Teeth don’t possess regenerative ability found in most other tissues. Therefore, once enamel & dentin are lost as a result of caries, trauma or wear, restorative material must be used, to reestablish form & function. Teeth require preparation to receive restoration & these preparations must be based on fundamental principles, which are discussed in this presentation, from which basic criteria can be developed to help predict the success of prosthodontic treatment.

1. PRINCIPLES OF TOOTH PREPARATION
PRESENTED BY:
Dr. SONALI HARJANI
III M.D.S.

2. Teeth don’t possess regenerative ability found in most other
tissues. Therefore, once enamel & dentin are lost as a result of
caries, trauma or wear, restorative material must be used, to
reestablish form & function.
Teeth require preparation to receive restoration & these
preparations must be based on fundamental principles from
which basic criteria can be developed to help predict the success
of prosthodontic treatment.

3. The scope of fixed prosthetic treatment can range from the
restoration of a single tooth to the rehabilitation of the entire
occlusion.
Restorations can help in achieving a full function and improved
esthetic outcome.
Missing teeth can be replaced with fixed prosthesis that will
improve patient comfort and masticatory ability, maintain
health and integrity of dental arches and also improve patient’s
self-image.

4. TOOTH PREPARATION:
Tooth preparation may be defined as the
mechanical treatment of dental disease or
injury to hard tissue that restores a tooth
to the original form.
(Tylman)
The process of removal of diseased and/or
healthy enamel, dentin and cementum to
shape a tooth to receive a restoration.
(GPT-9)

5. Crown:
An artificial replacement that restores missing
tooth structure by surrounding part or all of the
remaining structure with a material such as a cast-
metal alloy, metal-ceramics, resin or a
combination of materials. (GPT-9)
Fixed partial denture:
Any prosthesis that is securely fixed to a natural
tooth or teeth, or to one or more dental
implants/implant abutments, that replaces one
or more missing teeth, which cannot be removed
by the patient. (GPT-9)

6. OBJECTIVES OF TOOTH PREPARATION:
1.Reduction of the tooth in miniature to provide retention.
2.Preservation of healthy tooth structure to secure resistance form.
3.Provision for acceptable finish lines.
4.Performing pragmatic axial tooth reduction to encourage favorable
tissue responses from artificial crown contour.

7. GUIDELINES OF TOOTH PREPARATION:
Total occlusal convergence
Occluso-cervical/ inciso-cervical dimension
Ratio of OC and FL dimension
Circumferential form of the prepared tooth
Reduction uniformity
Reduction depths
Finish line location
Line angle form

8. PRINCIPLES OF TOOTH PREPARATION:
1. Biologic considerations,
which affect the health of
the oral tissues
2. Mechanical considerations,
which affect the integrity
and durability of the
restoration
3. Esthetic considerations,
which affect the
appearance of the patient
REFERENCE:
CONTEMPORARY FIXED PROSTHODONTICS,
ROSENTIEL, 5TH EDITION

9. PRINCIPLES OF TOOTH PREPARATION:
PRESERVATION
OF TOOTH
STRUCTURE
RETENTION AND
RESISTANCE
FORM
STRUCTURAL
DURABILITY
MARGINAL
INTEGRITY
PRESERVATION
OF THE
PERIODONTIUM
REFERENCE:
FUNDAMENTALS OF FIXED PROSTHODONTICS,
SHILLINBURG, 3rd EDITION

10. PRESERVATION OF TOOTH STRUCTURE
1) PRESERVATION OF TOOTH STRUCTURE
• Adjacent teeth
• Soft tissue
• Pulp

11. 1) ADJACENT TEETH:
• Damage to adjacent teeth is prevented by positioning the diamond so that a
thin lip of enamel is retained between the bur and the adjacent tooth.
• The orientation of the diamond parallels the long axis.
• Metal matrix band around the tooth – can be perforated and the adjacent
tooth will not be accidentally niched.

12. 2) SOFT TISSUES:
• Damage to the soft tissues of the tongue and cheeks can be prevented
by careful retraction with an aspirator tip, mouth mirror , or flanged
saliva ejector.
• Great care is needed to protect the tongue when the lingual surfaces of
mandibular molars are being prepared.

