Biomechanics in fixed partial prosthodontics /certified fixed orthodontic courses by Indian dental academy

BIOMECHANICS IN FIXED
PARTIAL
PROSTHODONTICS

INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

INTRODUCTION:
Fixed Prosthodontics is concerned with the
replacement of large amounts of missing tooth
structure. The restorative procedures involved can
have a great effect on the forces transmitted to the
remaining tooth and its supporting structures. This
potential is greater than in many other treatment
modalities because of the magnitude of the
replaced missing structural form. For example, to
evaluate the significance of a simple full crown on
a mandibular molar tooth in a patient with relatively
normal occlusion, a full complement of teeth, and
normal bone support,

we see that the following parameter of form and
forces are within the control and responsibility of
the operator:
Number and area of occlusal contacts
Inclination and length of cusps
Axial contours
Interproximal contacts
These parameters are related to the surface
contours of the completed restoration.

Number and area of occlusal contacts:
The number and area of occlusal contacts
have a profound influence on the distribution of
occlusal forces. For example, the larger the total
area of contact over which a given occlusal force is
applied, the less stress is concentrated at any one
point. As the total number of occlusal contacts
increases in an occlusal scheme, the force is
applied over a greater number of locations, also
reducing the localized stress. In addition, the larger
number of contacts results in more cutting or
grinding surfaces to facilitate mastication.

Inclination and length of cusps:
The angles of inclines of individual cusps present a
dilemma that must be considered. Greater
chewing efficiency generally is attained with
steeper cusps. However, if these cusps are
allowed to come into contact in excursive
movements of the mandible, they become
interferences, which is a deleterious situation.
The operator has limited control over the
length of the cusps of restorations unless the
opposing teeth are also being treated. Where
possible, however, excessive length should be
avoided, because these cusps tend to transmit
greater force to the supporting structures due to
the longer lever arm involved.

Axial contours:
When axial contours are considered, the
concern is with forces of a much lower magnitude
and of a less predictable range. The food bolus
undergoing mastication can apply some small
amount of force to lateral walls as well as a greater
force to the occlusal surface. In addition, low-level,
persistent forces from the tongue or lips acting on
the lateral surfaces of teeth may result in tooth
movement. This kind of action is often
compounded when the contour of a restoration or
a natural tooth tends to trigger or encourage a
habit pattern that accentuates this effect.

Interproximal contacts:
The size and form of interproximal contacts can
have a striking effect on the forces applied to the
interseptal bone and in particular that area referred
to as the gingival col. This latter feature of the
periodontal supporting structures has been found
to be particularly vulnerable to adverse and
prolonged irritation. Any design parameters
incorporated into a restoration that reduce forces
acting in this region are desirable.
The preceding factors in restoration design
are concerned primarily with the outer surface of
the final restoration. Two other important factors
are concerned with the interaction between the
restoration and the tooth.

Tooth-restoration interface:
The tooth- restoration interface is not as
important to the final result from a design standpoint as those factors previously discussed.
However, it has a more definitive, all or nothing
effect. Unless the crown has at least a minimum
degree of required retention, occlusal forces tend
to dislodge the crown, making the discussion of all
other parameters academic.

Tooth-supporting structure interface:
The interface between the tooth and the
supporting structures is a region of special concern
and one where a sound knowledge of the
principles of restorative design must be applied. It
is here that the greatest chance for damage from
lack of foresight and poor judgment by the
operator can play a detrimental role in the future
health of the patient. The opportunities related to
the fabrication of a full crown on a single tooth are
great for both improvement of force distribution
and potential damage. It must then follow that
more extensive restorations can present greatly
increased problems or benefits.

Abutment Evaluation:
Abutment teeth are called upon to withstand
the forces normally directed to the missing teeth, in
addition to those usually applied to the abutments.
Whenever possible, an abutment should be a vital
tooth. However, a tooth that has been
endodontically treated and is asymptomatic, with
radiographic evidence of good seal and complete
obturation of the canal, can be used as an
abutment. The tooth must have some sound,
surviving coronal tooth structure to insure
longevity. However, some compensation can be
made through the use of a dowel core, or a pin
retained amalgam or a composite core.

Teeth that have been pulp capped in the process
of preparing the tooth should not be used as an
FPD abutment unless they are endodontically
treated. The supporting tissues surrounding the
abutment teeth must be healthy and free from
inflammation before any prosthesis can be
contemplated. Normally, abutment teeth should
not exhibit any mobility, since they will be carrying
an extra load. The roots and the supporting tissues
should be evaluated for three factors:
Crown-root ratio.
Root configuration.
Periodontal ligament area.

Crown-root ratio:
This ratio is a measure of the length of
tooth, occlusal to the alveolar crest of bone
compared with the length of root embedded in the
bone. As the level of alveolar bone moves apically,
the lever arm of that portion out of bone increases,
and the chances for harmful lateral forces is
increased. The optimum crown-root ratio for a
tooth to be
utilized as a FPD abutment is 2:3. A ratio of 1:1 is
the minimum ratio that is acceptable for a
prospective abutment under normal
circumstances.

However, there are situations in which a crown root
ratio greater than 1:1 might be considered adequate. If the
occlusion opposing a FPD is composed of artificial teeth,
occlusal forces will be diminished, with less stress on the
abutment teeth. The occlusal forces against prosthesis have
been shown to be considerably less than that against natural
teeth: 26 lb for removable partial dentures and 56 lb for fixed
partial dentures versus 150 lb for natural teeth.

For the same reasons, an abutment tooth with a
less than desirable crown-root ratio is more likely
to successfully support FPD if the opposing
occlusion is composed of mobile, periodontally
involved teeth than if the teeth are periodontally
sound.


Root configuration:
This is an important point in the
assessment of an abutment’s suitability from a
periodontal standpoint. Roots that are broader
labiolingually than they are mesiodistally are
preferable to roots that are round in cross-section.


Multirooted posterior teeth with widely separated
roots will offer better periodontal support than roots
that converge, fuse, or generally present a conical
configuration. The tooth with conical roots can be
used as an abutment for a short span FPD if all
other factors are optimal. A single rooted tooth with
evidence of irregular configuration or with some
curvature in the apical third of the root is preferable
to the tooth that has a nearly perfect taper.


Periodontal ligament area / Ante’s law:
When the normal complement of roots is
not available to distribute the forces of mastication,
pathologic stress concentrations may result in the
periodontal ligament and supporting bone. This
condition is the most fundamental problem with
which the prosthodontist must contend each time a
fixed prosthesis is designed to replace a missing
tooth.


Here we
must take into accounts Ante’s law, which states,
“The total periodontal membrane area of the
abutment teeth should equal or exceed that of the
teeth to be replaced’’. The
essential feature of this clinical guideline is that the
actual area of the interface between tooth and
supporting structures must be of a certain minimal
amount to withstand and
resist the forces that will now be transmitted to
those supporting structures. Realistic
determination of the area of good, healthy,
periodontal ligament available on a potential FPD
abutment is not an easy matter.

Maxillary
 Tooth

Area mm2

Mandibular
Ranking

Area mm2

Ranking

Central

139

7

103

8

Lateral

112

8

124

7

Canine

204

3

159

4

First premolar

149

5

135

6

Second
Premolar

140

6

135

5

First molar

335

1

352

1

Second molar

272

2

282

2

Third molar

197

4

190

3


The combined root surface area of the
second premolar and the second molar
(A2p+A2m) is greater than that of the first
molar being replaced (A1m).

first premolar and the second molar
abutment (A1p+A2m) is approximately
equal to that of the teeth being replaced
(A2p+A1m)

canine and the second molar (Ac+A2m)
is exceeded by that of the teeth being
replaced (A1p+A2p+A1m)

Also of extreme importance is the actual area of
contact between the periodontal structures and the
root in question as it relates to the normal amount
of contact area for that particular tooth in that
particular mouth. One can assume that in a given
case, without bone loss, the area of this contact is
optimal. Therefore, any loss of bone support
compared with the optimal situation decreases the
chances of this root being an adequate fixed
partial denture prosthesis abutment. If one looks at
the problem in this manner, it becomes apparent
that a root that appears adequate in one situation
may be inadequate in another. The planning and
design of a restoration of this type must have the
benefit of sound clinical judgement and knowledge
of basic biomechanical principles.

Table , which compares the root surface areas of
16 teeth, may aid in visualizing root areas. It is
interesting to note that the addition of abutment
roots will provide a wide variation of additional
support. The addition of such support is not
necessarily proportional to the number of
abutments supporting prosthesis. There fore, the
prosthodontist should not expect a proportional
increase in stress-bearing ability, particularly when
the amount of periodontal ligament is reduced. The
area of the normal periodontal ligament (PDL) for
teeth to be replaced by pontics should be less than
the actual PDL area of the existing abutment
candidates.

The values given in the table are averages for the
various teeth in the mouth under ideal conditions.
Two problems are evident when one attempts to
arrive at useful interpretation of such data.
Degree and nature of bone loss
In clinical practice, the decision making
process in which root surface area information is to
be used does not always involve ideal situations.
More often than not, there has been some bone
loss; thus, a tooth with a moderate amount of bone
loss may be still a better candidate for use as a
FPD abutment than another tooth with no bone
loss at all.


