Tooth preparation is the mechanical treatment intended to restore teeth lacking regenerative ability, aimed at preserving tooth structure while accommodating restorative materials. Key principles involve retention, resistance, structural durability, and maintaining the integrity of the surrounding periodontium. Proper preparation guidelines ensure effective restoration and minimize the risks of biological and mechanical failures.

1. PRINCIPLES OF TOOTH
PREPARATION
Dr Apurva Thampi
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2. 2

3. WHAT IS 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 mechanical preparation or the chemical treatment of the remaining
tooth structure, which enables it to accommodate a restorative material
without incurring mechanical or biological failure.
Marzouk
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4. NEED FOR TOOTH PREPARATION
• Teeth - no regenerative ability  restoration
• Teeth require preparation to receive restoration
• Conservative tooth preparation
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5. BROAD OBJECTIVES
Optimal tooth
preparation
According to Rosensteil,
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6. GUIDELINES OF TOOTH PREPARATION
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7. PRINCIPLES OF TOOTH PREPARATION
Preservation
of tooth
structure
Retention &
Resistance
Structural
durability
Marginal
integrity
Preservation
of the
periodontium
According to Shillingburg,
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8. PRESERVATION OF TOOTH
STRUCTURE
Preservation of
 Adjacent teeth
 Soft tissue
 pulp
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9. PRESERVATION OF ADJACENT TEETH
• Metal matrix band around the tooth – can be
perforated
• Thin layer of proximal enamel retained – protect
adjacent tooth structure
• Thin tapered diamond – “fin” of enamel
• Undesirable angulation to be avoided
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10. PRESERVATION OF SOFT TISSUE
• Careful retraction with mouth mirror or suction tip
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11. PRESERVATION OF PULP
Temperature Chemical action Bacterial action
• Friction • Materials – on
freshly cut dentin
• All carious dentin –
removed
• Water spray should
be used
• Cavity varnish or
dentin bonding
agents - prevents
• Bacteria gain access
- microleakage
• Retention features –
at low speeds
• Cleaning and
degreasing agents –
pulpal irritants
• Zinc phosphate
cement -
antibacterial
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12. 10 – 19
years
20-29
30-39
50-59
20-2910 – 19
years
30-39
50-59
10 – 19
years
20-2930-39
50-59
Maxillary central incisors with
metal ceramic crown
preparation
Maxillary lateral incisors with
metal ceramic crown
preparation
Maxillary canine with metal
ceramic crown preparation
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13. PULPAL TEMPERATURE RISE
IN RELATION TO TOOTH
CONTACT
Grp I : air turbine ; water cooled
Grp II : air turbine ; dry
Grp III : low speed ; water cooled
Grp IV : low speed ; dry
According to Zach and Cohen –
• Rise of 5.5 o C - 15% necrosis
• Rise of 11.1o C - 60% necrosis
• Rise of 16.6o C - 100% necrosis
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14. 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
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15. Partial coverage. Minimum convergence angle. Anatomic reduction
Axial surface reduction.Apical extension. Conservative margin
Thickness of remaining dentin inversely proportional to pulpal
response
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16. CONSIDERATIONS AFFECTING FUTURE
DENTAL HEALTH
• Insufficient axial reduction
• Inadequate occlusal reduction
• Poor choice of margin location
• Over contoured restoration –
periodontal disease
• Poor form – occlusal dysfunction
• Chipping of enamel or crown fracture
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17. RETENTION AND RESISTANCE
• Taper
• Freedom of displacement
• Length
• Substitution of internal features
• Path of insertion
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18. A preparation with good resistance
form will be retentive; the opposite is not
necessarily true.
Retention – A +B
Resistance – B+C+D
A B
C D
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19. RETENTION FORM
Retention prevents removal of
the restoration along the path
of insertion or long axis of the
tooth preparation.
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.
RESISTANCE FORM
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20. TAPER
• 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 - relationships between the two opposing
walls of a preparation.
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21. Nearly parallel sides gives maximum retention
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22. TAPER
• Initially – Ward – 5% - 20% taper (3-12 degrees)
• Recently – 3-5 ; 6 ; 10-14 degrees
• Several studies suggest – 16 degrees clinical taper
(TC)
• Range – 10 degrees on anteriors and 22 degrees on
molars
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23. Relationship between taper and retention – as taper increases, retention decreases
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24. 24

