The document discusses principles of tooth preparation for dental restorations. It summarizes that the all-ceramic crown preparation design for mandibular incisors requires the highest percentage of tooth structure reduction, around 65%, while ceramic veneers for maxillary incisors require the lowest, around 30%. Proper tooth preparation aims to preserve tooth structure, provide retention and resistance form, maintain structural durability and marginal integrity, and preserve the periodontium. Factors such as taper, path of insertion, surface roughness and luting agents influence retention while resistance is also impacted by the geometry and dimensions of the preparation.

1. Vinay Pavan Kumar K
2 nd year MDS student
Department of Prosthodontics
AECS Maaruti College of Dental Sciences

2. Principles of tooth preparation
Preservation of
tooth structure
Retention &
resistance form
Structural
durability
Marginal
integrity
Geometry
-taper
-freedom of
displacement
-path of insertion
-length
-stress
-preparation type
Materials
cemented
Roughness of
fitting surfaces
Dislodging
forces
Luting
agent used
Occlusal
reduction
Axial
reduction
Preservation of
periodontium

3.  Al-Fouzan etal quantified the volume of reduction of tooth
structure associated with different commonly used
preparation designs using microcomputed tomography
 The all-ceramic crown preparation design for the
mandibular central incisors had the highest percentage
(65.26% ± 4.14%) of tooth structure reduction, while the
lowest percentage of tooth structure reduction was
associated with the ceramic veneer preparation design for
maxillary central incisors (30.28% ± 5.54%)
Al-Fouzan A.F Volumetric measurements of removed tooth structure
associated with various preparation designs Int J Prosthodont 2013;26:545–8

4. Tooth preparation
The process of removal of diseased
and/or healthy enamel, dentin and
cementum to shape a tooth to receive a
restoration

5.  Biological -maintenance of pulp vitality,
adjacent teeth & soft tissues
-conservation of tooth structure
 Mechanical - retention & resistance
 Esthetic - minimal display of metal
- adequate thickness of porcelain
- proper shade matching

7.  Total occlusal convergence
 Occlusocervical/incisocervical dimension
 Ratio of OC and FL dimension
 Circumferential form of the prepared tooth
 Reduction uniformity
 Reduction depths
 Finish line location
 Line angle form
Goodacre C J. Designing tooth preparations for optimal success. Dent
Clin NAm 2004; 48: 359-85.

8.  Preservation of tooth structure
 Retention & Resistance
 Structural durability
 Marginal integrity
 Preservation of the periodontium

9.  Preserve the remaining tooth
structure
 Conservation guidelines-
 Coverage: Partial v/s complete
 Margin: Supragingival v/s
subgingival

10.  Preparation of teeth with the minimum practical
convergence angle between axial walls
 Occlusal surface reduction: follow anatomic planes
 Axial surfaces : if necessary, teeth should be
orthodontically repositioned.

11.  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.

12.  Dislodging forces
 Geometry of the tooth preparation
 Roughness of the fitting surface of the restoration
 Materials being cemented
 Luting agent being used

13.  Forces that tend to remove a cemented
restoration along its path of withdrawal
 FPD subject to dislodging forces-
 Flossing under the connectors
 Sticky food

14.  Restrained movement (eg. Nut and bolt )
 Sliding pair – two cylindrical surfaces constrained to
slide along one another

15.  Taper / Total Occlusal Convergence (TOC)
 Substitution of internal features
 Path of insertion
 Freedom of displacement
 Length and Surface area
 Stress concentration
 Type of preparation

16.  Inclination - relationship of one wall of a preparation to
the long axis of that preparation
 Tapered diamond bur: 2-3° inclination
 Opposing surfaces with 3° inclination= 6° taper
External walls
(converge)
Internal walls
(diverge)

17.  Parallel walls – maximum
retention
 Taper
 visualize preparation walls
 prevent undercuts
 permit more nearly complete
seating of restorations during
cementation
 Ideal taper: 6°

18.  More the taper, lesser the retention
Retention
Jorgenson KD. The relationship between retention and convergence angle in
cemented veneer crowns. Acta Odontol Scand 1955 Feb;59(2):94-8.