To determine as to which of the teeth in question
has adequate support for the anticipated loads
has, there are but few aids on which the
prosthodontist can rely, none of them capable of
giving all the information necessary to make a
decision.
These aids are
Radiographs
Periodontal probing, and
Mobility tests.


The radiograph is unquestionably the most useful
tool at the disposal of the prosthodontist in making
a determination of the integrity of the remaining
periodontal supporting structures as related to the
loads anticipated. The opportunity to make
radiographs from different projection angles should
not be overlooked, since the primary areas that
can be visualized on radiographs are the mesial
and distal surfaces. Any chance for seeing even a
little of the facial or lingual surfaces should be
taken advantage
of.

Periodontal probing is the second tool at the
disposal at the prosthodontist and should be used
extensively when attempting to determine the
efficacy of using a given tooth as an abutment.
Periodontal probing is a particularly important step
as related to the facial and lingual surfaces, since
these areas of the tooth to supporting structures
interface cannot be viewed adequately on the
radiograph.


Finally, standard clinical tests for mobility should
be employed. Any degree of mobility outside the
normal accepted physiologic range should be
suspected. It means
either that the loss of supporting structure, whether
or not fully appreciated from the
radiographs and probing, is severe enough to alter
physically the stability of the tooth or that the
occlusion has traumatically loosened the tooth. It
is very important to understand which of these
processes is at work in a given abutment
situations. Occlusal trauma is usually reversible,
and given the fact that the prosthodontist is going
to construct a restoration on the tooth in question,
there is ample opportunity to correct the situation.

A periodontal defect, however, is not always
reversible and depending on its severity may
require a drastic alteration of the plan of treatment
for the tooth in question. The important concept to
keep in mind is that the prosthodontist must
exercise a certain degree of good judgement in the
question of interpretation of the adequacy of
supporting structures in a given situations. Many
aspects of the final restoration and the chances for
success are basically technical and depend on the
skill of a particular prosthodontist. Not only good
judgment but a conservative approach must be
hall marks of the thought processes of the
prosthodontist in this all important consideration.

Ante’s law says that in a situation where the
preceding values are normal, a prosthesis to
replace the maxillary first molar would need
abutment teeth with at least 335 mm2 of
periodontal membrane. This requirement is
theoretically well satisfied in the case because of
the total average area of the second molar and
second premolar is 412mm2, however, has there
been enough loss of bone on these two teeth to
result in, for example, a total of only 300mm2, the
law would not be satisfied.

Preparations design as related to stress
and force distribution
A large number of stress distribution studies
have been performed on fixed prosthetic
restorations, particularly since the 1960s. These
studies have provided certain conclusions that are
useful in determining the proper design in a given
clinical situations. Many of the factors that often
cause restorations failure may be eliminated by
viewing preparations in the context of force and
stress distribution.

Several parameters of preparations design interact
with occlusal forces to determine restorative
success or failure. The reasons for near parallel
walls, grooves, offsets, counter bevels, pins, and
other design features must be viewed in light of
their functions. Each design features should
contribute to retention of the restoration,
resistance against displacement, protection of
remaining tooth structure, and harmony with the
restorative material.


Retention and resistance
Looking at these design features one at a
time, we must first consider the question of
retention and resistance. Retention is the ability of
the restoration to withstand forces acting in such a
manner as to dislodge the restoration from the
preparation along the line of draw. Resistance, on
the other hand, is the ability of the preparation and
the restoration to resist forces that would tend to
dislodge the latter in direction other than the line of
draw. In other words, the resistance to vectors of
occlusal forces encountered on laterotrusive
occlusal movements during either mastication or
grinding of the teeth is the important consideration
here.


Taper:

The relationship of one wall of a
preparation to the long axis of that preparation is
the inclination of that wall. The axial walls of the
preparation must taper slightly to permit the
restoration to seat. Theoretically, the more nearly
parallel the opposing walls of a preparation, the
greater should be the retention. Craig RG, et.al.
(1969) suggested a taper of 2.5-6.5 degrees as
optimum to minimize stress in the cement interface
between the preparation and restoration, but there
is only a slight increase in stress as taper is
increased from 0-15 degrees.

However, at 20 degrees, stress concentration was
found to sharply increase. Cement creates a weak
bond, largely by mechanical interlocks., between
the inner surface of the restoration and the axial
wall of the preparation. Therefore, the greater the
surface area of a preparation, the greater its
retention.


Freedom of displacement:
Retention is improved by geometrically
limiting the numbers of paths along which a
restoration can be removed from the tooth
preparation. Maximum retention is achieved when
there is only one path.


The walls of a groove that meet the
axial wall at an oblique angle do not
provide necessary resistance (A). The
walls of a groove must be
perpendicular to rotating forces to
resist displacement (B).

If the buccal and lingual walls of a proximal
box forms oblique angle with its pulpal floor,
there will not be adequate resistance to
rotating forces. The buccal and lingual walls
must meet the pulpal wall at angles near 90
degrees so that these walls will be
perpendicular to any forces which tend to
rotate the restoration.

Length:

Occlusogingival length is an important
factor in both retention and resistance. Longer
preparations will have more surface area and will
therefore be more retentive. Because the axial wall
occlusal to the finish line interferes with
displacement, the length and inclination of that wall
become factors in resistance to tipping forces. For
the restoration to succeed, the length must be
great enough to interfere with the arc of the casting
pivoting about a point on the margin on the
opposite side of the restoration.



The preparation with longer walls interferes with
the tipping displacement of the restoration better
than the short preparation.




A preparation on a tooth with a smaller diameter
resists pivoting movements better than a
preparation of equal length on a tooth of larger
diameter because smaller teeth will have a short
rotational radius for the arc of displacement and
the incisal portion of the axial wall will resist
displacement.



Resistance to displacement for a short walled
preparation on a large tooth can be improved by placing
grooves in the axial walls. In effect, this reduces the
rotational radius, and that portion of the walls of the
grooves near the occlusal surface of the preparation that
will interfere with displacement.

Substitution of internal features:
The basic unit of retention for a cemented
restoration is two opposing axial walls with a
minimal taper. It may not always be possible to
use opposing walls for retention: one may have
been destroyed previously, or it may be desirable
to leave a surface uncovered for a partial veneer
restoration. It may also be that the walls are
present, but with a greater than desirable
inclination. At such times, internal features such as
the groove, the box form and the pin hole can be
substituted for an axial wall or for each other.

The forces to be applied by the opposing
occlusion in the situations depicted in (a)
would dictate a preparation with relatively
strict adherence to the basic principles of
length and taper.


The lesser angle of the application of force in the
situations in (b) would permit the use of a
preparation design with less retentive features, as
seen in (c). This conclusion is based on the
premise that once a crown is cemented on a tooth
preparation, the retention will be adequate until
such times as a force is applied with proper
magnitude and direction to overcome the
resistance to that force. The resistance to force
referred to here is that which is afforded by the
preparation design, the fit of the casting on the
preparation, and the cementing medium. If this
combination of factors proves inadequate to resist
the force, retention will be lost. Conversely, if
these factors are adequate for a given application
of forces, retention will endure.


Path of insertion:
The path of insertion must be an imaginary
line along which the restoration will be placed onto
or removed from the restoration. It is of special
importance when preparing teeth to be fixed partial
denture abutments, since the paths of all the
abutment preparations must be parallel to each
other. The path of insertion must be considered in
two dimensions: faciolingually and mesiodistally.
The faciolingual orientation of the path can affect
the esthetics of metal-ceramic or partial veneer
crowns. The mesiodistal inclination of the path
must be parallel to the contact areas of the
adjacent teeth

Few forces applied to teeth can dislodge a
restoration in an occlusal direction. Sticky foods,
chewing gum, and the like have been known to
remove restorations in the line of draw. However,
these situations are relatively rare and usually
occur after prior fracturing of the cement film which
results in a loss of retention. Most restorations
that are properly designed and that fit well
generally are not subject to this type of occlusal
failure.


The primary function of a crown placed on a tooth
is to restore lost contour and function, but we must
also consider the necessity for the crown to
distribute forces and loads applied to it in a
manner that will assure its ability to satisfy the
primary purpose over a reasonably long period of
time. If a crown were placed on a tooth and
expected only to serve this function without being
subjected to any external forces, all crowns would
look much more similar than they in fact do.
However, we known this not to be the case, and
therefore find it necessary to logically and
predictable adjust the previously mentioned
mechanism of retention to meet the needs of a
great variety of stresses and strains.

The magnitude and quality of the retentive effect of
a restoration should resist the forces that will be
applied to it in specific clinical situations.
Therefore, a preparation on the same tooth in two
different patients could look quite different, yet
both could be entirely satisfactory. Furthermore,
the degree of retention afforded by fit of one crown
might be twice as great as another or stronger
cement may be used on one case than on another,
and yet both could perfectly adequate for the
respective situations.


