Prosthodontic Lecture

4\10

Basic Principles of RPD Design

The objective of partial denture designing is to control the denture movement
without exceeding the physiological tolerance of the oral structure

Response of the Denture to Various Forces Acting on it

Introduction
The forces acting on the artificial teeth of the denture can be transferred
:through
the metal framework of the RPD (e.g. rest) to remaining teeth (1
.the denture base to the supporting soft tissues (2

Here the forces acting on the occlusal

tables of the artificial teeth are
distributed to the supporting soft
tissue and the to the abutment tooth

: The partial denture can be

Tooth supported partial denture (1
The denture is supported completely by the remaining teeth, as the soft tissue
are covered for aesthetic and hygienic purposes, e.g. class III Kennedy's
.classification
Here the soft tissue, that are covered, dont provide any support; hence the
mucosa in this context is not for load bearing, BUT the remaining teeth are, so
.the forces are transmitted along the long axis of the abutment teeth
In tooth supported partial denture, the remaining teeth are the ones to be
loaded, thus, they act as stoppers preventing denture's tissue-ward
.movement, eventually they will prevent compression of the soft tissue
Tooth-tissue supported partial denture (2
The denture is supported by both the soft tissues and the remaining teeth.
e.g. in class I, class II Kennedy's classification and extensive Kennedy class
IV (long span class IV) cases. The mucosa or the soft tissues under the
denture are not designed for load bearing, but in this case the forces acting on
:the denture are shared between the teeth and the soft tissues because

Prosthodontic Lecture
4\10
First, the number and the location of the remaining teeth only dont provide
sufficient support for the denture, so we depend on the soft tissues to obtain
.the support the denture needs
Second, since the teeth are more rigid components than the mucosa, so they
are more susceptible to be overloaded when the forces are transferred by the
partial denture to the supporting tissue, so this teeth's overloading is
.prevented via directing the forces more toward the soft tissues
Hence, the occlusal loads are distributed, on the remaining teeth and on the
soft tissues, so the prosthesis design should be wisely established in order to
.distribute these loads properly
Note : Forces that act on the artificial teeth of the denture are going to be
transferred to the remaining supporting teeth (via the metal framework) and
to the soft tissues (via the denture base) in case we have tooth-tissue
supported partial denture. In case we have tooth supported partial
denture, the forces are dissipated to the remaining teeth via the metal
.framework
In tooth-tissue supported partial denture, since the denture is partially
supported by the resilient soft tissues, so logically this kind of support allow
some movement of the denture, and the amount of movement (especially
tissue-ward movement) corresponds to the amount of compression that's
happening on the soft tissue. Such movement should not be prevented,
because as the previous objective states "control the movement of the
denture"; "control" means reducing the denture movement in a way that
.permits the loads to be distributed
So, in tooth-tissue supported denture, some movements of the denture are
allowed, and as result, the soft tissues are loaded and compressed in order to
.distribute the loads
:In order to achieve that, we have to provide 3 characteristics
Stability -1
It's the resistance to horizontal forces and rotational tendencies acting on the
RPD, such tendencies would result in unstable denture, in other words, the
.denture is rocking or moving, so we need to get rid of these tendencies

These movements in a
buccolingual direction
(horizontal) affect the stability of
.the denture, making it to rock

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Retention -2
It's resistance to dislodging forces acting on the RPD, these dislodging forces
:originating from
I- Gravity
.Affects the upper denture only
II- The surrounding soft tissues
.They provide some push on the denture to get it out from its place
III- Sticky food
The most important dislodging forces. We should know that these forces act
on the occlusal surfaces of the artificial teeth of the RPD, not on the rests, that
is due to the small surface area provided by the rest in contrast to the surface
area provided by the artificial teeth, making the sticky food dislodging forces
acting on the rest neglitiable in comparison to the one acting on the occlusal
.surface of the artificial teeth

Support -3
It's the resistance to vertical forces acting on the RPD or on the artificial teeth.
.It's provided by the occlusal, lingual, and incisal rests

