Design consideration in reducing stress in rpd/ oral surgery courses

Design consideration inDesign consideration in
the control of stresses inthe control of stresses in
removable partial dentureremovable partial denture
INDIAN DENTAL ACADEMY
Leader in continuing Dental Education

IntroductionIntroduction
Maxwell -“common observation
indicates the ability of living
things to tolerate force is largely
dependent upon the magnitude or
intensity of the force.”
www.indiandentalacademy.com

Basic ConceptsBasic Concepts
Stress Equalization
Physiologic Basing
Broad Stress Distribution

Stress EqualizationStress Equalization
The rigid connection between the denture bases
and the direct retainer on the abutment teeth is
damaging and that some type of stress director or
stress equalizer is essential to protect the
vulnerable abutment teeth.
Stress EqualizerStress Equalizer-: According to GPT 7According to GPT 7
A device which relieves the abutment teeth of all
or part of occlusal forces.

Types of Stress
Equalizers
Type I:
Hinges,sleeves and
cylinders,and ball –and-
socket devices.
Type II:
Designs having flexible
connection which include
the use of wrought wire
connection,divided major
connectors or other
flexible devices for
movement of distal
extension. www.indiandentalacademy.com

AdvantagesAdvantages
Reduced horizontal force.
Forces are distributed equally on abutment
and the soft tissue.
Stimulating effect on the underlying tissue
thus reduced tissue changes.
Frequency of relining is reduced.

DisadvantagesDisadvantages
Fragile,complex and costly.
Vertical and horizontal forces are concentrated on
the residual ridge ,thus causing resorption.
Requires constant maintenance.
Difficult to repair.
Bulky and annoying to patient.

Physiologic BasingPhysiologic Basing
The equalization of stress can be best and
most simply be accomplished by some form
of physiologic basing, or lining, of the
denture base.
Mucosa is recorded in its functioning form.
Soft tissue offer more resistance to further
compression.This increases resistance
provided by mucosa equates to that of
periodontal ligament of abutment tooth.
Artificial teeth will be above occlusal plane
in rest position.www.indiandentalacademy.com

RequirementsRequirements
Direct retainer/retentive clasps designed
with minimal retention
A rigid frame work
Well adapted, broad coverage denture
bases.

Stimulating effect on
underlying tissue
Simplicity in design
and construction
Reduced stresses on
abutment tooth

Not well stabilized against lateral
forces.
Premature contacts.

Broad Stress DistributionBroad Stress Distribution
Advocates believe that excessive trauma to
remaining teeth and the residual ridge
can be prevented by distributing occlusal
forces over as much the available area of
teeth and soft tissue as possible.

Broad stress distribution.
Excellent horizontal stabilization.
Removable splinting.
Easier and less expensive to construct.
Less danger of distortion and breakage of denture.

Less comfortable.
Requires good maintenance of oral
hygiene.

ROLE OF RPDROLE OF RPD
COMPONENTS IN STRESSCOMPONENTS IN STRESS
CONTROLCONTROL

Major Connector:Major Connector:
Rigid
Lingual plate properly
supported by rest can aid
in stress distribution and
support periodontally
weakened anterior teeth.
It also adds rigidity thus
increases cross arch
stabilization.
Complete palatal major
connector that contacts
lingual surfaces of all
remaining teeth distributes
functional stress widely as
possible.

Minor ConnectorMinor Connector
Offers horizontal
stability to partial
denture against lateral
forces of prosthesis.
Maximum contact of
proximal plate of
minor connector with
guiding plane provides
better horizontal stress
distribution.

RestsRests
Properly prepared rest
seats control stress by
directing the forces
transmitted to
abutment teeth down
the long axis of teeth.

Direct RetainerDirect Retainer
RPD should be designed such that the clasp
retention is kept to minimum yet provide
adequate retention to prevent the
dislodgement of the denture by unseating
forces.

Clasp DesignClasp Design
Flexible clasp produce
least stresses than rigid
clasp.
Longer the clasp more
will be its flexibility.
But they contribute less
resistance to horizontal
forces.
Cross section of clasp
arm.
Material used for
construction of clasp.

Circumferential ClaspCircumferential Clasp
Engage mesiobuccal
undercut-shouldn’t be
used in distal extension.
Reverse circlet clasp that
engage distobuccal
undercut is acceptable.
Wedging effect of reverse
circlet clasp can be
overcome by occlusal
rests.