19.  Angle between two opposing prepared axial surfaces
 Historically TOC : 2°-6°
 Clinical goal : 10°-22°
 TOC beyond 10-22° – auxilliary features needed
 Resistance testing was found to be more sensitive to
changes in the TOC than retention testing
Goodacre C J. Designing tooth preparations for optimal success. Dent
Clin NAm 2004; 48: 359-85.

20. Goodacre C J. Designing tooth preparations for optimal success. Dent
Clin NAm 2004; 48: 359-85.

21. Esteves HJ, Costa N, Esteves IS, Clinical determination of angle convergence in a
tooth preparation for a complete crown. Int J Prosthodont. 2014 Sep-Oct;27(5):472-4.

22.  Basic unit of retention-opposing walls with minimal taper
 Opposing walls not available for use-
 Destroyed previously (severe attrition)
 Partial veneer restorations
 Greater than desirable inclination
Groove Box Pinhole

23.  Imaginary line along which the restoration will be placed
onto and removed from the preparation
 Paths of all FPD abutments must parallel each other

24.  Visual survey - ensures preparation is
neither undercut or overtapered
 Center of the occlusal surface of the
preparation is viewed with one eye
from a distance of 30 cm (12”)
 Binocular vision avoided- undercut
preparation can appear to have an
acceptable taper

25.  In patient’s mouth – mouth mirror is held at an angle
approximately ½ inch above the preparation
 Image viewed with one eye

26.  FPD abutments– common path of insertion
 Firm finger rest established – mirror maneuvered until
one preparation is centered– mirror moved by pivoting
on the finger rest without change in angulation till the 2nd
preparation is centered

27.  Path of insertion considered in 2 dimensions-
mesiodistally and faciolingually.
 Mesiodistal inclination - parallel to contact areas of
adjacent teeth

28.  Faciolingual orientation - affects esthetics of metal
ceramic and partial veneer crowns
Facially inclined path of insertion
prominent facio-occlusal line angle
overcontouring or opaque show-through
 For full veneer crowns
 parallel to long axis of the tooth

29.  Posterior ¾ crown
 parallel to long axis of the tooth
 Anterior ¾ crown
 parallel to incisal ½ of the labial surface

30.  Numbers of paths along which a restoration can be
removed from the tooth preparation
 Only one path – maximum retention

31.  Longer preparation – more surface area – more retentive
 Length must be great enough to interfere with the arc of
the casting pivoting about a point on margin on opposite
side of restoration
 Short preparations – inclination critical

32.  Smaller tooth - short
rotation radius
 Grooves in the axial
walls- reduce the
rotation radius

33.  Retentive failure occurs - cohesive failure
in cement
 Stress concentration- around the junction
of axial and occlusal surfaces
 Rounding the internal line angles

34.  Complete crown> partial coverage crowns
 Adding groove/ boxes increases retention

35. Potts RG, Shillingburg HT Jr, Duncanson MG Jr,Retention and resistance of
preparations for cast restorations. J Prosthet Dent. 1980 Mar;43(3):303-8

36.  Roughening/grooving the restoration -
retention increased
 Prepared by air-abrading the fitting
surface with 50 µm of alumina
 Airborne particle abrasion - increase in
vitro retention by 64%
 Roughening the tooth preparation- not
recommended

37.  Retention affected both by the casting alloy and
the core build-up material
 The more reactive the alloy is, the more
adhesion there will be with certain luting agents
 Type I and II gold alloys- intracoronal
restorations
 Type III and IV gold alloys- crowns and FPD
 Ni-Cr alloys- long span FPD

38.  Adhesive cements- most retentive
 Film thickness of luting agent- effect not
certain
 Adhesive resin> Glass ionomer> Zinc
Phosphate= Polycarboxylate> ZnO-E

40.  Dislodging forces
 Luting agent being used
 Geometry of the tooth preparation

41.  Mastication and parafunctional activity - substantial
horizontal or oblique forces
 Lateral forces displace the restoration by causing
rotation around the gingival margin

42.  Resistance to deformation affected by
compressive strength and modulus of
elasticity
 Adhesive resin> Glass ionomer> Zinc
Phosphate> Polycarboxylate> ZnO-E

43.  Type of preparation
 Freedom of displacement
 Occlusocervical/incisocervical dimension
 Ratio of OC and FL dimension
 Circumferential form of the prepared tooth