The support is the resistance to

movement of the denture tissue
ward

Support is provided by the

rest that act as vertical stop

Obtaining the Support for the RPD

The support gained for the RPD should be uniform, it's not a problem for the
tooth supported partial denture, since the support is provided by the teeth
only, BUT it's a problem for tooth-tissue supported partial denture; since the
support provided by the soft tissues is not as the same as the one provided by
.the remaining teeth
Thus, since two tissues of different resiliency support the denture, stress is
precipitated within the denture due to uneven settlement during occlusal
.loading

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Tooth-tissue supported dentures are subjected to leverage forces due to the

differences in the compressibility of the supporting structures (periodontal
ligament of the abutments is less resilient compared to the mucosa overlying the
residual ridge hence the denture will settle more in the tissue support areas)

That is, if we exert a force of equal amount on the soft tissues and on the
teeth, the response of the soft tissues differs from that of the teeth, e.g. this
graph below shows that exerting a force will result in a displacement of the
soft tissues (500 micron) that is much more than that of the teeth (25 micron),
that means that the soft tissues exhibit different behavior from that exhibited
by the teeth; so the support provided by the soft tissues is not similar to the
.one provided by the teeth
This difference is not coincide with our goal of support; that is to be uniform,
so we try to minimize this difference either by increase the displacement of
the tooth, but this will damage the tooth and cause it's mobility, or by
decreasing the displacement of the soft tissue; and this the effective
.maneuver for obtaining the uniform support we seek

The soft tissues exhibit different

behavior than that of the teeth
under stress, so the support
obtained for the tooth- tissue
supported denture is not
uniform

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:Note
This minimal displacement or compressibility (25 micron) of the teeth is
because of the stiffness of the periodontal ligaments surrounding the teeth
which has more collagen content than the soft tissues, so these ligaments are
.the ones that allow such a minimal movement or displacement of the teeth

The periodontal ligaments allow minimal tooth movement

:So, Obtaining the support for the RPD can be gained from
.The soft tissues (1
.The remaining teeth (2

Obtaining Support from the soft tissues

To control the movements of the denture, first thing we do is to dissipate the
forces acting on the denture, including the tissue-ward forces, these harmful
forces have to be distributed and resisted in order to get rid of any unwanted
movement that may affect the denture. Obtaining the support from the soft
:tissues is based on
I- Reducing the occlusal loads acting on the occlusal table of the
:artificial teeth. This can be achieved via
Reducing the size of the occlusal tables of the artificial teeth. It will
:result in
If the occlusal table is large and wide, the patient may bites parafunctionally; not on the functional cusps, then these biting forces may
act buccally or lingually away from the centre\axis of the artificial teeth
causing the RPD to rotate. Thus, if we reduce the occlusal table size, the
occlusal forces is then directed at the axis\centre of the artificial tooth and
no off-centric forces acting on the occlusal tables anymore [Note: the
.centre\axis of the artificial teeth resembles the crest of the ridge]

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With a wide large occlusal table, there

a great possibility that the patient bites
parafunctionally away from the center,
or the axis, of the artificial teeth
causing the rotational movement of the
RPD and lack of stability

With a small narrow occlusal table, the

possibility that the patient will bite
parafunctionally buccally or lingually is
limited, the biting forces are now more
directed toward the center or the axis
of the artificial tooth and no off-centric
forces acting on the occlusal tables
anymore; the forces are centered by
.the small size of the occlusal table

The food penetration forces or the masticatory functional loads exerted to crush the food per masticatory cycle is going to be reduced when reducing
the occlusal table, such as the sharp and the blunt edge of a knife; if the
occlusal table is wide, the cusps of the artificial teeth have the effect of a blunt
edge of a knife and more forces must be exerted to crush the food, BUT if the
occlusal table is small, the cusps have the effect of the sharp edge of the
.knife, thus the force exerted to crush the food is decreased
Note: Recalling the equation: pressure = force\area, Someone may say that
if we decrease the size of the occlusal table (the area), the pressure will
increase, but the increased pressure affects the occlusal surface, not the
.resilient supporting tissues under the denture

