The Indian Dental Academy is the Leader in continuing dental education , training dentists in all aspects of dentistry and
offering a wide range of dental certified courses in different formats.for more details please visit
www.indiandentalacademy.com
2. 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
www.indiandentalacademy.com
3. 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
www.indiandentalacademy.com
4. 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
www.indiandentalacademy.com
5. 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
www.indiandentalacademy.com
6. 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
www.indiandentalacademy.com
7. 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
www.indiandentalacademy.com
8. Conservation of tooth structure
• More the remaining dentin – lesser the pulpal damage
• Must be considered relative
• GUIDELINES:
1. Partial coverage
2. Minimum taper
www.indiandentalacademy.com
9. 3. Planar occlusal reduction
4. Even reduction of axial surfaces
www.indiandentalacademy.com
10. 5. Selection of conservative margin
6. Avoidance of unnecessary apical extension of
preparation
www.indiandentalacademy.com
11. 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
www.indiandentalacademy.com
12. 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:-
www.indiandentalacademy.com
13. • 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.
www.indiandentalacademy.com
15. 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
www.indiandentalacademy.com
16. 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…
www.indiandentalacademy.com
17. • 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
www.indiandentalacademy.com
18. 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
www.indiandentalacademy.com
19. 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.
www.indiandentalacademy.com
20. 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
www.indiandentalacademy.com
21. 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
www.indiandentalacademy.com
22. 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
www.indiandentalacademy.com
23. 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
www.indiandentalacademy.com
24. 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
www.indiandentalacademy.com
25. 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
www.indiandentalacademy.com
26. 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
www.indiandentalacademy.com
27. 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
www.indiandentalacademy.com
28. 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
www.indiandentalacademy.com
29. 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 ……
www.indiandentalacademy.com
30. 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°
www.indiandentalacademy.com
31. 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
www.indiandentalacademy.com
32. 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
www.indiandentalacademy.com
34. 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
www.indiandentalacademy.com
35. 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
www.indiandentalacademy.com
36. 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
www.indiandentalacademy.com
37. 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
www.indiandentalacademy.com
38. 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
www.indiandentalacademy.com
39. 2) Internal surfaces:- eg. Buccal & lingual walls of the
proximal box of a proximo-occlusal inlay.
An intracoronal restoration resists displacement by
wedge retention.
www.indiandentalacademy.com
40. 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.
www.indiandentalacademy.com
41. • 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.
www.indiandentalacademy.com
42. • 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”.
www.indiandentalacademy.com
43. 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.
www.indiandentalacademy.com
44. • 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.
www.indiandentalacademy.com
45. 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.
www.indiandentalacademy.com
46. 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.
www.indiandentalacademy.com
47. • 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.
www.indiandentalacademy.com
48. • 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.
www.indiandentalacademy.com
49. • 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.
www.indiandentalacademy.com
50. • 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.
www.indiandentalacademy.com
51. • 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.
www.indiandentalacademy.com
52. 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
www.indiandentalacademy.com
53. 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
www.indiandentalacademy.com
54. 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.
www.indiandentalacademy.com
55. • 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 increased
www.indiandentalacademy.com
56. • 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.
www.indiandentalacademy.com
57. • 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.
www.indiandentalacademy.com
58. • 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.
www.indiandentalacademy.com
59. • 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.
www.indiandentalacademy.com
60. 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
www.indiandentalacademy.com
61. 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
www.indiandentalacademy.com
62. 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
www.indiandentalacademy.com
63. 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
www.indiandentalacademy.com
64. • 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
www.indiandentalacademy.com
65. • 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.
www.indiandentalacademy.com
66. • 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.
www.indiandentalacademy.com
67. • 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).
www.indiandentalacademy.com
68. • 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
69. • 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
www.indiandentalacademy.com
70. • 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.
www.indiandentalacademy.com
71. • 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.
www.indiandentalacademy.com
72. • 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.
www.indiandentalacademy.com
73. • 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.
www.indiandentalacademy.com
74. 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
www.indiandentalacademy.com
75. 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”.
www.indiandentalacademy.com
77. 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
www.indiandentalacademy.com
78. - 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.
www.indiandentalacademy.com
79. • 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
www.indiandentalacademy.com
80. 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.
www.indiandentalacademy.com
81. • 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.
www.indiandentalacademy.com
82. - 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.
www.indiandentalacademy.com
83. 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.
www.indiandentalacademy.com
84. • 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.
www.indiandentalacademy.com
85. 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…
www.indiandentalacademy.com
86. 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
www.indiandentalacademy.com
87. 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
88. 7. Laboratory performance:
Factors like casting accuracy, surface roughness,
strength, metal-ceramic bond strength should be
considered.
www.indiandentalacademy.com
89. 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
www.indiandentalacademy.com
90. 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.www.indiandentalacademy.com
91. 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.
www.indiandentalacademy.com
92. 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.
www.indiandentalacademy.com
93. 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
www.indiandentalacademy.com
94. 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
www.indiandentalacademy.com
95. - 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
www.indiandentalacademy.com
96. 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
www.indiandentalacademy.com
97. 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
www.indiandentalacademy.com
98. 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
www.indiandentalacademy.com
99. - Anterior partial coverage facial margin is
extended between highest contour of the incisal
edge and incisolabial line angle
www.indiandentalacademy.com
108. ENDODONTICALLY TREATED TEETH
• Conservation of tooth structure
• Retention form
• Resistance form
www.indiandentalacademy.com
109. 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
www.indiandentalacademy.com
110. 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
www.indiandentalacademy.com
111. • Retention form
– Preparation geometry
– Post length
– Post diameter
– Post surface texture
– Luting agent
www.indiandentalacademy.com
112. • Resistance form
– Stress distribution
– Rotational resistance
www.indiandentalacademy.com
113. 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.
www.indiandentalacademy.com
114. 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 www.indiandentalacademy.com
115. 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.
www.indiandentalacademy.com