2. 2
Introduction
Biomechanics of the RPD
Factors influencing the magnitude of stresses
Design considerations
Additional considerations influencing design.
Essentials of partial denture design.
Differentiation b/w the two main types of
removable partial dentures
Review of literature
Conclusion
References
3. 3
INTRODUCTION:
The primary objective of partial denture design
is the preservation of the remaining teeth, their
supporting structures, the residual alveolar ridges
and the oral mucosa in a healthy condition, while at
the same time replacing the missing teeth for
improving aesthetics, mastication and speech.
Emphasis must thus be placed first on the
biological aspects of Partial Denture restorations,
rather than upon the purely technical aspects.
4. 4
Concepts of framework design
Static-dynamic, biologic, esthetic and
comfort consideration.
Static -dynamic concept of framework
design considers the distribution of
forces b/w the abutments and b/w the
abutments and the mucosa and what
precaution to take not to overload the
periodontal membrane and to maintain
functionally stable dentures.
5. 5
Biologic concepts have been
established to minimize the harmful
long term effects of wearing RPDs, such
as caries or periodontal disease.
The esthetic considerations in framework
are mainly concerned with keeping
parts of the framework out of sight.
6. 6
BIOMECHANICS-: the relationship between
the biologic behavior of oral structures and
the physical influence of a dental restoration
The design of the typical removable partial
denture must differ from its fixed counterpart in
several aspects and for various reasons.
The edentulous area that it restores are
usually bilateral, the spans are generally longer,
and more importantly, the typical prosthesis must
rely for a portion of its support on a resilient,
displaceable foundation,- the oral mucosa.
7. 7
A RPD in the mouth can perform the
action of two simple machines, the
lever and the inclined plane.
It may also act like a wedge but
not in the normal course of events
Class I, II and IV RPD are subjected
to greater stresses because their
support is a combination of tooth
and soft tissue.
8. 8
The lever is a rigid bar supported at some point
along its length.
If the lever rests against its support and a weight
is applied at another point, rotation or movement will
occur around the support – the support is known as
the fulcrum.
9. 9
Levers are classified according to the location of
the fulcrum,the power and the resistance in
respect to each other
Class 1
Fulcrum is in b/w the power and the resistance.
The resulting mechanical advantage can be
greater or less than one depending upon the
location of the fulcrum along the lever.
10. 10
Class 2---
Has the resistance b/w the fulcrum and
the power,and the mechanical
advantage that's greater than one.
11. 11
Class 3---
The power is applied b/w the fulcrum
and the resistance.
Its mechanical advantage is always less
than one.
12. 12
A cantilever is a beam supported only at one
end and acts as a first class lever.
13. 13
Inclined plane:
Forces against the inclined plane may result in
deflection of that which is applying the force or may result
in movement of the inclined plane., neither of these results
are desirable.
Inclined planes are not a factor when the partial denture
is tooth supported.
The distal extension PD on the other hand is subjected to
rotation around three-principle fulcrum. During the
formulation of a design these three fulcrums and the
movement that may take place around them must be
considered.
14. 14
Components of the denture may then be positioned to
counteract as much of the rotation as possible.
Components of a partial denture must be selected
and positioned to perform a specific function.
Fibers of the pdl are so arranged that vertical forces
are resisted to a greater degree than are horizontal or the
torsionalforces. .
15. 15
Forces acting on the partial denture
The forces acting on the partial denture are a result of a
composite of forces arising from three principle
fulcrums.
First fulcrum is on the horizontal plane that extends
through the two principle abutments.
This fulcrum controls the rotational movement of the
Denture in the sagittal plane.
The resulting force on the Abutment teeth is usually
Mesial or distal-apical with the greatest vector in the
Apical direction.
16. 16
Second fulcrum is on the sagittal plane and extends
through the occlusal rest on the terminal abutment and
along the crest of the residual ridge on one side of the
arch. In class I situation there would be two of these
fulcrums. This fulcrum controls the rotational
movement of the denture in the vertical plane (rocking,
side-to-side movements over the crest of the ridge).
The main direction of the resulting force is more
nearly horizontal and not well resisted by the
tissues.
