Principles of removable partial dentures / orthodontic seminars

PRINCIPLES OFPRINCIPLES OF
REMOVABLE PARTIALREMOVABLE PARTIAL
DENTUREDENTURE
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com
www.indiandentalacademy.comwww.indiandentalacademy.com

IntroductionIntroduction
 Designing a removable partialDesigning a removable partial
prosthesis which optimally satisfies theprosthesis which optimally satisfies the
prosthodontic requirements of support,prosthodontic requirements of support,
function, and esthetics is a dauntingfunction, and esthetics is a daunting
challenge.challenge.
 A denture should be designed to obtain aA denture should be designed to obtain a
balance between both the mechanical andbalance between both the mechanical and
biological factors.biological factors.

 Keeping the KISSKeeping the KISS PrinciplesPrinciples—“KISS” is—“KISS” is
an acronym meaning “Keep It Safe andan acronym meaning “Keep It Safe and
Simple”—in mind and utilizing theseSimple”—in mind and utilizing these
principlesprinciples as a handy reference tool willas a handy reference tool will
assist in meeting this challenge.assist in meeting this challenge.

 ““Safety”— those design elements whichSafety”— those design elements which
promote maximum support withinpromote maximum support within
physiologic limits, resistance to movement,physiologic limits, resistance to movement,
and resistance to degradation / wear of bothand resistance to degradation / wear of both
hard and soft tissues.hard and soft tissues.
 ““Simple”— those design elements whichSimple”— those design elements which
promote function, esthetics, comfort, easepromote function, esthetics, comfort, ease
of fabrication, and ease of maintenance,of fabrication, and ease of maintenance,
cleaning, and repair.cleaning, and repair.

UNDERSTANDING THE PRINCIPLESUNDERSTANDING THE PRINCIPLES
INVOLVED IN DESIGNING AN R P DINVOLVED IN DESIGNING AN R P D
I.I. Different forces act on the dentureDifferent forces act on the denture
inside the oral cavityinside the oral cavity
II.II. The response of the denture to theThe response of the denture to the
forces acting on it.forces acting on it.
III.III. Design methods, which help to limit theDesign methods, which help to limit the
effects of these harmful forces.effects of these harmful forces.

Different forces acting on theDifferent forces acting on the
denture inside the oral cavitydenture inside the oral cavity
 Occlusal forces – in a tissue wardOcclusal forces – in a tissue ward
directiondirection
 Forces from musculatureForces from musculature
 Sticky foodSticky food
 GravityGravity
 Dead zone or zone of equilibriumDead zone or zone of equilibrium

Biomechanical considerationBiomechanical consideration
 Maxfield stated that common observationMaxfield stated that common observation
clearly indicates that the ability of the livingclearly indicates that the ability of the living
things to tolerate force is largelythings to tolerate force is largely
dependent upon the magnitude anddependent upon the magnitude and
intensity of the force.intensity of the force.
 dentist should consider thedentist should consider the
direction, duration, frequency anddirection, duration, frequency and
magnitude of the force.magnitude of the force.

SIMPLE MACHINESSIMPLE MACHINES
 InIn physicsphysics, a simple machine is any, a simple machine is any
device that only requires thedevice that only requires the
application of a single force to work. Itapplication of a single force to work. It
is a tool used to make work easier.is a tool used to make work easier.
Machines use aMachines use a forceforce (push or pull) to(push or pull) to
move amove a loadload..
 An understanding of simple machinesAn understanding of simple machines
will enhance our rationalization of thewill enhance our rationalization of the
design of RPD.design of RPD.

Machines may be classified intoMachines may be classified into
SimpleSimple
compoundcompound

 Lever is a rigid object that is used with anLever is a rigid object that is used with an
appropriateappropriate fulcrumfulcrum or pivot point to multiplyor pivot point to multiply
the mechanical force that can be applied tothe mechanical force that can be applied to
another objectanother object..

