Introduction Teeth do not possess the regenerative ability found in most other tissues. Therefore, once the enamel or dentine is lost as a result of caries, trauma, or wear, restorative materials must be used to re-establish the form and function. Teeth require preparation to receive restorations and these preparations must be based on fundamental principles from which basic criteria can be developed to help predict the success of the prosthodontic treatment. Definition Objectives of tooth preparation Principles of tooth preparation Biological considerations Mechanical considerations Esthetic considerations Conclusion Each tooth preparation must be measured by clearly defined criteria that can be used to identify and correct problems. It is important to understand the pertinent theories underlying each step is crucial. Successful preparation can be obtained most easily by systematically following the steps which will ensure optimal quality of final restoration, which will serve the patient for a long time.

1. PRINCIPLESOFTOOTH
PREPARATION
Dr Kumari Kalpana
PG - 1
1

2. CONTENTS
✔ Introduction
✔ Definition
✔ Objectives of tooth preparation
✔ Principles of tooth preparation
✔ Biological considerations
✔ Mechanical considerations
2

3. ✔ Esthetic considerations
✔ Conclusion
✔ References
3

4. INTRODUCTION1
Teeth do not posses the regenerative
ability found in most other tissues.
Therefore, once the enamel or dentine is
lost as a result of caries, trauma or wear,
restorative materials must be used to re-
establish the form and function.
1. Rosensteil SF, Land FM, Fujimoto J. Contemporary fixed prosthodontics.5thed. St.Louis:Mosby;2016.
4

5. 5
Teeth require preparation to receive
restorations and these preparations must be
based on fundamental principles from which
basic criteria can be developed to help predict
the success of the prosthodontic treatment.

6. DEFINITIONS 2,3
TOOTH PREPARATION - The process of
removal of diseased and/or healthy enamel
and dentin and cementum to shape a tooth to
receive a restoration.2
(GPT -9)
6
2. Glossary of prosthodontic terms. J Prosthet Dent 2017;117(5):10-92.

7. DEFINITIONS3
The mechanical treatment of dental disease or
injury to hard tissue that restores a tooth to the
original form.3 (Tylman)
7
3. Malone FP, Koth LD in ed. Tylman’s theory and practice of fixed prosthodontics.8th ed.
All india publisher and distributor;1997.

8. OBJECTIVES OF TOOTH PREPARATION 3
● Reduction of the tooth in miniature to provide
retainer support.
● Preservation of healthy tooth structure to
secure resistance form.
● Provision for acceptable finish lines.
8
3. Malone FP, Koth LD. Tylman’s theory and practice of fixed prosthodontics.8th ed. All
india publisher and distributor;1997.

9. ● Performing pragmatic axial tooth reduction to
encourage favorable tissue responses from artificial
crown contours e.g., fluting of molars.
9

10. PRINCIPLES OF TOOTH
PREPARATION 1,4
10
4. Shillingburg HT. Fundamentals of tooth preparation. 1991. Quintessence. China.

11. According to shillingburg 4
11
Preservation
of tooth
structure
Retention
and
resistance
Structural
durability
Marginal
integrity
Preservation
of the
periodontium

12. ACCORDING TO ROSENSTEIL1
12
Biologic
Affect the
health of the
oral tissues
Mechanical
Affect the
integrity and
durability of
the restoration.
Esthetic
Affect the
appearance of
the patient

13. ESTHETIC
• Minimum display of metal
• Maximum thickness of
porcelain
Porcelain occlusal surfaces
Subgingival margins
MECHANICAL
Retention form
Resistance form
Deformation
BIOLOGIC
• Conservation of tooth
structure
• Avoidance of over
contouring
• Supragingival margins
• Harmonious occlusion
• Protection against tooth
fracture
13

14. 14

15. PREVENTION
OF DAMAGE
DURING TOOTH
PREPARATION1
Adjacent teeth
Iatrogenic damage to the
adjacent tooth is a common
error while doing tooth
preparation. The technique
of tooth preparation must
avoid and prevent damage to
the adjacent tooth surfaces.
15

16. :
✔ Use of metal matrix band around the adjacent
tooth for protection.
16

17. ✔ A thin tapered diamond bur can be used that
passes through the inter-proximal contact
area to leave a slight lip or fin of enamel
without resulting in excessive tooth
reduction or necessitating undesirable
angulation of the rotary instrument.
17

18. SOFT TISSUE
By careful retraction with
✔aspirator tip
✔mouth mirror
✔flanged saliva ejector.
18

19. ● Moisture Control5
✔rubber dam
✔suction devices
✔absorbents
19
5. Roberson TM, Heymann HO, Swift EJ. Sturdevant’s art and science of operative dentistry.
5th Edition. St. Louis, Missouri:Mosby;2006.