13. 3) PULP:
• Great care also is needed to prevent pulpal
injuries during fixed prosthodontic procedures,
especially complete crown preparation.
• Pulpal degeneration can also occur many years
after tooth preparation.
• Extreme temperatures, chemical irritation, or
microorganisms can cause an irreversible pulpitis
particularly when they occur on freshly sectioned
dentinal tubules.

14. THERMAL ACTION CHEMICAL ACTION BACTERIAL
ACTION
FRICTION MATERIALS- ON
FRESHLY CUT DENTIN
ALL CARIOUS DENTIN
NOT REMOVED
WATER SPRAY CAVITY VARNISH OR
DENTIN BONDING
AGENTS
REMOVAL OF CARIOUS
DENTIN FOLLOWED BY
RESTORATION IF
NEEDED
RETENTION FEATURES-
AT SLOW SPEEDS
CLEANING AND
DEGREASING AGENTS
AVOIDED
ZINC PHOSPHATE
CEMENT-
ANTIBACTERIAL

15. THERMAL ACTION:

17. Zach L, Cohen G: Pulp response to externally applied heat.
Oral Surg Oral Med Oral Pathol 19:515, 1965

18. Pulpal temperature rise during
tooth preparation
Group I: air turbine, water cooled
Group II: air turbine, dry
Group III: low speed, water cooled
Group IV: low speed, dry
Zach L, Cohen G: Pulp response to externally applied heat. Oral Surg Oral Med Oral Pathol 19:515, 1965
Acc. To Zach and Cohen:
• Rise of 5.5 °C – 15% necrosis
• Rise of 11.1 ° C – 60% necrosis
• Rise of 16.6 ° C – 100% necrosis

19. CONSERVATION OF TOOTH STRUCTURE:
• Extensive amount of reduction – every dentinal tubule exposed –
communicated directly with the dental pulp.
• Any damage to the odontoblastic process would adversely affect the cell
nucleus at the dentin-pulp interface

20. Use of partial coverage rather than
full coverage restorations
Preparation of tooth with minimal practical
convergence angle (Taper) between axial walls
Preparation of occlusal surface following the
anatomical contour to give uniform reduction
Avoidance of unnecessary apical extension of the
preparation
Selection of marginal geometry which is
conservative and compatible with other principles
Reduction of axial walls with maximal thickness
of remaining dentin surrounding the pulp

21. CONSIDERATIONS AFFECTING FUTURE
DENTAL HEALTH:
INSUFFICIENT
REDUCTION
Over contoured
crown and
periodontal diseases
Occlusal
dysfunction

22. OVER REDUCTION
Retention And
Resistance Are
Reduced
Hypersensitivity,
pulpal inflammation
and necrosis
Tooth fracture

23. RETENTION AND RESISTANCE
• Taper
• Freedom of displacement
• Length
• Substitution of internal features
• Path of insertion

24. RETENTION FORM:
Retention prevents removal of the restoration along the path of
insertion or long axis of the tooth preparation
RESISTANCE FORM:
Resistance prevents dislodgment of the restoration by forces
directed in an apical or oblique direction and prevents any
movement of the restoration under occlusal forces.

26. i) Taper:
• Taper is defined as the convergence of two
opposite-facing external walls of a crown
preparation as viewed in a given plane.
• The axial walls of the preparation must taper
slightly.
• Two opposing external walls must gradually
converge or
• Two opposing internal surfaces of tooth
structure must diverge occlusally.
• The terms angle of convergence and angle of
divergence – state relationships between the
two opposing walls of a preparation.
Nearly parallel sides give
maximum retention

27. Recommended convergence angle is 6 degrees.
For illustrative purposes, one can visualize the
hands of a clock at 12:01, which produces an
angle of 5.5°
To produce 6° taper, each opposing axial
wall must have an inclination of 3°

28. Tilting the bur away from the tooth creates
an undercut
Tilting the bur towards the tooth results
in over reduction and excessive taper

29. Jørgensen KD: The relationship between retention and convergence angle in cemented veneer crowns. Acta
Odontol Scand 13:35, 1955
As the degree of taper of a
preparation increases, its ability
to retain a restoration
decreases

30. ii) Freedom of Displacement:
• More is the freedom of displacement, less is the retention.
Retention is enhanced by
restricting the paths of
withdrawal.
The excessively tapered cone has
infinite paths of withdrawal– Fig. A
Addition of parallel sides grooves
limits the path to one thus
reducing possibility of
dislodgement– Fig. B

31. In an anterior three quarter crown, retentive grooves are made, lingual walls of the grooves
must be perpendicular to the path of displacement

32. iii) Length:
• Longer preparations – more surface area – more retention
• Length must be enough to interfere with the arc of the casting
pivoting about a point on margin on opposite side of restoration.