17. 17
The third fulcrum is located in the vicinity of the
midline just lingual to the anterior teeth.
This fulcrum line is vertical, and it controls the
rotational movement of the denture in the
horizontal plane or the flat circular movement of
the denture.
18. 18
Factors influencing magnitude of stresses
transmitted to abutment teeth:
1)Length of the span
2)Quality of the ridge
3)Qualities of the clasp-design,
- length of the clasp,
- materials used,
-occlusal harmony
19. 19
1) Length of the span:
The longer the edentulous span, the longer
will be the denture base. The longer the base,
the more will be the leverage factor, and thus
the greater will be the stress that is transmitted
to the abutment teeth.
20. 20
The fulcrum is located at or near the
occlusal rest on the terminal abutment
tooth.
A base that begins next to the cuspid will
have a greater degree of movement than
will the one that begins distal to the
second bicuspid.
21. 21
2) Quality of support of ridge:
Large, well-formed ridges are capable of absorbing greater
amount of stress than small, thin, or knife-edge ridges.
The type of mucoperiosteum influences the magnitude of
stresses transmitted to the abutment teeth.
A healthy mucoperiosteum approximately 1mm thick is
capable of bearing a greater functional load than is a thin
atrophic mucosa.
22. 22
Soft, flabby, displaceable tissue contributes
little to the vertical support and nothing to the
lateral stability of the denture base.
This type of tissue allows excessive
movement of the denture, with resultant
transmission of stress to the adjacent
abutment tooth.
23. 23
3) Qualities of clasp:
a) Clasp design:
A clasp that is designed so that it is passive
when it is completely seated on the abutment
tooth will exert less stress.
Only when the framework is completely
seated will the retentive clasp arms be passive.
If a retentive tip is designed and constructed
to lie in a 0.010 inch undercut and the
framework does not go completely to place, that
retentive clasp will exert a continuous force to
the abutment tooth.
24. 24
Round section and the half round section clasps flex
easily in the horizontal plane but the half round form
is more resistant to movement in the vertical plane.
If the thickness is reduced by half the flexibility is
increased by a factor of eight.
25. 25
b) Length of clasp:
The more flexible a clasp, the less stress it
will exert on the abutment tooth. Flexibility can
be increased by lengthening the clasp.
Doubling the length of the clasp will increase its
flexibility five times.
26. 26
c) Materials used in clasp construction:
A clasp constructed of chrome alloy will normally
exert greater stress than a gold clasp.
Similarly a retentive clasp arm made of wrought
alloy is more resilient than one made of cast alloy and
thus will transfer less stress to the abutment.
The greater tooth to metal contact between the
clasp and the tooth, the more will be the stress.
27. 27
d) Occlusal harmony:
A disharmonious occlusion, one in which deflective
occlusal contacts between opposing teeth are present,
generates horizontal forces that can transmit destructive
forces to both abutment teeth and the residual ridges.
Ideally the occlusal load should be applied in the center of
the denture bearing areas.
If the occlusal load is applied to the base adjacent to the
abutment tooth, there will be less movement of the denture
base than if it is applied at the distal end of the denture
base.
28. 28
Design
considerations:
1)DIRECT
RETENTION:
A removable partial
denture should always be
designed to keep clasp
retention to a minimum
yet provide adequate
retention to prevent
dislodgement of the
denture.
29. 29
Factors that influence the retention of
RPD
Physiologic retention
Is proportional to the tissue covered by
the denture base .Maximum area
possible within the limits of the health
and function of the tissues should be
covered.
30. 30
Mechanical retention
Flexible clasp tips placed in undercuts
on teeth provide mechanical retention.
Applegate stated that a clasp should
generate only an acceptable minimum
of retention.
McCracken and Hickey wrote that
retention on all abutment teeth should
be as nearly equal as possible.
31. 31
Retention is provided by the terminal
portion of the retentive arm. In essence,
for a clasp to be an effective retainer its
terminal tip must pass over the
maximum contour of the tooth and enter
an undercut area.
32. 32
Depth of horizontal undercut engaged
The greater the depth of horizontal undercut
engaged by the retentive portion of clasp arm, the
greater will be the force required to flex the arm
over the maximum contour of the tooth.
