISSUE

SUPPORTED PROSTHESIS• TOOTH SUPPORTED:-

• CLASS 3,CLASS 4• DERIVE SUPPORT FROM TEETH• THE MOVEMENT POTENTIAL IS

LESS BECAUSE THE TEETH PROVIDE RESISTANCE TO FUNCTIONAL LOADING.

• SINGLE UNIVERSAL DESIGN CAN BE FOLLOWED FOR TEETH SUPPORTED PROSTHESIS

• THE DENTURE BASE IS MADE UP OF METAL BASES.

• TISSUE SUPPORTED:-• SEEN IN CLASS1 AND CLASS2

CASES.• TISSUE MAXIMUM AMOUNT

OF SUPPORT (TISSUE PROVIDES PRIMARY SUPPORT AND TEETH PROVIDE SECONDARY SUPPORT).

• TOO MUCH TISSUE MOVEMENT BECAUSE OF THE DYNAMIC STATE OF THE TISSUE.

• MULTIPLE COMPLEX DESIGN HAVE TO FOLLOWED.

• DENTURE BASE ACRYLIC RESIN

FACTORS CONTRIBUTING TO THE AMOUNT OF STRESS TRANSMITTED TO THE ABUTMENT

1. LENGTH OF EDENTULOUS SPAN:- • THE LONGER THE EDENTULOUS

SPAN THE LONGER THE DENTURE BASE GREATER THE LEVERAGE FORCE TRANSMITTED TO THE ABUTMENT TEETH.

• LOAD ARTIFICIAL TEETH THE LENGTH OF EDENTULOUS RIDGE(DENTURE BASE) DETERMINES THE FORCE ASSOCIATED WITH ABUTMENT.

• TRY RETAINING POSTERIOR TEETH TO PREVENT THIS PROBLEM.

• OTHER FACTOR LIKE THICKNESS OF MUCOSA, AMOUNT OF OCCLUSAL FORCE ALSO CONTRIBUTES TO IT.

2. QUALITY OF RIDGE SUPPORT:-• LARGE WELL ROUNDED RIDGES

BETTER STRESS DISTRIBUTION• SMALL THIN, KNIFE LIKE RIDGES ARE

VERY POOR.• TALL BROAD RIDGE LONGER

DENTURE FLANGE WITHSTAND LATERAL FORCES.

• THICKNESS AND HEALTH OF THE MUCOPERIOSTEUM ALSO INFLUENCE THE LOADS TRANSFERRED TO ABUTMENT TEETH.

• 2 MM THICK HEALTHY MUCOSA BEAR GREATER FUNCTIONAL LOADS.

• THIN FRIABLE, FLABBY MUCOSA ARE VERY POOR IN WITHSTANDING STRESSES.

Factors influencing magnitude of stress

3. CLASP FLEXIBILITY:-• A FLEXIBLE CLASP IS MORE SOUGHT IN CASE OF TOOTH TISSUE

SUPPORTED DENTURES BECAUSE IT TRANSMITS LESS LOAD TO THE TOOTH STRUCTURE.

• WROUGHT WIRE CLASP IS MOST COMMONLY USED.• IT HAS TENDENCY TO PRODUCE VERY HIGH AMOUNT OF LATERAL

STRESS.• BUT NOT INDICATED IN CASES WITH POOR RIDGES AS IT CANNOT

WITH STAND LATERAL STRESSES LEADING TO HEAVY STRESSES ON THE RIDGES.

4. CLASP DESIGN:-• IT SHOULD BE PASSIVE.• IT SHOULD BE SUCH THAT IT DOESN'T APPLY FORCE ON THE TEETH

WHILE IT IS SEATED ON THE TEETH.• SO COMPLETE SEATING OF PROSTHESIS IS MANDATORY.• RECIPROCAL ARM SHOULD DESIGNED THAT IT LIES IN THE ABOVE

THE HEIGHT OF CONTOUR.

Factors influencing magnitude of stress

5. LENGTH OF THE CLASP• A LONGER CLASP IS MORE FLEXIBLE.• AND A CURVED PATH THAN A STRAIGHT

PATH ON THE TOOTH SURFACE WILL PROVIDE MORE FLEXIBILITY

6. MATERIAL USED FOR CLASP CONSTRUCTION:-• CO CR ALLOYS BASED CLASP APPLY MORE

FORCE ON THE ABUTMENT TEETH THAN GOLD BASED ALLOY.

• SO THINNER DIAMETER CO CR CLASP CAN BE USED TO REDUCE THE AMOUNT OF FORCE APPLIED.

7. SURFACE CHARACTERISTICS OF AN ALLOY:-• GOLD CROWNS MORE RESISTANCE TO

CLASP THAN ENAMEL.• ABUTMENT RESTORED WITH GOLD

EXPERIENCES MORE FORCES THAN INTACT ENAMEL.