33. iv) Substitution of internal features:
The basic unit of retention is two
opposing walls with a minimum
taper.
•But in cases such as, when one or
more than one walls are lost due
to decay, it may not be possible to
use opposing walls for retention.
•In such cases, internal features
such as grooves, box forms or
pinholes can be substituted for an
axial wall.
• The addition of a groove limits the
paths of placement, thus the
retention is increased.

34. Resistance of a short preparation can be
improved by addition of grooves

36. v) Path of Insertion:
Path of insertion is an imaginary line along which the restoration will be
placed into or removed from the preparation.
• This is important in preparing bridge abutments, because multiple paths
of insertion must be parallel.
•Path of insertion should be such that it allows the margins of the
retainers to fit against their respective finish line with the removal of
minimum sound tooth structure.

37. •The path of insertion for a posterior full and
partial veneer crown is usually parallel to the long
axis of the tooth.
•However, the path of insertion of an anterior
three-quarter crown is parallel to the incisal two-
thirds of the facial surface (Fig. B), to prevent
unaesthetic display of the metal on the facial
surface (Fig. A).

38. A. Path of insertion of a full veneer
crown parallels the long axis of
tooth.
B. A tipped tooth.
C. If the path of insertion of a tipped
tooth is kept parallel to its long
axis, the seating will be prevented
by adjacent teeth which protrude
into path of insertion.
D. Correct path of insertion for such
a tooth is perpendicular to the
occlusal plane.

39. TAPER
Minimum
6 degrees
excessive
LUTING AGENT
GIC
Zinc phosphate Polycarb-
oxylate
Zinc oxide eugenol
PREPARATION TYPE
complete
partial
onlay
HEIGHT
long
average
short
RESISTANCE FORM

40. STRUCTURAL DURABILITY
• Occlusal Reduction
• Functional Cusp Bevel
• Axial Reduction

41. • A restoration should contain bulk of material
that is adequate to withstand the forces of
occlusion.
• Sufficient tooth structure must be removed to
create space for an adequate bulk of
restorative material to accomplish this without
departing from the normal contours of tooth.
•Occlusal surface should be prepared
anatomically to aid in adequate clearance and
prevent excessive amount of reduction.
i) Occlusal Reduction:

43. Gold crown: 1.5 mm (FC)
1 mm (NFC)
Metal ceramic crown:
1.5-2 mm(FC), 1-1.5 mm (NFC)
Ceramic crown: 2 mm
(entire occlusal surface)
Recommended occlusal reduction for restorations with
different alloys:

44. ii) Functional Cusp Bevel:
• Integral part of occlusal reduction.
• Wide bevel on functional cusps
provides space for adequate bulk
of metal in area of heavy occlusal
loads.

45. A. Function bevel allows for adequate
bulk of restoration.
B. If bevel not given, restoration is
likely to be too thin in the stress-
bearing area
C. If restoration thickness is achieved
by over-tapering axial wall,
retention is compromised.
D. In absence of bevel, to provide
sufficient bulk, lab technicians may
over-contour the restoration which
may result in super-occlusion of the
restoration

46. iii) Axial Reduction:
• Important for securing space for an adequate thickness of restorative
material.
• Inadequate axial reduction will have thin walls which will be subject to
distortion (Fig. B).
• To compensate that, lab technicians may try to provide the necessary
bulk for the restorative material by overcontouring the axial surfaces. This
in turn has disastrous effect n the perodontium (Fig. C).

47. MARGINAL INTEGRITY
• Bevels
• Finish line configuration

48. i) Bevels:
Even the restorations with
high degree of precision in
fitting show some
discrepancy between the
margin and the restoration
d

49. Bevel allows closer approximation of a crown
margin to the tooth.
More acute the angle of the margin-m, or more
obtuse the angle of finish line-p, less is the
discrepancy at the margin, m.