The increased pressure

affect the occlusal
surfaces

The increased pressure

has no effect on the
mucosa underneath

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.II- Reducing the saddle movement under occlusal loads
This strategy is implemented during impression making procedure; that is ,
during impression making procedure, we depend on the "Mucofunctional
Concept" in order to compress the soft tissues. Thus, this concept aims to
fabricate a denture that fit accurately against functionally displaced mucosa,
so the denture, at the insertion and function, will sink less under masticatory
.loads
That is, as we mentioned previously the compressibility of the soft tissues
equals to 500 microns, at the impression making procedure, and according to
the Mucofunctional Concept, we use an non-elastic rigid impression material;
impression compound, in order to compress the soft tissues by 300 micron,
the remaining 200 micron of the soft tissues thickness is compressed later at
the time of insertion and functioning. So, this 200 micron thickness, is of a low
magnitude causing minimal movement of the denture under the masticatory
.loads
This intended movement of the denture is referred to "Stress Breaking".
Stress Breaking is one of the concepts that are used to distribute the forces
.evenly along the soft tissues and the supporting tooth structure
Stress Breaking is defined as relieving the abutment teeth of all or part of the
occlusal forces.
We know that the soft tissues are more compressible than the abutment teeth.
In a tooth-tissue supported partial denture, when an occlusal load is applied
the denture tend to rock due to the difference in the compressibility of the
abutment teeth and the soft tissues. As the tissues are more compressible,
the amount of stress acting on the abutment increased. This can produce
harmful effects on the abutment teeth, so we are breaking the stress, that is
concentrated on the on the tooth, in order to distribute the loads to the other
.areas which are more flexible
:Note
The anatomy of the ridge is important to be considered when obtaining the
support. As the picture below shows; in B , this ridge provide support but no
stability, because this ridge posses horizontal surfaces but no vertical
surfaces. In C, this ridge provide stability but not support because the ridge
owns vertical surfaces but no horizontal surfaces. In D, the ridge is flappy and
has neither support nor stability; hence, the soft tissues either they are left or
removed surgerically, so that they become thinner to transfer the forces to the
.underlying bone more efficiently

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B : support but no stability

C : stability but no support
D : flappy soft tissues
provide neither support
nor stability

Obtaining the Support from the Remaining Teeth

The support for the RPD is obtained from the remaining teeth through using
the rests. These primary supportive elements are placed adjacent to the
.edentulous space on the primary abutment
A dilemma may arise is to decide where to place the rest; mesially or distally
on the abutment tooth, in case of class I Kennedy's classification for example.
Thus, if the rest is:
Distally located:
This resembles class I lever system, that is when the load is applied on the
artificial teeth in a tissue-ward direction, the rest will act as a fulcrum, and the
clasp is going to move upward and engage with the undercut with each
masticatory loads, even at the initial closure of the mouth, so the tooth is
going to be compromised through this rotational forces acting on it. As a
result, the tooth is going to move distally; this has a destructive potential on
the tooth and it's eventually going to be destroyed.
Mesially located:
It's advantages: the abutment tooth is going to be move closer to the adjacent
tooth, it's more favorable than that if the rest is distally located. The soft
tissues are more loaded, since the fulcrum of rotation is far away from the
force applied; that is if the distance increase, that will allow more movement
of the denture and the soft tissues will be loaded more. In contrast to the
distally-located rest in which the soft tissues are loaded less.

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Mesially-located rest resembles class II lever system, and the most famous
system to be used in the RPD fabrication is the RPI system by which the I-bar
clasp won't engage with the undercut with each masticatory force.
RPI System
It consist of a mesial Rest, Proximal plate and an I-bar clasp. The significance
of the RPI system is that being a stress breaker; the I-bar is being disengaged
with the undercuts with each masticatory cycle, the proximal plate is designed
in away that serves as a stress breaker, and although the rest still makes a
contact with the abutment, it resembles the hinge of the door that allowing the
denture to move, but not out of its place, permitting the soft tissues to be
loaded.