Factors influencing magnitude of stress

8. OCCLUSAL HARMONY:-• IT PLAYS AN IMPORTANT ROLE.• DEFLECTIVE CONTACTS SHOULD BE AVOIDED.• PROSTHESIS OPPOSING NATURAL DENTITION FACE MORE

FORCES THAN FROM A NATURAL DENTITION.• OCCLUSAL FORCE SHOULD BE DIRECTED TO MIDDLE OF

RESIDUAL ALVEOLAR RIDGE CLOSER TO THE ABUTMENT.

Factors influencing magnitude of stress

DESIGN CONSIDERATION- CONTROLLING STRESS

• DIRECT RETENTION:-• THE RETENTIVE CLASP ARM

DESTRUCTIVE FORCES -ABUTMENT• SO RETENTIVE CLASP ARM SHOULD

PROVIDE ADEQUATE RETENTION WITH LEAST FORCES.

• THIS CAN BE DONE BY PROVIDING RETENTION FROM OTHER COMPONENTS OF DENTURE.

• THE SUPPORT AND STABILITY OF THE PROSTHESIS ALSO MAY BE IMPROVED.

• OTHER COMPONENTS THAT PROVIDE ADDITIONAL RETENTION ARE:-• ADHESION COHESION.• FRICTIONAL GRIP.• NEUROMUSCULAR CONTROL.

• CLASP POSITION :-• OFTEN, THE SPATIAL DISTRIBUTION OF RETENTIVE CLASPS IS

MORE IMPORTANT TO RETENTION THAN THE NUMBER OF CLASPS.

• THE FOLLOWING CONFIGURATIONS CAN BE FOLLOWED WHILE DETERMINING CLASP POSITION.

1. QUADRILATERAL CONFIGURATION:- IT IS USED IN CLASS 3 SITUATION WITH MODIFICATION SPACE.

2. TRIPODAL CONFIGURATION:- CLASS 2 MODIFICATION 13. BILATERAL CONFIGURATION

Design considerations

1. QUADRILATERAL CONFIGURATION:-• CLASS 3 WITH MODIFICATION SPACE.• CLASP ASSEMBLY ON BOTH ABUTMENT TEETH

ADJACENT TO EDENTULOUS SPACE.• IN CASE OF ABSENCE OF MODIFICATION SPACE

CLASP ASSEMBLY ANTERIORLY AND POSTERIORLY ARE GIVEN ON THE DENTULOUS OPPOSITE ARCH.

2. TRIPODAL CONFIGURATION:-• CLASS 2 WITH MODIFICATION SPACE.• CLASP PRESENT ADJUSTMENT TO THE

EDENTULOUS SPACE.• ON THE MODIFICATION SPACE SIDE CLASP ON

BOTH THE ABUTMENT TEETH.• IF MODIFICATION IS ABSENT CLASP PLACED AS FAR

ANTERIORLY AND POSTERIORLY ON THE TEETH.• NOT AS EFFECTIVE AS QUADRILATERAL

CONFIGURATION BUT BETTER IN CLASS 2 CASES.3. BILATERAL CONFIGURATION:-

• CLASS 1 SITUATIONS.• PROVIDES LEAST STRESS REDUCTION.

• CLASP DESIGN:-• CAST CIRCUMFERENTIAL CLASP:-• CLASS 2 AND CLASS 1 CASES CLASP

ASSEMBLY INVOLVING DISTO OCCLUSAL REST AND RETENTIVE TIP INVOLVING MESIOFACIAL UNDERCUT IS PREVENTED.

• TERMINAL END OF SUCH CLASP TIPPING FORCES ON THE ABUTMENT TEETH.

• A CLASP THAT ORIGINATES FROM THE MESIOOCCLUSAL REST AND ENGAGES THE DISTOFACIAL UNDERCUT OR A REVERSE CIRCULATE CLASP SHOULD BE USED.

• VERTICAL PROJECTION CLASP:-• T CLASP OR MODIFIED T CLASP CAN BE USED ON

AN ABUTMENT ADJACENT TO THE DISTAL EXTENSION SPACE.

• IT IS USED WHEN THE A DISTOFACIAL UNDERCUT IS SEEN ON THE ABUTMENT.

• IT IS NOT INDICATED IN CASE OF MESIOFACIAL UNDERCUT.

• I CLASP IS BETTER USED INVOLVING MESIOFACIAL UNDERCUT AND MESIOOCCLUSAL REST SEAT.

• IT DOESN’T APPLY ANY STRESS ON TEETH.

