Principles of Tooth Preparation for Restorations

PRINCIPLES
OF
TOOTH PREPARATION
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

CONTENTS
 INTRODUCTION
 BROAD CATEGORIES OF PRINCIPLES
 BIOLOGICAL CONSIDERATIONS
 MECHANICAL CONSIDERATIONS
 ESTHETIC CONSIDERATIONS
 FEATURES OF DIFFERENT TYPE OF PREPARATIONS
 ENDODONTICALLY TREATED TEETH
 SUMMARY AND CONCLUSION
 REFERENCES

INTRODUCTION
Teeth - no regenerative ability
Restorative material must be used if….
Teeth require preparation to receive restoration
These preparation must be based on fundamental
principles from which basic criteria must be developed to
help predict the success of prosthodontic treatment
Careful attention to every detail is imperative during tooth
preparation

3 Broad categories :
1. BIOLOGIC CONSIDERATIONS,
which affect the health of oral tissues
2. MECHANICAL CONSIDERATIONS,
which affect the integrity and durability of
the restoration
3. ESTHETIC CONSIDERATIONS,
which affect the appearance of the patient

BIOLOGIC
Conservation of tooth
structure
Avoidance of
overcontouring
Supragingival margins
Harmonious occlusion
Protection against tooth
fracture
MECHANICAL
Retention form
Resistance form
Deformation
ESTHETIC
Minimum display of metal
Maximum thickness of
porcelain
Porcelain occlusal surfaces
Subgingival margins
Best combination of compromises

BIOLOGIC CONSIDERATIONS
1. Prevention of damage during tooth preparation
-Adjacent teeth
-Soft tissue
-Pulp
2. Conservation of tooth structure
3. Avoidance of overcontouring
-Axial reduction
4. Marginal integrity and preservation of periodontium
-Margin location
-Margin adaptation
-Margin geometry or Finish line configuration
5. Occlusal considerations
6. Preventing tooth fracture

Prevention of damage during tooth preparation
Adjacent teeth - iatrogenic damage
- damaged proximal contact
- To avoid
Soft tissues - tongue and cheeks
- careful retraction
Pulp - excessive temperature
- chemical irritation
- microorganisms

Conservation of tooth structure
 More the remaining dentin – lesser the pulpal damage
 Must be considered relative
 GUIDELINES:
1. Partial coverage
2. Minimum taper

3. Planar occlusal reduction
4. Even reduction of axial surfaces

5. Selection of conservative margin
6. Avoidance of unnecessary apical extension of
preparation

Avoidance of overcontouring
A crown should duplicate the contours and profile of the
original tooth
Axial reduction
Undercontoured restoration is better than overcontoured
restoration
Anterior teeth, increasing the proximal contour is better

Marginal integrity and preservation of
periodontium
Margin location-
Biologic width
It’s the dimension of space that the healthy gingival tissues occupy
above the alveolar bone.
It refers to the combined connective tissue-epithelial attachment from
the crest of the alveolar bone to the base of the sulcus(2mm;connective
tissue-1.07mm and epithelium-0.97mm).
Evaluation of the biological width
Radiographs
Probing
Sounding of bone
Violation of biological width:-

Nevins and Sukrow in 1984 recommended that for the
maintenance of healthy periodontium, no restoration
should violate the attachment ,even though its not possible
for a clinician to identify the most coronal extent of the
junctional epithelium.
Wilson and Majnard cautioned against extending
restorations so far subgingivally that the attachment
complex is damaged.They stated that “Some distance of
unprepared tooth structure should remain between the
finished line and junctional epithelium and this distance
should be ideally 0.5mm.