44.  Partial coverage restoration< complete crown
(no buccal resistance areas in partial coverage)
 Adding groove/ boxes increases resistance
(greatest if walls are perpendicular to direction
of force)

45. GROOVE
 Lingual wall
perpendicular to the
direction of force
Oblique angle
V-shaped groove
PROXIMAL BOX
 Buccal and lingual walls
must meet the pulpal wall
at 90°
Oblique angle

46. Minimal OC dimension:
 Anteriors - 3mm
 Premolars - 3mm
 Molars - 4mm
Goodacre C J. Designing tooth preparations for optimal success. Dent
Clin NAm 2004; 48: 359-85.
Occlusocervical
dimension
Total occlusal
convergence
1mm <6°
2mm <12°
3mm <17°

47.  Should be 0.4 or higher for all teeth
Goodacre C J. Designing tooth preparations for optimal success. Dent
Clin NAm 2004; 48: 359-85.
OC/FL ratio Total occlusal
convergence
0.1 <6°
0.2 <12°
0.3 <18°
0.4 <24°

48.  Should possess circumferential irregularity
 Maxillary molars – rhomboidal form
 Mandibular molars – rectangular form
 Premolars and anteriors – oval form
Goodacre C J. Designing tooth preparations for optimal success. Dent
Clin NAm 2004; 48: 359-85.

49.  Preserve corners of a tooth preparation
 No axial grooves, boxes should be provided in corners
Chewing and parafunctional habits
Dislodging forces largely faciolingual
So, grooves and boxes on the proximal surfaces
Goodacre C J. Designing tooth preparations for optimal success. Dent
Clin NAm 2004; 48: 359-85.

50.  A restoration must contain a bulk of material that is
adequate to withstand the forces of occlusion
 Bulk should be confined to the space created by the
tooth preparation
 To provide adequate bulk:
▪ Occlusal reduction
▪ Functional cusp bevel
▪ Axial reduction

51.  Full metal restoration:
 1.5 mm – functional cusp
 1mm – non functional cusp
 Metal-ceramic crowns :
 1.5 to 2mm – functional cusp
 1 to 1.5mm – non functional cusp
 All ceramic crowns :
 2mm over all

52. Adequate reduction Inadequate clearance Overpreparation

53.  Wide bevel on-
 Lingual inclines of the maxillary lingual cusps
 Buccal inclines of mandibular buccal cusps
 Adequate bulk of metal in area of heavy occlusal contact

54.  Lack of functional cusp bevel:
Thin area in casting Overcontouring Overinclination

55.  Thin walls of casting– subject to distortion
 Overcontouring- disastrous effect on the
periodontium

56.  Closely adapted margins to finish lines of preparation-
survival of restoration in the oral environment
 Configuration of finish line-
 dictates the shape and bulk of metal at the margins
 affects the marginal adaptation
 affects degree of seating

57.  Chamfer
 Heavy chamfer
 Shoulder
 Sloped shoulder
 Radial shoulder
 Shoulder with a bevel
 Knife edge

58.  Indications-
 Cast metal crowns
 Metal-only portion of PFM crowns
 Distinct, easily identified
 Least stress
 Round end tapered diamond
 Half the tip of the diamond

59.  Indicated for all-ceramic crowns
 90 degree cavosurface angle with a large radius
rounded internal angle
 Round end tapered diamond
 Better than conventional chamfer but not shoulder
 Bevel added - to use with metal restoration

60.  All-ceramic crowns
 Facial margin of PFM crowns
 Wide ledge-
 resistance to occlusal forces
 minimizes stresses which leads to fracture of porcelain
 Flat-end tapered bur
 Healthy contours
 Maximum esthetics

61.  Destruction of more tooth structure
Sharp 90° internal line angle
concentrates stress on tooth
Coronal fracture
 Not used for cast metal restorations

62.  120° sloped shoulder margin
 Facial margin of a metal-ceramic crown
 No unsupported enamel, yet sufficient bulk to allow
thinning of the metal framework to a knife-edge for
acceptable esthetics

63.  Modified shoulder
 Cavosurface 90°
 Shoulder width lessened with rounded internal angles
 Lesser stress concentration
 Good support for porcelain