33. 33
Direction of approach of the clasp arm :
Clasp arms may approach the undercut area of a tooth
from the occlusal (occlusally approaching) or gingival
aspects (gingivally approaching).
The retentive action of the gingivally approaching arm
is said to be better because it wedges against the tooth
surface as it moves towards the maximum contour of the
tooth as the denture is displaced during function.
The occlusally approaching clasp arm does not have
this‘trip’action.
34. 34
Forces of adhesion and cohesion :
The denture base should cover the
maximum area of available support and must
accurately adapt to the underlying mucosa.
Adhesion : is the affinity of the denture base,
as well as the palatal connector, to the
mucosa when an intervening layer of fluid is
interposed between them.
Cohesion:
Is the internal attraction of the molecules of
saliva for each other.
35. 35
Atmospheric pressure:
Not a major factor in retention of partial
prosthesis. The retentive potential of atmospheric
pressure is exploited by sealing the peripheries of
the denture from the ingress of air between the
denture base and the mucosa.
Frictional contact:
Properly planned and prepared guiding planes
contribute to the retention as a result of frictional
contact with adjacent tooth surfaces.
37. 37
Clasp position :
The position or the relation of the retentive
clasp to the height of contour is more important
in retention and in controlling stress than is the
number of clasps.
Quadrilateral configuration :
Is indicated most often in class III situation a
retentive clasp should be positioned on each
abutment tooth adjacent to the edentulous
spaces.
38. 38
This results in the denture being confined
within the outline of the four clasps, and
leverage on the denture is effectively
neutralized.
39. 39
Tripod configuration :
Used primarily for class II arches. If there
is a modification space on the dentulous side,
the teeth anterior and posterior to the space
are clasped to bring about the tripod
configuration.
40. 40
Bilateral configuration :
Ideally, single retentive clasp on each side of
the arch should be located near the center of the
dental arch or denture bearing area.
The terminal abutment one each side of the
arch must be clasped regardless of where it is
positioned.
41. 41
Reciprocation :
Every retention clasp should have a
rigid reciprocating component located
on the diametrically opposed tooth
surface.
As a retentive clasp arm is displaced
in the occlusal direction, it exerts,
horizontally directed force, if no
reciprocating component is present, the
tooth is displaced laterally.
42. 42
Clasp design :
Circumferential clasp :
The design of the clasp can greatly
affect the type of stress that is
transmitted to the abutment tooth.
43. 43
The conventional circumferential clasp originating from a
distal occlusal rest and engaging a mesiobuccal retentive
undercut should not be used on a distal extension RPD. The
terminal of this clasp reacts to movement of the denture
base toward the tissue by placing a distal tipping or
torquing, force on the abutment tooth.
44. 44
The reverse circlet clasp that
approaches the distobuccal undercut
from the mesial surface of a abutment
is acceptable.
45. 45
Bar clasp :
It is indicated when the retentive
undercut is located on the distobuccal
surface. It is never indicated when the tooth
has a mesiobuccal undercut. As the denture
base is loaded toward the tissue, the
retentive tip of the T-clasp rotates gingivally
to release the stress being transmitted to the
abutment tooth.
46. 46
Combination clasp :
When a mesiobuccal undercut exists on
an abutment tooth adjacent to a distal
extension edentulous ridge, the combination
claps can be employed wrought alloy wire is
more flexible than a cast clasp.
The wrought wire retentive arms has a
stress breaking action that can absorb
torsional stress in both vertical and horizontal
planes.
47. 47
2)INDIRECT RETENTION :
An indirect retainer is a part of the removable partial
denture that helps the direct retainer prevent
displacement of the distal extension denture by
resisting the rotational movement of the denture
around the fulcrum line established by the occlusal
rests.
48. 48
In class I arch the indirect retention must always
be used.
The indirect retainer or retainers must be
positioned as far anterior to the fulcrum line as
possible.
If a modification space exists on the tooth
supported side, abutment teeth on both sides of
the space should be selected.
49. 49
For class III arch, indirect retention is not
ordinarily required, because there is no distal
extension denture base to create a lever arm.
The consideration for the class IV arch is the
reverse of that for class I and class II arches.
