Introduction Principles of tooth preparation Preservation of tooth structure Retention form taper Surface area Freedom of movement Length Resistance form Height/width ratio path of insertion Structural durability Occlusal reduction Functional cusp bevel Axial reduction Preservation of periodontium Types of margin Biological consideration Conclusion References

1. Principles of
Tooth preparation
Dr Urvashi Sodvadiya

2. ContentIntroduction
Principles of tooth preparation
Preservation of tooth structure
Retention form
taper
Surface area
Freedom of movement
Length
Resistance form
Height/width ratio
path of insertion
Structural durability
Occlusal reduction
Functional cusp bevel
Axial reduction
Preservation of periodontium
Types of margin
Biological consideration
Conclusion
References

3. Principles of tooth preparation
Preservation
of
tooth structure
Retention
&
Resistance form
Structural
durability of
the restoration
Marginal
Integrity
c
Preservation
of
periodontium

4. Preservation of tooth structure

5. Retention & Resistance form
Retention: Ability of the preparation to impede
removal of restoration along its path of insertion
Cement bond: subjected to tensile & shear stress
Resistance: Ability of the preparation to prevent
dislodgement of restoration by forces directed in
an apical, oblique, or horizontal direction
Cement bond: subjected to compressive forces
Whether
restoration will
remain cemented
or not?
Enough
capability to
withstand the
dislodging
forces, encounter
in function

6. Parallel opposing walls
greater will be retention
(Jorgensen et al; 1955)
Taper allows:
 Visualization of prepared walls,
 prevent undercuts,
 compensate for inaccuracies in the
fabrication process,
 Permit more nearly complete seating
of restorations during cementation
Theoretically most
retentive: one with parallel
walls
To avoid undercuts and
allow complete seating of
restoration: 2-6.5 degrees
(Turner et al;1977)
Student: taper of 13-29
degrees
(Eames et al; 1978)
Retention form
“Ability of cement bond to withstand a force
depends largely on the direction of the force in
relation to the cemented surfaces”
Experienced operators:
taper of 8.6- 26.6 degrees
(Kent et al)
Clinically achievable:
taper of 16 degrees
(Weed et al; 1980)
Taper & Retention

7. Recommendations for
degree of taper for
specific teeth

8. Size of the tooth
Extent of coverage by
restoration
Features: grooves and
boxes
Retention formSurface area & Retention
Greater the surface area
Greater will be the retention
(Lorey et al; 1968)

9. To obtain greatest area of cement under shear, the directions in
which a restoration can be removed must be limited to essentially
one path
Retention formArea under shear
• More important for retention
than total surface area
Full porcelain-fused-to-metal crown verses preparation with one
unprepared wall
Retention : grooves, boxes or pinholes for missing wall
Freedom of movement

10. Direction and angulation of
grooves
Retention formLimit the freedom of displacement

11. Partial coverage crown not as retentive as full coverage crown
(a) Definite lingual walls resist displacement.
(b) An oblique lingual wall offers poor
resistance. (c) An undermined facial enamel
plate may fracture. (d) A groove that is too far
lingual does not provide bulk of metal to
support the margin.
Retentive groove

12. Rotation around a vertical axis

13. Same diameter with double
height
Retention formLength
Same height with double
diameter

14. Retention formSurface roughness
Prepared tooth surface should not be highly polished
Same degree of taper with different irregularities
10-degree taper: 40 µm versus 10 µm
(Øilo & Jorgensen; 1978)
Smith et al (1970): didn’t find any difference

15. Resistance form
VS
Through the margins Outside the restoration
Compressive stresses in a perpendicular direction

16. Resistance form
Force applied at an oblique angle
Line of
action
Lever arm
Fulcrum
point
Tangent line
Target point

17. Resistance form
Hight/ width ratio

18. Resistance form

19. Preparation length & Resistance
The preparation with longer
walls (a) interferes with the
tipping displacement of the
restoration better than the
short preparation (b).
A preparation on a tooth
with a smaller diameter (a)
resists pivoting movements
better than a preparation of
equal length on a tooth of
larger diameter (b).
The resistance of a short
preparation (a) can be
improved by adding grooves
(b).
3 mm minimum : for anteriors and premolars
4 mm minimum : for molars
Height to base ratio: 0.4

20. Path of insertion

21. Path of insertion

22. Structural Durability
Occlusal
reduction
Axial
reduction
Functional
cusp bevel
A restoration must contain a bulk of material that is adequate to
withstand the forces of occlusion