Protection of tooth structure
In the area of protection of the remaining
tooth structure, the prosthodontist is faced with
even more variables than those of retention and
resistance. It is also a fact that serious errors in
judgement here would more likely result in loss of
the tooth at some future time. Although the most
probable failure that would occur if retention is
inadequate would be loss of the crown from the
tooth, with no real damage to the remaining tooth,
the most likely failure in the case of unprotected
tooth structure, which obviously is a much more
serious type of failure.

Important factors to be considered related to
protection of the existing tooth structure are:
Amount of enamel supported by adequate viable
dentin
Amount of adequate viable tooth structure after the
preparation for the restoration is made.
Accordingly, the first step in any crown
preparation should be to remove all old restorative
material and new carious lesions. At this point in
the procedure, nothing should be left but sound
dentin and enamel supported by dentin.


The importance of the principle of adequate
removal of carious lesion cannot be over
emphasized. An apparent minimally damaged
tooth presents a totally different situation after the
clean out procedure. The remaining tooth
structure must be the primary concern in the
design decision making process, not the false
appearance of the tooth previous to removal of the
unsupported enamel of the occlusal surface.

As is often seen in these situations where the
carious lesion was initiated in the occlusal grooves
of a posterior tooth, the structure remaining for use
in developing a preparations is quite different after
all of the unsupported overlying enamel is removed
and the carious lesion is excavated.

The second step must be to establish the correct
occlusal clearance for the restoration to be
constructed on the tooth. By definition, any crown
requires occlusal clearance since coverage of this
surface is a part of any such restoration except
inlays. Therefore, it follows that before any
decisions can be made regarding other features of
the preparation, the occlusal clearance must be
created to properly visualize the parameters with
which these decisions must be made.

The next step is to determine whether this
remaining dentin and the enamel supported by it
will still be available after the axial surfaces are
prepared for the type of crown being done on the
tooth. This step seems to be one of the more
frequently over looked aspects of preparation
planning and design. Continuing consideration of
the molar seen, one may assume that it requires a
full crown.


One can see that after completion of
the
aforementioned
cleanout
procedure (left) and occlusal clearance
(right) the remaining dentin one the
axial walls is minimal to say the least.
Only enamel that is supported by
adequate sound dentin will provide
strength in the resulting preparation. It
is the dentin, due to its elasticity,
greater
toughness,
and
lesser
brittleness, that provides the sound
foundation
needed
for
cast
restorations, www.indiandentalacademy.com
not the enamel.

After reduction of these two axial surfaces there is
not adequate dentin remaining to allow
development of retention and resistance form and
still have sound tooth structure that would not be
prone to failure, even under the most minimal
loading. Therefore auxiliary intra coronal retention
such as pin retained build ups or retentive bases is
required.


The essential point to be considered is that the
operator must visualize the remaining sound
dentin in the context of the final restoration. To
carry this principle further, consider the result if the
tooth required even greater reduction of the lingual
surface to attain a particular line of draw not
commensurate with savings as much tooth
structure as possible.


One situation in which we often find the remaining
tooth structure predisposed to failure is the case of
endodontically treated teeth. Let us first consider
posterior teeth, which is generally where greater
forces are applied to the occlusal surfaces as well
as where more controversy seems to exist at the
point of preparation and restoration design. As a
basic principle, it would seem reasonable to
suggest that any posterior tooth that has had
endodontic treatment and has had both
interproximal surfaces involved with carious
lesions or previous restorations should ultimately
be restored by casting that provides full occlusal
coverage.


Regarding the aforementioned principle that
protection of remaining tooth structure is
predicated on the presence of sound dentin, it is a
fact that in those teeth that have undergone
endodontic treatment, there has often been
removal of a significant amount of this all important
dentin in the area between the pulpal floor of the
previous restorations (or carious lesion) and the
superior aspect of the pulp chamber.


Depending on the ability of the operator performing
the endodontic access opening as well as the
presence of anomalies of the position of the
chamber, the access opening may significantly
reduce the strength of the remaining cusps and
their ability to withstand lateral forces. It is clear
that a fracture in such a situation is considerably
more likely than if the area above the pulp
chamber were still sound dentin.


Integrity of the restoration
The fixed restoration must be able to
withstand forces generated by centric contact,
eccentric movements, mastication against hard
and soft foods, and mild accidental trauma. The
chosen restorative material should not deform
permanently or fail under these conditions.
In terms of centric contact, it is generally an
easier matter to design and execute the restoration
in a way that will adequately resist excessive
deformation and therefore any possibility of
ultimate failure of the material.

Only an occasional error in casting or preparation
reduction on the occlusal surface will result in a
casting that will later fail due to inadequate amount
of material on the occlusal surface. However,
often insufficient attention is paid to the problem of
potential wear when one decides how much
occlusal clearance is needed in a given situation.
Normally, it is accepted that about 1 to 1.5mm is
adequate for most situations. However, in cases
where some wear in anticipated this amount may
not be enough clearance and 2 or 2.5 mm might
be needed.

Consideration also must be given to the
restorations in eccentric movements, because an
increasing number of cast metal restorations have
become subject to wear due to chronic bruxism. In
spite of attempts to control this problem so that
wear does not occur on the occlusal surfaces of
the teeth, these attempts are not always
successful. The operator should take the problem
of chronic bruxism into account when designing
restorations.
Besides wear, there are other
considerations. The force, and therefore, the
resulting strain, induced in the casting in such a
case can be of considerable magnitude.

As a result of wear the occlusal contacts often take
on the form of very flat surfaces, no longer
possessing the ideal cuspal form that will provide
for efficient cutting interaction with the food bolus.
The flatter occlusal contacts can cause a
decreased effectiveness of the masticating
surfaces and a concomitant increase in force
needed to properly masticate a bolus of food. It is
also possible to have enough wear to result in a
perforation through the casting into the cement film
and then into the underlying dentin.

Another consideration is the possibility of
deformation of the casting due to these forces.
The restoration could, and often does, lose
retention because the deformation of the casting
leads to either adhesive or cohesive failure of the
cement layer. Exceeding the yield strength of the
casting can result in an open margin and recurrent
caries. If the restoration in question is porcelain
fused to metal crown or bridge, excessive
deformation will result in failure of the porcelain
bond or in a fracture of the porcelain at some
point.
Studies using stress analysis have
demonstrated the need for proper framework
design, particularly related to the problem of
resisting occlusal forces.

Flexure of long span bridges during mastication
can be a problem. A very long span may present
no real problem in terms of flexure during centric
closure with no food in the mouth, because there is
distribution of forces among all the occlusal
surfaces around the arch. However, when there is
a bolus of food interposed between the pontic area
of an excessively long span bridge and it’s
opposing occlusal surfaces, the effect can be quite
different since the occlusal contacts on the
opposite side of the arch have not yet come into
play. In this case, all of the force is concentrated
on the bridge span, inducing a strain that the
restoration and / or the abutment teeth may not be
capable of withstanding.


In these situations, the case should be designed to
provide adequate beam strength in casting. For
example: it would be questionable to construct a
bridge from the mandibular first premolar to the
mandibular third molar when the available inter
occlusal distance is only 2 mm. Such a casting is
of such a long span that very high flexural stresses
would be generated even by the low loads
generated during mastication of a bolus of soft
food. Then, we must consider the possibility of a
rather hard object suddenly finding it ways into the
interocclusal space, such as a piece of bone.

The problem in these situations is that the force
with which the masticatory muscles are functioning
at that moment is the force that was needed and
that felt comfortable to the individual for the
purpose of chewing the soft food. When suddenly
and unexpectedly a small hard object is interposed
between the occlusal surfaces, tremendous forces
is concentrated at that point. By the time of
proprioceptive or pain reflexes of the individual can
take over and stop the action of the neuromuscular
complex, the damage is often done. The result
may be loss of cement bond, porcelain fracture,
failure of a build up under a retainer, failure of a
connector, and so forth.

When designing dental restorations of any type,
we are generally concerned only with resistance to
those forces generated within the stomatognathic
system. It is not possible to predict damage to our
restorations from such causes as automobile
accidents, or blows, but there are a limited number
of situations where the planning for such
eventualities is at least to some extent a
possibility. In these instances, the design of
restorations can take into account certain type of
potential damage and therefore obviate the need
for a new restoration, possibly a more extensive
one.

One such situation would be that of the epileptic
patient who is subject to seizures. These patients
often have missing maxillary or mandibular
anterior teeth, lost in just such a seizure in which
the patient fell. In these cases, it is often wise to
modify the usual treatment to avoid use of
porcelain fused to metal restoration. Such a
restoration is prone to fracture when subject to
impact, and if it does not fracture, there is the even
more serious risk of fracture of the teeth. In such
situations, it is usually wise to consider the use of
a more flexible gold alloy with plastic facing. The
restoration can be designed and constructed in
such a manner that the facing could easily be
replaced in the mouth.

This same type of contingency planning can
logically be applied to those who are regularly
engaged in sports, where it can be predicted that
they are likely to repeatedly encounter this type of
trauma.


