Biomechanics in fixed partial prosthodontics/ orthodontics training

BIOMECHANICS IN FIXEDBIOMECHANICS IN FIXED
PARTIALPARTIAL
PROSTHODONTICSPROSTHODONTICS
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com
www.indiandentalacademy.comwww.indiandentalacademy.com

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

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

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

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

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

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.www.indiandentalacademy.comwww.indiandentalacademy.com

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

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

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

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

Crown-root ratio:Crown-root ratio:
This ratio is a measure of the length ofThis ratio is a measure of the length of
tooth, occlusal to the alveolar crest of bonetooth, occlusal to the alveolar crest of bone
compared with the length of root embedded in thecompared with the length of root embedded in the
bone. As the level of alveolar bone moves apically,bone. As the level of alveolar bone moves apically,
the lever arm of that portion out of bone increases,the lever arm of that portion out of bone increases,
and the chances for harmful lateral forces isand the chances for harmful lateral forces is
increased. The optimum crown-root ratio for aincreased. The optimum crown-root ratio for a
tooth to betooth to be
utilized as a FPD abutment is 2:3. A ratio of 1:1 isutilized as a FPD abutment is 2:3. A ratio of 1:1 is
the minimum ratio that is acceptable for athe minimum ratio that is acceptable for a
prospective abutment under normalprospective abutment under normal
circumstances.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 aFor the same reasons, an abutment tooth with a
less than desirable crown-root ratio is more likelyless than desirable crown-root ratio is more likely
to successfully support FPD if the opposingto successfully support FPD if the opposing
occlusion is composed of mobile, periodontallyocclusion is composed of mobile, periodontally
involved teeth than if the teeth are periodontallyinvolved teeth than if the teeth are periodontally
sound.sound.

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

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

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

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


Maxillary Mandibular
Tooth Area mm2
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 theThe combined root surface area of the
second premolar and the second molarsecond premolar and the second molar
(A(A2p2p+A+A2m2m) is greater than that of the first) is greater than that of the first
molar being replaced (Amolar being replaced (A1m1m).).

first premolar and the second molarfirst premolar and the second molar
abutment (Aabutment (A1p1p+A+A2m2m) is approximately) is approximately
equal to that of the teeth being replacedequal to that of the teeth being replaced
(A(A2p2p+A+A1m1m))

canine and the second molar (Acanine and the second molar (Acc+A+A2m2m))
is exceeded by that of the teeth beingis exceeded by that of the teeth being
replaced (Areplaced (A1p1p+A+A2p2p+A+A1m1m))

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

Table , which compares the root surface areas ofTable , which compares the root surface areas of
16 teeth, may aid in visualizing root areas. It is16 teeth, may aid in visualizing root areas. It is
interesting to note that the addition of abutmentinteresting to note that the addition of abutment
roots will provide a wide variation of additionalroots will provide a wide variation of additional
support. The addition of such support is notsupport. The addition of such support is not
necessarily proportional to the number ofnecessarily proportional to the number of
abutments supporting prosthesis. There fore, theabutments supporting prosthesis. There fore, the
prosthodontist should not expect a proportionalprosthodontist should not expect a proportional
increase in stress-bearing ability, particularly whenincrease in stress-bearing ability, particularly when
the amount of periodontal ligament is reduced. Thethe amount of periodontal ligament is reduced. The
area of the normal periodontal ligament (PDL) forarea of the normal periodontal ligament (PDL) for
teeth to be replaced by pontics should be less thanteeth to be replaced by pontics should be less than
the actual PDL area of the existing abutmentthe actual PDL area of the existing abutment
candidates.candidates.

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

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

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

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

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

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

Ante’s law says that in a situation where theAnte’s law says that in a situation where the
preceding values are normal, a prosthesis topreceding values are normal, a prosthesis to
replace the maxillary first molar would needreplace the maxillary first molar would need
abutment teeth with at least 335 mmabutment teeth with at least 335 mm22
ofof
periodontal membrane. This requirement isperiodontal membrane. This requirement is
theoretically well satisfied in the case because oftheoretically well satisfied in the case because of
the total average area of the second molar andthe total average area of the second molar and
second premolar is 412mmsecond premolar is 412mm22
, however, has there, however, has there
been enough loss of bone on these two teeth tobeen enough loss of bone on these two teeth to
result in, for example, a total of only 300mmresult in, for example, a total of only 300mm22
, the, the
law would not be satisfied.law would not be satisfied.

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

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

Retention and resistanceRetention and resistance
Looking at these design features one at aLooking at these design features one at a
time, we must first consider the question oftime, we must first consider the question of
retention and resistance. Retention is the ability ofretention and resistance. Retention is the ability of
the restoration to withstand forces acting in such athe restoration to withstand forces acting in such a
manner as to dislodge the restoration from themanner as to dislodge the restoration from the
preparation along the line of draw.preparation along the line of draw. Resistance, onResistance, on
the other hand, is the ability of the preparation andthe other hand, is the ability of the preparation and
the restoration to resist forces that would tend tothe restoration to resist forces that would tend to
dislodge the latter in direction other than the line ofdislodge the latter in direction other than the line of
draw. In other words, the resistance to vectors ofdraw. In other words, the resistance to vectors of
occlusal forces encountered on laterotrusiveocclusal forces encountered on laterotrusive
occlusal movements during either mastication orocclusal movements during either mastication or
grinding of the teeth is the important considerationgrinding of the teeth is the important consideration
here.here.www.indiandentalacademy.comwww.indiandentalacademy.com

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Biomechanics in fixed partial prosthodontics/ orthodontics training

Biomechanics in fixed partial prosthodontics/ orthodontics training

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (20)

Similar to Biomechanics in fixed partial prosthodontics/ orthodontics training

Similar to Biomechanics in fixed partial prosthodontics/ orthodontics training (20)

More from Indian dental academy

More from Indian dental academy (20)

Recently uploaded

Recently uploaded (20)

Biomechanics in fixed partial prosthodontics/ orthodontics training