The lever arm is anterior to the fulcrum line so
the indirect retainer must be located as far
posteriorly as possible.
50. 50
Indirect retention is an application of class
2 lever.
Tooth surface on which the indirect
retention rests is the fulcrum.
The retentive part of the direct retainer
clasp becomes the resistance and power is
represented by any force that tends to
move the denture base away from the
underlying tissue.
51. 51
3)DENTURE BASE :
The denture base should be designed to
cover as extensive an area of supporting
tissue as possible.
The denture base flanges should be made
as long as possible to help stabilize the
denture against horizontal movements.
The distal extension denture bases must
always extend onto the retromolar pad of the
mandible and cover the entire tuberosity in
the maxilla.
52. 52
4)Occlusion :
A smoothly functioning occlusion that is in
harmony with the movements of both the TMJ
and the neuromusculature will minimize the
stress transferred to the abutment teeth and
the residual ridge.
Steep cuspal inclines on the artificial teeth
should be avoided because they tend to
introduce horizontal forces that can produce
torsional stresses on the abutment.
53. 53
5)Major connector :
In mandibular arch the lingual plate major
connector that is properly supported by rests can
aid in the distribution of stresses.
The lingual plate also adds rigidity. In the
maxillary arch the use of broad palatal major
connector that contacts several of the remaining
natural teeth through a lingual plating can
distribute stress over a large area.
A major connector that uses maximum
coverage can contribute greatly to the support,
stability and retention of the prosthesis
54. 54
6)MINOR CONNECTOR :
The most intimate tooth-to partial denture
contact takes place between the minor
connector joining the claps assembly to the
major connector.
This serves two purposes – first it offers
horizontal stability against lateral forces.
Second through the contact of the minor
connector and the abutment tooth, the tooth
receives stabilization against lateral stress
55. 55
7)RESTS :
One of the most critical points of the rest
seat is that the floor of the preparation must
form an angle of less than 90 degrees with a
perpendicular line dropped down the long
axis of the tooth.
If the angle formed is greater than 900 an
inclined plane action is set up and the stress
against the abutment tooth is magnified.
56. 56
Splinting of abutment teeth :
Adjacent teeth may be splinted by means of
crowns to control stress transmitted to a weak
abutment tooth.
Fixed splint may be indicated when some loss
of periodontal attachment has occurred.
Splinting is also indicated when the proposed
abutment tooth has either a tapered or short
roots such that there is not an acceptable amount
of pdl attachment present.
57. 57
One of the most important and frequently
indicated needs for splinting is when the
terminal abutment tooth on the distal
extension side of the arch stands alone, i.e.,
an edentulous space exists both anterior and
posterior to it.
59. 59
Jpd 1989
An analysis of rotational movement of
asymmetrical distal extension Rpd
The distal extension removable partial denture
derives its support from two different types of tissue.
These are 1) tooth representing a relatively
immovable support and 2)the soft tissue overlying
the residual ridge. These two have different degrees
of displaceability.
When pressure is applied to the denture base ,an
axis of rotation is created around the most distal
abutment teeth causing a torquing effect on the
clasped teeth.
60. 60
In a symmetric bilateral distal extension RPD,
the long axis of the denture base is
perpendicular to the axis of rotation.
Occluding forces tend to move the denture
base framework in an arc almost parallel to
the longitudinal axis of the residual ridge and
toward it.
61. 61
In the asymmetric bilateral distal-extension RPD, where
residual ridges are of unequal length, the axis of rotation is
not perpendicular to the residual ridge.
The resultant vector is directed in a buccal-to-lingual
direction on the longer edentulous side and from a lingual-
to-buccal direction on the shorter edentulous side as the
framework rotates eccentrically.
62. 62
A unilateral distal extension RPD is an extreme situation
where the axis of rotation is at an angle to the long axis
of the residual ridge instead of parallel to it.
The tissue ward movement of the denture base may
result in elevation of the occlusal rests on the opposite
side of the arch.
The only occlusal rest to remain functional will be the
rest adjacent to the longer denture base.
63. 63
REST LOCATION
The advantages of the mesial rest on the
tooth adjacent to the edentulous ridge in the
bilateral extension RPD is well documented.
