dr.devi, MDS , Amrita school of dentistry

1. 31-7-17
1

2. CONTENTS
 Introduction
 Definitions & Classifications
 Principles
- Preservation of tooth structure
- Retention & Resistance
- Structural durability
- Marginal integrity
- Preservation of the Periodontium
 Conclusion
 References
2

3. Introduction 3

4. 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. Maintenance of pulp vitality,
adjacent teeth & soft tissues
conservation of tooth
structure
Minimal
display of
metal ,
adequate
thickness of
porcalain
,proper shade
matching
Retention &
resistance
Biological
1
2 Mechanical .
3 Esthetic
According to Rosenstiel ,
5

6. 5
4
3
2
1
Preservation
of the tooth
structure
.
Structural
durability
Preservation
of the
periodontium
Retention &
Resistance
Marginal
integrity
According to Shillingburg, 6

7. 01
Replacing
lost tooth
structure.
02
Preserve
remaining
tooth
03
Coverage:
Partial v/s
complete
04
Margin:
Supragingival
v/s
subgingival
Preservation Of the Tooth
structure
7
1

8. 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.
Depth orientation grooves
, use of proper diamond
points
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9. 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)
9

10. Retention & Resistance
Ideal taper: 6°
Taper
Length
Freedom of
Displacement
Grooves
/proximal
boxes/ pinhole
Substitution of
internal
featuresLimited
numbers of
path
Path of
insertion
102

11. Retention prevents removal
of the restoration along the
path of insertion or long axis
of the tooth preparation.
11

12. 12
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.

13. Retention form 13

14. 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

15. Geometry of the tooth preparation
 Restrained movement (eg. Nut and bolt )
 Sliding pair – two cylindrical surfaces constrained to
slide along one another
15

16. 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

17. 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

18. Parallel walls – maximum retention
Taper :
• visualize preparation walls
• prevent undercuts
• permit more nearly complete seating of
restorations during cementation
Ideal taper: 6°
18

19.  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

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

21. 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
21

22.  Posterior ¾ crown
 parallel to long axis of the tooth
 Anterior ¾ crown
 parallel to incisal ½ of the labial surface
22

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
23

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
24

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

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
26

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

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

29. Freedom of displacement
 Numbers of paths along which a restoration can be
removed from the tooth preparation
 Only one path – maximum retention
29

30. 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
30

31.  Smaller tooth - short rotation radius
 Grooves in the axial walls- reduce
the rotation radius
31

32. Stress concentration
 Retentive failure occurs - cohesive failure in cement
 Stress concentration- around the junction of axial and
occlusal surfaces
 Rounding the internal line angles
32

33. Type of preparation
 Complete crown> partial coverage crowns
 Adding groove/ boxes increases retention
33

34. 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
34

35. 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
35

36. 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
36

37. Resistance form
37
Dislodging forces
Luting agent being
used
Geometry of the
tooth preparation
01
02
03

38. Dislodging forces
 Mastication and parafunctional
activity - substantial horizontal or
oblique forces
 Lateral forces displace the
restoration by causing rotation
around the gingival margin
38

39. Luting agent being used
 Resistance to deformation affected by
compressive strength and modulus of
elasticity
 Adhesive resin> Glass ionomer> Zinc
Phosphate> Polycarboxylate> ZnO-E
39

40. 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
40

41. 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)
41

42. Freedom of displacement
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
42

43. 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: 359-85.
43

44.  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 N Am 2004; 48: 359-85.
44

45. 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
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3

46. Occlusal reduction
 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
46

47. Adequate reduction Inadequate clearance Over preparation
47

48. 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
48

49.  Lack of functional cusp bevel:
Thin area in casting Overcontouring Overinclination
49

50. Axial reduction
 Thin walls of casting– subject to
distortion
 Overcontouring- disastrous effect on
the periodontium
50

51. 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
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4

52. Finish line configurations
 Chamfer
 Heavy chamfer
 Shoulder
 Sloped shoulder
 Radial shoulder
 Shoulder with a bevel
 Knife edge
Acceptable margin
adaptation
Tissue tolerant
surface
Sufficient strength
Fundam
ental
criteria
Adequate contour
52

53. shoulder
•Obtuse angled
•Distinct
•Conservative
•Sinking half of
the bur
•Heavy chamfer
•All ceramic
•External
line angle
per. To LA
•Wide ledge
•Shoulder
with bevel
•Modified
shoulder
•FETD
•End cutting
parallel
sided
carbide bur
•Bin angled
chisel
•Obtuse
•120 degree
•Reduces
unsupported
enamel
•Bulk
•Acute
angled
•Thin
•Adolescent
pts/ tilted/
mand molars
53

54. 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: 359-85.
54

55. 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
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5

56.  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
56

57.  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
57

58.  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
58

59. 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
59

60. Conclusion 60

61. References
 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
61

62.  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
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63. Thank you !
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