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

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. Requirements of tooth preparation
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. Guidelines for tooth preparation
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 N Am 2004; 48:

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

9. Preservation of tooth structure
Preserve the remaining tooth structure
Conservation guidelines-
Coverage: Partial v/s complete
Margin: Supragingival v/s subgingival
Conservative preparation- shoulder: less conservative than chamfer

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 & Resistance 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-Prevents rotation of casting about a fixed point

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

13. Dislodging forces
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. Geometry of the tooth preparation
Restrained movement (eg. Nut and bolt )
Sliding pair – two cylindrical surfaces constrained to slide along one another

15. Geometry of the tooth preparation
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. Taper
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
Jorgenson KD. The relationship between retention and convergence angle in cemented veneer crowns. Acta Odontol Scand 1955 Feb;59(2):94-8.

19. Total occlusal convergence
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
When retention and resistance forms were tested by cementing crowns on metal dies, resistance testing was found to be more sensitive to changes inthe TOC than retention testing
Goodacre C J. Designing tooth preparations for optimal success. Dent Clin N Am 2004; 48:

20. Total occlusal convergence gauge
Goodacre C J. Designing tooth preparations for optimal success. Dent Clin N Am 2004; 48:

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

22. Substitution of internal features
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. Path of insertion
Imaginary line along which the restoration will be placed onto and removed from the preparation
Paths of all FPD abutments must parallel each other
Determined before the preparation.
Tipped tooth- perpen to the occlusal plane

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. Facially inclined path of insertion
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 Anterior ¾ crown parallel to long axis of the tooth
parallel to incisal ½ of the labial surface
Minimises display of metal and also allow the groove to be longer and more retentive

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

31. Length and Surface area
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. Stress concentration
Retentive failure occurs - cohesive failure in cement
Stress concentration- around the junction of axial and occlusal surfaces
Rounding the internal line angles

34. Type of preparation 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 Mar;43(3):303-8

36. Roughness of the fitting surface of restoration
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. Materials being cemented
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
Too soft for crowns and bridges
Ni-Cr harder and more reactive…thus more retentive

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

39. Factors influencing retention of cemented restorations

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

41. Dislodging forces
Mastication and parafunctional activity - substantial horizontal or oblique forces
Lateral forces displace the restoration by causing rotation around the gingival margin
Rotation is prevented by any areas of the tooth preparation that are placed in compression, called resistance areas

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

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

44. Type of preparation
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. Freedom of displacement
Groove
Proximal box
Lingual wall perpendicular to the direction of force
Oblique angle
V-shaped groove
Buccal and lingual walls must meet the pulpal wall at 90°
Oblique angle
Horizontal cross section of premolar..
Placing grooves—limits the freedom of displacement and interfere with rotational movement

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

47. Ratio of occlusocervical to faciolingual dimension
Should be 0.4 or higher for all teeth
OC/FL ratio
Total occlusal convergence
0.1
<6°
0.2
<12°
0.3
<18°
0.4
<24°
Goodacre C J. Designing tooth preparations for optimal success. Dent Clin N Am 2004; 48:

48. Circumference form of prepared tooth
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 N Am 2004; 48:

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
When teeth have no corners due to their round morphological form.
Proximal grooves – complete resistance, Facial/lingual grooves – partial resistance
Goodacre C J. Designing tooth preparations for optimal success. Dent Clin N Am 2004; 48:

50. Structural durability
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. Occlusal reduction Full metal restoration: Metal-ceramic crowns :
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. Functional cusp bevel 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. Axial reduction Thin walls of casting– subject to distortion
Overcontouring- disastrous effect on the periodontium

56. Marginal integrity
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. Finish line configurations
Chamfer
Heavy chamfer
Shoulder
Sloped shoulder
Radial shoulder
Shoulder with a bevel
Knife edge

58. Chamfer Indications- Distinct, easily identified Least stress
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. Heavy chamfer 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. Shoulder 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. Sloped shoulder 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. Radial shoulder Modified shoulder Cavosurface 90°
Shoulder width lessened with rounded internal angles
Lesser stress concentration
Good support for porcelain

64. Shoulder with a bevel 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. Knife edge 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. Reduction depths All metal crowns – Metal ceramic crowns –
Chamfer depth: mm
Axial surface reduction: mm
Occlusal reduction: mm
Metal ceramic crowns –
Finish line depth: 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 N Am 2004; 48:

68. Reduction uniformity 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 N Am 2004; 48:

69. Line angle form 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 N Am 2004; 48:

70. Preservation of the periodontium
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. Margin 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

75. Prevention of Damage During Tooth Preparation
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 – C
2mm depth – 10 C
3 mm depth C
Drop in temperature was noted with coolants
1mm depth – C
2mm depth – C
3mm depth – C
Chhatwal N. Effect of tooth preparation and coolants on temperature within the pulp chamber. TPDI 2010;1(2):45-48.

78. A research tool for determination of tooth structure loss both in vitro and in vivo studies

79. References
Shillingburg HT, Fundamentals of Fixed Prosthodontics, 4th edition, USA, Quintessence publications,2012, pp
Rosenstiel SF, Contemporary Fixed Prosthodontics, 4th edition, USA, Mosby, 2006, pp
Goodacre C J. Designing tooth preparations for optimal success. Dent Clin N Am 2004; 48:
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
Parker MH. Resistance form in tooth preparations. Dent Clin N Am 2004; 48:
Owen CP, Retention and resistance in preparations for extracoronal restorations. Part II: Practical and clinical studies, J Prosthet Dent 1986;56(2):
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.