Principles of design./ orthodontic seminars

Principles of designing
removable partial dentures
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

Contents
• Introduction
• Principles of design
• Philosophies of designing of RPD
• Factors influencing design
• Systematic approach to desiging of RPD
• Essentials of design in Kennedy class I, II, III, IV
situations
• Surveying and steps involved in surveying
• Summary
• Conclusion
• references

Introduction
• Partial or complete loss of natural teeth is
the result of disease of the calcified tooth
surfaces ( dental caries) or disease of the
supporting tissues (periodontal disease).
When the individual looses some of his
teeth, the remaining teeth and the
periodontium, muscles, ligaments, and
temperomandibular joint may also be
affected.

• The dentists obligation is to intercept these
processes and prevent them from running their
full course. Our purpose is to assist the patient
to attain and maintain the best physiologic oral
health possible.
since a high percentage of patients who are
partially edentulous require RPD’s, the most
reasonable course of action is to plan dental
treatment from the perspective of how the
design of the prosthesis can best preserve and

• Maintain the residual oral tissues. The
dentist must have a logical goal, must plan
a systematic approach to achieving the
most satisfactory RPD design for the
dental conditions individual to that patient,
and must develop, in sequence, the
methods and means of achieving that
design.

Principles of design
• Dentists must have a working knowledge
of both the mechanical and the biologic
factors involved in removable partial
denture design and construction.
• Any plan of restoration must be based on
a complete examination and diagnosis of
the individual patient.

• The dentist, not the technician should
correlate the pertinent factors and
recommend a proper plan of treatment.
• A removable partial denture should
restore form and function without injury to
tissue
• A removable partial denture is a form of
treatment, not a cure.

Philosophy of design
• Stress equalization
• Physiologic basing
• Broad stress distribution

Principles of partial denture
construction
• Provide adequate support on the
remaining natural teeth to resist vertical
stress, and to direct those stresses in the
direction of the long axis of the abutment
teeth
• Provide broad coverage of saddle areas
so that the stresses borne by the soft
tissue are distributed over a large area.

• Provide adequate tooth borne resistance
to lateral stresses
• Provide adequate retention
• Provide for distribution of stress between
the relatively rigid abutment teeth and the
relatively resilient saddle areas.
• Have rigid bars and connectors

• Avoid covering free gingival margin
around the remaining teeth with denture
base
• Decrease the occlusal table by using
narrower artificial teeth
• Provide harmony of the occlusal surface of
opposing teeth and fewer teeth

Factors influencing design
• The arch to be restored with removable
partial denture. If both arches are to be
restored, following considerations are to
be considered.
– Orientation of the occlusal plane
– Space available for restoration of the missing
teeth
– Occlusal relationship of the remaining teeth

– Tooth morphology
2. Periodontal condition of the remaining teeth
3. The amount of abutment support remaining
4. Need for splinting
5.If denture is entirely tooth supported or tissue
supported.

If one or more distal extension bases are
present, the following are to be considered.
• Clasp design that will minimize the forces applied
to the abutment tooth
• Secondary impression to be used
• Need for indirect retention
• Need for later rebasing, which will decide the type
of base material to be used

6. Need for abutment tooth modification or
restoration, which may influence the type
of clasp arms to be used and their specific
design
7. Type of major connector indicated
8. Material to be used for framework and
bases
9. Type of teeth used for replacement

A systematic approach to designing
of partial dentures.
• Designing of partial denture framework
should be systematically developed and
outlined on an accurate diagnostic cast.
1. Determine how the partial denture is to
be supported
2. Connecting the tooth and tissue
supporting units

3. Retention for the partial denture
4. Connecting the retention units to the
supporting units
5. Design the outline and join the edentulous
area to the already established design
components

Establishing support

• Tooth support
– In an entirely tooth supported RPD , rests
should be located in such a manner that any
load applied to the prosthesis is directed
along the long axis of the abutment tooth so
that the movement of the prosthesis in an
apical direction can be effectively resisted.

• Direct retainers and minor connectors
must be designed so that the least
possible lateral or torquing stress will be
transmitted to the abutment tooth.
• Location of placing rests
• Occlusal
• Cingulum
• Incisal surfaces of the abutment tooth

• Rests do act as point of rotation of the
prosthesis. The closer the rest is to the
edentulous area, the greater is the arc of
rotation around the rest.

• In a distal extension RPD, it is advisable to
place the rest on the mesial aspect of the
primary abutment tooth. The resultant
force on the prosthesis will tend to move
the tooth mesially and be reciprocated by
the dental arch.

Considerations to be considered
when evaluating potential support
that an abutment tooth can provide
• Periodontal health
• Crown and root morphology
• Crown to root ratio
• Length of edentulous span
• Opposing dentition

Tissue support
• Distal extension RPD derives a great deal of its
support from the residual ridge.
• Considerations in evaluating the potential of
tissue support for RPD’s
– Length and contour of the residual ridge
– Contour of the edentulous base area
– Quality of the supporting bone and overlying mucosa
– Forces on supporting tissues
– Previous response to stress

Length and contour of residual
ridge
• Denture base in close approximation to
the abutment tooth is primarily supported
by the abutment teeth. Proceeding away
from the abutment teeth, support for the
distal extension base is primarily derived
from the underlying tissue.