50. Smaller the angle
between the prepared
tooth surface and the
path of insertion, less is
the marginal
discrepancy.
However, angles less
than 25 degrees
produce a margin
which is too thin and
weak.

51. • Acute margin should be continued to be used for metal restorations and
the angle should be kept in 30- 45 degree range.
• Tapered edge in a wax pattern produced by beveling is more readily
adaptable to a die than butt joint.

52. ii) Finish Line Configuration:
• Prosthesis should fit as closely to the
finish line to minimize exposed cement.
• Sufficient strength to withstand the
forces of mastication.
• Should be located where the dentist can
inspect them and the patient can clean
them.

53. FUNCTIONS OF FINISH LINE:
Measure of tooth structure that has been removed
Used to measure the accuracy of an impression
Helps to evaluate a die and trim accurately
Proper fabrication of a wax pattern
Evaluation of a restoration
Helps in determining if the restoration is seated completely

54. • Easily finished w/o soft
tissue trauma
• Easily kept plaque free
• Impressions easily made,
with less tissue damage
• Restorations can be
easily evaluated
• Dental caries, cervical erosion,or
restoration extend subgingivally
• Crown lengthening procedure is
contraindicated
• Proximal contact area extends
apically to level of gingival crest
• Margin of an aesthetic
restoration is to be hidden
• For control of severe root
sensitivity
ADVANTAGES OF
SUPRAGINGIVAL MARGIN:
INDICATIONS OF
SUBGINGIVAL MARGIN:

56. CHAMFER MARGIN:
• Least stress
• Torpedo – less likely to produce a butt
joint
• Formed as a negative image of a round-
ended tapered diamond.
•Indicated: all metal restorations, lingual
margins of ceramic facing restorations

57. HEAVY CHAMFER:
• Rounded internal line angle
• Better support than Chamfer
• Bevel can be added to
provide an even better
support

58. SHOULDER MARGIN:
• Healthy restoration contours and
maximum esthetics
• Minimizes stress that may lead to
fracture of porcelain
• Sharp line angle – stress concentration –
coronal fracture
•Indicated: facial margins of metal-ceramic,
margins of all ceramic restorations

59. RADIAL SHOULDER:
• Essentially same as the shoulder
•Internal line angle rounded
•Cavo surface margin – 90 degrees
•Stress concentration less than in
classic shoulder

60. KNIFE EDGE MARGIN:
• Permits an acute margin of metal
• Thin margin – difficult to wax up
• Susceptible to distortion when
subjected to occlusal forces
• May result in over contoured
restoration to compensate for bulk
• Not indicated.

63. PRESERVATION OF THE PERIODONTIUM
• Placement of margins
• Preservation of Biological
width

64. i) Margin placement:
• Margin placement has direct effect on ultimate
success of the restoration.
• Margins should be as smooth as possible.
• They should be placed in areas that can be
finished well by the dentist and kept clean by
the patient.
• They should be placed in enamel wherever
possible.
• Should be kept supragingival whenever
possible.

65. ii) Preservation of biological width:
• Biologic width describes the combined heights of that the healthy gingival tissue occupies
above the alveolar crest i.e. the connective tissue and epithelial attachments to a tooth.
• The connective tissue attachment having an average height of 1 mm, and the epithelial
attachment also having an average height of 1 mm, leading to the 2 mm dimension.
• Biologic width is essential for — the preservation of periodontium and removal of irritation
that might damage the periodontium.

66. • Violation of the biologic width in
case of subgingival restoration
margins which are less than 2mm
away from the alveolar crest —
lead to ultimate failure of the
restoration
• Bone loss and gingival recession
occur as the body attempts to
recreate room between the
alveolar bone and the margin to
allow space for tissue
reattachment.