Location of restorative margins:-
 Supragingival
 Equigingival
 Subgingival

Supragingival margin:-
 Least traumatic to the soft tissues
 Margin placement…
 Easily finished
 Impressions are easily made
 Most accessible for cleansing
 Restorations can be easily evaluated

Subgingival margins:-
 Clinical crown is short…
 Old restorations with subgingival margin
 Esthetic appearance
 Caries, abrasion or erosion…
 Root sensitivity
Disadvantages of subgingival margins
 Plaque retention…
 Margin placement…
 Mechanical irritation…
 Violation of biological width
 Soft tissue injury…

Criteria for subgingival margin placement:-
 Emergence profile
 Margins are closed and properly finished
 Adequate band of attached gingiva
 Margin should not violate the biological width

Crown lengthening procedures:-
It’s a procedure similar to the apical repositioning of the flap
with concomitant osteoplasty.
Indications –
 Short clinical crown
 Restoration impinge on the biological width
 Hopless teeth with extensive subgingival caries, subgingival
fracture and root perforation
Crown lengthening procedure -
 Surgical methods
 Orthodontic method

D.A.Felton in 1991 conducted a study on “Effects of in vivo
crown margin discrepancies on the periodontal health” in
his study he strongly supported the placement of
supragingival margins for artificial crowns and FPD’s.
William.G.Reeves in his review article concluded that more
supragingivally a restorative margin is placed,the less
chance that the margin will contribute to gingival
inflammation.

MARGINAL GEOMETRY
OR FINISH LINE CONFIGURATION
shape and bulk of the restorative material in the margin of
the restoration
Marginal adaptation
Degree of seating of restoration
Guidelines for evaluation for margin design :
 Ease of preparation without overextension or unsupported
enamel
 Ease of identification in the impression and on the die
 Sufficient bulk of the material
 Conservation of the tooth structure
 A distinct boundary to which wax pattern can be finished

Finish lines
 The finish line is the peripherel extension of a tooth preparation
 The most important consideration in selecting a cervical margin design
is its ability to consistently and predictably provide excellent marginal
integrity.
 Knife edge
 Chisel edge
 Chamfer
 Shoulder
 Sloped shoulder
 Shoulder with bevel
 Radial shoulder
 Heavy chamfer

KNIFE EDGE OR FEATHER EDGE
Advantages
 Conservation of the tooth structure
 Permits an acute margin of the metal
Disadvantages:
 The axial reduction may fade out
 Thin margin may be difficult to accurately wax and cast
 More susceptible to distortion
 Results in overcontouring
Indications:
 Not recommended

CHISEL EDGE
Variation of feather edge
Larger angle between the axial surface and
unprepared tooth structure
Associated with excessively tapered preparation
Historic advantage-impression making with rigid
impression compound in coper bands
Indications:
Lingual surface of mandibular posterior teeth
The surfaces towards which tooth has tilted
Cementum

Chamfer
It’s a finish line design for tooth preparation in which gingival aspect
meets the external axial surface at an obtuse angle
Advantages
 It provides distinct margin
 Adequate bulk to the restoration
 Easier to control
 Exhibits least stresses
 Disadvantage -Care needed to avoid unsupported lip of enamel
Indications:
 Cast metal restorations
 Lingual margin of the metal ceramic crowns

Shoulder
 Bulk of the restoration
 Wide ledge provides resistance to occlusal forces
 Provides space for healthy restorative contours
 Maximum esthetics
 It offers resistance against distortion during processing
Disadvantages:
 Less conservative of tooth structure
 The sharp 900
internal line angle…
Indications:
 All ceramic restorations
 Facial magins of metal-ceramic crowns

SLOPED SHOULDER
Cavosurface margin - 120°
Reduces the possibility of leaving unsupported enamel and
yet leaves sufficient bulk to allow thinning of the metal
framework to a knife edge for acceptable aesthetics.
Indicated for facial margin of metal ceramic crowns

SHOULDER WITH BEVEL
Removes unsupported enamel, allows finishing of metal
Disadvantage – extends the preparation into the sulcus if
used on apical margin
Indications –
-facial margins of maxillary partial coverage restorations
- Inlay and onlay margin
-Shoulder is already present because of destruction by
caries or presence of previous restorations

RADIAL SHOULDER:
Shoulder with rounded internal line angle
Stress concentration is less in the tooth structure
HEAVY CHAMFER
Internal line angle is large radius rounded
Provides better support for a ceramic crown than chamfer,
but it is not as good as shoulder
Easier to prepare than shoulder