20. ● Retraction and Access
PROVIDES
✔ maximal exposure of the operating site
✔ usually involves maintaining an open mouth
✔ depressing or retracting the gingival tissue,
tongue, lips, and cheek
✔ rubber dam, high-volume evacuator, absorbents,
retraction cord and mouth prop are used for
retraction and access.
20

21. Local Anesthesia –
Local anesthetics play a role in eliminating the
discomfort of dental treatment and controlling
moisture.
✔ Reduces salivation
21

22. Pulp 1,6,7
Prevention of pulp injury is of
great importance during crown
preparation.
Extreme temperatures, chemical
irritation, or microorganisms can
cause irreversible pulpitis.
22
6. Ottl P, Lauer HC. Temperature response in the pulpal chamber during ultrahigh-speed tooth
preparation with diamond burs of different grit. J Prosthet Dent 1998;80:12-9.
7.Lauer HC, Kraft E, Rothlauf W, Zwingers T. Effects of the temperature of cooling water during
high-speed and ultrahigh-speed tooth preparation. J Prosthet Dent 1990;63:407-14.

23. Temperature
✔ Considerable heat is generated by friction
between a rotary instrument and the surface
being prepared .
✔ Excessive pressure, higher rotational speeds, and
the type and condition of the cutting instrument
may result in increased generation of heat.
✔ Prevention- With high speed hand piece, a
feather light, intermittent touch along with
continuous water spray. 1
23

24. ● Sato and Schuchard reported that excessive heat
adduction can result in structural changes of the hard
dental tissues and damage the dental pulp.
● Zach and Cohen recorded in animal studies that 15% of
dental pulps became necrotic when the temperature in
the pulpal chamber increased by 5.6° C and 60% became
necrotic if the temperature increases by 11.1° C.
24

25. ✔ Pohto and Scheinin documented increased capillary
permeability, that is a first sign of heat related pulp
damage, when the temperature increased between 5°
and 7° C.
✔ Coarser the grit of diamond bur, the more
pronounced the temperature elevation within a pulpal
chamber during tooth preparation.
25

26. ● The relative temperature within the pulp increases by
a maximum of 2.5° C for fine, 2.6° C for coarse, and
3.2° C for ultra coarse diamond burs.
● Preparation with cooling water, temperatures of
29.8°C to 33.7° C with the turbine or the high-speed
angle did not produce an increase in temperature in
the pulp chamber after three grinding steps7
26

27. Air-water spray when accurately
directed :
○Reduces the heat generated
○Prevents clogging
○Increases the cutting efficiency of the
bur.
○While preparing grooves or pin holes, as
the coolant cannot reach the cutting
edge of the bur, prevention can be done
by using low rotational speed. 27

28. Chemical action
✔ The chemical action by materials like restorative
resins, solvents, bases & luting can cause pulpal
damage particularly when applied to freshly cut
dentin.
Prevention- Cavity varnish or dentin bonding agents
form an effective barrier but their effect on
retention of the restoration is controversial.
28

29. Bacterial Action
✔ Pulpal damage under restorations- left behind
bacteria or gained access to the dentin due to
microleakage.
✔ Prevention- All Carious dentin must be removed
before placement of the restoration.
29

30. CONSERVATION OF TOOTH
STRUCTURE 1
● One of the basic tenets of restorative dentistry
is to conserve as much tooth structure as
possible consistent with the mechanical and
esthetic principles of tooth preparation.
● This will reduce the harmful pulpal effects of the
various procedures and materials used.
30

31. Tooth structure is conserved by using the
following guidelines:
1. Use of partial-coverage rather than
complete coverage restorations.
1. Preparation of teeth with the minimum
practical convergence angle (taper)
between axial walls.
31

32. 3. Preparation of the occlusal surface so reduction
follows the anatomic planes to give uniform
thickness in the restoration.
32

33. 4. Preparation of the axial surfaces so that a maximal
thickness of residual tooth structure surrounding
pulpal tissues is retained, if feasible , teeth may be
orthodontically repositioned which allows less axial
convergence than necessary when tooth alignment is
less than optimal to accommodate preparations for
fixed dental prosthetic retainer
33

34. 5. Selection of a conservative
margin compatible with the other
principles of tooth preparation.
6. Avoidance of unnecessary
apical extension of the
preparation. 34

35. CONSIDERATIONS AFFECTING FUTURE DENTAL
HEALTH
● Improper tooth preparation may have adverse
effects on long-term dental health.
1. AXIAL REDUCTION 1,4 :
● Axial reduction also plays an important role in
securing space for an adequate thickness of
restorative material.
● Gingival inflammation is associated with excessive
axial contours. 35