Whenever effort arm is longer than resistance arm,
mechanical advantage is in favor of effort arm,
proportional to difference in length of the two arms.

Types of leverTypes of lever

Forces acting on partial dentureForces acting on partial denture
 Distal extension partial denture willDistal extension partial denture will
rotate in relation to 3 cranial planesrotate in relation to 3 cranial planes
HorizontalHorizontal
SagittalSagittal
VerticalVertical

Horizontal fulcrumHorizontal fulcrum
 Passes thro’ terminal abutments.Passes thro’ terminal abutments.
 Controls rotational movements in the sagittalControls rotational movements in the sagittal
plane.plane.

Sagittal fulcrumSagittal fulcrum
 Extends thro’ the occlusal rest on the terminalExtends thro’ the occlusal rest on the terminal
abutment and along the crest of residual ridgeabutment and along the crest of residual ridge
on the same side.on the same side.
 It controls rotational movement in vertical plane.It controls rotational movement in vertical plane.

Vertical fulcrumVertical fulcrum
 Located in the vicinity of the midline justLocated in the vicinity of the midline just
lingual to the anterior teeth.lingual to the anterior teeth.
 It controls rotational movements in horizontalIt controls rotational movements in horizontal
plane.plane.

Inclined planeInclined plane
 Two inclined surfaces in close alignment toTwo inclined surfaces in close alignment to
one another.one another.
 The action of inclined planes when force isThe action of inclined planes when force is
applied may beapplied may be
1.1. Deflection of object itself (denture)Deflection of object itself (denture)
2.2. Movement of inclined plane itself ( tooth)Movement of inclined plane itself ( tooth)

Factors influencing the magnitude of stressesFactors influencing the magnitude of stresses
transmitted to abutment teethtransmitted to abutment teeth
 Length of edentulous span.Length of edentulous span.
 Quality of support of ridgeQuality of support of ridge..
• Large, well formed - ↑ stress distributionLarge, well formed - ↑ stress distribution
• Parallel walls - ↑ stabilityParallel walls - ↑ stability
• Thick muco periostium - ↑ supportThick muco periostium - ↑ support

 Qualities of claspQualities of clasp..
 Type of claspType of clasp
 Flexible retentive arm - ↓ stressFlexible retentive arm - ↓ stress
 Wrought wire > vertical projectionWrought wire > vertical projection
claspclasp
 Clasp designClasp design
 Should be passiveShould be passive
 ↑↑ Length - ↑ flexibilityLength - ↑ flexibility
 ↑↑ diameter - ↓ flexibilitydiameter - ↓ flexibility
 ↑↑ taper - ↑ flexibilitytaper - ↑ flexibility
 ↑↑ curvature - ↑ flexibilitycurvature - ↑ flexibility
 Clasp materialClasp material
 Chrome alloy - ↑ stressChrome alloy - ↑ stress

 Abutment tooth surfaceAbutment tooth surface..
 Gold crowns or restoration produce moreGold crowns or restoration produce more
resistance to clasp arm movement than naturalresistance to clasp arm movement than natural
toothtooth
 Occlusal HarmonyOcclusal Harmony..
 Improper occlusal relationshipImproper occlusal relationship
 steep occlusal plane - ↑ stresssteep occlusal plane - ↑ stress
 22ndnd
premolar and 1premolar and 1stst
molar – best area tomolar – best area to
withstand masticatory load.withstand masticatory load.

