Types of Fixed Partial Denture Failures

Failures in Fixed
Partial Dentures
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Fixed prosthodontic treatment can offer exceptional
satisfaction for both patient and dentist. It can transform an
unhealthy, unattractive dentition with poor function into a
comfortable, healthy occlusion capable of giving years of
further service while greatly enhancing esthetics. To achieve
such success, however, requires meticulous attention to every
detail from initial patient interview, through the active
treatment phase, to a planned schedule of follow-up care.
Failure to achieve the desired specifications of design for
function and esthetics would result in failure of the prosthesis.


The causes of FPD failures were summarized as early as
in 1920 when Tinker wrote “ Chief among the causes for such disappointing results have
been:
First : Faulty, and in some cases, no attempt at diagnosis and
prognosis.
Second: Failure to remove foci of infection in attention to
treatment and care of the investing tissues and mouth
sanitation.
Third: Disregard for tooth form
Fourth: Absence of proper embrasures
Fifth: Inter-proximal spaces
Sixth: Faulty occlusion and articulation”


CLASSIFICATION


Bennard G. N. Smith
1. Loss of retention
2. Mechanical failure of crowns or bridge components
a. Porcelain fracture
b. Failure of solder joints
c. Distortion
d. Occlusal wear and perforation
e. Lost facings
3. Changes in the abutment tooth
a. Periodontal disease
b. Problems with the pulp
c. Caries
d. Fracture of the prepared natural crown or root
e. Movement of the tooth

4. Design failures
a. Under-prescribed FPDs
b. Over-prescribed FPDs
4. Inadequate clinical or laboratory technique
a. Positive ledge
b. Negative ledge
c. Defect
d. Poor shape and color
4. Occlusal problems


John F. Johnston
1. Discomfort
a. Malocclusion or premature contact
b. An oversized or poorly positioned mastication area, with
retention of food by pontics or retainers.
c. Torque produced from the seating of the bridge or from
occlusion
d. An excess of pressure on the tissue
e. Plus or minus contact area
f. Over protected or under protected gingival and ridge tissue.
g. Thermal shock

2. Looseness of FPD
a. Deformation of the metal casting on the abutment
b. Torque
c. Technique of cementation
d. Solubility of cement
e. Caries
f. Mobility of one or more abutments
g. Lack of full occlusal coverage
h. Insufficient retention in the abutment preparation
i. Poor initial fit of the casting.


3. Recurrence of caries
a. Over extension of margins
b. Short castings
c. Open margins
d. Wear
e. A retainer becoming loose
f. Pontic form that fills the embrasure
g. Poor oral hygiene
h. Use of wrong type of retainer, which will promote caries
susceptibility
i. Permanent displacement of the gingiva due to temporary
protection

4. Recession of supporting structure
a. Length of the span
b. Size of the occlusal table
c. Embrasure form
d. Few extensions of the cervical margins
e. Impression technique can also stimulate recession of the
gingiva.
4. Degeneration of Pulp
5.

Fractures of bridge components
a. A faulty solder joint
b. Incorrect casting technique
c. Overwork of the metal due to length of the span or parts
that are too small.

7. Loss of veneers
a. Little retention
b. Badly designed metal protection
c. Deformation of the protecting metal
d. Malocclusion
e. Improper fusing or technique
7. Loss of function
a. They don’t function in occlusion
b. They have no contact with opposing teeth
c. They have permanent contact
d. Over carved or under carved occlusal surface may impair
efficiency
e. Loss of opposing or approximating teeth

9. Loss of teeth tone or form
a. Pontic design
b. Position and size of the joints
c. Embrasure form
d. Over contouring or under contouring of retainers
e. Oral hygiene practiced by the patient
9. Failure to seat
a. The abutment preparations may not be near parallel
b. Soldering assembly may have been incorrect, or relationship
of the retainers may have been altered during soldering.


TYPES OF BRIDGE FAILURE
I. Cementation failure
II. Mechanical failure
III. Gingival and periodontal breakdown
IV. Caries
V. Necrosis of pulp
VI. Biomechanical failure
VII.Esthetic failure

I. CEMENTATION FAILURE


Cementation failures can be broadly divided into:
1. CEMENT FAILURE
2. RETENTION FAILURE
3. OCCLUSAL PROBLEMS
4. DISTORTION OF FPD


1. CEMENT FAILURE
The primary function of the luting agent is to provide a
seal preventing marginal leakage and pulp irritation. The
luting agent should not be used to provide significant retentive
and resistive forces.
An ideal luting agent would have the following properties:
1. Adequate working time
2. Adhere well to both tooth structure and metal surface
3. Provides a good seal
4. Non toxic to the pulp
5. Have adequate strength properties
6. Be compressible into thin layers
7. Have low viscosity and solubility
8. Exhibit good working time and setting properties