The RPI System and Stress Breaking

The Proximal plate design and the Tilt used : In Kennedy's class I the tilt
that is made is an anterior tilt, and this tilt aids in the stress breaking. That is,
the picture below shows:

In 1: the occlusal forces are seating in nature, and the fulcrum of rotation is
the closest rigid contact with the abutment tooth, and that is the long proximal
plate. As the force is applied, the cervical part of the proximal plate will still
contact the tooth structure at all phases of it's movement. So, there is no
stress breaking in here; since it contact the tooth in it's phases of movement.
. ,
.

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So, this long proximal plate has the tendency to transfer the force to the tooth
by it's cervical part, and hence, so, there is no stress breaking; since it contact
the tooth in it's phases of rotation, where as the occlusal part of the proximal
plate goes up away from the tooth
In 2: the occlusal forces are seating in nature, and the fulcrum of rotation is
the closest rigid contact with the abutment tooth, and that is the mesial rest.
Here, the proximal plate is shortened and makes no contact with the
abutment tooth when the denture is fully seated, however, there is a contact,
between the guiding plane and the abutment, at the moment that the denture
is being inserted guiding the denture to it's place, after that the proximal plate
is out off-contact with abutment. So, this kind of stress breaking.
In 3: the occlusal forces are seating in nature, and the fulcrum of rotation is
the closest rigid contact with the abutment tooth, and that is the proximal
plate. Here, we cut part of the cervical segment of the proximal plate that
make the engagement with the tooth structure making no contact between the
proximal plate and the abutment cevicallly. When the forces are applied, the
occlusal part of the proximal plate goes up and there is no engagement of the
cervical part with the tooth surface as well. So, this is kind of stress breaking.
Note: Here, there is space between the proximal plate and the tooth
cervically and the I-bar that engages the mid buccal undercut is free and it's
not limited to move mesially or distally, so that it is not going to transfer the
load to the teeth when the denture is being loaded vertically.
So, in 2 and 3 design of the RPI system have stress breaking potential; the
denture is allowed to move to distribute the loads to the tooth only by the
rests, and such movement will allow the soft tissues to be loaded.
Note:

If the lifting forces are applied, the fulcrum of rotation will be shifted to the
proximal plate, (the original fulcrum of rotation was the rest when the
denture is fully seated; since it was the proximal rigid contact and no
contact was present between the abutment and the proximal plate; this kind
of stress breaking prior to the rotational stresses in the denture) and there
would be a contact between the abutment and the proximal plate. That is,
retention here is provided by the occlusal part of the proximal plate, thus,

Prosthodontic Lecture
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the cervical part is moving away from the tooth, and the occlusal part is
moving into the tooth.
If a posterior tilt is made, thus, more stress breaking potential is provided,
since the loads on the abutments are reduced via increasing the retentive
potential. [ Note: here the stress breaking does not mean that there is no
contact between the abutment and metal framework only, but it also means
directing the loads in term of low magnitude retentive forces decreasing the
undesirable forces acting on the abutment ].
Transferring the forces in term of retention is more favorable than in term of
support that is due to the retentive forces will be in a low magnitude in
contrast to the supportive forces that will be in a high magnitude that the
tooth can not tolerate, without the soft tissues are being shared in the
process of load bearing.

Essentials of design
In this section we will discuss about the key factors to be considered while
designing a partial denture for common clinical situations.