Support areas for RPD in mandible
• Buccal shelf
area( primary)
• Retromolar pad area
• Slopes of the residual
ridge (secondary)

Support areas for an RPD in
maxilla
• Slopes of the residual
ridge (secondary)
• Horizontal portion of
the hard palate
( primary support)
• Crest of the posterior
residual ridge
( primary support)

Contour of the edentulous base
area
• The ideal contour ridge contour is one that
has a broad, smooth, rounded ridge crest
with nearly vertical buccal and lingual
slopes.
• Residual ridge crests that are parallel to
the opposing ridges or the occlusal plane
provide the most advantageous position
for distribution of stress.

• Residual ridges that are flat might offer
secondary support but offer little lateral
stability
• Residual ridges that are sharp and spiny
provide the poorest anatomy for both the
support and stability of a prosthesis.

Quality of the supporting bone and
overlying mucosa.
• Absence of cortical bone in specific areas
usually indicate poor response of residual
bone to stress.
• The mucosal covering of the residual ridge
plays an important role in support. Tissues
covering residual ridges must be recorded
in a dynamic form, that is functional form
of the supporting areas and anatomic form
of the anatomic areas.

Forces acting on
supporting tissues
• Force activated by Resultant force Counteracted by
Sticky foods Vertical lift Retention
tongue & muscle forces Vertical lift Adequate
denture base coverage
Gravity Vertical lift indirect retainers
Occlusal load Movt towards Occlusal, cingulum,
residual ridge and incisal rests
Adequate denture base

Forces around the
longitudinal axis
Force activated by Resultant force Counteracted by
Occlusal force on one Twisitng, tilting Rigid connectors
Side of the arch causes Direct retauner design
Lifting forces on the Denture base coverage
Contrlateral side of Denture tooth placement
the arch Contour of the denture base

Forces around
the perpendicular axis
Force activated by Resultant force Counteracted by
Masticatory stress Twisting and spreading Adequate Bracing
of the RPD Rigid connectors
Denture base coverage
Occlusal balance
Contour of the denture base

Connection of supporting units

• The connection of the support units is
facilitated by major and minor connectors.
• A Major and minor connector should be
designed meeting the following
requirements
– They should be rigid and strong enough to
withstand masticatory forces

– They should conform to and not interfere with
the normal anatomic structures of the mouth
– Should avoid food entrapment
– They should not interfere with mastication or
occlusion.

Design considerations for maxillary
major connectors
• Should be designed with the comfort of the
patient in mind.
• To use as much as support from the hard palate
as possible.
• Free gingival margins and gingival crevices that
are traversed by the major connectors should be
relieved.
• Coverage of the anterior part of the hard palate
should be avoided whenever possible

• A bead on the tissue side of the major
connector should be prepared along the
peripheral outline, so that the major
connector will slightly displace the
underlying soft tissue to provide a
peripheral seal.

Function of beading
• Prevent food debris from collecting beneath the
major connector
• Additional thickness along the bead permits the
edges of the polished surface of the connector
to be tapered so that a smooth junction with the
soft tissues is created
• Serves as a finish line for the technician during
finishing and polishing of metal framework

Minor connectors
• Should be rigid to withstand masticatory stress
• Should be positioned in interproximal spaces to
avoid tongue interference and should pass
vertically from major connector to other
components
• Should be thicker towards lingual surface and
taper towards the contact area.
• There should be a minimum of 5 mm space
between the vertical minor connectors

• Types of minor connector
• Open latticework
• Mesh type
• Metal base
• Finish lines
• Internal finish line and external finish line

Providing retention

• Retention achieved by two means
• Tissue seal
• Mechanical retention
• Requirements of a clasp
– Support
– Bracing (stabilization)
– Retention
– Reciprocation
– Encirclement ( more than 180 degrees)
– passivity

General rules concerning clasp
retention
• Clasp assembly should be designed so
that only one retentive arm is used on an
abutment teeth. The retentive arm should
be opposed by a reciprocal component of
the clasp assembly on the opposite side of
the abutment tooth

• An abutment tooth that has a definite
taper occlusally on which the reciprocal
component of the clasp assembly is
located should be restored or recontoured
to provide for a vertical surface to oppose
the retentive arm if adequate bracing for
the retentive arm is to be achieved

• The clasp assembly design selected for
an abutment tooth should be based on the
location and depth of the available
undercut area on the abutment tooth.
• Complicated clasp assembly designs
should be avoided

Guidelines for location of retentive
areas
• The closer the retentive area to the
edentulous area, the greater the potential
for retention
• Retention areas should be located as
widely as possible through the remaing
natural teeth in the dental arch to provide
eqaulized retention and stability to the rpd

• Whenever possible, retentive areas on
one side of the dental arch should be
opposed by similar retentive areas on the
opposite side of the dental arch.

Factors influencing the
effectiveness of indirect retainers
• The indirect retainer should be placed at
right angles to and as far from the fulcrum
line as possible.
• An indirect retainer should be always
placed on a prepared rest seat on an
abutment tooth that is capable of
withstanding stresses placed on it,
preferably a canine or premolar.

• In distal extension RPD’s, indirect retainers
moves the fulcrum line anteriorly to the
abutment tooth or teeth contacted by the indirect
retainer. This prevents the base from lifting of
the soft tissue.
• Indirect retainers serve as a third point of
reference for the cast RPD framework and thus
aid in locating the correct framework position
during relining procedures or when altered cast
impression technique is used.

Principles of design./ orthodontic seminars

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