Besides an inadequate retainer, failure can also occur because of a
poor cementation technique. This maybe due to the wrong
choice of material, failure to observe the manufacturer’s mixing
instructions, the use of old or contaminated material, an
inadequate powder/liquid ratio, or the insertion of the
prosthesis when the cement has started to set.
An inadequately cemented restoration may cause
1. An increased vertical dimension of occlusion
2. A loosening of the crown or FPD after a relatively short time
3. Leakage and decay under the abutment
4. The unsightly appearance of a metal margin where originally
the metal was concealed under the gingiva
5. Sensitivity to sweets or brushing due to exposure of the cervical
end of the tooth

Causes of cement failure
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)

Cement selection
Old cement
Prolonged mixing time
Thin mix
Cement setting prior to seating
Inadequate isolation
Incomplete removal of temporary cement
Thick cement space
Inclusion of cotton fibers
Insufficient pressure

2. RETENTION FAILURE
For a restoration to accomplish its purpose, it must stay in
place on the tooth. No cements that are compatible with living
tooth structure and the biologic environment of the oral cavity
possess adequate adhesive properties to hold a restoration in
place solely through adhesion. The geometric configuration of
the tooth preparation must place the cement in compression to
provide the necessary retention and resistance.
CAUSES FOR RETENTION FAILURE
1) Excessive taper
2) Short clinical crowns
3) Mis-fit
4) Misalignment

Excessive taper : As a cast metal or ceramic restoration is placed
on or in the preparation after the restoration has been fabricated in
its final form, the axial walls of the preparation must taper slightly
to permit the restoration to seat
Theoretically, the more nearly parallel the opposing walls
of the preparation are, the greater should be the retention.
Recommendations for optimal axial wall taper of tooth
preparations for cast restorations ranged from 10 to 12 degrees.
Tooth preparation taper should be kept minimal because of
its adverse effect on retention, but Mock estimates that a minimum
taper of 12 degrees is necessary just to insure the absence of
undercuts.


Short clinical crown : Cement creates a weak bond largely by
mechanical interlocks between the inner surface of the restoration
and the axial wall of the preparation. Therefore, the greater the
surface area of the preparation the greater is its retention. The
preparations on large teeth are more retentive than preparations on
small teeth.
A short, over-tapered or short clinical crown would be
without retention as there would be many paths of removal.
For the restoration to succeed, the length must be great
enough to interfere with the arc of the casting pivoting about a point
on the margin on the opposite side of the restoration. A shorter wall
cannot afford this resistance. The walls of short preparations should
have as little taper as possible.

Clinical conditions with excessive taper and short clinical crowns
should be treated with :1. In case of excessive taper:
a. Incorporation of proximal grooves.
b. Additional retentive grooves (should be along with the path
of insertion).
c. Additional pins
2. In case of short crowns:
a. Crown lengthening procedure
b. Modification of supra-gingival margin to sub-gingival
margin
c. Additional retentive grooves and proximal box
d. Incorporation of pins
e. Addition of extra abutments

Misfit : The fit of casting can be defined best in terms of the
“misfit” measured at various points between the casting surface
and the tooth.
The measurement of misfit at different locations and geometrically
related to each other and defined as :
1. Internal gap
2. Marginal gap
3. Vertical marginal discrepancy
4. Horizontal marginal discrepancy
5. Over-extended margin
6. Under-extended margin


Causes for misfit :
a. Expansion of the metal substructure
b. Improper water / powder ratio
c. Improper mixing time
d. Improper burnout temperature
e. Distortion of the margins (towards the tooth surface)
f. Distortion of the metal substructure
g. Metal bubbles in occlusal or marginal regions
i. Inadequate vacuum during investing
ii. Improper brush technique
iii. No surfactant
h. Porcelain flowed inside the retainer
i. Excessive oxide layer formation in inner side of the retainer (due
to contaminated metal or repeated firing of porcelain)
j. Tight contact points
k. Thick cement space
l. Insufficient pressure during cementation procedure

Misalignment : In case of the fixed FPD, it is more difficult to
differentiate whether a FPD is not seating because of a faulty fit,
or the alignment of the retainers relative to each other is incorrect.
The only difference which may sometimes be apparent is that, in
the case of misalignment the FPD will have some ‘spring’ in it and
tend to seat further on pressure due to the abutment teeth moving
slightly, whereas in the case of a defective fit, the resistance felt
will be solid.