Vertical projection, or Bar,Vertical projection, or Bar,
clasp:clasp:
The vertical projection ‘T’
clasp releases tortional stress
on the terminal abutment
tooth.This releasing action is
accomplished when retentive
clasp tip rotates gingivally
into a greater undercut as
tissueward forces are applied
to denture base .
Rotation takes place around
distal occlusal rest.

Combination ClaspCombination Clasp:(Kotowicz:(Kotowicz
1973)1973)
Circumferential clasp with
retentive arm made of wrought
metal which is more flexible and
flex in all spatial plane.It has a
stress breaking action.
Rest of the clasp are rigid and
better resist the lateral force.
Indicated:Indicated:-presence of
mesiobuccal undercut
-Presence of cervical
or buccal undercut.

Kratochvil in 1963Kratochvil in 1963
Explained the system
with mesial rest.
Long guide planes that
extend onto the tooth-
tissue junction.
I-bar retainer.

RPI system: Krol ,1973Krol ,1973
Mesial rest,proximal
plate,I- bar.
Minimal tooth
contact,minimal gingival
coverage and stress
control.
Mesioocclusal rest with
minor connector placed into
the mesiolingual embrasure
area but no tooth contacting
the adjacent tooth.
Distal guide plane,
extending from the
marginal ridge to the
junction between middle
and the gingival third of the
abutment tooth.www.indiandentalacademy.com

RPA system: Krol ,1980Krol ,1980
Rest, proximal
plate,Aker’s retentive
clasp
Modified
circumferential clasp

Clasp PositionClasp Position
Position of clasp in
relation to height of
contour is more
important than the
number of clasp.
Quadrilateral
configuration
Tripod configuration
Bilateral configuration

Quadrilateral ConfigurationQuadrilateral Configuration
Most often used in class III
arches particularly in
presence of modification
space.
Retentive clasp placed on
each abutment tooth adjacent
to the edentulous area.

Tripod ConfigurationTripod Configuration
Primarily for class II
If there is a modification
space on the edentulous
side,the teeth anterior
and posterior to the space
are are clasped to bring
about the tripod
configuration.

Bilateral ConfigurationBilateral Configuration
Most of RPD’s fall into
this category.
The clasp exert little
neutralizing effect on the
leverage induced .
Must use other means of
reducing stress.

Indirect Retainer:Indirect Retainer:
It helps the direct retainer prevent
displacement of the distal extension
denture by resisting the rotational
movement of the denture around fulcrum
line established by the occlusal rests.

Located as far anterior and
opposite side of the fulcrum
line.
They counteract the forces
attempting to move the
denture base away from the
residual ridge by moving the
fulcrum far from the force.
In class I arch, indirect
retainer is placed as far
anterior to the fulcrum line as
possible.

In class II arch, single
indirect retainer on
side of arch opposite
distal extension

In class IV, indirect
retainer placed
posterior to the
fulcrum line.
lingual plate major
connector supported at
each end of anterior
teeth by rests.

Denture Base:Denture Base:
Should cover extensive area of supporting
tissue as possible.
Should satisfy the “snow shoe” principle
Denture flanges should be as long as
possible.
Accurate adaptation
No overextension

Forces of Adhesion and CohesionForces of Adhesion and Cohesion
To enhance quality of retention throughTo enhance quality of retention through
adhesion and cohesion denture base-adhesion and cohesion denture base-
Must fit edentulous ridge accurately
Must cover maximum area of available
support.
Quality of saliva.

Atmospheric Pressure:
Accurately casted major
connector with beaded
margins can contribute
slight amount of
peripheral seal and
retention.

Neuromuscular Control
Properly contoured
borders of a denture base
aid in retention by
permitting patient to use
neuromuscular skills to
avoid dislodging base.
Overextended denture
bases will constantly be
unseated by the action of
border tissues.

Frictional ControlFrictional Control
Guide planes are placed
on the proximal surfaces
of the teeth adjacent to the
edentulous spaces or it can
be created on the
restorations placed on the
teeth.
Guide planes should be
created on as many teeth
as possible.
Guide planes increase
retention of partial denture
through frictional contact.