Pontic Selection
The pontic is the part of the restoration that
replaces the missing tooth. From a biomechanical
view point, pontic present some unique problems
that must be considered in the design and
construction of these restorations. First, there is
less bone support for a given number of
functioning occlusal surfaces. It is therefore
advisable to increase the efficiency of the occlusal
surface as a masticatory device.


This task usually involves creating an occlusal
form that has the following features:
Maximum angle of cusp inclines,
Narrow cusp ridges,
Sharp cusp tips,
A greater number of small occlusal contacts.


As can be seen in the cusp form on the left would
tend to be more efficient. The same magnitude
applied to the occlusal surfaces of the teeth by the
elevating musculature can cut through the bolus of
food easier if the cusps are narrower and sharper
because the available force is more concentrated.


The degree to which modifications to the present
occlusal scheme of the patient can be
accomplished depends on the controlling factors in
the occlusion, such as the amount and angle of
canine disocclusion, the angle of the eminentia,
and the amount of enamel available for
adjustment.
Long spans and periodontally
involved cases are in particular need of efficient
occlusal design but only to the point that no lateral
or protrusive interferences are introduced.


Any potential gain in efficiency that was attained
while causing traumatic occlusion would not be
justified. An attempt should not be made to reduce
the magnitude of the load on a posterior prosthesis
pontic by lightening the occlusal contacts. When
the occlusal contacts are lightened, the opposing
tooth usually supererupts into the same tight
occlusion in centric as exists on the abutment
teeth.


Another factor in pontic design is the width of the
occlusal table. The faciolingual width of the
occlusal table is the portion of the occlusal surface
delineated by the occlusal contacts. This width
should be made as narrow as possible to create a
greater concentration of force where the work is
being done. There is limitation to attaining this
goal since the width of the occlusal table is
dictated by the anatomy of the opposing dentition.


The occlusal table of the pontic may be narrowed if
a cast restoration is indicated on the opposing
tooth and if it is possible to narrow the distance
between the contacts by equilibration.
Any
adjustment in the design of the pontic that would
distribute the force at the site of the work over a
smaller total area of the occlusal surface of the
pontic would create a better concentration of force.


Over all, faciolingual width is another important
feature of pontic design. This width refers to the
greatest faciolingual width of the pontic, which is
usually gingival to the occlusal table. It should be
a basic goal of all posterior pontic design to keep
the overall faciolingual width as narrow as possible
to promote oral hygiene in the area of the pontic.


The smaller the total surface of the pontic that
faces the residual ridge, the better will be the
ability of the patient to clean this surface of
bacterial plaque. By making the faciolingual width
of the pontic as narrow as possible, the operator
may also make a greater percentage of the facial
and lingual surfaces more vertical and therefore
more easily reached by the tooth brush or other
aids.
Finally, the forces applied by the pontic on
the residual ridge must be considered. There have
been various schools of thought concerning this
aspect of pontic design.

These opinions range from placing the pontic in a
very definite positive contact with the ridge tissue
to leaving a 2 to 3 mm space between the pontic
and the ridge. Virtually all possibilities between
these extremes have been advocated. It is easy to
understand that the pontic that does not touch the
tissue cannot directly apply any force to the ridge.
This design is most often accomplished to promote
the best possible hygiene. At the other extreme,
the pontic that is placed in heavy contact with the
ridge tissues is going to apply a direct force on the
tissue and then to the underlying bone and its
periosteum.

This type of pontic is no longer advocated to any
great degree because of its poor oral hygiene
potential.
Probably the greatest controversy
regarding this problem in recent years has been
whether to place the pontic in very light contact or
just out of contact with the ridge tissue. Although
the great majority of bridge pontics placed in light
contact with the tissue show no changes in either
tissue or bone, occasionally one can see the
proliferation of bone. It is advisable to create
pontics that do not actually contact the tissue from
the stand point of both oral hygiene and
transmission of detrimental forces to the residual
ridge.

Connectors
All fixed bridges must be united by some
type of connector that must satisfy certain
structural requirements. It must provide enough
strength to resist forces of occlusion that cause
flexure of the joint, producing stress in the solder,
the interface, and the parent casting. Functional
forces applied to the pontic result in a more severe
stress condition than when the patient closes into
centric occlusion without a bolus of food. In centric
closure the force is uniformly shared by the pontic
and the retainers.

Studies have demonstrated that the greatest
chewing action takes place with the bolus over the
second premolar and first molar, with the food
being masticated primarily over the premolar at
first, and then gradually move farther posteriorly as
the degradation of the bolus progresses. Since the
restorations in question are usually replacing one
or more posterior teeth in the area of greatest
chewing function, often a force will be applied to
the pontics alone by the bolus. The magnitude as
well as the concentration of force can be quite
great and may exceed the ability of the abutments
to adequately resist it, resulting in failure. This
failure often takes place at one of the connectors,
which is the thinnest and therefore, the weakest
link in the restorations.

In anterior bridges it is of paramount importance to
design the connectors so that they are esthetic,
that is, so they appear as close as possible to a
natural embrasure between two separate teeth. To
provide adequate thickness of porcelain in the
area of facial embrasure, one must provide the
clearance at the expense of what might be
considered a more ideal amount of metal in the
area. It is also necessary to provide natural lingual
embrasure form for phonetics, patient comfort and
hygiene potential. All of these requirements tend
to limit the faciolingual thickness available.

The other variable in the equation is incisogingival
dimension of the connector. Again, we are limited
by
functional,
esthetic,
and
hygiene
considerations. It is necessary from an esthetic
stand point to create both an incisal and a gingival
embrasure that will match the contra lateral ones.
The classic exception is the embrasure between
the two central incisors. Since there is no contra
lateral embrasure, there is a little more freedom.


The height of the gingival tissue in the area
between the teeth will limit the incisogingival height
of the connector. This excess tissue can often be
adjusted using conventional or electrosurgical
methods. Connector design in anterior bridges is
dependent on esthetic considerations as well as
structural ones.
Other means should be
considered before relying on increased size to
solve the problem of strength.


The proper manipulation of the alloys being used
in the connector is very important. If the connector
is cast, correct spruing is vital to assure that the
alloys is cast and cooled correctly for a porosity
free joint. The presence of porosity is probably the
most common cause of failure in the cast joint.
When soldered joints are used, good principles of
soldering must be scrupulously adhered to;
cleanliness, access, and heat control. Fractures
tend to occur in the parent metal, not the solder to
parent metal interface or the solder itself, probably
because of changes in the parent metal during the
soldering procedures.
Another factor, which
contributes to the ability of the prosthesis
connectors to maintain rigidity of the restorations,
is the design of the joint contours.

Photo elastic studies have shown that V-shaped
embrasures produce high stress concentrations in
the connector area, whereas lower concentrations
of stress result with U-shaped embrasures. The
need for rounded connector design often conflicts
with the esthetic requirements in anterior teeth,
where sharp, deep embrasures are preferred
because they mimic the natural embrasure.


In the case of posterior prosthesis, the situation is
a little less troublesome. There are minimal or no
esthetic considerations, depending on location.
Also, there is usually a greater area available for
bridge connectors by virtue of the fact that the
proximal surfaces of the teeth are larger than the
anterior teeth. Since the strength of the connector
is related directly to the cross-sectional
configurations, one can readily see that it is easier
to attain strength in the posterior part of the mouth.


In most cases, greater strength is required
probably, due to the fact that greater biting forces
are applied there. It has been shown that the
proprioceptive reflex arc is more active and
sensitive in the anterior teeth than in the posterior
teeth. What this differences means is that when
an individuals bites on an unexpectedly hard
object in the food with the anterior teeth, the reflex
arc tends to cause the muscles to react and to
open the jaws quickly before a great deal of force
has been applied.
This reaction is often
accompanied by a fair amount of discomfort in the
periodontal tissues of the involved teeth.

In the posterior teeth, however, it is not uncommon
to have a patient bite with enough force on the
same unexpected hard object in the food to
fracture a cusp. As the posterior teeth acted on
the object, the proprioceptive reflex arc was not so
sensitive, and the muscles continued to apply the
force for a longer period. This force, then, is
transmitted from the point of contact to the
connectors, the weakest portion of the fixed partial
prosthesis and most prone to fracture. In addition,
the biting forces are simply of greater magnitude
on the posterior teeth than on the anterior teeth.

Comparison between inlay and onlay:
Some discussion is in order regarding the
use of inlays to restore posterior teeth. Both two
and three dimensional photoelastic investigations
have shown that stress concentrations occur in
critical areas of the tooth when a mesiocclusodistal
inlay is loaded by occlusal forces or when the
cusps of the tooth are loaded in a tooth so
restored.
According to Fisher DW et.al; high
concentrations of stress were found at the
faciopulpal and linguopulpal line angles when the
inlay was loaded in centric occlusion as well as
when the cusps were loaded in a three point
occlusal contact scheme.