Causes for misalignment
a. Abutment displacement due to improper temporization.
b. Distortion of wax pattern while sprueing and investing.
c. Casting defects.
d. Distortion of metal frameworks in porcelain firing.
e. Porcelain flow inside the retainers.
f. Misalignment of soldering points.
g. Insufficient pressure in cementation.
h. Thick cement film.
i. Excessive metal or porcelain in tissue surface (ridge lap) of
pontic prevents the proper seating of FPD and open margin
(can be detected by observing the blanching of the tissue or
patient may complain of pressure on the pontic region).

3. OCCLUSAL PROBLEMS
Following the placement of a dental restoration, a patient
might report discomfort ranging from a feeling of ‘lameness’ to
‘severe and constant pain’. Sensitivity, in most cases, is due to
pulp irritation from traumatic contact or greater leverages. When
the occlusion has been adjusted, each type of discomfort may be
relieved almost instantly and should disappear shortly.


Causes in occlusal problems
1. Immediate problems
 Occlusal interference
 Marginal ridges at different levels
 Supra eruption of the opposing tooth
 Parafunctional habits
2. Delayed problems
 Wearing of occlusal surface
 Loss of occlusal contacts
 Perforation of occlusal surface due to
• Porcelain Vs resin
• Porcelain Vs gold
 Food lodgment due to plunger cusp
 Fracture of facing due to defective occlusal contact
 Periodontal or gingival breakdown due to improper occlusal
contacts
 Tenderness due to www.indiandentalacademy.com
food lodgment

4. DISTORTION OF FPD
Margin Integrity
The completed restoration should go into place without
binding of its internal aspect against the occlusal surface or the
axial walls of the tooth preparation. In other words, the best
adaptation should be at the margins. If the indirect procedure is
handled properly, there should be no noticeable difference between
the fit of a restoration on the die and that in the mouth.


Causes for failure in marginal integrity:
1) Bending of FPD (wax patterns and metal substructure)
• In waxing stage
• Removal from the die
• Spruing stage
• Investing stage (thick mix of investment distort or displace
the wax pattern)
1) Incomplete casting
• Wax patterns too thin
• Incomplete wax elimination
• Cold mold or melt
• Inadequate metal

3) Rough casting
• Improper finishing of wax pattern
• Excess surfactant
• Improper water powder ratio
• Excessive burnout temperature
• Improper devesting (direct hit on the metal framework)
3) Bending of long span FPDs
• Thin crown
• Soft metal
• Heat treatment not being done
• Porosity in the metal
• Distortion of the metal substructure during the porcelain
firing
• Contaminated metal

Inadequate FPD design
Designing FPDs is difficult. It is neither a precise science nor a
creative art. It needs knowledge, experience and judgment, which
takes years to accumulate.
So it is not surprising that some designs of FPDs, even though
well intentioned and consciously executed, fail. A simple
classification of these failures is as; under-prescribed and overprescribed FPDs.
Under-prescribed FPDs – these include designs that are unstable or
have too few abutment teeth. E.g., a cantilever FPD carrying pontics
that cover too long a span or a fixed movable FPD where again the
span is too long or where abutment teeth with too little support has
been selected.
Another ‘under-design’ fault is to be too conservative in
selecting retainers. E.g., intra coronal inlays for fixed FPDs. With
these design faults, little can be done other than to remove the FPD
and use another type of replacement.

Over-prescribed FPDs – cautious dentists will somewhere include
more abutment teeth than necessary, and fate usually dictates that it is
the unnecessary retainer that causes fault. Some use the upper canines
and both premolars on each side in replacing the four incisor teeth. As
well as being destructive, this gives rise to unnecessary practical
difficulties in making the FPD. This, in turn, reduces the chance of the
FPD being successful.


Several suggestions have been proposed scientifically to explain the
distortion resulting in metal frameworks after the various stages
of the porcelain firing schedule; these include:
a)
b)
c)
d)
e)
f)
g)
h)

Contraction of the porcelain with subsequent metal deformation
Contamination of the casting, reducing its melting temperature
Grain growth of the alloy, constricting the diameter of the crown
Plastic flow and creep of the porcelain gold alloy at high
temperatures
Reduction in the resiliency of the metal due to the rigidity of
porcelain
Improper support of the framework during firing
Inadequate framework design at the gingival level
inadequate design of the framework as a whole

Shillingburg stated that ceramic metals require a certain amount
of
bulk in the cervical area to resist distortion when subjected to the

II MECHANICAL FAILURES


In the design of fixed partial denture pontics, if insufficient
attention is given to mechanical principle, the prognosis will be
compromised. Mechanical problems might be due to poor diagnosis
and treatment plan, improper choice of materials, poor framework
design, poor tooth preparation, or poor occlusion. These could lead
to fracture of the prosthesis or displacement of the retainers.
It is therefore important to evaluate the likely forces on a
pontic and to design it accordingly. For example, a strong all metal
pontic may be needed in situations of high stress rather than a metal
ceramic pontic which could be more susceptible to fracture.