36. • If restorations are made with normal contours over
preparations with inadequate axial reduction, they will
have thin walls that will be subject to distortion.
• The crown should duplicate the contours & profile of
the original tooth unless it is malformed or
malpositioned. 1
36

37. MARGIN PLACEMENT1
Three types of margins-
1. Supragingival
2. Subgingival
3. Equigingival
37

38. 38
The margins should be placed supra-gingival
whenever possible as sub-gingival margins are a
major etiologic factor in periodontal disease
SUPRAGINGIVAL
Advantages :
1. Easily finished without soft tissue trauma
2. More easily kept plaque free
3. Impressions are more easily made, with less potential soft tissue damage
4. Restorations can be easily evaluated at recall appointments

39. CONTRAINDICATIONS8
39
Supragingival placement is not indicated when
(1) the clinical crown is short,
(2) old restorations with subgingival margins are present
(3) crowns with facings are to be placed in areas requiring an esthetic
appearance.
(4) caries, abrasion, or erosion has degraded the gingival third of the
clinical crown.
8. Richer WA, Ueno H. Relationship of crown margin placement to gingival
inflammation. J Prosthet dent 1973:30(2);156-161

40. SUBGINGIVAL
Indications:1
40
Dental caries , cervical erosion and restorations extend subgingivally/ Crown
lengthening not indicated
The proximal contact area extends to the gingival crest
Additional retention / resistance is needed
Margin of metal ceramic crown is to be hidden behind labio-gingival crest
Root sensitivity cannot be controlled by more conservative procedures
Modification of axial contour is indicated to provide an undercut to provide
retention for the clasp of partial removable dental prosthesis

41. ● Gottelib stated “The practice of extending the
margin of any metal restoration underneath the
gingiva must prove detrimental. It is well known that
restorations placed underneath the gingiva are
commonly ill-fitting and, in that way, serve as an
additional source of irritation.” 7
41

42. MARGIN ADAPTATION 1
● The junction between a cemented
restoration and the tooth is always a
potential site for recurrent caries
because of dissolution of the luting agent
and inherent roughness.
42

43. ● A well-designed preparation has a smooth and even
margin.
● Rough, irregular, or "stepped“ junctions greatly
increase the length of the margin and substantially
reduce the adaptation of the restoration.
● Smooth accurately placed preparation margins are
particularly important when restoration are fabricated
with CAD/CAM process.
43

44. MARGIN INTEGRITY 4
● The restoration can survive in the biological
environment of the oral cavity only if the margins
are closely adapted to the cavo-surface finish line
of the preparation.
● The configuration of the preparation finish line
dictates the shape and bulk of restorative
material in the margin of the restoration.
44

45. ● It also can affect both marginal adaptation and
the degree of seating of the restoration.
■Historically, the bevel was used as a device for
compensating for the solidification shrinking of alloys
used in fabricating cast restorations
45

46. TO BEVEL…
● Bevels have been advocated as means of
diminishing marginal discrepancy..
● D – vertical discrepancy in fit
● d - closest distance between margin & surface
of preparation
d = D sin µ
d = D cos ø
46

47. 47

48. 49

49. MARGIN GEOMETRY1,4
A. Feather edge
B. Chamfer
C. Beveled
D. Chisel edge
50

50. E. Shoulder
F. Sloped shoulder
G. Shoulder with a
bevel
51

51. Feather edge:1
✔Advantages: conservative of tooth
structure.
✔Disadvantages: does not provide
sufficient bulk.
✔Indications: not recommended.
52

52. Chisel Edge 1
✔ A variation of the feather edge.
✔ Occasionally on tilted teeth
✔ Location of margin difficult to control
✔ Unfortunately, this margin is frequently
associated with an excessively tapered
preparation or one in which the axial reduction is
not correctly aligned with the long axis of the
tooth.
53

53. Chamfer 1,4
● It is distinct and easily identified, provides room
for adequate bulk of material, and can be placed
with precision, although care is needed to avoid
leaving a ledge of unsupported enamel.1
● This finish line has been shown experimentally to
exhibit the least stress, so that the cement
underlying it will have less likelihood of failure.
54

54. ✔ The chamfer should never be
prepared wider than half the tip of
the diamond; otherwise, an
unsupported lip of enamel could
result. 1
✔ Indication- cast metal restoration,
lingual margin of the metal-
ceramic restoration.4
55

55. Heavy Chamfer / deep chamfer 4
✔ A heavy chamfer is used to provide a 90-degree
cavo-surface angle with a large-radius rounded
internal angle
✔ The heavy chamfer provides better support for a
ceramic crown than does a conventional chamfer
✔ Stress concentration is less in the tooth structure.
56

56. Bevel 1,4
✔ Wide, shallow bevels that are nearly parallel
with the outer surface of the tooth should be
avoided. They are likely to lead to over
contouring. 4
57