Higher concentrations of stress were seen on the
walls of the isthmus in the models prepared for
inlays than in those for onlays. It appears from the
results of these studies that it is particularly
dangerous from a stress standpoint to restore
these teeth with inlays under which a cement base
has been placed. The practice of placing a cement
base on the pulpal floor is often done in the
interest of the creating a more ideal preparation
form. This practice should be avoided because of
stress concentrations that result under load.

It is considerably more defensible in terms of
protection of the remaining tooth structure to
design the preparation to have the restoration
seated on as much solid tooth structure that is
perpendicular rather than nearly parallel to the line
of draw. The problem with inlays is that this
requirement is not satisfied in a great many
situations. First, too much of the preparation
involves the walls of the isthmus and the walls of
the boxes, and too little of the preparation involves
the pulpal floor and the floors of the boxes. When
this deficiency is aggravated by the placement of a
cement base on the pulpal floor, thus rendering it
an ineffective vehicle for resisting forces, the inlay
becomes a wedge.www.indiandentalacademy.com

The onlay, on the other hand, counteracts these
shortcomings of the inlay because its increased
occlusal coverage more effectively distributes
forces to the tooth substructure. Consequently, in
teeth where there is a need for an intra coronal
restoration of some type, the material and
technique of choice will more often be an amalgam
for two main reasons. First, in general, less sound
tooth structure will need to be removed to
accomplish a proper preparation. Less removal of
tooth structure will result in a better opportunity for
the remaining cusps of the tooth to resist forces
applied during contacts that might occur in lateral
excursions.

Second, in centric occlusion, the softer (lower
modulus) amalgam will tend to deform more and
therefore cause less stress concentrations at the
walls and line angles of the preparations than in
the case of the inlay that is made from a much
higher modulus alloy. Clinical experience has
shown that the amalgam materials tend to wear,
flow, or even fracture under these loads, whereas
the inlays tend to cause high stress concentrations
to develop with in the tooth structure.


However, if proper application of the principles
were followed, there will be some indications for
the use of very conservative inlays in vital posterior
teeth, though never in non vital posterior teeth.
These indications would normally involve two
surface defects that would then leave one of the
interproximal surfaces and the corresponding
marginal ridge intact to afford a greater degree of
structure integrity.


According to the basic principle, the best way to
provide strength for the tooth is to leave enamel
supported by sound dentin wherever possible.
When a cast inlay replaces both proximal surfaces,
this principle is not satisfied. However, when only
a disto-occlusal or mesio-occlusal inlay is done,
the remaining natural tooth structure on the other
interproximal surface is usually enough to resist
the wedging action referred to earlier, because the
facial and lingual cusps remain united by the
marginal ridge.


There is, however, no justification for using an
inlay of any design in a posterior tooth that has
had endodontic therapy. In these cases, even a
two surface inlay can transmit enough undesirable
force to the remaining tooth structure, already
compromised, to cause a fracture.


Multiple unit restorations
Fixed partial dentures present certain
problems that are unique by virtue of the fact that
more than one unit is involved. It is possible in
these cases to transmit forces to the abutments
even when the forces are not directed over these
teeth. This indirect force application to the
abutments can result in forces on the preparation
and the supporting structures that are related in
some complex manner to the direction of the
original force.

A relatively common situation in which occlusal
forces produce very complex structural responses
is the situation involving an abutment in the middle
of the multiple unit fixed restoration. A typical
example would be a restoration replacing the first
premolar and the first molar. This restoration
configuration results in three separate abutments
in locations where reactive forces are not always
conductive to maintaining retention.
Such a
reactive force is particularly critical on an abutment
at one end of the bridge that may have
questionable retention to begin with. Here the
problem arises from the fact that a certain degree
of movement always occurs in the periodontal
membrane of teeth in function.

Parfitt GJ (1960), have shown that the faciolingual
movement ranges between 56-108 µm, and
intrusion of 28 µm. Teeth in different segments of
the arch move in different directions. Because of
the curvature of the arch, the faciolingual
movement of an anterior tooth occurs at a
considerable angle to the faciolingual movement of
molar.
These movements of measurable magnitude and
in divergent directions can create stresses in a
long span prosthesis that will be transferred to the
abutments.

Because of the distance through which it occurs,
the independent direction and magnitude of
movements of the abutment teeth, and the
tendency of the prosthesis to flex, stress can be
concentrated around the abutment teeth as well as
between retainers and abutment preparations.


When a force is applied to the occlusal surface of
one of the retainer of a three unit bridge, that
abutment tooth tends to be displaced from its
original position to a degree related to the
magnitude of the force and the resistance of the
supporting structures. The direction of movement
is determined by the direction of the applied forced
dictated by the occlusal anatomy of the teeth
involved.


In these situations, the abutment tooth at the
opposite end of the restoration will tend to rotate
slightly in its socket. Again, the movement is
permitted by the flexible nature (low modulus) of
the periodontal membrane. By this mechanism,
retention
is
maintained.
However, when there is an abutment tooth in
the middle of the restoration, a different set of
problems occur. When an occlusal load is applied
to the retainer on the abutment tooth at one end of
such a restorations, the abutment tooth in the
center can act as a fulcrum. Tensile forces would
then be generated between the retainer and
abutment at the other end of the bridge.

This abutment would be required to under go an
extrusive movement to react to the force. If the
periodontal support for this abutment is sound,
such a movement is well resisted by the root. The
result of the tensile force would be at the retainer
to abutment interface, namely, the cement layer.
The end result is frequently a loss of retention on
one of the terminal abutments; the abutment with
the least retention fails.


Therefore, the goal in planning and constructing
restorations where there is more than a single
missing tooth to be replaced should be to create a
series of one tooth replacement restorations. The
principle here is that the span involving the first
premolar replacement is a simple three unit bridge
cemented to the canine and second premolar,
resulting in the advantages described for a one
tooth replacement.


Then, the span involving the first molar
replacement is another simple three-unit bridge
replacing one missing tooth. In this case, the
anterior unit is actually a nonrigid connector that
will modify the transmission of forces applied to the
anterior section that might be detrimental to the
abutment-retainer complex of the molar or vice
versa.
By incorporating various available
attachments, the operator may use this basic
principle in the planning and construction of
restorations involving nearly any combination of
missing teeth and abutments.

The non-rigid connector is a broken-stress
mechanical union of retainer and pontic, instead of
the usual rigid connector. The most commonly
used non-rigid design consists of a T-shaped key
that is attached to the pontic, and a dove-tail
keyway
placed
within
the
retainer.
Use of non-rigid connector is restricted to a short
span FPD replacing one tooth. Prosthesis with
non-rigid connectors should not be used if
prospective abutment teeth exhibit significant
mobility. There must be equal distribution of
occlusal forces on all parts of FPD.

The location of the stress breaking device in the
five unit pier abutment restoration is important. It
usually is placed on the middle abutment, since
placement of it on either of the terminal abutments
could result in the pontic acting as a lever arm.
The keyway of the connector should be placed
within the normal distal contours of the pier
abutment, and the key should be placed on the
mesial side of the distal pontic. The long axes of
the posterior teeth usually lean slightly in the
mesial direction, and vertically applied occlusal
forces produce further movement in this direction.

If the keyway of the connector is placed on the
distal side of the pier abutment, mesial movement
seats the key into the keyway more solidly.
Placement of the keyway on the mesial side,
however, causes the key to be unseated during its
mesial movements.


Cementation is another factor related to the
problem of multiple abutment restorations in which
there is an intermediary tooth. For example, a
restoration with minimal taper and a large flat
occlusal surface on the preparation is more difficult
to seat than one with excessive taper and a very
small occlusal surface. Because of the flexibility of
the PDL, which allow for some small movement to
take place in the abutments at each end of the
simple three unit bridge, it is easy to see how it is
possible to completely seat this type of
restorations.
However, the hydraulic back
pressure of the cement makes it more difficult to
seat a solid bridge with more than two abutments.

Valves reported in the literature for acceptable
marginal opening range from 25 to more than
200μm.It is possible that while two of the retainers
are seated with a marginal discrepancy of 25μm,
the middle retainer is seated less completely, say
to an opening 300 µm. This could be due to the
effect of the hydraulic pressure exerted by the
cement acting on an abutment which can be
displaced in an apical direction more than the
other abutments because of its lessened bone
support. Various techniques have become
accepted for improving seating of castings. A few
examples of these methods are die spacers,
venting, stripping of the casting internal surface,
and variations in the liquid /powder ratio of the
investment.


In this situation, it would seem impossible to
completely seat the third retainer, which is often
the case. This problem is magnified many times in
the design, construction, and cementation of
periodontal splints because the supporting
structures have been degraded beyond normal
limits. The design of this type of restorations must
take into account the difficulty to be encountered
during cementation. Here, the use of venting or
any other means of reducing hydraulic back
pressure of the cement must be considered.

There is another consideration with respect to
multiple unit restorations with an abutment in the
center of the span. The restoration should be
designed so that no part of the occlusal surface of
the middle retainer leaves tooth structure
uncovered and in occlusal function. As can be
seen occlusal forces acting directly on the tooth
structures of the middle abutment could easily
displace this tooth apically out of its retainer while
the bridge is held in position by the other two
abutments. This situation often results in loss of
retention of the middle retainer on its abutments.