Design Consideration for Kennedy's Class III Case

This is a modified class III Kennedy's classification; class III modification 1. it's
.referred to as Quadrilateral or Trapezoidal Configuration

As a rule in designing, the design of the RPD should be as simple as possible;

extra components means extra cost and this cost can be: coverage of the soft
tissue, patient discomfort, and more preparations, that means biological and
.psychological stresses on the patient

Quadrilateral Configuration
This design involves the use of 4 clasps and 4
rests. It's used commonly for Kennedy's class
III specially when there is a modification space
on the opposite side of the arch. The rests and
clasps should be positioned on each abutment
.tooth adjacent to the edentulous area

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Clasp
.Four clasps should be placed to obtain Quadrilateral design

Rest
The rests should be placed adjacent to the edentulous space; on the primary
.abutments

Major Connector
:It should fulfill the ideal requirements, and they are
Rigid -1
A major connector should not be flexible. It should be rigid enough to
uniformly distribute the occlusal forces acting on any portion of the prosthesis
without undergoing distortion.
2- It should be comfortable to the patient.
3- It should not allow any food accumulation and it should be self-cleansing,
so it's designed with open angles, where it connect the minor connectors, for
hygienic purposes.
4- Provide minimal tissue covering.
5- Does not compress the sensitive tissues underneath.
:Note
If there were more modification areas, the number of the rests would increase
But the number of the clasp won't, thus, The maximum number of the clasps
in Kennedy class III cases is 4, and they are usually 4 in number at average.
Even in an unmodified class III Kennedy's classification; with double Akar's
clasps that are made on the dentulous side for cross arch stabilization the
clasp number still 4. The minimal number of the rests to be placed is 4, even if
there is no modification areas, with double Akar's clasps would double rest on
.the abutment

Unmodified class III Kennedy's

classification
With the double Akar's clasp for
cross arch stabilization, the
number of clasps is 4 and the
number of the rest is 4 as well; by
which the Akar's clasp provides 2
clasps and 2 rests as well

:Note
In class III Kennedy's classification, class III lever system is applied; the most
.favorable lever system

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Design Considerations for Kennedy's class I and II Cases

.First, I should introduce the lever system for better understanding
Occlusal Forces acting on the RPD (e.g. biting, incising, sticky food dislodging
forces) develop internal stresses within the denture, which ultimately lead to
lever action; the denture tends to rotate around a Fulcrum Line (axis of
rotation).
The Lever is defined as a long bar with a single support around which it
rotates when the load is applied to any one of it's ends. The support around
which the lever rotates is called as the fulcrum.
The Fulcrum Line is defined as : an imaginary line around which a partial
denture tends to rotate. It's formed at the terminal abutment axis (line joining
the two posterior-most rests).
levers can be of 3 types:
1st - order lever
2nd-order lever
3rd-order lever
In 1st -order lever:
the fulcrum is in the center, resistance is at one end and effort (force) is at the
other end. This type of lever system can occur in patient with distal extension
partial denture.

1st order lever

E=effort , F=fulcrum, L=load

A distal extensions denture

base is an example for a 1storder lever where in the
masticatory force (E) lift the
anterior part of the denture (L)
using a direct retainer as a
fulcrum (F)

Adding an auxiliary rest anterior

to the fulcrum prevents the lever
action by acting as an indirect
retainer. When an indirect
retainer is given, the 1st lever
system is converted to a line
order lever which is more
beneficial for the prosthesis

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2nd-order lever system
In this lever the fulcrum is at one end, effort is at the opposite end and the
resistance or the load is at the center.

2nd order lever

E=effort , F=fulcrum, L=load

3rd-order lever
In this lever, the fulcrum is at one end, resistance is at the opposite end and
the effort is at the center.

3rd order lever

E=effort , F=fulcrum, L=load

a tweezer is a typical example

of the 3rdorder lever

Lever action in a Kennedy's class I prosthesis:

In Kennedy class I, there is no posterior abutments, so the support is shared
between the soft tissues and the remaining teeth as mentioned previously.
In a distal extension partial denture, rotation occurs around 3 principles
fulcrums. They are:
1- Saggital fulcrum line.
2- Vertical fulcrum line.
3- Horizontal fulcrum line.

The vertical fulcrum line

The saggital or the

Anteroposterior fulcrum line

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The Horizontal fulcrum line passing between 2 principal abutment teeth.
This controls the rotational movement of the denture, toward or away from the
supporting soft tissues.