57. ✔ Advantages: removes unsupported enamel, allows
finishing of metal.
✔ Disadvantages: extends preparation into sulcus if
used on apical margin.
✔ Indications: facial margin of maxillary partial
coverage restorations and inlay/ onlay margins.1
58

58. Shoulder 4
✔ The shoulder has long been the finish
line of choice for the all-ceramic crown.
✔ The wide ledge provides resistance to occlusal
forces and minimizes stresses that might lead to
fracture of the porcelain.
✔ It produces the space for healthy restoration
contours and maximum esthetics.
✔ However, it does require the destruction of more
tooth structure than any other finish line.
59

59. Radial shoulder
✔ modified shoulder finish
line
✔ Initial instrumentation of
ledge (coarse ,flat end
tapered diamond)
✔ A small radius rounded
internal angle (fine, flat
end tapered diamond)
60

60. Sloped Shoulder 1
✔ Advantages: bulk of material,
✔ Disadvantages: less conservative of tooth
structure.
✔ Indications: facial margins of metal
ceramic crowns.
61

61. Shoulder with a Bevel 4
✔ This design can also be used for the facial finish line of
metal-ceramic restorations
✔ Proximal box & occlusal shoulder of inlays and onlays
✔ Mandibular three- quarter crowns
✔ It can be used in those situations where a shoulder is
already present, either because of destruction by caries or
the presence of previous restorations.
✔ It is also a good finish line for preparations with extremely
short walls, since it facilitates axial walls that are nearly
parallel. 62

62. MECHANICAL
CONSIDERATION
63

63. Mechanical considerations
can be divided into1 :
1. Providing retention form,
2. Resistance form
3. Preventing deformation of the
restoration.1,11
64

64. RETENTION FORM1,2,4,11
● Retention is that quality inherent in the dental
prosthesis acting to resist the forces of dislodgment
along the path of placement.2 (GPT-9)
● Retention form is the feature of a tooth preparation
that resists dislodgment of a crown in a vertical
direction or along the path of placement.2 (GPT-9)
65
11.Narula S, Punia V, Khandelwal M, Sharma V, Pamecha S. Retention in Conventional
Fixed Partial Dentures: A Review. Journal of Clinical and Diagnostic Research.
2011;5(5):1128-1133.

65. 4. Surface area
3. Taper
2. Geometry of the tooth preparation
1. Magnitude of dislodging force
The following factors must be considered in deciding whether retention
is adequate for a given fixed restoration. These include:
66

66. 9. Film thickness of luting agent
8. Materials being cemented
7.Roughness of the fitting surfaces of the restorations
6. Type of preparation
5. Stress concentrations
67

67. MAGNITUDE OF DISLODGING FORCES 1,11
● Forces that tend to remove a cemented
restoration along path of withdrawal are small as
compared to those that tend to tilt it.
● The magnitude of the dislodging forces depend
on the stickiness of the food and the surface
area and texture of the restoration being pulled.68

68. GEOMETRY OF THE
TOOTH PREPARATION 1,11
✔Fixed prosthesis
depend on the
geometric form of the
preparation rather than
on adhesion for
retention.
✔Cement is effective
only if the restoration
has a single path of
placement.
69

69. ● For effective retention, a tooth preparation must
constrain the movement of a restoration. For this to
occur, it must be cylindrical.
70

70. ● If the preparation has multiple path of withdrawal,
then the particles of the cement will tend lift away
from rather than slide along the preparation and
the only retention will be a result of the limited
adhesion of the cement
71

71. TAPER 1,4,11
It is defined as the convergence of two opposing external
walls of tooth preparation as viewed in a given plane.
Theoretically, maximum retention is obtained if a tooth
preparation has parallel walls.
72

72. ● A slight convergence, or taper, is necessary in
the completed preparation.
● As long as this taper is small, the movement of
the cemented restoration will be effectively
restrained by the preparation and will have what
is known as a limited path of withdrawal.
● As the taper increases, however, so does the
free movement of the restoration, and retention
will be reduced.
73

73. ● The axial walls of the preparation must taper slightly to
permit the restoration to seat; i.e., two opposing
external walls must gradually converge.
● Too small taper may lead to unwanted undercuts and too
large will no longer be retentive.10
● To minimize stress in the cement interface between
the preparation and restoration, a taper of 2.5 to 6.5
degrees has been suggested as optimum.4
74

74. 75

75. Reports by various studies
● Nordlander et al 12- 4 to 10 degrees
● Gilboe and teteruck 13- 2 to 5 degrees
● El-Ebrashi and Craig 14 – 2.5-6.5 degrees
● Ghafoor et al 15 - 11.13±8.3 degrees
● El-Mubarak 16 – 39.98 degrees
76