Philosophies ofPhilosophies of RPDRPD Design &Design &
Stress DistributionStress Distribution
 a) Stress Breakersa) Stress Breakers
 b) physiologic basingb) physiologic basing
 c) Broad Stress Distribution - the mostc) Broad Stress Distribution - the most
commonly subscribed todaycommonly subscribed today

Design consideration to control stressDesign consideration to control stress
 Direct retentionDirect retention
 Retentive clasp arm – transmitsRetentive clasp arm – transmits
destructive forcesdestructive forces
 Clasp should be active only duringClasp should be active only during
insertion and removal.insertion and removal.
 Forces of adhesion and cohesionForces of adhesion and cohesion
 Should cover maximum areaShould cover maximum area
 Atmospheric pressureAtmospheric pressure
 May contribute slight amount of retentionMay contribute slight amount of retention

 Frictional controlFrictional control
 Frictional contact between theFrictional contact between the
prosthesis and guide plane – aid inprosthesis and guide plane – aid in
retentionretention
 Guide planes – on as many teeth asGuide planes – on as many teeth as
possiblepossible
 Neuromuscular controlNeuromuscular control
 Patients ability to control the action ofPatients ability to control the action of
lips, cheeks and tongue - ↑ retentionlips, cheeks and tongue - ↑ retention

 Clasp positionClasp position
 Relation of the retentive clasp to theRelation of the retentive clasp to the
ht of contour is more important thanht of contour is more important than
the no of clasps.the no of clasps.
 Number and placement of claspsNumber and placement of clasps
1) Quadrilateral configuration1) Quadrilateral configuration

3) Bilateral configuration
2) Tripod configuration

 Clasp designClasp design
 Circumferential cast claspCircumferential cast clasp
• Should not engage mesio-buccalShould not engage mesio-buccal
undercutundercut
• Reverse circlet clasp is advisableReverse circlet clasp is advisable
 Vertical projection or bar claspVertical projection or bar clasp
 Combination claspCombination clasp

 Splinting of abutment teethSplinting of abutment teeth
Loss of periodontal attachmentLoss of periodontal attachment
Tapered rootTapered root
Short rootShort root
An isolated terminal abutmentAn isolated terminal abutment
 Advocated byAdvocated by
CrownsCrowns
multiple rests and claspsmultiple rests and clasps

 Indirect retentionIndirect retention
 Resist movement of theResist movement of the RPDRPD away fromaway from
the tissue basethe tissue base
 Resists rotational movements of theResists rotational movements of the
denture around the fulcrum linedenture around the fulcrum line

 OcclusionOcclusion
 in harmony with TMJ and neuromusculaturein harmony with TMJ and neuromusculature
 Steep cuspal inclines avoidedSteep cuspal inclines avoided
 Denture baseDenture base
 Cover maximum areaCover maximum area
 Maxilla – entire tuberosityMaxilla – entire tuberosity
 Mandible – retromolar padMandible – retromolar pad
 Type of impression will influence the amount ofType of impression will influence the amount of
stressstress

 Major connectorMajor connector
 Should be rigid ,cover broad area for support ,Should be rigid ,cover broad area for support ,
stability, retention, which in turn reduces thestability, retention, which in turn reduces the
stresses transferred to the abutment teeth.stresses transferred to the abutment teeth.
 Design principleDesign principle
 L-bar principleL-bar principle
 Circular configurationCircular configuration

 Minor connectorMinor connector
 It connects the clasp assembly to theIt connects the clasp assembly to the
major connector.major connector.
 Should not interfere with the placementShould not interfere with the placement
of artificial teeth.of artificial teeth.
 OFFERSOFFERS
 Horizontal stability to partial dentureHorizontal stability to partial denture
 Stabilization against lateral stressesStabilization against lateral stresses
to the abutment tooth.to the abutment tooth.

 RestsRests
Directs forces all along the long axis ofDirects forces all along the long axis of
the abutment teeth.the abutment teeth.
Floor of the rest seat should be lessFloor of the rest seat should be less
than 90°than 90°
Seat should be saucer-shaped andSeat should be saucer-shaped and
completely devoid of any sharp anglescompletely devoid of any sharp angles
or ledges.or ledges.