Horizontal fulcrum line

passing through the terminal
abutment axis in a
Kennedy's class I partial
denture

If the forces were unseating in nature (dislodging forces, e.g. sticky food dislodging forces) , the rest would resemble the fulcrum of rotation; it's the
.fulcrum about which the clasp is going to exert their retentive function
That is, if consider a long bar which has a single support. When the bar is
pulled up on one end, the other end goes down. Now, if the same bar has
another support on the other end, then the bar will not go down on that end.

It is easy to lift a bar with a single support as it will act

like a fulcrum and allow free rotation of the bar

The same bar supported by more than one support cannot be easily lifted at one end
because the support away from the effort (E) will prevent downward movement of the
bar. If additional force is applied to lift the bar, the support away from the effort will act
as a fulcrum of rotation. Since the fulcrum of rotation is away from the effort, additional
force is required to destabilize the bar.

Prosthodontic Lecture
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This is the same mechanism present in an indirect retainer. When the denture
tends to rotate along the fulcrum line, the denture rotates around the single
support i.e. the direct retainer. providing an additional support away from the
fulcrum line in the form of occlusal rest can prevent the rotation of the denture
and function as an indirect retainer.

When the denture is lifted

away from the tissues, it
tends to rotate around the
direct retainer. Adding an
auxiliary rest anterior to the
point of rotation of the
denture will function as an
indirect retainer and prevent
the rotation of the denture

- If the forces acting on the artificial teeth, were seating in nature (tissue-ward
forces), the fulcrum of rotation would be the closest rigid contact; that is, either
they would be the rest or the proximal guiding plates, if they are the one that
make the closest rigid contact.
Note: If the fulcrum of rotation is far away from the force applied, then the
clasp would exert more retentive force (R) exerted by the indirect retainers
that are placed anterior to the fulcrum line, that is, D1*force =D2*R. Thus,
supposing that the force and the D2 is constant, when D1 increases; D1: the
distance between the fulcrum of rotation and the force applied, R would
increase; that means that the clasp exerts more retentive force on the
abutment tooth (in the picture below, the abutment tooth that the indirect
retainers (clasp) exert it's retentive force is the canine).

Prosthodontic Lecture
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These indirect retainers that are placed anterior to the fulcrum line have
destructive potential, because with each masticatory cycle, loading of the
artificial teeth on downward direction, will cause the anterior clasp to move
upward; if it goes upward that means, the tooth is going to be loaded upward,
so the tooth will be compromised by this unfavorable loading, thus, any clasp
anterior to the fulcrum line is going to be contraindicated in Kennedy's class
I and II , and that's due to:
1- The unfavorable loads that are exerted by the retentive claps, as
mentioned previously
2- Not acceptable aesthetically; since they are placed on the anterior
teeth.
BUT if we consider class II modification 1, in which the fulcrum line runs
from the left second premolar to the right second molar. Here the supportive
element of the direct retainer (rest) on the right fist premolar can act as an
indirect retainer because it is far enough from the axis of rotation.
That is, if we imagine here that we have two fulcrum lines; one that runs from
the from the left second premolar to the right second molar, and such a
rotation occurs a round this fulcrum line is controlled via the direct retainer
(rest) that is located on the right first premolar, that is also act as an indirect
retainer. The other fulcrum line runs from the left second premolar to the right
first premolar, and such a rotation tendency around this fulcrum line is
controlled via the direct retainer that is located on the right second molar, that
is also acts as indirect retainer.

The direct retainers of the

modification space may
also act as an indirect
retainers provided they
are far enough from the
fulcrum line

Summary of Design Considerations for Kennedy's class I and II

Cases
Clasp
For class I case, two clasps on each terminal abutment are needed (the
maximum number of clasps needed in class I is 2). For a class II case three
retentive clasps are required; one clasp is placed on the edentulous side and
the two clasps are placed on the dentulous side.