25. ACCORDING TO CHARLES GOODACRE…
Posterior teeth were prepared with greater taper than anterior (mandibular molars)
FL surfaces had greater convergence than MD surfaces
FPD abutments were prepared with greater taper
Monocular vision created greater taper than binocular vision
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26. FREEDOM OF DISPLACEMENT
• Limiting path of insertion
• Limiting torqueing forces in a horizontal plane –
grooves with definate lingual walls
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27. Walls of the groove must be perpendicular to the
rotating forces
Buccal and lingual walls will not resist
displacement if they meet the pulpal wall at
oblique angles
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28. LENGTH
• Longer preparations – more surface area – more
retention
• Short walls – decrease taper – increase resistance
• Preparation on smaller tooth – smaller arc of
displacement
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29. Preparattion of a tooth with smaller diameter
resists displacement better than in a larger tooth
with same height
Resistance of a short preparation can be improved
by adding grooves
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30. SUBSTITUTION OF INTERNAL FEATURES
• Minimal taper may always not be possible
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31. Adding grooves or boxes to a preparation with a
limited path of withdrawal does not markedly affect its
retention because the surface area is not increased
significantly
(Rosenstiel)
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32. A flat gingival seat and adequate axial depth will aid in resisting
forces by increasing surface area
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33. PATH OF INSERTION
Path of insertion is an imaginary line along which the
restoration will be placed into or removed from the
preparation
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34. 34

35. PATH OF INSERTION
• Considered FL and MD
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36. STRUCTURAL DURABILITY
• Occlusal reduction
• Functional cusp bevel
• Axial reduction
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37. OCCLUSAL REDUCTION
Preparation Occlusal reduction
Gold crown 1.5 mm
Metal ceramic crown 1.5 – 2mm
All ceramic crown 2 mm
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38. OCCLUSAL REDUCTION
• Occlusal anatomy must be maintained
• Flat occlusal anatomy – does not allow good
functional morphology
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39. FUNCTIONAL CUSP BEVEL
• Wide bevel (on functional cusps)
• Lingual cusps of maxillary teeth
• Buccal cusps of maxillary teeth
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40. 40

41. • If no bevel – excessive axial inclination – destroy
excessive tooth structure – lessens retention
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42. AXIAL REDUCTION
• Space for adequate restoration material
• Inadequate axial reduction – over contoured
restorations – disastrous periodontal effects
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43. 43

44. “TRUSS EFFECT”
To maximise space –
• Offset
• Occlusal shoulder
• Isthmus
• Proximal groove
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45. MARGINAL INTEGRITY
• Finish line
configuration
• Bevels
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46. FINISH LINE CONFIGURATION
• Fit as closely to the finish line to minimise 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
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47. 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
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48. 48