76. ● A negative taper or
undercut must be
eliminated or it will
prevent the seating of the
restoration
● Preparation taper can be
evaluated by viewing it
with one eye from a
distance of approximately
30 cm or 12 inches 77
VIEWING
PREPARATION TAPER

77. ● In this way it is
possible to see all
the axial walls with
an ideal taper of 60
● An undercut as
great as 80 can be
overlooked if both
the eyes are used
78
BINOCULAR VISION
SHOULD NEVER BE
EMPLOYED

78. SURFACE AREA 1,4,13
● If the restoration has a limited path of placement,
its retention is dependent on the length of this path
or more precisely on the surface area in sliding
contact.
● Therefore, crowns with tall axial walls are more
retentive than those with short axial walls and molar
crown of same taper are more retentive than
premolar crown of the same taper.1
79

79. ● Features such as grooves & boxes that are
placed in the preparation increase the surface
area thereby increasing retention.
● These are substituted for the missing axial
wall.
● These features are also useful for augmenting
retention on severely damaged teeth.
80

80. ● These secondary factors should be located
(1) with the line of withdrawal
(2) as far as possible from its reciprocal retentive
feature, and
(3) at a point which permits the maximum length.
● The appropriate factor to be incorporated is the one
which resists the displacing forces while conserving
the greatest amount of structure.13
81

81. ● Groove- A groove is the secondary factor which
best achieves resistance form while conserving
maximum tooth structure. It is, thus, the most
commonly incorporated secondary factor.
● Box- A box may be regarded as a wide groove with
an increased surface area. 82

82. ● Pinhole- The pinhole may have tapered or parallel
walls. Both types contribute more toward retention
than resistance. The amount of retention is directly
related to the area of the pin contacting the tooth
and to the intimacy of the contact.
● Retention is a function of the length and diameter of
the pin. 13
84

83. ● Secondary factors should be placed within 2
to 5 degrees of parallelism with a primary or
another secondary factor for maximum
effectiveness.13
85

84. FREEDOM OF DISPLACEMENT 1,4
Retention is improved by geometrically limiting the
number of paths along which a restoration can be
removed from the tooth preparation.
• Maximum retention is achieved when there is only
one path.
86
More the number of paths ,
less will be the retention

85. STRESS CONCENTRATION 1,11
● When a retentive failure occurs, cement often adheres to
both the tooth preparation and the fitting surface of the
restoration. In these cases, cohesive failure occurs
through the cement layer because the strength of the
cement is less than the induced stresses.
● Changes in the geometry of the preparation (e.g. rounding
of the internal line angles) reduces stress concentrations
and hence increases the retention of the restoration.
87

86. TYPE OF PREPARATION
● Retention of a complete crown is about double that of
partial-coverage restorations.
● Adding grooves or boxes to a preparation will increase
retention because they will limit the paths of withdrawal.
88

87. ROUGHNESS OF THE FITTING SURFACES OF
THE RESTORATIONS
● When the internal surface of a restoration is very smooth,
retentive failure occurs not through the cement but at
the cement-restoration interface.
● Under these circumstances, retention will be increased if
the restoration is roughened or grooved .
● The casting is most effectively prepared by air-abrading
the fitting surface with 50 um of alumina
● Retention increases by 64%
89

88. MATERIALS BEING CEMENTED
● Retention is affected by both the casting alloy
and the core or build up material.
● The more reactive the alloy is, the more
adhesion there will be with certain luting agents.
Therefore, base metal alloys i.e, Ni, Co & Cr are
better retained than less reactive high-gold
content metals
90

89. ROLE OF LUTING CEMENTS IN
RETENTION
Type of luting agent chosen affects the
retention of cemented restoration
✔ Five kinds of luting agents are most
commonly used:
1. Zinc Phosphate
2. Zinc Polycarboxylate
3. Glass ionomer
4. Zinc oxide eugenol
5. Resin bonded cement.
91

90. FILM THICKNESS OF LUTING
AGENT1
92
CATEGORY HIGHER RESISTANCE LOWER RESISTANCE
TAPER MINIMUM EXCESSIVE
HEIGHT LONG SHORT
TYPE OF PREPARATION COMPLETE COVERAGE PARTIAL COVERAGE/ ONLAY
LUTING AGENT GLASS IONOMER (24 µm)
ZINC PHOSPHATE( 20µm)
Zinc Polycarboxylate ( 21µm)
A uniform thickness of cement between restoration and tooth
provides more retention than a non-uniform thickness.
A film thickness of 25 μm or less has been preferred for successful
restoration.11

91. ADDITIONAL
METHODS OF
GAINING
RETENTION
93
Preparing a suitable gingival finish
linePreparing
Contouring and placing suitable
contact areas
Contouring and
placing
Incorporating occlusal locks i.e.
Dovetail, boxes and groovesIncorporating
Adding tapered or parallel pinsAdding