The restorations will usually remain in position,
held there by the other retainers, while the middle
tooth is allowed to slowly but surely succumb to
caries.
This problem is often seen where
prosthesis with pier abutments have been
constructed in one solid piece rather than in
sections.


In addition to the increased load placed on the
periodontal ligament by a long span fixed partial
denture, longer spans are less rigid. Bending or
deflection varies directly with the cube of the
length and conversely with the cube of the
occlusogingival thickness of the pontic. Compared
with a fixed partial denture having a single tooth
pontic span, a two tooth pontic span will bend 8times as much. A three tooth pontic span bends
27-times as much as a single pontic.


A pontic with a given occlusogingival dimension
will bend 8-times as much if the pontic thickness is
halved. Longer pontic spans also have the
potential for producing more torquing forces on the
fixed partial denture, especially on the weaker
abutment. To minimize flexing caused by long and/
or thin spans, pontic designs with a greater
occlusogingival dimension should be selected.
The prosthesis may also be fabricated of an alloy
with higher yield strength, such as nickel
chromium.


Tilted molar abutment
Titled abutment teeth are a common
problem that must be addressed in construction of
fixed partial prostheses. The tooth to be replaced
by the restoration frequently has been missing for
a long time. Therefore, the tooth distal to the
missing one often will have tilted into the space. It
is impossible to prepare the abutment teeth for a
fixed partial denture along the long axis of the
respective teeth and achieve a common path of
insertion. There is further complication if the third
molar is present. It will usually have drifted and
tilted with the second molar.

Because the path of insertion of the fixed partial
denture will be dictated by the smaller premolar
abutment, it is probable that the path of insertion
will be nearly parallel to the former long axis of the
molar abutment before it tilted mesially. As a
result, the mesial surface of the tipped third molar
will encroach upon the path of insertion of the fixed
partial denture, thereby preventing it from seating
completely.


Some possible solutions to these problems are:
Preparation modifications: The design of the
preparation could be modified to be in harmony
with the line of draw requirements of the other
abutment and adjacent teeth while at the same
time satisfying all other preparation criteria, such
as retention and protection of the pulp. A proximal
half crown can be used as a retainer on the distal
abutment.


This retainer can be used only if the distal
surface itself is untouched by caries or
decalcification and if there is very low
incidence of proximal caries throughout the
mouth. If there is a severe marginal ridge
height discrepancy between the distal of the
second molar and the mesial of the third
molar as a result of tipping, the proximal half
crown is contraindicated.


Telescopic crown designs: A two piece
restorations is constructed whereby the line
of the draw of one component (seated on the
tipped tooth preparations) is such that it
favors the tooth. The line of the draw of the
component is then in harmony with the other
abutment preparation.


Broken connectors: In these situations it is
desirable to connect units of fixed bridges in
some manner that will allow the various
components of the prosthesis to be seated
separately.


Pre-prosthetic orthodontics.
Some degree of
uprighting the tooth may be accomplished
orthodontically.


Composite
resin
bonded
prosthesis:
The most recent innovation in multiple unit
restorations is the composite resin bonded
prosthesis. Utilization and popularization of this
technique is based on the ability to etch certain
high modulus, non precious alloys. After etching,
the metal can be placed after only a minimum of
tooth reduction. To accomplish the goals of this
conservative restoration, one must make the metal
frame work thin and in-conspicuous which has led
to FPD’s with minimal structural integrity.

The essential features of this type of restoration
have included:
Minimal axial reduction lingually at the height of
contour.
1 mm deep occlusal rests inclined toward the
center of the abutment teeth.
180-degree proximal wraparounds approximately
0.4mm thick.
A distinct path of insertion.
For anterior abutments, bonded cingulum rests
have been advocated.

When these composite resin bonded prosthesis
are subjected to occlusal loadings, very high
complex stresses are generated at the connector
areas and extend into the high flexure of the
wraparound arms. These high flexural stresses
are transmitted to the resin adhesive. During
function, the bridge is subjected to a large number
of chewing cycles, which may be translated into
fatigue failure of the adhesive layer
When the thickness is increased, a
substantial decrease in the level of stress
concentration results. Another means to
substantially reduce the level of stresses within the
frame work is to include occlusogingival
extensions adjacent to the extraction site.

The occlusal rests are also important structural
elements in the transmitting of forces from the
pontic to the abutment teeth. A similar structural
support may be obtained by preparing a ledge on
which the occlusogingival extension rests. This
support is, in essence, a very minor box
preparation. There are pros and cons to both
approaches, but one of these two rest concepts
should be used.


Structural considerations for the success of this
technique should include:
Wraparound arms as thick as possible consistent
with reasonable tooth contour.
Occlusogingival proximal extensions and
A sound rest, whether it is on the occlusal surface
or in the form of a gingival box.


Canine replacement fixed partial dentures
Fixed partial dentures replacing canines
can be difficult because the canine often lies
outside the interabutment axis. The prospective
abutments are the lateral incisors, usually the
weakest tooth in the entire arch, and the premolar,
the weakest posterior tooth.


A fixed partial denture replacing a maxillary canine
is subjected to more stresses than that replacing a
mandibular canine, since forces are transmitted
outward (labially) on the maxillary arch, against the
inside of the curve (its weakest point).


On the mandibular canine, the forces are directed
inward (lingually), against the outside of the curve
(its strongest point). Any fixed partial denture
replacing a canine should be considered a
complex a fixed partial denture. No fixed partial
denture replacing a canine should replace more
than one additional tooth. An edentulous space
created by the loss of a canine and any two
contiguous teeth is best restored with a removable
fixed partial denture.


Cantilever
fixed
partial
dentures
A cantilever fixed partial denture is one
that has an abutment or abutments at one end
only, with the other end of the pontic remaining
unattached. This is a potentially destructive design
with the lever arm created by the pontic. In a
routine three-unit fixed partial denture, force that is
applied to the pontic is distributed equally to the
abutment teeth. If there is only one pontic and it is
near the interabutment axis line, less leverage is
applied to the abutment teeth or to the retainers
than with a cantilever.

When a cantilever pontic is employed to replace a
missing tooth, forces applied to the pontic have an
entirely different effect on the abutment tooth. The
pontic acts as a lever that tends to be depressed
under forces with a strong occlusal vector.


Prospective abutment teeth for cantilever fixed
partial dentures should be evaluated with an eye
towards lengthy roots with a favorable
configuration, long clinical crowns, good crownroot ratios, and healthy periodontium. Generally,
cantilever fixed partial dentures should replace
only one tooth and have at least two abutments. A
cantilever can be used for replacing a maxillary
lateral incisor. There should be no occlusal contact
in either centric or lateral excursions. The canine
must be used as an abutment, and it can serve in
the role of solo abutment only if it has a long root
and good bone support.

There should be a rest on the mesial of the pontic
against a rest seat preparation in an inlay or other
metallic restoration on the distal of the central
incisor to prevent rotation of the pontic and the
abutment. The mesial side of the pontic can be
little ‘wrapped around’ the distal portion of the
uninvolved central incisor to stabilize the pontic
faciolingually. The root configuration of the central
incisor does not make it a desirable cantilever
abutment.


A cantilever pontic can also be used to replace a
missing first premolar. This scheme will best work
if occlusal contact is limited to the distal fossa. Full
veneer retainers are required on both the second
premolar and first molar. These teeth must exhibit
excellent bone support. This design is acceptable
if the canine is unmarred and if a full veneer
restoration is required for the first molar in any
event.


Cantilever fixed partial denture can also be used to
replace molars when there is no distal abutment
present. When used judiciously, it is possible to
avoid the insertion of a unilateral removable partial
denture. Most commonly, this type of fixed partial
denture is used to replace the first molar, although
occasionally it is used to replace a second molar to
prevent supereruption of opposing teeth.


When pontic is loaded occlusally, the adjacent
abutment tends to act as a fulcrum, with a lifting
tendency on the farthest retainer. To minimize the
leverage effect, the pontic should be kept as small
as possible, more nearly representing a premolar
than a molar. There should be absolutely no
contact in any excursion. The pontic should
possess maximum occlusogingival height to
ensure a rigid prosthesis.


A posterior cantilever pontic places maximum
demands on the retentive capacity of the retainers.
Its use, therefore, should be reserved for those
situations in which there is adequate clinical crown
length on the abutment teeth to permit
preparations of maximum length and retention.
The success of cantilevers in the restoration of the
periodontally compromised dentition is probably
due, at least by part, to the fact that periodontally
involved abutments do have extremely long clinical
crowns. While cantilever fixed partial dentures
appears to be a conservative restoration, the
potential for damage to the abutment teeth
requires that they be used sparingly.