64.  Indications:
 Proximal box of inlays, onlays
 Occlusal shoulder of onlays and mandibular ¾ crowns
 Facial finish line of metal-ceramic restorations (gingival
esthetics not critical)
 Situations where a shoulder is already present
(destruction by caries, previous restorations)

65.  Bevel:
 allows the cast metal margin to be bent or
burnished against the prepared tooth structure
 minimizes the marginal discrepancy
 removes unsupported enamel

66.  Permit acute margin of metal
 Axial reduction may fade out
 Thin margin - difficult to wax and cast
 Susceptible to distortion
 Indications:
 Mandibular posterior teeth with very convex axial
surfaces
 Lingually tilted lower molars

67.  All metal crowns –
 Chamfer depth: 0.3-0.5 mm
 Axial surface reduction: 0.5 -0.8 mm
 Occlusal reduction: 1- 1.5 mm
 Metal ceramic crowns –
 Finish line depth: 1-1.5 mm
 Occlusal reduction: 2mm
 All ceramic crowns–
 Finish line and facial reduction depth: 1mm
 Incisal/occlusal reduction: 2mm
Goodacre C J. Designing tooth preparations for optimal success. Dent
Clin NAm 2004; 48: 359-85.

68.  Uniformly reduced :
 normal crown form
 improved aesthetic
 Makes easier for laboratory technician to create
esthetic restorations
 Best achieved by placing depth grooves
Goodacre C J. Designing tooth preparations for optimal success. Dent
Clin NAm 2004; 48: 359-85.

69.  Should be rounded (increases crown strength)
 Sharp line angles – stress concentration
 Facilitates laboratory fabrication and fit
 Ease to pour impressions
Goodacre C J. Designing tooth preparations for optimal success. Dent
Clin NAm 2004; 48: 359-85.

70. 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

71.  Supragingival margins
 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

72.  Subgingival margins:
 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

73.  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

74.  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

75.  Adjacent teeth :
 Iatrogenic damage
 Metal matrix band
 Leave a slight lip or fin of proximal enamel
 Soft tissues:
 Careful retraction of lips, cheeks
 Care to protect tongue when lingual surfaces of mandibular
molars prepared
 Pulp
 Temperature
 Chemical action of cements
 Bacterial action (microleakage)

76. Borelli etal In vitro analysis of residual tooth structure of maxillary anterior teeth
after different prosthetic finish line preparations for full-coverage single crowns
Journal of Oral Science, Vol. 55, No. 1, 79-84, 2013

77.  Different preparation depths
 With/without coolants
 Rise in temperature was noted without coolants
 1mm depth – 0.540 C
 2mm depth – 10 C
 3 mm depth - 1.840 C
 Drop in temperature was noted with coolants
 1mm depth – 0.400 C
 2mm depth – 0.820 C
 3mm depth – 1.130 C
Chhatwal N. Effect of tooth preparation and coolants on temperature
within the pulp chamber. TPDI 2010;1(2):45-48.

79.  Shillingburg HT, Fundamentals of Fixed Prosthodontics, 4th
edition, USA, Quintessence publications,2012, pp119-137.
 Rosenstiel SF, Contemporary Fixed Prosthodontics, 4th
edition, USA, Mosby, 2006, pp 166-201.
 Goodacre C J. Designing tooth preparations for optimal
success. Dent Clin N Am 2004; 48: 359-85.
 Borelli etal In vitro analysis of residual tooth structure of
maxillary anterior teeth after different prosthetic finish line
preparations for full-coverage single crowns Journal of Oral
Science, Vol. 55, No. 1, 79-84, 2013
 Al-Fouzan A.F Volumetric measurements of removed tooth
structureassociated with various preparation designs Int J
Prosthodont 2013;26:545–8

80.  Parker MH. Resistance form in tooth preparations. Dent
Clin N Am 2004; 48: 387-96.
 Owen CP, Retention and resistance in preparations for
extracoronal restorations. Part II: Practical and clinical
studies, J Prosthet Dent 1986;56(2):148-153.
 Gilboe DB, Teteruck WR. Fundamentals of extracoronal
tooth preparation. Part I-Retention and resistance form.
J Prosthet Dent 2005;94:105-7.
 Chhatwal N. Effect of tooth preparation and coolants on
temperature within the pulp chamber. TPDI
2010;1(2):45-48.