92. FACTORS
AFFECTING
RETENTION
IN
FIXED
PARTIAL
DENTURES
1. Length of Span
2. Curvature of Arch
3. Type of Bridge
4. Occlusion
5. Periodontal Condition
6. Tooth or Teeth Being Replaced
94

93. 95
7. Type of
Retainer
Used
8. Materials
Employed in
the
Construction
of Retainers
9. Arch
Position of
the
Abutment
Teeth and
Retention
10. Spring
Cantilever
Bridges
and
Retention

94. 1. Length of Span
In addition to the increased load placed on the
periodontal ligament by long span bridge, the longer
spans are less rigid and so less retentive.
2. Curvature of Arch
Arch curvature has its effect on stresses occurring in a
fixed bridge.
When pontic lie outside the inter abutment axis line , it
acts as lever arm producing torquing movement.
96

95. ● 3. Type of Bridge
There are two types of bridges made according to
the prevalent condition and position of abutments
in the arch.
a. Rigid connector
b. Non-rigid connector
4. Occlusion
Interference with undesirable occlusal contacts
produce deviation during closure of maximum
intercuspation, hinder smooth passage to and from
the intercuspation position and lead to deflective
occlusal force on the bridges which may lead to
damaging effects on abutment and also on the
retention of the casting.
97

96. 5. Periodontal Condition
● The abutment tooth must be able to provide good
support for the bridge. This support is related to
both the amount of root and the amount of bone
present.
6. Tooth or Teeth Being Replaced
A bridge replacing a maxillary canine is subjected to
more stresses than the mandibular since forces are
transmitted outward (labially) on the maxillary arch
against the inside of the curve (its weakest point).
98

97. 7. Type of Retainer Used
There are two types of retainers which are
generally used
• Intra coronal
• Extra coronal
8. Materials Employed in the Construction of
Retainers
● The material used in the construction of the fixed
partial dentures calls for certain requirements
which help to increase the longevity of the
restoration.
99

98. • Cobalt chromium or nickel chromium alloys
generally used for making fixed bridges fulfill
majority of these ideal requirements.
• On the other hand acrylic is generally weak, is
not rigid and cannot provide strong connectors.
It also has lower compressive and tensile
strength compared to other alloys and is thus
easily subjected to fracture. Hence acrylic is used
for interim or temporary restorations in the
mouth.
100

99. 9. Arch Position of the Abutment Teeth and Retention
● When the abutment teeth are more or less parallel to
each other, complete or partial crown retainers can
be made.
10. Spring Cantilever Bridges and Retention
● This bridge provides a method of supporting a pontic
at some distance from the retainers. This type of
bridge is both tooth and tissue supported.
● A gold bar which fits in contact with the palatal
mucosa connects the pontic to the retainers. 101

100. DIFFERENT TOOTH
PREPARATION
AND WAYS OF ACHEIVING
RETENTION
102

101. COMPLETE CAST CROWN
PREPARATION
● Ways of gaining retention while
tooth preparation:
✔guiding grooves on the axial walls
✔ a taper of 3-6°
✔width of chamfer should be approximately
0.5 mm which will allow adequate bulk of
metal at the margin 103

102. THE METAL CERAMIC CROWN
PREPARATION
Factors affecting retention that should be
taken into consideration while preparation
✔ Reduction of the incisal edge on an anterior
tooth should allow 2 mm of adequate
material thickness to permit translucency in
the completed restoration.
104

103. ✔ Labial reduction of 1.5 mm
✔ 90° butt joint
✔ taper of 6 degree
✔ all the line angles and
point angles should be
rounded .This will help in
reducing the stress
concentration and thus will
enhance retention.
105

104. THE PARTIAL
VENEER
CROWN
PREPARATION
POSTERIOR
TEETH-THREE
QUARTER
CROWN
✔ During axial reduction,
place grooves for axial
alignment in the centre of
the lingual surface and in
the mesio-lingual and
distolingual transitional
line angles. These grooves
should be made parallel to
the long axis of tooth.
106

105. • During proximal reduction the proximal grooves are
placed parallel to the path of withdrawal. The groove
should not be deeper than 1mm and is best done with
a tapered carbide bur.
• If additional bulk is needed to ensure rigidity of the
restoration, it can be provided with an occlusal offset.
This V-shaped groove extends from the proximal
grooves along the buccal cusp.
107

106. RETENTIVE FEATURES FOR ALL CERAMIC
RESTORATION
● An all ceramic restoration remains the most
aesthetic restoration for duplicating
individual anterior teeth.
● Adequate tooth reduction is created to
achieve space for the porcelain bulk required
for the strength of the restoration.
108