““It is more importantIt is more important
to preserve whatto preserve what
already exists than toalready exists than to
replace what isreplace what is
missing”missing”

References
Clinical Removable Partial Prosthodontics-
Stewart ,Rudd,Kuebker,2ND
edition.
Removable Partial Prosthodontics-
McCracken,9th
edition.
Removable Partial Denture-Robert
P.Renner Louis Boucher

Removable Partial Prosthodontics-Joseph
E.Grasso, Ernest L.Miller,3rd
edition.
Removable Denture Prosthodontics-
A.A.Grant W.Johnson,2nd
edition.
Designing of Partial Denture-Osbourne

Advanced Removable Partial Denture,
James S. Brudvick.
Partial Dentures, Fritz Singer.
Kim et al,Comparison of cast Ti-Ni alloy
clasp extension with conventional RPD
clasps – JPD 2004;91:374-82
Mona K Marei,Measurement(in vitro)of the
amount of force required to dislodge
specific clasps from different depths of
undercut – JPD 1995;74:258-63.

Arthur R Frechette,Influence of partial
denture design on distribution of force to
abutment teeth – JPD 1996;622
Hussein G El Charkawi,Effect of splinting
of load distribution of extracoronal
attachment with distal extension prosthesis
in vitro- JPD 1996;76:315-20
RPD research and its clinical significance –
JPD 1978;39:203

THANKTHANK
YOUYOU

Discussion
Frechette (1956),was the first one to investigate
the effect of RPD design on forces imparted to the
abutment teeth.
Recent studies shows new materials used in cast
frame work,different clasp designs.
At least two teeth on each side should be splinted
when extracoronal distal extention attachment
prosthesis is used.(JPD 1996)
Its also documented that cast round clasp has more
flexibity and when incorporated in restproximal
plate of Akers design for distal extension resulted
in better distribution of torquing force.(JPD 1996)

• Ti-Ni alloy cast frame work is been
introduced because of its greater flexibility
and accuracy of fit.(JPD 2004)

OcclusionOcclusion
A smoothly functioning occlusion that is in
harmony with the movements of both the TMJ and
the neuromusculature will minimize stress
transferred to the abutment teeth and residual
ridge.
Neither the metal of the framework nor the
artificial teeth of the partial denture should receive
initial occlusal contact as the jaws come together.
Artificial posterior teeth should have reduced
buccolingual width.
Number of teeth being should be reduced.
Teeth should have sharp cutting surfaces and
sluiceways.
Reduce the steep cuspal inclines.www.indiandentalacademy.com

Splinting of abutmentSplinting of abutment
Adjacent teeth may be
splinted by means of crowns
to control stress transmitted
to a weak abutment tooth.
Splinting two or more teeth
actually increases the
periodontal ligament
attachment area and
distributes the stress over a
larger area of support.

Periodontally used teeth can
be stabilized against
horizontal forces by RPD,
multiple rests and clasps are
used to contribute this
horizontal stabilization.
Curved arch splinting results
in both bucco-lingual and
mesiodistal stabilization.
Splinting with removable
RPD result in cross arch
stabilization.

conclusionconclusion
As we understand that that the stresses
created by the RPD’s cannot be nullified
completely,so attempts has to be made to
control stresses by incorporating proper
design consideration as discussed.Thus
the longevity of the supporting hard and
soft tissues can be increased.

Thank you
Thank
you

Methods of reducing stress toMethods of reducing stress to
abutment teeth and supportingabutment teeth and supporting
tissues:tissues:
According to Kratochvil (1971)According to Kratochvil (1971)
Decreasing the mobility of the abutment teeth.
Improving the supporting ability of the edentulous
area.
Reducing the amount of force applied to the
abutment teeth and residual ridges.
Creating favorable distribution of the load
between the abutment teeth and the ridges.

Decreasing the mobility of theDecreasing the mobility of the
abutment teethabutment teeth
Perform an occlusal equilibration.
Splinting the teeth
Preparing and positioning of guiding planes
properly
Designing and positioning of rest seat
properly.

Improving the supporting ability ofImproving the supporting ability of
the edentulous areathe edentulous area
Proper impression procedures
Maximum coverage of residual ridges
Relining and rebasing the prosthesis in a
timely fashion

Reducing the amount of force appliedReducing the amount of force applied
to the abutment teeth and residualto the abutment teeth and residual
ridgesridges
Reducing the number of posterior teeth
Reducing the buccolingual width
Maintaining sharpness of the cusps
Adequate sluiceways

Creating favorable distribution of theCreating favorable distribution of the
load between the abutment teeth andload between the abutment teeth and
the ridgesthe ridges
Provide physiologic influence on
edentulous denture base area.
Extensive stress distribution by use of rigid
RPD design
Select clasps that provide direct retention
Use of stress breaking prosthesis.

Design consideration in reducing stress in rpd/ oral surgery courses

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