Few articles to quote….Few articles to quote….
 Bridgeman et al (1997)….Bridgeman et al (1997)….
For the 0.75 mm undercut specimens, thereFor the 0.75 mm undercut specimens, there
was less loss of retention for clasps made from purewas less loss of retention for clasps made from pure
titanium and titanium alloy than for cobalt-chromiumtitanium and titanium alloy than for cobalt-chromium
clasps after 3 years. Porosity was more apparent in theclasps after 3 years. Porosity was more apparent in the
pure titanium and titanium alloy clasps than in thosepure titanium and titanium alloy clasps than in those
made from cobalt-chromium, but the amount of porositymade from cobalt-chromium, but the amount of porosity
did not correspond to evidence of fractures or permanentdid not correspond to evidence of fractures or permanent
deformation.deformation.

 Itoh et al (1998)Itoh et al (1998)
Under the same load conditions, the highestUnder the same load conditions, the highest
stresses developed in the model with the largeststresses developed in the model with the largest
osseous defect. Increasing the number of splinted teethosseous defect. Increasing the number of splinted teeth
did not provide a proportional decrease in maximumdid not provide a proportional decrease in maximum
stress levels. The more severe the osseous defect, thestress levels. The more severe the osseous defect, the
greater assistance was provided by splinting togreater assistance was provided by splinting to
periodontally sound teeth. This simulation studyperiodontally sound teeth. This simulation study
suggests that routine cross-arch splinting may not besuggests that routine cross-arch splinting may not be
appropriate.appropriate.

 Petridis (2001)Petridis (2001)
Removable partial dentures do not cause anyRemovable partial dentures do not cause any
adverse periodontal reactions, provided thatadverse periodontal reactions, provided that
preprosthetic periodontal health has been establishedpreprosthetic periodontal health has been established
and maintained with meticulous oral hygiene. Frequentand maintained with meticulous oral hygiene. Frequent
hygiene recalls and prosthetic maintenance are essentialhygiene recalls and prosthetic maintenance are essential
tools to achieve a good long-term prognosis.tools to achieve a good long-term prognosis.

 Ben-Ur et al (1999)Ben-Ur et al (1999)
-- In the maxillary arch, the most rigid majorIn the maxillary arch, the most rigid major
connector was the anteroposterior palatal barconnector was the anteroposterior palatal bar
combination placed on different horizontal and verticalcombination placed on different horizontal and vertical
planes. The most flexible was the U-shaped design. Inplanes. The most flexible was the U-shaped design. In
the mandibular arch, the most important factor inthe mandibular arch, the most important factor in
achieving rigidity was the cross-sectional shape of theachieving rigidity was the cross-sectional shape of the
major connector. The half pear-shaped cross sectionmajor connector. The half pear-shaped cross section
proved to be the most rigid.proved to be the most rigid.

 Sato et al (2001)Sato et al (2001)
studied the effects of buccolingual width andstudied the effects of buccolingual width and
position of occlusal rest seats on load transmission toposition of occlusal rest seats on load transmission to
the abutments for tooth-supported removable partialthe abutments for tooth-supported removable partial
dentures. And concluded that buccal shifting of the restdentures. And concluded that buccal shifting of the rest
seats seems to be advantageous for load transmissionseats seems to be advantageous for load transmission
to the abutmentsto the abutments..

 Gapido et al (2003)Gapido et al (2003)
the cast Co-Cr rests are more rigid and fatiguethe cast Co-Cr rests are more rigid and fatigue
resistant than Ag-Pd-Cu-Au rests. The fatigue resistanceresistant than Ag-Pd-Cu-Au rests. The fatigue resistance
of 0.8-mm-thick occlusal rests may be adequate if castof 0.8-mm-thick occlusal rests may be adequate if cast
with Co-Cr alloy. An increased number of casting defectswith Co-Cr alloy. An increased number of casting defects
may hasten the fatigue failure of occlusal rests.may hasten the fatigue failure of occlusal rests.

Principles of removable partial dentures / orthodontic seminars

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