In the uni-lateral distal extension RPD on the
dentulous side the occlusal rest should be
placed on the most distal tooth, enabling the
indirect retainer to be placed at a greater
distance from the axis of rotation and
therefore to be more effective
64. 64
THE DIRECT RETAINER-
To prevent torquing of the abutment tooth when
the forces are applied to the denture base, the
retentive tip of the clasp arm in the RPI clasp
assembly is placed on or just anterior to the
greatest linguo-buccal curvature of the
abutment tooth.
Tissue ward functional movement of the denture
base will disengage the retainer tip from the
undercut.
65. 65
THE RPL CLASP ASSEMBLY.
A method of recreating a more desirable class
2 lever effect on the abutment tooth requires
the substitution of L-shaped direct retainer for
the I bar.
The occlusal rest is mesially located and the
proximal plate placed at the linguodistal
surface of the abutment tooth.
The L bar emerges from the framework, rises
vertically to the height of contour of the
abutment tooth,and passes distally to engage
the disto-buccal undercut.
66. 66
When the denture base is loaded the
forward movement of the retainer will be
minimized by placing the retentive clasp
tip on the most distal part of the tooth
near the same horizontal level as the
occlusal rests.
67. Frank and Nicholls (JPD 1977:38;494)11
-did a study on the effectiveness of indirect retainer and
concluded that use of a mesial rest instead of a distal rest
on the terminal abutment tooth does not decrease
indirect retention.
Thus the choice of indirect retainer location should be
made mostly on the basis of abutment tooth support, a
crown form favoring adequate rest seat preparation, and
the patients esthetic desires.
68. PHILOSOPHIES OF DESIGN
• There are three basic design philosophies:
• Stress Equalization
• Physiologic Basing
• Broad stress distribution
69. STRESS EQUALIZATION
• The resiliency of the tooth supported by periodontal ligament in
an apical direction is not comparable to the greater resiliency
and displacement of the mucosa covering the dentulous ridge.
• It is the belief of this school of thought that the rigid connection
between the denture base and the direct retainer on the
abutment teeth is damaging
70. • Thus some form of stress director or stress equalizer is
essential to protect the abutment teeth.
• The most commonly used ones are composed of a hinge
device interposed between the minor connector of the
abutment tooth and the denture base.
71. PHYSIOLOGIC BASING
The belief is that the equalization can best and most simply
be accomplished by some form of physiologic basing.
• The physiologic basing is produced either by
• Displacing or depressing the ridge mucosa during the
impression making procedure
• Relining the denture base after it has been constructed.
72. Displacing the mucosa during the impression procedure
records it in its functioning and not the anatomic form.
This denture base formed over displaced tissue, will adapt
more readily to the depressed tissue when occlusal force
acts and will be better able to withstand the force that is
generated
73. BROAD STRESS
DISTRIBUTION
distributing the forces of
occlusion over as many teeth
and as much of the available
soft tissue area as possible.
This is accomplished by the
use of additional rests,
indirect retainers, clasps and
broad coverage denture
bases
74. STEP BY STEP DESIGNING
Diagnostic cast area of recontouring
Black-mark survey lines desired undercut is measured
77. 77
Every denture must be designed from first principles,
which involves the application of a simple sequence of
stages.
Looking through a textbook until a design is
found in which the distribution of the natural teeth is
similar to that of the patient and then copying the
configuration of the components ignores the unique
status of the patient whose oral health may thereby
be irreparably damaged.
Conclusion
78. 78
References
1. McCracken's removable partial
prosthodontics...11 edition.
2. Stewart Rudd kuebker-clinical removable
prosthodontics.....2nd edition
3. Joseph E Graso and Ernest
Miller.....Removable partial
prosthodontics—3rd edition
4. Jpd July 2003 vol 90 no 1....Indirect
retention in partial denture design.
79. 79
6. Jpd Feb. 1989 vol 61 pg 211....an
analysis of rotational movement of
asymmetrical distal extension rpd.
7. Removable partial dentures –Davenport,
Basker,Ralph
8. Dental technology for students eight
edition ..H.J.Wilson,M.A.Mansfield