Classification of mechanical failure
1. Retainer failure
2. Pontic failure
3. Connector failure


1. RETAINER FAILURE
1) Perforation
2) Marginal discrepancy
3) Facing failure
Fracture
Wearing
Discoloration


1) Perforation
Causes
a) Insufficient occlusal reduction
b) Insufficient occlusal material
c) High points in opposing dentition (plunger cusp)
d) Premature contacts
e) Contaminated metal
f) Porosity in metal work (subsurface, back pressure, suck
back)
g) Due to improper melting temperature
h) Improper pattern position
i) Improper sprue (too thin)
j) Improper location
k) Parafunctional habits

2) Marginal discrepancy
Causes
a) Selection of margin
b) Improper preparation and failure to establish the margin
properly
c) Failure to do gingival retraction prevents definite margin
location and subsequently in impression
d) Selection of the impression material
i. Shrinkage in material (condensation silicon)
ii. Distortion of material (alginate)
e) Improper impression procedures
f) Voids in the impression
g) Variation in pressure application in wash technique
h) Delayed pouring of die material
i) Distortion of wax patterns at margins

j)
Insufficient flow of metal
k) Shrinkage of metal
l) Nodules in margins and inner side of coping
i. Due to inadequate vacuum during investing
ii. Improper brushing technique
iii. No surfactant
l) Excessive sand blasting
m) Distortion due to degassing procedure
n) Open margins due to porcelain shrinkage (opaque porcelain)
o) Thick cement
p) Cement setting prior to seating
q) Insufficient pressure application during cementation

3. Facing failure
Types of veneer failures
a) Fracture
b) Wearing of facing (resin veneers)
c) Discoloration


Causes for veneer fracture:
i. Too little retention (mechanical)
ii. Badly designed metal protection
iii. Deformation of the protecting metal
iv. Malocclusion
v. Micro-leakage between metal and facing
vi. Improper curing or fusing technique
vii.Excessive oxide layer formation


Cause of wearing of facing:
i.
ii.
iii.
iv.
v.

Improper curing or fusing technique
Deep bite (decreased overbite in lower anteriors)
Acrylic veneering opposing porcelain teeth
Faulty brushing techniques and flossing
Parafunctional habits


Causes of discoloration:
i. Absorption of oral fluids
ii. Absorption of artificial food colouring agents through
micro-cracks or microleakage in metal and facing
interfaces
iii. Tarnish of underlying metal and facing (greening of
porcelain in silver alloys)
iv. Micro-cracks due to malocclusion


2. PONTIC FAILURE
Factors affecting selection and failure of pontics
1) Pontic space
2) Residual ridge contour
3) Biological consideration
a. Ridge relation
b. Dental plaque
c. Gingival surface of pontic (Contact with mucosa)
i. Mucosal contact
ii. Non mucosal contact
4) Pontic ridge relationship
5) Pontic material
6) Biocompatibility
7) Occlusal forces
8) Metal substructure support

1) Pontic space
One function of an FPD is to prevent tilting or drifting of
the adjacent teeth into the edentulous space. If such unwanted
movement has already occurred the space available for the
pontic may be reduced and its fabrication may be complicated.
Under these circumstances it is often impossible to create an
acceptable appearance without repositioning the abutment
teeth orthodontically where aesthetics is important.
Even with a less aesthetic requirement, as for posterior
teeth, overly small pontics are unacceptable because they trap
food and are difficult to clean. When orthodontic repositioning
is not possible, it may be better to increase the proximal
contours of adjacent teeth than to make an FPD with
undersized pontics.

2) Residual ridge contour
The contour texture of the edentulous ridge should be
carefully evaluated during the treatment planning phase. An ideally
shaped ridge will be smooth, for this is the easiest to maintain
plaque free. Unfortunately, many patients present with irregular
hypoplastic tissue, particularly where an ill fitting RPD has been in
place, and under these circumstances , surgical removal of the
excess fibrous tissue may be recommended.
Some patients suffer severe bone resorption following tooth
loss, particularly if the loss occurred due to trauma. These patients
can present a significant aesthetic challenge. Surgical ridge
augmentation (e.g., with hydroxyl apatite) may be one solution.
Another surgical procedure that has been proposed is to create a
role of soft tissue labial to the pontic site, which will enhance the
illusion that a tooth is growing out of the gingival tissue.