MARGINAL ADAPTABILITY
The restoration can survive in the biological environment of
the oral cavity only if the margins are closely adapted to the
cavosurface finish line
Junction between a cemented restoration and tooth….
More accurate the adaptation – lesser the chance of
recurrent caries
A well designed preparation should have a smooth and even
margins
Rough, irregular and stepped junctions ……

TO BEVEL OR NOT TO BEVEL4
Controversial
Trignometric analysis-
more acute the bevel-
-lesser the marginal discrepancy
-lesser the seating with cement
So, bevel improves the marginal adaptation but
reduces the seating of restoration
Empirical clinical results dictate that acute margin
of metal should continue to be used on metal
restorations but that angle should be in 30-45°

OCCLUSAL CONSIDERATIONS
Occlusal scheme should not be traumatic
Tooth preparation should allow sufficient space
for developing a functional occlusal scheme in
the finished restoration
Supraerupted or tilted teeth – reduce the teeth
Sometimes even the endodontic treatment is
necessary to make enough room
Careful judgment is needed and diagnostic tooth
preparation and waxing procedures are essential
to determining the exact amount of reduction
required to develop an optimum occlusion

PREVENTING THE TOOTH FRACTURE
The likelihood that a restored tooth will fracture can be
lessened if the tooth preparation be designed to minimize
the potentially destructive stresses
Inlay- greater potential for fracture
Onlay – lessens the chance of fracture
Complete crown –greatest protection against fracture

MECHANICAL CONSIDERATIONS
1. RETENTION FORM
2. RESISTANCE FORM
3. STRUCTURAL DURABILITY

RETENTION FORM
DEFINITION-The feature of a tooth preparation that
resists dislodgement of a crown in a vertical direction or
along the path of placement.
In practice retention and resistance are closely related and
they are not always clearly distinguishable.
Only dental caries and porcelain failure outrank lack of
retention as a cause of failure of crown and FPD

FACTORS AFFECTING RETENTION
Magnitude of dislodging forces
Geometry of tooth preparation
-Taper
-Surface area
-stress concetration
-Type of preparation and secondary retentive features
Roughness of surfaces being cemented
Materials being cemented
Type of luting agent
Film thickness of luting agent

MAGNITUDE OF DISLODGING FORCES
small compared to those that tend to seat or tilt it
 by pulling the FPD with floss under the connectors,
 when exceptionally sticky food is eaten
Depends on the stickiness of food and the surface area and
texture of the restoration being pulled

GEOMETRY OF TOOTH PREPARATION
Essential elements of retention:-
opposing vertical surfaces in same preparation.
Path of insertion
Area under shear stress
Freedom of displacement
FACTORS
Taper
Surface area
stress concetration
Type of preparation and secondary retentive features

Opposing vertical surfaces in same preparation:
1) External surfaces:- eg. Buccal & lingual walls of
full veneer crown.
An extracoronal restoration is an example of
veneer, or sleeve retention.
Scan0001.jpg

2) Internal surfaces:- eg. Buccal & lingual walls of the
proximal box of a proximo-occlusal inlay.
An intracoronal restoration resists displacement by
wedge retention.

PATH OF INSERTION:-
It is an imaginary line along which the restoration will
be placed onto or removed from the preparation.
It is of special importance when preparing teeth to be
fixed partial denture abutments, since the paths of all
the abutment preparations must parallel each other.
Surveying visually, since it is the primary
means of insuring that the preparation is neither
undercut nor over-tapered.

The path of insertion must be considered in two
dimensions:-
- Faciolingually
- Mesiodistally
The faciolingual orientation of the path can affect
the esthetics of metal-ceramic or partial veneer
crowns.

The mesiodistal inclination of the path must parallel
the contact areas of adjacent teeth.
So if path is inclined mesially or distally, the
restoration will be held up at the proximal contact
areas & be “locked out”.

Area under Shear Stress:
 Most important feature for retention is that the total
surface area of cement which will experience shearing
rather than tensile stress.
 To achieve this the preparation must have opposing
walls nearly parallel to each other.

To obtain the greatest area of cement under shear
stress, the direction in which a restoration can be
removed must be limited to one path.