49. TYPES OF FINISH LINE
• Least stress
• Round end diamond
• Torpedo – less likely to
produce a butt joint
CHAMFER MARGIN
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50. TYPES OF FINISH LINES
• 90 degree cavo-surface
angle
• Rounded internal line
angle
• Better support than
Chamfer
• Bevel can be added to
provide better support
HEAVY CHAMFER
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51. TYPES OF FINISH LINES
• Healthy restoration
contours and maximum
esthetics
• Minimises stress that
may lead to fracture of
porcelain
• Sharp line angle – stress
concentration – coronal
fracture
SHOULDER
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52. TYPES OF FINISH LINES
• Essentially same as the
shoulder
• Internal line angle
rounded
• Cavo surface margin –
90 degrees
• Stress concentration
less than in classic
shoulder RADIAL SHOULDER
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53. TYPES OF FINISH LINES
• 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
KNIFE EDGE
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54. TO BEVEL OR NOT TO BEVEL….
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55. TO BEVEL…..
• Diminish marginal inaccuracy
d = D sin m
or
d = D cos p
• As the angle m is reduced its sine
value also reduces and so does the
value of d…thus reducing the
marginal discrepancy.
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56. ….NOT TO BEVEL
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57. PRESERVATION OF THE
PERIODONTIUM
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58. MARGIN PLACEMENT
• Direct effect on ultimate success of restoration
• Margins should be as smooth as possible
• Placed in area that can be finished well by the dentist
and kept clean by the patient
• Placed in enamel whenever possible
• Should be supragingival whenever possible
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59. SUPRA GINGIVAL MARGIN
• Less potential for soft
tissue damage
• Easily prepared and
finished
• More easily kept clean
• Impressions are more
easily made
• Restorations easily
evaluated at recall
appointments
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60. SUBGINGIVAL MARGIN
:
• Esthetics
• Existing caries, cervical erosion, or restorations extend
subgingivally, and crown-lengthening is not indicated
• Proximal contact area extends to the gingival crest
• Additional retention is needed
• Margin of a metal-ceramic crown is to be hidden behind the
labiogingival crest
• Root sensitivity cannot be controlled by more conservative
procedures, such as the application of dentin bonding agents
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61. • Finish line should not be closer than 2mm to the alveolar
crest
• Placement in this area –
• gingival inflammation
• loss of alveolar crest height
• pocket formation
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62. MARGINAL ADAPTATION
• Junction between a cemented restoration and the tooth -
potential site for recurrent caries
• Casting- fits within 10 µm
• Porcelain margin- 50 µm
• Stepped irregular margin- poor adaptation
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63. CURRENT CONCEPTS
Tooth preparations for complete crowns: An art
form based on scientific principles
Charles Goodacre
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64. • The total occlusal convergence - between 10 and 20 degrees.
• 3 mm should be the minimal occlusocervical /incisocervical dimension
of incisors and premolars prepared within the recommended 10 to 20
degrees of total occlusal convergence.
• The minimal occlusocervical dimension of molars should be 4 mm when
prepared with 10 to 20 degrees total occlusal convergence.
• 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.
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65. • Teeth should be prepared so that the facioproximal and
linguoproximal corners are preserved.
• 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.
• Many molars need auxiliary grooves or boxes to enhance resistance
form because of their short occlusocervical dimensions and the
unfavorable ratio of the occlusocervical dimensions to the faciolingual
dimensions.
• Axial grooves/boxes should be used routinely when mandibular
molars are prepared for fixed partial dentures, and they should be
located on the proximal surfaces.
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66. • When tooth conditions and esthetics permit, finish lines should be located
supragingivally.
• When subgingival finish lines are required, they should not be extended to
the epithelial attachment.
• Chamfer finish lines approximately 0.3 mm deep are well suited for all-
metal crowns.
• Both shoulder and chamfer finish lines can be used with all-ceramic crowns
if the crowns are bonded to the prepared teeth. Depths greater than 1 mm
are not required when a semitranslucent type of allceramic crown is used.
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67. • 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 reductions in excess of 1 mm can
compromise the remaining tooth structure external to the pulp, whereas
2.0 mm of occlusal reduction is commonly achievable even on a young
tooth.
• With all-ceramic crowns, it is not necessary to exceed 1 mm of axial
reduction with semitranslucent systems and higher value, lower chroma
shades.
• 2 mm incisal/occlusal reduction for allceramic crowns.
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68. 14. Line angles should be rounded on all-ceramic tooth preparations to
reduce stress in the definitive restoration.
With crowns that use metal, the primary purpose of line angle rounding
is to facilitate pouring impressions and investing wax patterns without
trapping air bubbles and to facilitate removing casting modules.
15. Smooth tooth preparation appears to enhance the fit of restorations.
Surface roughness generally increases retention with zinc phosphate
cement, but its effect with adhesive cements (polycarboxylate, glass
ionomer, resin) has not been as definitely determined.
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69. CONCLUSION
Successful restoration
Accurate diagnosis
Thoughtful Rx planning
Preparation design
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