Double abutment
Many clinical situations require the use of
double abutments in the fixed bridges. The term
as used here refers to the use of two adjacent
teeth at one or both ends of a fixed prosthesis
joined by a solid connector. The usual reasons for
use of double abutment are:
Increase retention of the restorations as a whole
Splint and stabilize periodontally compromised
teeth and
Increase the area of the supporting PDL and bone.


Improvement of the retentive aspects of the
restoration would seem to be a reasonable
justification for including an extra abutment. This
rationale is not always true. As seen, the second
premolar has insufficient coronal dentin to provide
the necessary retention for use as an abutment.
The assumption was made that adding the extra
premolar abutment would give the bridge adequate
retention of the anterior end. This abutment would
allow retention of the second premolar root to
reduce future bone loss, which would occur if this
tooth were extracted. This latter point would
certainly add credibility to the rationale, but at least
two other more conservative methods could be
considered to render the second premolar a sound
abutment.

First, a pin retained intra coronal casting or build
up could be made for the second premolar if
maintenance of the vitality of this tooth is a prime
concern.
Second, endodontic therapy and a
retentive post and core could be done on the
second premolar. The latter method would usually
be the method of choice in this situation due to the
greater chance of long term success compared
with the pin buildup.


Either of these options, particularly the post and
core, could obviate the need for double abutting
this restoration.
The reason being that by
correcting the problem involving the second
premolar, which is lack of retention, the operator
has created a typical three unit prosthesis
situation. The preceding example considered the
use of double abutment strictly on the basis of lack
of retention of the primary abutment choice. A
discussion of other reasons for the use of multiple
abutments follows. However, before proceeding, it
is advisable to consider some of the ramifications
of using double abutment as a solution for lack of
abutment retention.

During function, the prosthesis often develops a
cement failure at the second premolar because of
poor retention characteristics. The other units will
often be retained adequately. Breakdown of the
cement layer of this abutment tooth leads to slow
destruction by action of the saliva and its acidic
components. Had this same loss of retention
occurred in the case of a single unit restoration, it
would have simply come away from the
preparation and the patient would have sought
treatment for an obvious problem. Dislodgement
of the restoration does not occur, however, when
other retainers of a multiple unit restoration remain
in place on their respective abutment teeth.

This problem is difficult to diagnose because
“loose” retainer is still held in its correct position in
relation to the abutment tooth, though no longer
cemented. The patient complains of pain. Since
the retainer is still held in its correct position
relative to the abutment tooth preparation, no
marginal opening can be detected, nor can any
looseness or movement. As can be easily seen,
diagnosis of the patient’s complaint can be difficult,
if not impossible, without removal of the entire
restoration.


Due to these problems, it is imperative that
precautions be taken in the design and
construction of multiple unit restorations to
preclude the loss of retention on any abutment.
Further it is strongly recommended that the use of
double abutments to compensate for lack of
retention on one of the abutment teeth of a fixed
prosthesis be discouraged. The procedure may be
justified from the view point of maintaining bone,
but it is less justifiable when considered in the light
of resistance to the forces to which the restoration
will ultimately be subjected. The alternative of pins
or posts will usually be found to be the treatment of
choice to permit saving of the root.

Splinting and stabilization of a periodontally
compromised tooth can be more valid reasons for
the use of double abutments on a fixed bridge.
However, a fundamental decision must be made
early in the planning of the case; is the mobility the
result of a continuing process of periodontitis, or
occlusal trauma. If the mobility of the tooth is only
the result of occlusal trauma, stabilization of such
a tooth in this manner may be perfectly justified,
providing that the trauma can be eliminated in the
occlusal scheme of the restorations.

When a tooth is subjected to occlusal forces that
cannot be controlled, the adjacent tooth might be
added to the restoration as a double abutment to
provide the needed resistance to lateral forces. A
classic example on this situation would be a bridge
replacing a missing maxillary canine. In such a
case, the lateral occlusal forces generated on the
canine pontic are such that the lateral incisor is
seldom an adequate abutment due to its short root
form. It is then justified to add the central incisor to
such design. It has been shown that mobility
resulting from occlusal trauma is reversible once
the cause for the trauma is removed.

On the other hand, if the lack of bone support is
due to periodontal disease, and if the disease is
not totally controlled, using this tooth as part of
double abutment is contraindicated. In such a
situation, bone loss on the affected abutment tooth
continues, with the end result being that this tooth
eventually becomes simply another pontic in the
bridge. Also, pockets become less cleanable after
the placement of the restoration due to poorer
access, compounding the problem.


Finally, the best justification, for using double
abutments is to satisfy Ante’s law. If there are not
enough periodontal ligaments for a given number
of missing teeth, there is no better solution than to
add one or more teeth that do have sound support.
When many missing teeth are replaced by a fixed
restoration using a limited number of abutments,
most of which do not even possess the normal
amount of bone support, failure is assured.


One must make decision whether the addition of
more abutments in the design of the restoration is
more important than satisfying the concomitant
requirement for conservatism. There may be no
choice if the restoration is to be made at all. If it is
not possible to satisfy Ante’s law in this regard, a
removable partial denture should be considered so
that occlusal forces may be distributed cross arch
and to the edentulous ridges.


From the viewpoint of mechanical principles, the
advantage of adding a second abutment at one
end of a fixed prosthesis is that in so doing, we are
better able to distribute the forces that would be
applied to the prosthesis. Nothing would be gained
if a crown were placed on the added abutment
were it not connected rigidly to the remainder of
the prosthesis.


When the added tooth is made an integral part of
the prosthesis, its periodontal ligaments provide
resistance to forces transmitted by the other
abutment at this end of the bridge. This shared
load-bearing responsibility is the essence of Ante’s
law. An additional abutment tooth, or teeth, is used
to replace the missing tooth. Other wise, only two
abutment teeth would be performing the function of
resisting forces applied to three occlusal surfaces.

There is a common problem in replacing all four
maxillary incisors with a fixed partial denture and
the problem is more pronounced in the arch that is
pointed in the anterior. This occurs because the
pontics lie outside the interabutment axis line and
thus acts as a lever arm, which can produce a
torquing movement. In order to offset the torque,
additional retention is obtained in the opposite
direction of the lever arm and at a distance from
the interabutment axis equal to the length of the
lever arm.

The first premolars sometimes are used as
secondary abutments for a maxillary four-pontic
canine to canine fixed partial denture. Because of
the tensile forces that will be applied to the
premolar retainers, they must have excellent
retention.


Fixed

versus

removable

restorations

When it is necessary to replace missing
teeth, a choice must be made whether to use a
fixed or a removable restoration. This decision is
best made from the standpoint of force distribution
in relation to the ability of the supporting structures
to withstand those forces. The number of fixed
restorations since the late 1950s has greatly
increased due to improvements in high-speed
instrumentation, developments in impression
materials, and advancements with the porcelainfused-to-metal technique.

prosthodontists are now able to construct rather
extensive
and
complicated
multiple
unit
restorations replacing many missing teeth with
relative ease. There has been a trend in recent
years towards better esthetics and what the public
considers to be better prosthodontistry (i.e. more
extensive
fixed
restorations)
However, from the viewpoint of proper
distribution of forces in the gnathologic system,
particularly lateral forces, more consideration
should be given to the use of removable
restorations in many of the more extreme cases of
multiple missing teeth.

The most important factor in this consideration is
that the removable partial denture provides cross
arch stabilization, whereas individual fixed partial
dentures normally do not. Since many patients
have some degree of bone loss, it is often
desirable to distribute lateral forces to other teeth
in the arch that may have the benefit of more
adequate bone support. The bilateral removable
partial denture will accomplish this and also aid in
the improved distribution of vertical forces in the
periodontally compromised patient. Occlusal loads
on tipped or isolated teeth can better be directed
along their long axes. Again, Ante’s law is relevant.

The total area of remaining good periodontal
ligament on the teeth to be used as abutments for
the removable partial denture will be increased by
including teeth in other areas of the arch. Another
advantage of removable prosthesis is the
opportunity to distribute functional loads to the
gingival tissues and supporting bone of the
edentulous ridges and palate. When there is
inadequate PDL to accommodate fixed prosthesis,
the use of ridges becomes mandatory.


It has been shown that the periodontal ligament
cushions the shock of tooth-to-tooth contact by
moving as much as 0.05 mm. If it is assumed that
the degree to which this cushioning effect is
brought into play may be related to the hardness of
the occlusal surfaces of the restoration, the
removable partial denture may afford yet another
advantage. The occlusal surfaces of the
replacement pontics on removable partial denture
prosthesis are usually made of an acrylic resin
rather than gold or porcelain, as in the case of
most fixed prosthesis.

It would be reasonable to assume that shock
transfer to abutment teeth and their periodontal
ligaments will be less than in the case of a fixed
restoration. Therefore, a little less periodontal
ligament would suffice in such a situation and
would adequately withstand occlusal forces. It
would seem to follow, then, that many patients
might benefit from a design of fixed prostheses
that incorporate some type of plastic material on
the occlusal surface rather than gold or porcelain.