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

TAPER
The axial walls of the preparation must taper
slightly to permit the restoration to seat.
i.e. 2 opposing external walls must gradually
converge – ANGLE OF CONVERGENCE.
2 opposing internal surfaces of the tooth structure
must diverge occlusally – ANGLE OF
DIVERGENCE.
 The relationship of one wall of a preparation to the
long axis of that preparation is the INCLINATION
of that wall.

The more nearly parallel the opposing walls of a
preparation, the greater should be the retention.
Most retentive preparation should be one with
parallel walls, but the parallel walls are impossible to
create in the mouth without producing preparation
undercuts.
An UNDERCUT is defined as a divergence between
opposing axial walls, or wall segments, in a
cervical- occlusal direction.

Tooth preparation taper should be kept minimal
because of its adverse effects on retention.
ed taper ed retention
ed taper ed retention.
 Jogensen said as retention decreases taper increases.

Ideal taper should
be within the range
of 2-6.5°.
A taper of 6° have
been proposed as
being clinically
achievable while
affording adequate
retention.

William A. Kent, Herbert T. Shillingburg,
Manville G. Duncamon, Quintessence Int. vol. 19,
1988. - Conducted a study to evaluate the degree of taper
of 418 dies of preparations, cut over a time span of 12 years.
The dies were categorized by location and type of
preparation. The degree of taper of six opposing sets of
surfaces were measured and compared. They concluded the
mean of tapers of preparations ranged from 8.6 – 26.6°.
Internal features such as grooves and boxes have a lower
degree of taper compared with external axial walls and
preparations in the anterior segment have a lower degree of
taper than in posterior segment.

 Jeffrey Nodlander, Dennis Weir, Warren Stoffer
and Shigaro Ochi, JPD, vol. 60, 1988. – Conducted a
study to measure the convergence angles of a full coverage
preparations performed in a clinical environment. Teeth
were prepared by the participants to attempt a 4-10°
convergence angle. The convergence angles of all
preparations were determined by projecting the faciolingual
and mesiodistal silhouttes of the dies with an overhead
projector. They concluded that, the ideal convergence angle
of 4-10° is seldom achieved. The convergence angle for
mandibular preparations were greater than maxillary.
Auxillary retention should be used in molar region because
these preparations were found to have larger convergence
angle.

SURFACE AREA
Provided the restoration has limited path of withdrawal,
Greater the surface area of a preparation, greater is its
retention.
Length…..
Crowns with long axial walls are more retentive…
Molar crowns are more retentive than premolar crowns of
same taper

STRESS CONCENTRATION
If line angle between axial and occlusal surface is sharp, it
leads to concentration of stresses around that junction
Induced stresses exceeds the strength of the cement
Leads to cohesive failure of cement
Rounding the internal line angles reduces the stress
concentration and thus increases the retention of
restoration

TYPE OF RESTORATION AND SECONDARY RETENTIVE
FEATURES
Full veneer crown has excellent retention when compared
to partial veneer crown because reducing the path of
insertion to a narrow range.

If axial wall left unveneered, retention is achieved by
substituting a grooves, boxes or pinholes for the missing
wall.
• Secondary retentive features doesnot significantly affect the
retention because the surface area is not increased significantly.
• But where these features limits the path of withdrawal, retention
is increasedwww.indiandentalacademy.com

Surface roughness:
 Adhesion of dental cements depends primarily on
projections of the cement into microscopic
irregularities.
 Jorgensen found retention of castings cemented with
ZnPO4 cement on test dies with a 10° taper to be twice
as great on preparations with 40µm scratches than
10µm.
 Retention increases when restoration is roughened or
grooved.
 Failure rarely occurs at the cement tooth interface. So
roughening tooth preparation hardly influences
retention.

Materials being cemented:
 Retention is affected by both the casting alloy and the core
material.
 More reactive the alloy is more adhesion.
 Base metal alloys are better retained than less reactive high
gold content metals.
Type of luting agent:
 Studies show that adhesive resin cements are more retentive
than compared to conventional ZnPO4 and GIC cements.