• COMBINATION CLASP:-• USED WHEN A DISTAL EXTENSION IS PRESENTED WITH A

MESIOFACIAL UNDERCUT.• FLEX MORE AND IN MULTIPLE SPATIAL PLANES..• COMBINATION OF BOTH CAST AND WROUGHT WIRE IS

USED.• WROUGHT WIRE IS USED AS THE RETENTIVE ARM AND

CAST METAL IS USED AS THE RECIPROCAL ARM• MORE FLEXIBLE AND PRODUCES LESS STRESS ON THE

ABUTMENT.

SPLINTING OF ABUTMENT TEETH:-• TOOTH WITH DECREASED PERIODONTAL SUPPORT CAN BE SPLINTED

TOGETHER• IT STABILIZES THE ABUTMENT IN A MESIO-DISTAL DIRECTION.• SPLINT EXTENDS ANTERIORLY BEYOND THE CANINE THE STABILIZATION

EFFECT IS PRESENT EVEN IN FACIOLINGUAL DIRECTION.• ON OF THE MOST COMMON CONSIDERATION FOR FIXED SPLINTING IS LONE

STANDING ABUTMENT ADJACENT TO DISTAL EXTENSION EDENTULOUS SPACE.• SUCH A TOOTH CAN GET DAMAGED DUE TO ROTATIONAL FORCES.• FIXED SPLINTING IN THIS SITUATION SERVES THE PURPOSE.

• SPLINTING REMOVABLE PROSTHESIS

• NOT DONE IF FIXED SPLINTING IS POSSIBLE.

• THE SPLINTING CONSISTS OF CLASPING MORE THAN ONE TOOTH ON EACH SIDE OF THE ARCH AND USING ADDITIONAL RESTS FOR INCREASED SUPPORT.

• NOT ALL CLASPS TO BE RETENTIVE.

• PREPARED GUIDING PLANES MAY PROVIDE ADDITIONAL HORIZONTAL STABILITY.

• RESULTS IN DECREASED MOBILITY.• CROSS ARCH STABILIZATION

INDIRECT RETENTION

• IT PREVENTS RESIST ROTATION AND OR DISPLACEMENT OF A REMOVABLE PARTIAL DENTURE.

• IT IS LOCATED ON THE OPPOSITE SIDE OF THE PREVIOUS FULCRUM LINE AS FAR AS POSSIBLE.

• USED IN CLASS 1 REMOVABLE PARTIAL DENTURES.• NOT AS USEFUL IN CLASS2 SITUATION, IT IS USED ONLY IF A

MODIFICATION SPACE IS PRESENT. • CLASS 3 SITUATION USUALLY DOESN’T NEED A INDIRECT RETAINER

AS THERE IS NO LEVER EFFECT.• CLASS 4 SITUATION IT JUST OPPOSITE TO THE CLASS 1 SITUATION

WITH INDIRECT RETAINER PRESENT AS FAR POSTERIORLY AS POSSIBLE.

• OCCLUSION:- • A SMOOTHLY FUNCTIONING OCCLUSION TMJ• THE CONTACTS OF THE REMAINING NATURAL TEETH

SHOULD BE THE SAME WHETHER THE REMOVABLE PARTIAL DENTURE IS IN MOUTH OR NOT.

• THE NUMBER OF TEETH REPLACING THE NATURAL TEETH SHOULD BE REDUCED TO DECREASE THE AMOUNT OF FORCE FALLING ON THE RIDGE.

• ARTIFICIAL POSTERIOR TEETH SHOULD HAVE SHARP CUSP WITH LOW INCLINE PLANE IN ORDER TO INCREASE THE CUTTING EFFICIENCY AND PREVENT HORIZONTAL INTERFERENCES FORCE.

DENTURE BASES:-• THE DENTURE BASE SHOULD EXTEND AS MUCH DENTURE BEARING AREA AS

POSSIBLE IN ORDER TO DISSIPATE THE FORCES FALLING ON THE PROSTHESIS.• DENTURE FLANGES SHOULD BE AS LONG AS POSSIBLE.• MAXILLARY DENTURE BASE EXTEND MAXILLARY TUBEROSITY.• MANDIBULAR DENTURE BASE RETROMOLAR PAD.• OVEREXTENSION OF DENTURE BASE SHOULD BE AVOIDED.• ACCURATE ADAPTATION OF DENTURE BASE IS NECESSARY FOR PROPER

RETENTION OF DENTURE BASE.• THE EXTERNAL POLISHED SURFACE OF THE DENTURE SHOULD BE

CONTOURED PROPERLY IN ORDER TO AID IN RETENTION OF THE DENTURES.