3) Biological consideration
The biologic principles of pontic design pertain to the
maintenance and preservation of the residual ridge, abutment
and opposing teeth and supporting tissues.
Factors of specific influence are:
a) Pontic ridge contact
b) Removal of dental plaque
c) Gingival surface of the pontic


a)Pontic ridge contact
Pressure free contact between the pontic and the underlying
tissues is indicated to prevent ulceration and inflammation of the soft
tissues. If any blanching of the soft tissue is observed at try in, the
pressure areas should be identified with pressure indicating paste and
the pontic re-contoured until tissue contact is entirely passive.
b)Dental plaque
The chief cause of ridge irritation is the toxins that are released
from microbial plaque, which accumulates between the gingival
surface of the pontic and the residual ridge causing tissue
inflammation and calculus formation.
Unlike a RPD, a FPD cannot be taken out of the mouth daily
for cleaning. To enhance plaque control, the patient must be taught to
perform efficient oral hygiene techniques, with particular emphasis
on cleaning the gingival surface of the pontic. The shape of the
gingival surface, its relation to the ridge, and the materials used in its
fabrication will influence the success of these measures.

c) Gingival surface of the pontic
Where aesthetics is of concern in the anterior region of the
mouth, the pontic should contact the gingival tissue on the labial
or buccal aspect to give an appearance of ‘emerging from the
tissue’. In the posterior region, like the mandibular premolar and
molar areas more attention should be given to occlusion, function
and hygiene. Considering these aspects, pontic contacts may be
classified into different groups: mucosal and non mucosal contacts
based on the shape of the gingival surface and its relationship with
the underlying tissue.
Normally, where tissue contact occurs, the gingival surface
of a pontic is inaccessible for cleaning with a tooth brush.
Therefore, the patient must develop excellent hygiene habits and
the use of devices such as proxibrushes, pipe cleaners and dental
floss.

A pontic with a concave fitting surface that overlaps the
residual ridge bucally and lingually is called a saddle. This is
avoided because the gingival surface cannot be easily cleaned.
An egg shaped or bullet shaped pontic is probably easiest
for the patient to keep clean. It should be made as convex as
possible, with only one point of contact at the center of the residual
ridge. This design is recommended for the replacement of
mandibular posterior teeth because aesthetics is of less concern
here.


4) Pontic ridge relationship
Since 1918 it has been a popular concept that the tissue
surface of a mandibular posterior pontic should sometimes be left
well clear of the residual ridge. This design was often called
‘hygienic’ or ‘sanitary’.
The hygienic design permits easier plaque control by
allowing gauze strips and other cleaning devices to be passed
under the pontic and seesawed in shoeshine fashion. There are
disadvantages to the design as well. Food particles tend to
become trapped, which may lead to tongue habits that are
annoying to the patient. The hygienic design also is
contraindicated if minimum vertical space exists and where
esthetics is important; tissue proliferation can occur when the
pontic is too close to the residual ridge, forgoing the originally
intended advantages.

5) Pontic material
Any material chosen to fabricate the pontic should provide
good aesthetic results where needed, biocompatibility, rigidity and
strength to withstand occlusal forces, and the desired longevity.
FPDs, during mastication or parafunction, may impinge upon the
gingiva and also the veneering material may fracture. In the
fabrication of metal-ceramic FPDs, the porcelain on the occlusal
surfaces should be carefully evaluation. Porcelain is a brittle
material and may fracture easily.
When a metal-ceramic restoration is chosen, it is of
paramount importance to design the metal substructure properly if
flexure and porcelain fracture is to be avoided. Occlusal contacts
should not fall on the junction between metal and porcelain during
centric and eccentric contacts.

6) Biocompatibility
Glazed porcelain is generally considered to be the most
biocompatible of the available pontic materials and clinical data
tends to support this opinion, although the critical factor seems
to be the material’s ability to resist accumulation of plaque
rather than the material itself. Highly glazed porcelain is
relatively easy to clean, making plaque removal from it easier
than from other materials. For ease of plaque removal, it is
recommended that the tissue surface of the pontic be made in
glazed porcelain whenever possible.
Well-polished gold is smoother, less prone to corrosion, and
less retentive of plaque than an unpolished or porous casting.


7) Occlusal forces
Reducing the buccolingual width of the pontic by as much
as 30% has long been suggested as a means of lessening occlusal
forces on abutment teeth. Narrowing the occlusal table may
actually impede or even preclude the development of a
harmonious and stable occlusal relationship. Like a malposed
tooth, it may cause difficulties in plaque control as well as fail to
provide proper cheek support. For these reasons, pontics with
normal occlusal widths are generally recommended.
Mechanical failure of the pontic may occur because of
inadequate strength. Thus an all-porcelain occlusal pontic should
never be used unless the bite is favourable.