Film thickness of the luting agent.
Conflicting evidences
Studies showed that increased thickness of the cement film
will have some effect on a restoration.
This may be important if a slightly oversized casting is
made using die spacers.

Stanley G. Vermilyea, Michael J. Kuppler, Eugen
F. Hugger, JPD, 50, 1983. – Conducted a study to
determine the influence of die relief agent on the
retention of cast restorations using 3 cementing
mediums. They concluded that forces required to
dislodge unspaced castings were higher when ZnPO4 as
luting medium. Die relief agents resulted in a 32%
reduction in the dislodging forces.
Steven M. Carter, Peter R. Wilson, IJP, vol. 9,
1996.- Conducted a study to see the effect on die spacing
on pre and post cementation crown removal and crown
elevation. Different layers of die spacer was used. They
observed that the force required to remove a crowns
before cementation decreased with increased layers of die
spacers. Following cementation the mean crown
elevation decreased and the removal force is increased.

RESISTANCE FORM
 The features of a tooth preparation that enhance the stability of
restoration and resist dislodgement along an axis other than the path of
placement (GPT).
 It prevents dislodgement of a restoration by forces directed in an apical,
oblique or horizontal direction.
 The geometric configuration of tooth structure must place the cement
in compression to provide the necessary resistance.
 Lateral forces tend to displace the restoration by causing rotation
around gingival margin
 Rotation is prevented by any areas of the tooth preparation that are
placed in compression and are called as RESISTANCE AREA

FACTORS AFFECTING RESISTANCE FORM
Magnitude and direction of dislodging forces
Leverage
Length of the preparation
Width of the preparation
Taper
Type of preparation
Rotation about vertical axis
Physical properties of luting agent

Magnitude and direction of dislodging forces
Normal occlusion - axially directed forces
Habits (pipe smoking and bruxing) – large oblique forces to
restoration
Resistance decreases in following order:
- normal occlusion
- habits
- eccentric interferences
- anterior guidance

LEVERAGE AND RESISTANCE:
 Leverage occurs when the line of action of a force passes
out side the supporting tooth structure.
 If the force passes within the margin of a crown no
tipping of the restoration when compared to the line of
action passing outside the margins of the restoration

Forces are outside the margin in the following cases:
- wide occlusal table of restoration
- crowns on tipped teeth
- retainers for cantilever bridge
- force at an oblique angle
Fulcrum point – the point on margin that lies closest to
the line of action
Lever arm – the closest distance between line of action
and fulcrum
Torque - force × lever arm
In equilibrium, this torque is balanced by the sum of all
the resisting forces ( tensile, shear, compressive)
The farther these resisting forces lie from the fulcrum,
greater is their mechanical advantage

If a line drawn from the center of rotation perpendicular to
the cement film on the opposite wall of the preparation the
point where the line intersects the cement film is known as
tangent point.

If the tangent points of all the arcs of rotation around a
given axis are connected they form the tangent line. The
area above the tangent line is resisting area.
To have effective resistance the tangent line should extend
atleast halfway down the preparation.

Preparation length and resistance:
-Shortening of preparation
will reduce the resistance area.
-In short crown lifting force is
small when compared to long
crown.
-Small restoration is less
likely to fail through tipping
than long restoration (on
preparation of equal length).

Resistance and tooth width:
 A wider preparation has a greater retention but a
narrow tooth can have greater resistance to
tipping.
Because of smaller diameter
a tangent line falls low on the
wall opposite to axis of
rotation. Resulting in a large
resisting area.
Weak resistance can be
enhanced by placing
vertical grooves/ boxes/
pinholes.www.indiandentalacademy.com

A grooved lingual wall must be distinct and perpendicular
to the axial wall.
So U- shaped grooves or flared boxes provides more
resistance than V-shaped ones

Taper and Resistance:
 More tapered a preparation less is resistance.
No taper the resisting area cover half
the axial wall.
Ideal taper < ½ the axial wall.
Over tapered small resisting area
near the occlusal surface.