The potential for abrasion is the primary factor
obviating such an idealistic solution to the problem.
When the acrylic teeth on a removable partial
denture wear, they can easily be replaced without
great expense or extensive clinical procedures.
This would not be the case if a fixed restoration
and the plastic occlusal surfaces needed
replacement.


When the remaining attachment apparatus is
inadequate for distribution of occlusal loads, the
removable prosthesis has a decided advantage
over a fixed prosthesis. The abutment tooth,
periodontal ligament, and bone can accommodate
only a certain level of stress concentration. The
removable partial denture prosthesis can help by
transferring excessive stresses to the tissues of
the edentulous ridge – fixed partial denture
prosthesis cannot. At this point the objective is
only to decide which type of prosthodontic
replacement to use.

The significance of providing biomechanically
advantageous stress distribution in occlusal
loading becomes more evident when one
considers the amount of time the teeth are in
contact without the presence of food. It must be
pointed out that tooth contact, with and without
food interposed between the occlusal surfaces,
presents different situations.
When food is
present, forces are distributed over a broader area
and are generally assumed to be of lesser
magnitude since some of the energy is absorbed
in doing work, namely, masticating the food.
However, when no food is present, the tooth
contact is made directly with another tooth surface.

Therefore, forces are concentrated at the point of
contact, and all the energy is expended on
abrasive action or traumatic loading of the
supporting structures. Graf has indicated that the
average person exerts deglutition forces on the
teeth 25 times each hour during the day and 10
times each hour during the night while asleep.
During these 16 and 8 hour periods, respectively,
the person will make tooth to tooth contact (thus
applying force to the occlusal surfaces and
supporting tissues) of 8 minutes each day. Added
to this time are 30 seconds of contact during
swallowing of masticated food.

This amount of time may seem to be rather small,
but during these periods of tooth to tooth contact,
the magnitude of this force is rather high.
Also, this force is exerted between two very hard
surfaces. To emphasize the significance of the
point, consider that the tooth enamel is the hardest
tissue in the human body and further that now
where in the body do two hard surfaces come
together in this fashion without the protection of
cartilage. The effects of this force are cumulative
over long periods of time. For example, in 1
month, the average patient will have the teeth in
heavy occlusal contact for about 4 hours.

In light of the facts that in the oral environment we
have other debilitating factors at work such as the
presence of periodontal disease, it is the best
interest of the patient to avoid loading these
various structures beyond the normal limits. Tooth
to tooth contact will damage the occlusal surfaces,
exacerbate pre-existing periodontal disease
processes, or both when it is excessive, as in
bruxism. Another factor that should be taken into
account in this matter is the periodontal
maintenance of case.
The removable partial
dentures can be used, if designed properly, as a
periodontal splint, even though the tooth or teeth
requiring the splinting are not in the same area of
the arch as the teeth being replaced.

Obviously, the fixed partial denture cannot provide
this benefit, at least not one that is properly
designed. It is true that in theory some of the
same splinting effect could be provided by
roundhouse, or full, arch fixed bridge, but the other
disadvantage of such a restoration more than
outweigh the possible benefits. In addition, the
oral hygiene difficulty in the area of the missing
teeth, particularly surfaces of the abutment teeth
facing the pontic areas, is greater in the case of a
removable restoration. In actual fact, for all of its
advantages, the fixed restoration will nearly always
leave the patient with a greater problem of access
for cleaning than if the teeth had never been
replaced.


Review of literature
Aydinlik E, Dayangac B, Celik E. (1983),
investigated the effect of splinting on abutment
tooth movement and concluded that a significant
decrease in the magnitude of movement resulted
when the abutment teeth were splinted.


Revah A, Rehany A, Zalkind M, Stern N.
(1985) discussed the problem of achieving a
common path of insertion for a fixed partial denture
when a tilted posterior abutment is involved and
concluded that the problem can usually be solved
by well planned tooth preparation in conjunction at
times with intentional endodontic therapy. When
tooth preparation alone cannot solve the problem,
the mechanical solutions of the locked attachment
and the telescopic retainer are available and must
be considered.

Ziada HM, Orr JF, Benington IC. (1989),
analyzed the stresses induced in a pier retainer of
an anterior resin-bonded fixed partial denture and
concluded that the use of pier abutments should
be avoided and it is more favorable to use 3-unit
resin-bonded fixed partial dentures.


Farah JW, Craig RG, Meroueh KA. (1989),
used a two-dimensional finite element model of a
mandibular quadrant to examine differences in
magnitude of the principal stresses from the
placement of three- and four-unit bridges. No
significant differences in magnitude were observed
between the three- and four-unit bridge. From a
stress standpoint the bridges resulted in more
uniform stress distribution around the abutments
and an increase in the tensile stress distal to the
abutments. Such findings support the placement of
a fixed bridge to maintain bone in an edentulous
area.

Awadalla HA, Azarbal M, Ismail YH, elIbiari W. (1992), constructed a three-dimensional
mathematical model representing a three-unit
cantilever fixed partial denture and its supporting
mandibular structures. The results showed that a
cantilever pontic creates considerable
compressive stress on the abutment nearest to the
pontic and produces tensile stress on the
abutment farthest from the pontic.


Kerschbaum T, Haastert B, Marinello CP.
(1996), concluded that rebonded
resin-bonded fixed partial dentures developed a
greater risk of debonding. The risk of failure for
refabricated fixed partial dentures was similar to
that of the originally inserted resin-bonded fixed
partial dentures. There were no signs of greater
caries incidence after multiple recementation
procedures.


el-Mowafy OM. (1998), described an
uncomplicated clinical procedure to enhance the
retention of posterior resin-bonded fixed partial
dentures which involves some modifications to the
preparation and casting design and requires
slightly more time and attention at the cementation
stage of the prosthetic treatment.


Yang HS, Lang LA, Felton DA. (1999),
analyzed the stress levels in the teeth and
supporting structures of a fixed prosthesis and
ascertained how the addition of multiple abutments
in a fixed prosthesis modifies the stresses and
their deflection and concluded that Increasing the
number of the splinted abutment did not
compensate for the mechanical problems of a
long-span fixed partial denture sufficiently.


Nishimura RD, Ochiai KT, Caputo AA, Jeong CM.
( 1999), examined stress transfer patterns with
variable implant support and simulated natural
teeth through rigid and nonrigid connection under
simulated functional loads and concluded that
lower stresses apical to the tooth or implant
occurred with forces applied further from the
supporting abutment. Although the least stress
was observed when using a nonrigid connector,
the rigid connector in particular situations caused
only slightly higher stresses in the supporting
structure.

The rigid connector demonstrated more
widespread stress transfer in the 2 implantsupported restoration. Recommendations for
selection of connector design should be based on
sound clinical periodontal health of a tooth and the
support provided by implants.


Botelho M. (2000), concludes that the 2-unit
prosthesis is successful and adds value to the
clinical use of resin-bonded fixed partial dentures
because the single-abutment prosthesis is even
simpler and more cost effective than fixed-fixed
designs. However, there is no evidence-based
information relating to design principles for
abutment preparation and framework design for
the single-abutment, single-retainer prosthesis


Botelho MG, Nor LC, Kwong HW, Kuen BS
(2000), evaluated the clinical retention and
abutment movement of 2-unit cantilevered resinbonded fixed partial dentures (FPD) and
concluded that two-unit cantilevered resin-bonded
FPDs are successful in the short term, but further
research is required to determine if they offer a
viable alternative to fixed-fixed resin-bonded FPD
designs.


Koutayas SO, Kern M, Ferraresso F, Strub
JR. (2002), evaluated the influence of the
framework design on the fracture strength of allceramic resin-bonded fixed partial dentures
(RBFPD) in the mandibular incisor region and
concluded that
the clinical application of cantilevered all-ceramic
RBFPDs in the mandible may be an alternative to
all-ceramic RBFPDs with two retainers.


Hood JA, Farah JW, Craig RG. Investigated
the stresses induced in the supporting bone by a
tilted molar tooth under load and following
conclusions were reached:
1. Altering the angle of the load applied to the
unsupported molar from 0 (axial) to 30 degrees
resulted in a fourfold increase in compressive
stress in the supporting bone mesial to the tooth.
2. Increasing the load from 30 to 90 pounds while
maintaining a 30 degree angle of application
resulted in a linear increase in the shear stress on
the supporting bone mesial to the tooth.

3. Following the placement of a fixed partial
denture, the induced stress at a point on the
mesial aspect of the molar tooth, subjected to a 60
pound load at 30 degrees to the long axis, was
reduced from 241 to 43 p.s.i.
4. The introduction of a fixed partial denture
resulted in a decrease in the compressive stress in
the bone adjacent to the apex of the mesial root of
the molar from 481 to 174 p.s.i.
5. A distributed 120 pound load applied over the
length of the fixed partial denture compared
against individual tooth loadings of 60 pounds
revealed that placement of the fixed partial denture
favored the tilted molar at the expense of the
premolar.

Biomechanics in fixed partial prosthodontics /certified fixed orthodontic courses by Indian dental academy

Biomechanics in fixed partial prosthodontics /certified fixed orthodontic courses by Indian dental academy

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