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The indirect support is contraindicated in class I and II Kennedy's
classification, but it's indicated in cases of Kennedy's class IV cases. The
Indirect Support means: placement of indirect retainers far enough from the
fulcrum line to prevent the tissue-ward movement of the part of the denture
with the artificial teeth.

Class IV Kennedy's classification

The primary supporting elements (rest) are placed on the first premolars. When there is
incising force acting anteriorly, the components of the denture anterior to the fulcrum line will
move tissue-ward, and the other components that lies posterior to the fulcrum line will move
away from the tissues. Such rotational tendency is controlled by placing indirect retainers on
the second molars (stronger teeth) far enough from the fulcrum line, to prevent the tissueward movement of the part of the denture with the artificial teeth; this is referred to as
.""Indirect Support

Rest
The rests should be placed adjacent to the edentulous space.

Major and Minor connector

It should fulfill the ideal requirements.
In Kennedy's class II, I and II lever system are applied, which are less
favorable lever systems.

The Anticipated Movement Concept

Generally, In partial denture designing, we have to apply what is called the
"Anticipated Movement Concept", that is we have to imagine how the denture
will move under a specific kind of loading, and we will see if that movement is
prevented by components, and if the movement controlled without exceeding
the physiological tolerance of the tissues. It is also involves the studying of the
stress breaking that is If we want to make one, we have to imagine how the
movement of the components that are going to transfer the stress to the teeth,
in order to correct the design so that the stress is going to be broken, and as
we stated earlier that such a movement is going to be allowed, so that the soft
tissue are be loaded later on.

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Case Study using Anticipated Movement Concept
This case is class II modification 1 case. In this case , the I-bar clasp have
some specification in the design.

Retention That is, if sticky food dislodging forces acting on the occlusal
surface of the artificial teeth of the bounded saddle area, then the denture
tend to get out of it's place from that area. Thus, the I-bar clasp is going to
provide retention; this clasp will dislodge around the fulcrum line; the fulcrum
line would be the most distant retentive elements; the clasps on the other
side. So, this clasp will move vertically upward, this for
Support That is if a vertical force acting on the bounded saddle, the I-bar
clasp is not affected because the rests on the primary abutments will have the
load.
Stability The horizontal movement of the denture, either Anteroposterior
movement or the lateral movement , are prevented by the proximal plates and
clasps assembly respectively, regardless the undercut the clasp engages,
- If the occlusal force are acting as in the picture above, the denture tends to
rotate, because the loading is buccal to the access of resistance, the clasp
arm is going to be recruited for cross arch stabilization.
All of these function whether it's an I-bar or c-clasp , is going to fulfill this
function. Using such a clasps anteriorly arises an aesthetic problem, and this
may be solved if the lip covers the tooth, then we can use a c-clasp. BUT in
this case, the c-clasp is contraindicated, due to the clasp's movement upward
within each masticatory load acting on the distal extension.
Such a movement will result in either:
1- The abutment tooth will move mesially making more contact with the
adjacent tooth. This mesial displacement of the tooth is because of the
placement of the I-bar to engage the distobuccal undercut when it
move upward.

Prosthodontic Lecture
4\10
2- Compromised tooth. This is because the placement of c-clasp, thus
this c-clasp would goes upward and tooth might be pushed distally,
away from the adjacent tooth, and because that the c-clasp is rigid,
upward downward direction due to the half circle cross section, it is
flexible in outward inward direction, so with each movement the tooth is
doing to be compromised, the transfer of this load to the tooth, we call
it as indirect support, and the indirect support in this case is
contraindicated, so we use an I-bar engaging the distobuccal undercut,
unless it's contraindicated, and these contraindicators are:
a- Shallow sulcus, if it was less than 4 mm; 4mm is what you need
so that you can place an I-bar.
b- Deep soft tissues undercuts.
c- Deep tooth undercuts.
So if the I-bar is contraindicated, in such a case we use route wire clasp; the
route wire clasp is similar to the c-clasps but it's circular in cross section
providing more flexibility.

Done by : Rawan Hamdan