107. RETENTION IN ENDODONTICALLY TREATED
TEETH
✔ It has been demonstrated experimentally that
endodontically treated teeth are weaker and more
brittle than vital teeth. So for this reason attempts
have been made to strengthen the teeth by removing
part of the root canal filling and replacing it with a
metal post.
✔ Retention and support is derived from within the
root canal. 109

108. Canal Retention
✔It is recommended that the root canal should be
enlarged only to amount necessary to enable
the post to fit snugly for strength and retention
110

109. RESISTANCE FORM
It is the feature of a tooth preparation that enhance
the stability of a restoration and resist dislodgment
along an axis other than the path of placement. 2
(GPT-9)
111

110. ● Mastication and para functional activity may
subject a prosthesis to substantial horizontal
or oblique forces.
● Adequate resistance depends on the following:
1. Magnitude and direction of the dislodging
forces
2. Geometry of the tooth preparation
3. Physical properties of the luting agent
112

111. MAGNITUDE AND DIRECTION OF THE
DISLODGING FORCES 1
● In a normal occlusion, biting force is distributed
over all the teeth; most of it is axially directed.
● If a fixed prosthesis is carefully made with a
properly designed occlusion, the load should be
well distributed and favourably directed.
113

112. ● However, if a patient has a biting habit such as
pipe smoking or bruxing, it may be difficult to
prevent fairly large oblique forces from being
applied to a restoration.
● Consequently the completed tooth preparation
and restoration must be able to withstand
considerable oblique forces as well as the normal
axial ones
114

113. Length of the preparation 4
● Occluso-gingival length is an important factor
in both retention and resistance.
● Because the axial wall occlusal to the finish
line interferes with displacement, the length
and inclination of that wall become factors in
resistance to tipping forces.
115

114. ● The walls of shorter
preparations should
have as little taper
as possible to
increase the
resistance.
116

115. ● It may be possible to successfully restore a tooth
with short walls if the tooth has a small diameter.
● The preparation on the smaller tooth will have a
short rotational radius for the arc of
displacement.
● The longer rotational radius on the larger
preparation allows for a more gradual arc of
displacement.
117

116. • Resistance to displacement for a short-walled preparation
on a large tooth can be improved by placing grooves in the
axial walls.
This reduces the rotational radius, and that portion of the
walls of the grooves near the occlusal surface of the
preparation will interfere with displacement.
118

117. GEOMETRY OF TOOTH PREPARATION 1
● The tooth preparation must be shaped so
that particular areas of the axial wall will
prevent rotation of the crown.
● A partial-coverage restoration may have less
resistance than a complete crown because it
has no buccal resistance areas.
119

118. ● Resistance must be provided by boxes or grooves
and will be greatest if they have walls that are
perpendicular to the direction of the applied force
● The resistance form of an excessively tapered
preparation can be improved by adding grooves or
pinholes, because these interfere with rotational
movement and in so doing subject additional areas
of the luting agent to compression.
120

119. PHYSICAL PROPERTIES OF THE LUTING AGENT 1
● Resistance to deformation is affected by physical
properties of the luting agent, such as compressive
strength and modulus of elasticity.
● Zinc phosphate cements have a higher modulus of
elasticity than do polycarboxylate cements, which
exhibit relatively large plastic deformation.
● This may account for the observation that the
retentive ability of polycarboxylate cement is more
dependent on the taper of the preparation than is the
retention with zinc phosphate cement.
121

120. ● Method to analyse resistance form
122
17. Parker MH. Resistance form in tooth preparation. DCNA 2004,

121. ● For tilted molars
A- center of rotation
AB- distal wall
CD- mesial wall
123
NR
R
CENTRE OF
ROTATION

122. Adding a groove (HIJKLM) to the preparation affects the resistance form.
124
17. Parker MH. Resistance form in tooth preparation. DCNA 2004,
48:387-396

123. DEFORMATION 1
● A restoration must have sufficient strength
to prevent permanent deformation during
function. Otherwise, it will fail (typically at
the restoration-cement, or the metal-
porcelain, interface).
● This may be a result of inappropriate alloy
selection, inadequate tooth preparation, or
poor metal-ceramic framework design.
125

124. ESTHETIC CONSIDERATION 1
126

125. ✔Patients prefer their dental restorations
to look as natural as possible.
✔However, care must be taken that
esthetic considerations are not pursued at
the expense of a patient's long-term oral
health or functional efficiency.
127

126. METAL CERAMIC RESTORATION 1
● The poor appearance of some metal-ceramic
restorations is often due to insufficient porcelain
thickness.
● On the other hand, adequate porcelain thickness
is sometimes obtained at the expense of proper
axial contour (such overcontoured restorations
almost invariably lead to periodontal disease).
● In addition, the labial margin of a metal-ceramic
crown is not always accurately placed
128