William W. Dodge, Roger M. Weed,
Ramon J. Baez and Richard N. Buchanan
(Quintessence Int, Vol.3 1985) – conducted
a study to compare the effect of varying the
convergence angle on retention and resistance
in complete veneer crown preparations. 15
stainless steel dies were machined with 10°, 16°
and 22° taper. They concluded that resistance is
more sensitive to changes in a convergence
angle than retention form. There is no
significant difference in retention values
between preparations with 10° and 16°. 16° of
occlusal convergence was considered adequate
for retention and resistance.

 Permissible taper of a preparation is directly proportional to height : width ratio.
 Taper that permit an effective resisting area for a preparation in which height
equals width is double than in a preparation where height is only half width.

Rotation around a vertical axis:
A partial veneer
crown which has no
grooves offer little
resistance to rotation.
Axial symmetry
of a full veneer
crown preparation
may allow
rotation of the
restoration.

TYPE OF PREPARATION
Partial coverage restoration may have less resistance than a
complete crown because it has no buccal resistance area
PHYSICAL PROPERTIES OF LUTING AGENT
Resistance to deformation is affected by physical properties
of the luting agent, such as compressive strength and
modulus of elasticity
Adhesive resin >GIC > ZnPO4> Polycarboxylate > ZOE

STRUCTURAL DURABILITY
A restoration must have sufficient strength to prevent
permanent deformation during function
DEFINITION- “The ability of a restoration to withstand
destruction due to external forces is known as structural
durability”.

Factors affecting structural durability
1. Adequate tooth reduction.
- occlusal reduction
- functional cusp bevel
- axial reduction
2. Alloy selection.
3. Metal-ceramic framework design.
4. Margin design

ADEQUATE TOOTH REDUCTION
OCCLUSAL REDUCTION:-
An important feature for providing adequate bulk of metal &
strength to the restoration is occlusal clearance.
- Occlusal thickness varies with different restorative materials
Gold alloys – 1.5mm (FC) & 1mm(NFC)
Metal-ceramic crowns- 1.5-2mm(FC)&1-1.5mm(NFC)
All-ceramic crowns – 2mm of clearance on preparation

- Firstly, opposing occlusal equilibration is to be achieved
eg. Plunger cusps to be rounded.
- Round line and point angles, avoid deep grooves in the
center of the occlusal surfaces to prevent stress
concentration and to distribute the forces over a larger
surface area.
- Uniform and planar occlusal reduction.

This ensures:
-sufficient occlusal clearance
- preservation of tooth structure
- gives rigidity to crown because of
CORRUGATED EFFECT OF PLANE
OCCLUSAL OFFSET can be given on posterior
partial veneer crown preparation to provide
space for a TRUSS of metal to form reinforcing
strap
INCISAL OFFSET can be given on anterior
partial veneer crown preparation to provide
space for metal that helps to strengthen the
lingual-incisal margin

FUNCTIONAL CUSP BEVEL:-
It is an integral part of the occlusal reduction.
A wide bevel on the lingual inclines of the maxillary lingual
cusps & the buccal inclines of the mandibular buccal cusps
provides space for an adequate bulk of metal in an area of
heavy occlusal contact.

If a wide bevel is not placed on the functional cusp,
several problems may occur :
- If the crown is waxed & cast to normal contour it can
cause a thin area or perforation in the casting.
- To prevent this the crown may be waxed to optimal
thickness resulting in overcontouring & poor occlusion.

- If an attempt is made to obtain space for an
adequate bulk in a normally contoured casting
without a bevel, it will result in over inclination of
the buccal surface which will destroy excessive tooth
structure while lessening retention.

AXIAL REDUCTION:-
It plays an important role in securing space for an
adequate thickness of restorative material.
Inadequate axial reduction can cause thin walls & a
weak restoration subjected to distortion or a
bulbous, overcontoured restoration which will
strengthen the restoration but may have a disastrous
effect on periodontium.

Other features that provide space for metal and
improve the rigidity & durability of the restoration
are:
The offset, the occlusal shoulder, the isthmus,
the proximal groove & the box.

