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removable partial denture RPD Design part 1
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Dr.Mostafa Omran Hussein Assistant Professor of Prosthodontics
Faculty of Dentistry Qassim University
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1) Recognize how to asses clinically magnitude of stress falling
on the partial denture.
2) Describe basic principles and philosophies used to design
partial denture that make those stresses within the
physiologic tolerance of the tissues.
3) Understanding problems associated different classes of
partial denture cases and methods to solve them.
4) Discussing methods of designing different classes of partial
dentures in different situations.
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RPD design
The denture must be adequately designed following
bio-mechanical principles. Proper design helps in
reducing the harmful effects on the oral tissues
especially those supporting the appliance.
Successful RPD design tends to keep the stresses
evoked during function and parafunction within the
physiologic limit of the stressed tissues.
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How to asses these stresses clinically ?
General musculature and health of the patient
wear facets and attrition on the remaining natural teeth
The type of opposing occlusion
Teeth inclination and cusp inclines
Width of occlusal table
Length and location of the saddle
Absence of posterior abutment
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Possible movements of RPD
1. Tissue ward movement
2. Tissue away movement
3. Lateral movement
4. Rotational movement
1. Around axis passing through abutment teeth.
2. Around axis passing through crest of the ridge.
3. Around center of the arch (fish tail movement).
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Rotation around fulcrum passing between principle abutments
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Rotation around crest of the ridge
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Rotation around fulcrum passing through center of the arch
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I. Design for support
Support is designed to counteract vertical
tissue- ward movement of the denture.
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Design for support
Removable partial dentures are divided according
to the type of support into:
Tooth supported RPD
Tooth-tissue supported
Tissue supported
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II- design for Retention
Retention is necessary to counteract
vertical displacing forces.
This can be achieved by mechanical
means e.g. Retentive clasp arms,,
attachments, guiding planes, and by
physical means as adhesion, cohesion
and interfacial surface tension.
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Design for retention
The selection of Clasp form depends on:
- Position of the tooth: I-bar clasps are indicated for
premolars for better esthetics.
- Condition of the abutment tooth: For strong abutment tooth
rigid clasp is preferred.
- Position of retentive undercut
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Undercut:
An area of tooth or soft tissue which is beyond the survey line when viewed from a particular direction. An undercut is formed when the base of an object is smaller than its top.
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Undercuts
Soft or bony
tissue
undercuts
Undercut due to tooth
inclination
Bulbous shape of the crown
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All the Undercuts are Undesirable undercuts Except that used for Denture
Retention
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Paralleling Instrument Used to Determine the Survey
Line, delineate and Measure area of Undercuts. It
may also determine the Relative Parallelism of the
Surfaces of Teeth and Other Areas on the Cast.
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1- Horizontal plateform.
2- Vertical column.
3- Horizontal or cross arm.
4- Vertical spindle.
5- Surveying table.
6- Base equipped with a
lock screw and universal
joint.
7- Surveying tools.
8- storage compartment,
9- tool rack.
1
2
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Analyzing rod: is a rigid metal rod used for diagnostic purposes in the selection of the path of placement
Carbon marker: is used for the actual marking of the survey lines on the cast. A metal shield is used to protect it from breakage.
Undercut gauges: are used to measure the extent of the horizontal undercuts that are being used for clasp retention. Usually there are three sizes: 0.01, 0.02 and 0.03 of an inch
Wax trimmer is used to trim excess wax that may be inserted into those undercut areas, which are to be obliterated
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Design for retention
- Position of retentive undercut
If the undercut is on the mesio-buccal side, I- bar,
combination wrought wire clasp, RPI RPA clasp
or back action can be used.
If the abutment tooth exhibits an undercut, on the
disto-buccal side, then a reverse circlet or
modified T clasp can be used.
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III- Design for bracing and denture stability
Bracing is necessary to counteract horizontal forces
generated due to lateral movement of the mandible
during mastication and the presence of cuspal inclines.
Rigid components placed on one side of the arch
stabilizes the denture against horizontal forces acting
on the opposite side i.e. bilateral stabilization.
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Design for bracing & stability
Bracing & stability achieved by:
Rigid part of retentive arm
Proximal plate
Buccal & lingual
flanges
Distal area of tuberosity
Slope of palatal
connector
Proper occlusion
Reciprocal arm
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IV- design for Reciprocation
Reciprocation is necessary to counteract forces acting
on one side of the tooth by an equal and opposite force.
This can be achieved by reciprocal clasp arms
contacting the tooth prior to or at the same time the
retentive tip crosses the survey line of the tooth.
Cross arch reciprocation should also be provided.
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Retentive
Reciprocal
Reciprocal
Retentive
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VI- design for Indirect retention
Indirect retention should be designed in
free-end saddle cases to counteract
rotation of the partial denture away
from the underlying tissues.