8) Compromised metallic substructure
Causes
a. Limited edentulous space occluso-cervically due to supraeruption of opposing tooth.
b. Limited space mesiodistally due to migration or drifting of
adjacent tooth.


How to avoid:
a. The framework must provide a uniform veneer of porcelain
(approx 1.2mm). excessive thickness of porcelain contributes to
inadequate support and predisposes to eventual fracture. This is
often true in the cervical portion of an anterior pontic. A reliable
technique for ensuring uniform thickness of porcelain is to wax
the fixed prosthesis to complete anatomic contour and then
accurately cut back the wax to a pre-determined depth.
b. The metal surfaces to be veneered must be smooth and free of
pits. Surface irregularities will cause incomplete wetting by the
porcelain slurry, leading to voids at the porcelain metal interface
that reduces bond strength and increases the possibility of
mechanical failure.

c. Sharp angles on the veneering surface should be rounded.
They produce increased stress concentrations that could cause
mechanical failure.
d. The location and design of external metal porcelain junctions
need particular attention. Any deformation of the framework at
the junction can lead to chipping of the porcelain. For this
reason occlusal centric contacts must be placed at least 1.5mm
away from the junctions. Attention must be paid to excursive
eccentric contacts that might deform the metal ceramic
interface.


3. CONNECTOR FAILURE
The connector is that part of the FPD or splint that joins the
individual components (retainers and pontics) together.
Requirements of solders are their ability to resist tarnish and
corrosion, to be free flowing, to match the colour of the units to be
joined and to be strong. These factors also depend on the chemical
composition of the solder.
Casting can make a rigid connection as part of a multi unit
wax pattern or by soldering which involves the use of an
intermediate metal whose melting temperature is lower than that of
the parent metal. The parts being joined are not melted during
soldering, but they must be thoroughly wettable by liquefied solder.
Dirt or surface oxide can reduce wetting and impede successful
soldering.

Causes for connector failure
a. Improper selection of connector
b. Thin metal at the connector
c. Incorrect selection of solder
d. Solder gap – narrow or wide
e. Porosity
f. Insufficient metal around
g. Defective occlusal contacts over thin connectors


III GINGIVAL AND
PERIODONTAL PROBLEMS


Margins are one of the most important and weakest links in
the success of FPD restorations. One of the prime goals of
restorative therapy is to establish a physiologic periodontal health.
A successful prosthesis depends on a healthy periodontal
environment and periodontal health depends on the continued
integrity of the prosthodontic restoration.
All displacement techniques have the potential damage
gingiva, attachment apparatus and bone, especially if anatomic
forms are weak or if disease is present.
In healthy patients, properly used cord displacement or
copper band methods have proved to be atraumatic.


The margin is one of the components of the cast restoration
most susceptible to failure, both biologically and mechanically.
Most of the investigative proof shows that supragingival margins
are kinder to the gingiva than are subgingival margins. However,
practicality dictates that supragingival margins are not always
usable

There are three locations in which to prepare crown margins:
 Supragingival
 At the crest of the gingiva
 Subgingival


SUPRAGINGIVAL Vs SUBGINGIVAL MARGINS:
Whenever possible, the margin of the preparation should be
supragingival. Subgingival margins of cemented restorations have
been identified as a major factor in periodontal diseases, particularly
where they encroach on the epithelial attachment. Supragingival
margins are easier to prepare accurately without trauma of the soft
tissues. They can usually also be situated on hard enamel, whereas
subgingival margins are often on dentin or cementum.


SUPRAGINGIVAL MARGINS
ADVANTAGES:
 They can be easily finished
 They are more easily cleaned
 Impressions are more easily made, with less potential for soft
tissue damage
 Restorations can be easily evaluated at recall appointments
DISADVANTAGE:
 Aesthetically not indicated for anterior region
 Metal can be seen
 Not indicated in short clinical crowns
 The proximal contacts extend to the gingival crest
 In case of root sensitivity

SUBGINGIVAL MARGINS
SPECIFIC DEMANDS FOR SUBGINGIVAL MARGINS:
 Aesthetic demands
 Caries removal
To cover existing subgingival restorations
 To gain needed crown length
 To provide more favourable crown contour
DISADVANTAGES:
 Difficult for preparation
 Gingival management should be perfect
 Prone for soft tissue trauma
 More prone for gingival and periodontal pathosis
 Difficult to maintain oral hygiene
 Metal margins can be seen thru the gingiva

SOFT TISSUE PORBLEMS: GENERALIZED (Not due to bridge)
LOCALISED (May be due to
bridge)
Causes for soft tissue problems:
 Over / under contouring
 Narrow embrasures
 Over / under extended crowns
 Pressure of pontic over tissue
 Loss of contact
 Horizontal food impaction due to plunger cusp in the opposing
arch
 Marginal ridges at different levels
 Wide occlusal table
 Trauma from occlusion
 Parafunctional habits
 Acrylic facing in contact with gingiva

RESULTS OF IMPROPER CONTACT AREAS
 Cause displacement of teeth bucally, lingually, mesially and
distally.
 Exert a lifting force on the tooth when placed too high occlusally.
 Disturb the axial relation of the teeth, resulting in trauma.
 Cause rotation of the teeth.
 Cause injury to the investing structures by excessively opening or
closing the contact and interproximal embrasures.
 Disturb the coordination of the inclined planes and cusps causing
deflective occlusal contacts.
 Cause vertical or horizontal food impaction.