67. SUMMARY

68. PRESERVATION
OF TOOTH
STRUCTURE
RETENTION
AND
RESISTANCE
FORM
STRUCTURAL
DURABILITY
• Adjacent teeth
• Soft tissue
• Pulp
• Taper
• Freedom of displacement
• Length
• Substitution of internal features
• Path of insertion
• Occlusal Reduction
• Functional Cusp Bevel
• Axial Reduction

69. MARGINAL
INTEGRITY
PRESERVATION OF
THE
PERIODONTIUM
• Bevels
• Finish line configuration
• Placement of margins
• Preservation of Biological
width

70. Current Concepts of Tooth Preparation:
Tooth preparations for complete crowns:
An art form based on scientific principles
Charles J. Goodacre, Wayne V. Campagni, and Steven A. Aquilino, DDS.
The Journal Of Prosthetic Dentistry, April 2001.
1) The total occlusal convergence, formed between 2 opposing prepared axial surfaces,
ideally should range between 10 and 20 degrees.
2) Three millimeters should be the minimal occlusocervical/incisocervical dimension of
incisors and premolars prepared within the recommended 10 to 20 degrees of total
occlusal convergence.

71. 4) The ratio of the occlusocervical/incisocervical dimension of a prepared tooth to the
faciolingual dimension should be at least 0.4 or higher for all teeth.
5) Whenever possible, teeth should be prepared so that the facioproximal and
linguoproximal corners are preserved, thereby sustaining variation in the circumferential
morphology that enhances resistance form.
6) Teeth without natural circumferential morphology after tooth preparation (round
teeth) or teeth that lack adequate resistance form should be modified with the creation
of grooves/ boxes.
3) The minimal occlusocervical dimension of molars should be 4 mm when prepared with
10 to 20 degrees total occlusal convergence.

72. 7) Many molars need auxiliary grooves or boxes to enhance resistance form because of
their short occlusocervical dimensions.
8) Axial grooves/ boxes should be used routinely when mandibular molars, and they
should be located on the proximal surfaces.
9) When tooth conditions and esthetics permit, finish lines should be located
supragingivally.
10) When subgingival finish lines are required, they should not be extended to the
epithelial attachment.
11) Chamfer finish lines approximately 0.3 mm deep are well suited for all-metal crowns.

73. 12) Type of finish line for metal-ceramic crowns should not be based on marginal fit but
on personal preference, esthetics, ease of formation.
13) Both shoulder and chamfer finish lines can be used with all-ceramic crowns if the
crowns are bonded to the prepared teeth.
14) Axial and occlusal reductions for all-metal crowns should be at least 0.5 mm deep
and 1.0 mm deep, respectively.
For metal-ceramic crowns, facial/axial reduction of 1.5 mm is recommended, whereas
2.0 mm of occlusal reduction is commonly advised.
Two millimeters incisal/occlusal reduction is recommended for all ceramic crowns.

74. 15) Line angles should be rounded on the preparations to reduce stress in the definitive
restoration.
16) Smooth tooth preparation appears to enhance the fit of restorations. Hence the
preparations should be finished with a finishing bur.

75. CONCLUSION

76. The current focus is on conservative tooth preparation that is non-invasive
and that minimally involves dentin.
This trend is rational in the light of the reduction of the caries rate by
fluorides, nutritional counseling and oral hygiene programs.
Principles of tooth preparation are presently being modified to accommodate
modern approaches i.e., acid etching with minimum reduction.
Dentistry is changing from macro tooth preparation to an environment of
molecular chemistry i.e., esthetic bonding.

77. Despite these advances, traditional crowns are still indicated for
majority of patients.
The classic design of the preparation must be visualized so that
modifications, if required, can be instituted.
Diagnosis and disciplined tooth preparation are highly essential
to successful fixed prosthetics.

78. REFERENCES:
 Textbook of Fundamentals of Fixed Prosthodontics by Herbert T. Shillinburg.
Third edition.
 Textbook of Contemporary Fixed Prosthodontics by Rosentiel. Fifth edition.
Laforgia PD, et al: Temperature change in the pulp chamber during complete
crown preparation. J Prosthet Dent 65:56, 1991.
Proussaefs P, et al: The effectiveness of auxiliary features on a tooth preparation
with inadequate resistance form. J Prosthet Dent 91:33, 2004.

79. REFERENCES:
Tooth preparations for complete crowns: An art form based on scientific
principles
Charles J. Goodacre, Wayne V. Campagni, and Steven A. Aquilino, DDS.
The Journal Of Prosthetic Dentistry, April 2001.
Zach L, Cohen G: Pulp response to externally applied heat. Oral Surg Oral Med
Oral
Pathol 19:515, 1965
 Jørgensen KD: The relationship between retention and convergence angle in
cemented veneer crowns. Acta Odontol Scand 13:35, 1955

80. Thank you!!