Selection of the alloy:
-It is essential that there be sufficient clinical evidence of
superiority, before selecting a particular material.
-Type I and II gold alloys…

Factors considered when selecting an alloy:
1.Intended use:
Traditionally alloys for casting were classified on the basis of their
intended use-
-Type I: Simple inlays
-Type II: Complex inlays
-Type III: Crowns and fixed partial dentures
-Type IV: Removable partial dentures and pin ledges.
Porcelain: metal-ceramic alloys.
2. Physical properties: FDI (1965) classified casting alloys according to
their physical properties as:
Type I: Soft
Type II: Medium
Type III: Hard
Type IV: Extra-hard

3. Color: The patients view on the subject should be sought if
the metal will be visible in the mouth; otherwise the color is
irrelevant.
4. Composition: The percentage composition by weight of the
main ingredients must be mentioned.
5. Cost.
6. Clinical performance:
A) Biologic properties
-Gingival irritation.
-Recurrent caries.
-Plaque accumulation.
-Allergies.
B) Mechanical properties:
-Wear resistance and
strength.
-Marginal fit.
-Ceramic bond failure.
-Connector failure.
-Tarnish and corrosion.www.indiandentalacademy.com

7. Laboratory performance:
Factors like casting accuracy, surface roughness,
strength, metal-ceramic bond strength should be
considered.

Choice of material:
Gold:
Indications-
1.In situations of severe occlusal stress.
2.Following endodontic treatment of posterior teeth.
3.Full or partial coverage of posterior teeth where there has
been significant loss of coronal dentin.
4.For restoration of adjacent or opposing teeth to avoid
problems arising from use of dissimilar metals.
Contraindications:
-Aesthetics
--Cost

Porcelain (Ceramic):
Indications:
- Large inadequate restorations on the
anterior teeth provided there is enough
tooth substance.
- Severely discolored teeth.
- Over an existing post and core
substructure.
Contraindications:
-Teeth with short clinical crown
-Edge to edge occlusion
-Teeth which do not allow ideal preparation
form to support the porcelain.

Metal ceramic:
Indications:
- Esthetics
- Failure of porcelain jacket crowns.
-Posterior teeth where esthetics is necessary and partial
coverage gold crowns are contraindicated
Contraindications:
-Young patients at risk of pulp being exposed
-large pulp chamber
-Traumatic occlusion (heavy occlusal forces) , where wear of
opposing occlusal surfaces is expected.

Metal ceramic framework:
-A metal occlusal contact requires 1-1.5mm of reduction
-A porcelain contact requires 2mm of reduction
-Occlusal contacts need to be 1.5-2.0mm from the porcelain-
metal junction
-The substructure must support an even thickness of the
porcelain veneer (1mm minimum and 2mm maximum).
-The minimum thickness of the metal is 0.2-0.3mm.
-Cut-back: porcelain-metal junction should be 90° or greater.
-Metal should preferably be in the area of the centric stop to
enhance the durability of the restoration.

MARGIN DESIGN
- distortion of restoration margin is prevented by
designing the preparation outline to avoid
occlusal contact in this area
- tooth reduction should provide sufficient room
for bulk of metal at the margin to prevent
distortion

ESTHETIC CONSIDERATIONS
METAL-CERAMIC RESTORATION
- Usually poor appearance is due to insufficient porcelain thickness
- In addition, the labial margin of metal ceramic crown is not always
accurately placed
- To correct all these deficiencies, certain principles are recommended
during tooth preparation
FACIAL REDUCTION
- Adequate thickness of porcelain is needed to create a sense of color
depth and translucency
- Adequate reduction  sufficient bulk of porcelain for appearance
and metal for strength
- Minimum reduction of 1.5 mm is required
- Shade problems incisal and cervical 3rd
of restoration (here direct
light reflection from the opaque layer can make the restoration
appear very noticeable

- Opaque porcelain generally have a different shade from body
porcelain, they often need to be modified with special stains in these
areas.
- with very thin teeth like mandibular incisors……
INCISAL REDUCTION
- Incisal edge  no metal backing. So, it can be made with a
translucency similar to that of natural tooth structure
- reduction  2 mm
- Excessive reduction – reduces retention and resistance
PROXIMAL REDUCTION
- Esthetics depends on exact location of the metal ceramic junction in
complete restoration
- Proximal surface of anterior tooth will look most natural if they are
restored as the incisal edges without metal backing.
- In FPD, connectors makes it impossible