OVER EXTENDED CROWN
The over extended crown usually encroaches beyond the cut
of the preparation on the tooth and the excess beyond the margin of
the preparation is usually not in contact with the tooth surface. This
overhang impinges the gingival tissue, irritates and often causes
edema and proliferation of the gingival tissue, destruction of the
marginal alveolar bone and ultimate loss of the tooth. The overextension of the crown is usually due to inaccurate technique and /
or the dentists desire to ‘play safe’ by making it long enough to
cover the preparation or to extend beneath the gingival margin.


SHORT CROWN
The short crown fails to cover the cut surface of the prepared
tooth and often does not extend below the gingival margin. This
uncovered ground tooth surface is often sensitive to sweets and to
temperature changes and invites development of caries and causes
gingival irritation. Also, it is usually due to inadequate technique
and a willingness of the dentist to accept impressions that are
incomplete.
CONTOUR
The poorly contoured crown is one which may have an
excess contour that impinges on the gingival tissue and deflects food
over and away from this tissue, thereby depriving it of its normal
stimulation; or it may be under contoured and permit the impaction
of food into the gingival crevice, thereby stripping the gingival
tissue away from the tooth. Either will cause irritation of the
surrounding tissue and may lead to the loss of the tooth.

IV CARIES


CAUSES
Iatrogenic (dentists role)
 Failure to identify caries
 Incomplete removal of caries
 Rough abutment finishing margins
 Subgingival marginal placement in inaccessible areas or regions
 Burning of root dentin or cementum in electro surgical technique
(leads to damage or rough surface and causes plaque retention)
 Overhanging margins
 Rough margins of crowns or bridges
 Over contouring of the cervical thirds of crowns or bridges
prevents the physiologic too cleaning by tongue or muscles
 Marginal discrepancy
 Thick cement space in margins leads to cement dissolution.
 Narrow embrasures (inaccessibility to maintain hygiene)
 Wide connector
 Failure to motivate or educate the patient about oral hygiene

Patient role
 Systemic factors
 Xerostomia
 Due to radiation therapy
 Drug induced
 Endocrine disorders
 Epilepsy (difficult to maintain the oral hygiene)
 Rheumatoid arthritis
 Local factors
 Improper brushing and flossing
 Dietary habits
 Failure to understand importance of oral hygiene.


V PULP DEGENERATION


Supporting structures or root length may be lost owing to
periapical involvement brought about by the method of preparation, lack
of protection of the prepared abutment teeth during construction, hidden
caries, and malocclusion. The preparation of the abutment tooth and the
building of the FPD, irritation from temporary coverage, lack of
temporary coverage, or malocclusion can activate a latent, low grade pulp
infection. There is no method of discovering such pulp conditions and
discomfort or pulp degeneration may occur due to such infections.
A pulp may degenerate because of too rapid preparation of the
tooth or because of improper cooling during preparation. Teeth
unprotected during the construction of a FPD are exposed to saliva and the
resulting irritation.
Pulp reactions to various procedures
Each step in full crown preparation presents hazards, which may
injure the pulp. The result may be pulpitis or even necrosis. Among the
many essential procedures that may cause pulp injury are tooth
preparation, impression making, temporization and cementation. In
general, heat desiccation and / or chemical injury cause the insult.

VI BIOMECHANICAL FAILURE


Every restoration must be able to withstand the constant
occlusal forces to which it is subjected. This is of particular
significance when designing and fabricating an FPD, since the forces
that would normally be absorbed by the missing tooth are transmitted
through the pontic, connectors and retainers to the abutment teeth.
The abutment teeth are therefore called upon to withstand forces
directly to the missing teeth in addition to those usually applied to
them.
In addition to increased load placed on the periodontal
ligament by a long span FPD, longer spans are less rigid. Bending or
deflection varies directly with the cube of the length and inversely
with the cube of the occluso-gingival height of the pontic. Compared
with the FPD having a single tooth span, a two-toothed pontic span
will bend eight times as much, and a three-toothed pontic will bend
27 times as much.