This is mainly achieved by using rests
located on the opposite side of the
fulcrum axis and/or unrelieved
maxillary major connectors.
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VII- design for connection
Saddles are joined together by a suitable rigid
major connector.
Other components as clasps. additional rests or
indirect, retainers are joined to the saddle or to
the major connector by minor connectors.
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1. Lack of posterior abutment
2. Support is derived from both the residual ridge and abutment teeth
3. Major support is obtained from the residual ridge
4. If resorption occurs and relining of the denture is neglected further bone
resorption occurs with subsequent torque acting on the abutments.
Mucosa 2 mm
PDL 0.2 mm
bone
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Reduction of the load.
Distribution of the load between abutment teeth and residual ridges.
Wide distribution of the load.
Providing posterior abutments.
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I- Reduction of the load
This can be achieved by:
1- The use of small and narrow teeth to increase the masticatory
efficiency and reduce the masticatory load.
2-Developing harmonious occlusion and reducing the cusp angle of
artificial teeth.
3- Leaving a tooth off the saddle.
4-Placing the artificial teeth on the anterior two-thirds of the base (no
3rd molar).
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II. Distributing the load between abutments
and residual ridge
This could be achieved by:
Correct choice of the abutment tooth with suitable crown and root morphology and efficient alveolar
bone support.
Correct choice of direct retainer (clasps having stress releasing action) and using stress equalizing
design.
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II. Distributing the load between abutments
and residual ridge Clasps that have stress releasing action:
RPI clasp
Wrought wire clasp RPA clasp
Gingivally approaching clasp Back action clasp
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Mesial placement of the occlusal rest
provides the following advantages:
1. Changing the direction of torque on the abutment from the
distal to the mesial side of the tooth, where the resistance
to torque action will be applied to the neighboring teeth.
2. Achieving mechanical advantage from class I to class II lever
F
F
R
R
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3. Clasp disengagement from the tooth during function provides
less stresses on the abutment.
Buccal view
Occlusal view
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4. The farther the anterior placement of the rest, the more vertical will be the
forces, the less is the horizontal component of force falling on the ridge.
5. As rest is moved anteriorly, this will increase the area of, support (decrease
the force /unit area) and hence less stresses on the ridge and less torque
on the abutments.
6.The bone near the abutment will thus share the distal part of the ridge in
bearing the occlusal load.
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II. Distributing the load between abutments
and residual ridge The use of :
Physiologic impression
Selective pressure impression
Functional reline method
Before metal framework construction
After metal framework construction
After denture construction
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Ill- Wide Distribution of Load
Wide distribution of the load over the ridge. The broader the coverage, the greater the distribution of load,
that provides more denture ability to withstand vertical and horizontal stresses.
Wide distribution of load over the teeth
Placing additional rests on the teeth adjacent to the abutment.
Splinting of one or more teeth, either by fixed partial dentures or by an embrasure or multiple clasp.
Using a kennedy bar to distribute the lateral load on multiple teeth.
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IV- Providing Posterior Abutments
Using an implant at the distal part of the ridge.
Salvaging a hopeless posterior tooth to be used
as a partial overdenture abutment.
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1. Lack of posterior abutment
2. inadequate physical means of retention.
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Designing Kennedy class I
1- Denture base:
A combined metal-acrylic base is used to:
Allow further relining
Allow mechanical retention with the acrylic
base.
Make physiologic basing more applicable.
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Designing Kennedy class I
2- Rests :
Mesially placed saucer-shaped rest seats is used
to:
Allow dissipation of stresses
Allow transmission of stresses along the long
axes of teeth.
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Designing Kennedy class I
3- Direct retainers :
Diagonally placed clasp with stress releasing action to:
Reduce torque to the abutment tooth
Minimize interference with normal stimulation of gingiva.
Good stabilizing components.
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Designing Kennedy class I
4- Indirect retention:
In the form of rests or maxillary major connector
located as far anterior to the fulcrum axis as possible on a strong tooth.
Preferably bilateral.
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Designing Kennedy class I
5- connectors:
Lingual bar is preferred than lingual plate and sublingual bar?
Middle palatal strap is preferred than anteroposterior palatal strap?
Any condition prevent their use change to another choice.
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Designing Kennedy class I
6- Artificial teeth:
Small narrow teeth bucco-lingual.
Teeth with sharp cutting edges.
Centric occlusion is in harmony with centric relation.
Lower teeth over crest of the ridge.
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How to design this maxillary arch
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Class I Kennedy With Anterior Modification
It is wise to restore modification space with
fixed restoration?
When the remaining teeth are weak swing lock
design could be used to aid in splinting.