LABIAL MARGIN PLACEMENT
- Should follow the contour of gingiva
- High lip line  sub gingival margin
no discoloration of root surface – supragingival
porcelain margin
- Low lip line  metal supragingival collar
- Metal collars can be hidden below the gingival crest

PARTIAL COVERAGE RESTORATION
- No restorative material can achieve the appearance of
intact tooth enamel
- Esthetic depends on accurate placement of potentially
visible facial and proximal margins
PROXIMAL MARGIN
- Mesial margin  buccal to contact area
- Distal margin  can be extended beyond the contact
point
- Tooth preparation angulation long axis of posterior
teeth and incisal 2/3rd
of
facial surface of anterior

FACIAL MARGIN
- Maxillary  margin should be extended just beyond the
occlusofacial line angle
- If buccal margin is correctly shaped following the original
cuspal contour, it will not reflect light to an observer and
tooth will appear merely a little shorter than normal
- Mandibular  metal display is unavoidable because ….
- If unacceptable to the patient  metal ceramic
restoration with porcelain coverage on occlusal surface

- Anterior partial coverage  facial margin is
extended between highest contour of the incisal
edge and incisolabial line angle

FEATURES OF DIFFERENT TYPE OF PREPARATIONS

ENDODONTICALLY TREATED TEETH
Conservation of tooth structure
Retention form
Resistance form

Conservation of tooth structure
PREPARATION OF CANAL
Only minimum tooth structure should be removed from the
canal
Root canal should be enlarged only enough to enable the
post to fit accurately yet passively while ensuring strength
and retention

PREPARATION OF CORONAL TISSUE
Coronal tooth structure should be saved as much as
possible because…..
Ferrule:
- Extension of the axial wall of the crown apical to the
missing tooth structure provides what is known as a
ferrule.
- helps to bind the remaining tooth structure together,
preventing root fracture during function

Retention form
 Preparation geometry
 Post length
 Post diameter
 Post surface texture
 Luting agent

Resistance form
 Stress distribution
 Rotational resistance

SUMMARY AND CONCLUSION
Proper attention should be given during margin placement
and the principle of “Do no harm” to the soft tissues should
be followed.
All preparations require the incorporation of factors to
prevent the dislodgement of restoration by functional
stresses.
If too much emphasis is given on any one of the principles
then the success of the procedure may be limited by a lack
of consideration of the other factors.
An analysis of these principles and factors should enable
the dentist to effectively apply them during the design of
any preparation.

REFERENCES
1. Rosensteil (2001) Contemporary Fixed Prosthodontics.
3rd
edition.
2. Shillingburg (1981) Fundamentals of Fixed
Prosthodontics. 2nd
edition.
3. Tylman (1989) Theory & Practice of Fixed
Prosthodontics. 8th
edition.
4. Shillingburg, Richard Jacobi, S.E. Brackett. (1987)
Fundamentals of Tooth Preparations for Cast metal
and Porcelain restorations.
5. Carranza’s “ Clinical periodontology” 9th edition.
6. Anusavice “Phillips science of dental materials” 10th
edition

7. William A. Richter “Relationship of crown margin
placement to gingival inflamation” JPD. 30: 156-161,1973
8. W. Kent, H.T. Shillingburg, M.G. Duncansson: Taper of
clinical preparation for cast restoration. Quint. Int. 1988;
19: 339-345.
9. J. Nordlander, D. Weir, W. Stoffer, S. Ochi : The taper of
clinical preparations for fixed prosthodontics. JPD, 1988;
60: 148-151.
10.S.M. Carter, Peter Wilson: The effect of die-spacer on
crown retention. IJP, 1996, 9: 21-29.
11.S. Vermilyea, M.J. Buffler, E. Maget : The effect of die
relief agent on the retention of full coverage castings.
1983; 50: 207-210.

Principles of Tooth Preparation for Restorations

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