Owing to the fact that forces are being applied though the
pontics to the abutment teeth, the forces on the castings serving as
retainers are different in magnitude and direction from those
applied to a single restoration. The dislodging forces on an FPD
retainer tend to act in a mesio-distal direction as opposed to the
common bucco-lingual direction of forces on single restorations.
Preparations should be modified accordingly to produce greater
resistance and structural durability e.g. grooves on buccal and
lingual surfaces. Double abutments are sometimes used as a means
of overcoming problems created by unfavourable crown-root ratios
and long spans. A secondary abutment must have at least as much
root area and as favourable a crown-root ratio as the primary
abutment it is intended to bolster.

VII ESTHETIC FAILURES


REASONS FOR ESTHETIC FAILURE
 Failure to identify patient expectations regarding esthetics
 Improper shade selection
 Excessive metal thickness at incisal and cervical regions
 Thick opaque layer application
 Surface blistering (chalky appearance)
 Over glazing or too smooth a surface
 Metal exposure in connector, cervical and incisal regions
 Dark space in cervical third due to improper pontic selection
(anteriors)
 Failure to produce incisal and proximal translucency
 Improper contouring
 Failure to harmonize contra lateral tooth morphology
 Contour
 Color
 Position
 Angulation
 Discoloration of facing

CONCLUSION


Failures in FPD construction for the most part is due to
attempted short cuts or positive indifference and inexcusable
ignorance on the part of those concerned with building the
prosthesis. Also a FPD can just wear out and this cannot be called
as failure and no lifetime guarantee can be given.
Failures most often occur because of violation of principles
either collectively or individually. This may be due to reactions of
the soft tissue and reactions of the abutment.
It is better to speak of the level of acceptability to the
patient and the dentist and consider what needs to be done to
improve the treatment. The fundamentals of fixed prosthodontic
therapy modality have to be followed strictly, failure of which will
lead to the failure of the prosthesis itself.

REVIEW OF LITERATURE


1. I. Gerson (1957) – stated that inadequately cemented
restorations may cause an increased vertical dimension,
loosening of the crown or FPD after a relatively short time,
microleakage and decay under the abutments, exposure of metal
margins and sensitivity.
2. Kenneth C. Pruden (1957) – stated that the multiple abutments
should be resorted to only where the added support or root
length is needed, and only periodontally sound teeth should be
included.
3. Leonard I. Linkow (1962) – gave the importance of form, type,
shape and position of contact areas. He stated that the contacts
which are flat, open, improperly placed, rough, or poorly
polished will lead to displacement of teeth and exert a lifting
force, disturb, the axial relationship, injure supporting tissues,
produce a deflective occlusal contact and vertical or horizontal
food impaction.

4. Marvin Reynolds (1964) – reviewed the properties of porcelain,
acrylic resin and gold as related to their use in pontics. He stated
that acrylic resin does not permit the precise control of color and
light refraction as it absorbs oral fluids and is less resistant to wear
and abrasion. Gold alloys provide the best strength to withstand the
stresses of occlusion and resistance to wear. Porcelain is superior to
other materials as it maintains the color, is resistant to abrasion and
is dimensionally stable and insoluble to oral fluids.
5. Richter et al (1970) – studied the tensile strength and compressive
strength of cements. The results showed the tensile strength of zinc
phosphate, hydrophosphate and ZnOE cements are equal,
carboxylate cements had less strength properties. Compressive
strength is lowest with carboxylate and highest with zinc phosphate.
6. Guy M Newcomb (1974) – investigated the location of subgingival
margins and related that to gingival inflammation. The results
showed that the least inflammation is observed when subgingival
crown margins are placed at the gingival crest or just into the
gingival crevice.

7. Brule et al (1981) – conducted a study on placement of margins in
anterior veneer crowns. He concluded that the subgingival margin
placement of anterior crown for esthetics might be unnecessary
except in some cases like in short clinical crowns.
8. Abraham Revah et al (1985) – discussed the problems with tilted
posterior tooth in relation with path of insertion. He advised the
mesial half of the crown be with parallel seating grooves for better
seating and retention.
9. Aaron H Wilson et al (1994) – examined the relationship between
degree of convergence or a machined metal die and retention of its
casting. The retention was found to increase from 0 degree
convergence to a peak between 6 -12 degree convergences.
Convergence angles of less than 6 degrees are not advisable as it
causes incomplete seating of the crown due to increase in
hydraulic pressure in between the crown and cement.

10. Paulo Baldissara et al (1998) – conducted a comparative
study about the marginal microleakage of six cements in
fixed provisional crowns. Results showed Zinc phosphate
and cavity bas compound cements had the best sealing
properties than other cements.


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