MAJOR CONNECTORS:-• A MAJOR CONNECTOR MAXIMUM COVERAGE AREA DISSIPATE THE OCCLUSAL

FORCES FALLING ON IT.• IN MAXILLA A PALATAL FULL COVERAGE MAJOR CONNECTOR IS MORE

PREFERRED AS IT CONTACTS ALL THE REMAINING TEETH WITH A LINGUAL PLATE THUS HELPS IN DISSIPATING THE FORCES.

• IN MANDIBLE THE LINGUAL PLATE MAJOR CONNECTOR IS MOST PREFERRED BECAUSE IT COVERS THE WHOLE LINGUAL SURFACE OF ANTERIOR TEETH.

• THUS DISTRIBUTING THE FORCES TO ALL THE TEETH.• IT IS PARTICULARLY HELPFUL IN SPLINTING PERIODONTALLY WEAKENED TEETH.• IT ALSO PROVIDES RIGIDITY AND CROSS ARCH STABILIZATION.

MINOR CONNECTOR

• THE MINOR CONNECTOR CONNECTING THE GUIDE PLANE TO THE MAJOR CONNECTOR PLAYS A VERY IMPORTANT ROLE IN DISSIPATING FUNCTIONAL STRESSES. BECAUSE OF ITS CLOSE ADAPTATION TO THE ABUTMENT TEETH.

• THIS MINOR CONNECTOR HELPS IN TWO MAJOR FUNCTIONS.• IT PROVIDES A SINGLE PATH OF INSERTION.• IMPROVED STABILITY BY PROVIDING INCREASED RESISTANCE TO

HORIZONTALLY DIRECTED FORCES• ADDITIONAL GUIDE PLANES CAN BE INCORPORATED ON

OTHER TEETH TO HELP DISSIPATE THE LATERAL STRESSES FALLING ON THE SINGLE TOOTH.

REST AND REST SEATS:-• REST SEATS ARE ESSENTIAL AS THEY TRANSMIT FORCE VERTICALLY ALONG THE LONG AXIS OF THE TEETH.

• REST SEATS PREVENT FORMATION OF ANY LATERAL STRESSES.

• REST SEATS SHOULD BE DESIGNED IN SUCH A WAY THAT THEY ARE LESS THAN 90 DEGREES TO THE PATH OF INSERTION.

• THUS HELPING THE REST SEAT TO GRAB THE TOOTH SECURELY AND PREVENT ITS MIGRATION.

• OCCLUSAL REST SEATS SHOULD BE ROUNDED AND SOME AMOUNT OF SPACE SHOULD BE PRESENT BETWEEN THE REST AND REST SEAT TO ALLOW FREE MOVEMENT AS A MOVEMENT OF BALL AND SOCKET JOINT.

RPI RPA CONCEPT.• THIS DESIGN CONCEPT WAS PROPOSED TO ACCOMMODATE

FUNCTIONAL MOVEMENT IN DENTURE WHILE CHANGING THE FULCRUM LINE.

• RPI STAND FOR REST PROXIMAL PLATE I BAR• RPA IS A MODIFICATION OF RPI WHERE INSTEAD OF I BAR

ACKERS CLASP IS USED.

DESIGN PRINCIPLE FOR A OBTURATOR PROSTHESIS.

• MOVEMENT POTENTIAL WHEN COMPARED TO NORMAL DENTURE IS MORE IN OBTURATOR PROSTHESIS.

• SUPPORT, STABILIZATION AND RETENTION ARE PLACED ADJACENT TO AND FAR FROM DEFECT POSSIBLE.

• TEETH ADJACENT TO RESECTION MARGINS ARE INCISORS THEY SHOULD BE SPLINTED.

• CINGULUM REST FOR ANTERIOR TEETH ARE GIVEN.• MULTIPLE OCCLUSAL RESTS ARE USUALLY SUGGESTED TO IMPROVE STABILITY.• USUALLY A EMBRASURE CLASP IS GIVEN DISTALLY.• MAXIMUM COVERAGE OF PALATE SHOULD BE DONE FULL PALATAL COVERAGE IS

USUALLY PREFERRED

REFERENCES

• MC CRACKENS REMOVAL PARTIAL DENTURE. 11TH EDITION.• STEWARTS CLINICAL REMOVABLE PROSTHODONTICS. 3RD

EDITION• MAXILLARY RIDGE REHABILITATION WITH OBTURATOR

PROSTHESIS IJCD DEC 2013 2(6).• BEUMER J, CURTIS TA, FIRTELL DN. MAXILLOFACIAL

REHABILITATION, PROSTHODOTICS AND SURGICAL CONSIDERATIONS.

• NEXT PRESENTATION BY DR DEEPTHI SONIA• TOPIC :- PRINCIPLES OF TOOTH PREPARATION

• DATE:- !4-07-14