127. ✔ Shade problems are frequently encountered in
maxillary incisor crowns at the incisal and cervical
thirds of the restoration, where direct light reflection
from the opaque layer can make the restoration
appear very noticeable.
✔ The labial surfaces of anterior teeth should be
prepared for metal-ceramic restorations in two
distinct planes. If they are prepared in a single plane,
insufficient reduction in either the cervical or the
incisal area of the preparation will result. 129

128. Incisal Reduction
✔An incisal reduction of 2 mm is recommended for
good esthetics.
130

129. Proximal Reduction
● The proximal surfaces of anterior teeth will
look most natural if they are restored as the
incisal edges, without metal backing.
● This will allow some light to pass through the
restoration in a manner similar to what occurs
on a natural tooth
131

130. Labial Margin Placement
● Supragingival margin placement has many biologic
advantages.
● Nevertheless, subgingival margins may be indicated
for esthetic reasons, particularly when the patient
has a high lip line and the use of a metal collar
labial margin is contemplated.
132

131. ● Patients with a high lip line, which exposes
considerable gingival tissue, present the greatest
problem if complete crowns are needed.
● Where the root surface is not discoloured,
appearance can be restored with a metal-ceramic
restoration having a supra-gingival porcelain labial
margin.
133

132. ● If the patient has a low lip line, a metal supra-
gingival collar may be placed because the metal
is not seen during normal function.
● Metal margins generally have a more accurate
fit than porcelain margins.
● Metal collars can be hidden below the gingival
crest, although there will be some discoloration
if the gingival tissue is thin.
134

133. ● The margin should follow the contour of the
free gingiva, being further apical in the middle
of the tooth and further incisal inter-
proximally.
● A common error is to prepare the tooth so the
margin lies almost in one plane, with exposure
of the collar labially and irreversible loss of
bone and papilla proximally.
135

134. ALL CERAMIC RESTORATIONS
● Not conservative of tooth structure.
● 90 degree heavy chamfers margin has to be
prepared all around the natural teeth to ensure
material thickness.
● This limits the use of these restorations on facio-
lingually thin teeth.
136

135. PARTIAL-COVERAGE RESTORATIONS 1
● Esthetic partial-coverage restorations depend
on accurate placement of the potentially visible
facial and proximal margins.
137

136. Proximal margins
● Place the margin just buccal to the proximal
contact area, where metal will be hidden by the
distal line angle of the neighbouring tooth.
● Tooth preparation angulation is critical and
should normally follow the long axes of posterior
teeth and the incisal two thirds of the facial
surface of anterior teeth.
138

137. ● If a buccal or lingual tilt is given to the tooth
preparation, metal may be visible. The distal margin
of posterior partial-coverage restorations is less
visible than the mesial margin.
● Often in this area it is advantageous to extend the
preparation farther beyond the contact point for
easier preparation and finishing of the restoration
and better access for oral hygiene.
139

138. Facial Margin
● The facial margin of a maxillary partial-coverage
restoration should be extended just beyond the
occluso-facial line angle.
● A short bevel is needed to prevent enamel
chipping. A chamfer can be placed where
appearance is less important because this will
provide greater bulk of metal for strength.
140

139. PRESERVATION OF
THE PERIODONTIUM4
● Whenever possible, the finish line should be placed in
an area where the margins of the restoration can be
finished by the dentist and kept clean by the patient.
● In addition, finish lines must be placed so that they
can be duplicated by the impression, without tearing
or deforming the impression when it is removed past
them. 141

140. PRESERVATION OF PERIODONTIUM4
Finish line has a direct bearing on ease of
fabrication & ultimate success of restoration
Margins should be smooth as possible & fully
exposed to cleansing
- in enamal
- supragingival margins
143

141. 144
The deeper the restoration
margin resides in the
gingival sulcus, the greater
the inflammatory response.
Placement of a restoration
margin in this area
probably will result in
gingival inflammation, loss
of alveolar crest height, and
formation of a periodontal
pocket.

142. 145
Exercise caution if conditions require that the
finish line be placed any closer to the
alveolar crest than 2.0 mm, which is the
combined dimension of the epithelial and
connective tissue attachments.
“Crown lengthening" may be done to
surgically move the alveolar crest 3.0 mm
apical to the location of the proposed finish
line to guarantee the biologic width and
prevent periodontal pathology. This will allow
space for the connective and epithelial
attachments and a healthy gingival sulcus.

143. CONCLUSION
Each tooth preparation
must be measured by
clearly defined criteria
which can be used to
identify and correct
problems
146

144. Successful restoration
Accurate diagnosis
Thoughtful treatment planning
Preparation design
147

145. 148
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152

150. 153