23. Occlusal reduction
A restoration must contain a bulk of material that is adequate to
withstand the forces of occlusion
Occlusal clearanceReproduction of basic inclined planes

24. Functional cusp bevel
Integral part of tooth preparation
Crossbite

25. Axial reduction
Thin and
weak walls
Overcontoured
crown

26. Reduction of tooth during
preparation
Minimum metal thickness
 0.5 mm  at the margin
 1.0 mm  non contact area
 1.5 mm  contact area
Minimum ceramic thickness
 1.5 mm  overall
 1.5 mm  non contact area
(1.2 + 0.3)
 2 mm  contact area
(1.5 + 0.5)
Reduction
versus
Clearance

27. Marginal integrity

28. Classification of finish line..
Based on configuration of finish line
a. Feather edge
b. Knife edge
c. Bevel
d. Shoulder
e. Chamfer
Based on location of finish line
a. Supragigival
b. Equigingival
c. Subgingival
Based on margin angle by Kuwata et al
0 a. Margin angle b/w 0 and 30
Bevelled margins
0 b. Margin angle b/w 31and60
chamfer
0 c. Margin angle b/w 61 and 90
Shoulder
Pardo's classification:
a. Inclined vertical
Feather edge, shoulder with bevel
b. Horizontal margins
Shoulder, chamfer

29. Finish line configuration
Chamfer finish line (0.3-0.5 mm):
full metal crown
Deep Chamfer finish line:
Ceramic crown (but not as good
as shoulder finish line)

30. Finish line configuration
Shoulder finish line: full ceramic crown
Generally, not used as a finish line for
cast-metal restoration

31. 856bur
Width:1mm
Widthofchamfer:0.3-0.5
mm
6847 bur
1.2 mm: Width
1-1.2 mm: Width of shoulder

32. Radial Shoulder finish line: Decreased Stress
concentration: 7% decreases (Mullasseril PM;1994)
coarse, flat-end
tapered
diamond
fine, flat-end
tapared
diamond
binangle chisel

33. Bevel to an existing shoulder
A good finish line for preparations with
extremely short walls
It is possible to create an acute edge of
metal at the margin.
Indication: proximal box of inlays and
onlays and for the occlusal shoulder of
onlays and mandibular three-quarter
crowns
Knife edge
• Difficult to accurately wax and cast
• More susceptible to distortion in the
mouth when the casting is subjected to
occlusal forces
• May result in overcontoured
restorations
Indication: lingual surface of mandibular
posterior teeth, on teeth with very convex
axial surfaces

34. Riccitiello F, Amato M, Leone R, Spagnuolo G, Sorrentino R. In vitro evaluation of the marginal fit and internal adaptation of zirconia and lithium disilicate single crowns: micro-CT
comparison between different manufacturing procedures. The open dentistry journal. 2018;12:160.
Radial
shoulder

35. Maghrabi AA, Ayad MF, Garcia‐Godoy F. Relationship of Margin Design for Fiber‐Reinforced Composite Crowns to Compressive Fracture Resistance. Journal of Prosthodontics:
Implant, Esthetic and Reconstructive Dentistry. 2011 Jul;20(5):355-60.
Chamfer
versus
shoulder

36. Why marginal adaptation is different
for different margin type?
d = D sin μ
or
d = D cos ϕ
The more acute the angle of the
margin (μ) or the more obtuse the
angle of the finish line (ϕ), the
shorter the distance between the
restoration margin and the tooth

37. Finish line Advantages Disadvantages
Chamfer Minimal tooth destruction
Minimal stress
Reduces crown strength (ceramic)
Poor aesthetics (ceramic)
Deep chamfer Moderate tooth destruction
Minimal stress in tooth
Reduces crown strength
Potential lip formation
Classic shoulder Maximal esthetics
Maximal crown strength
Prevents overcontouring
Maximal destruction
Maximal tooth stress
Radial shoulder Maximal esthetics
Excellent crown strength
Less stress than classic shoulder
Destructive of tooth
More stress than chamfer
Radial shoulder with a
bevel
Excellent crown strength
Less stress than classic
shoulder
Destructive of tooth
More stress than chamfer
Poor esthetics (necessitates metal
collar)
Knife edge Minimal destruction Overcontouring (ceramic)
Poor esthetics
Weaker crown margin

38. Preservation
of
periodontium

39. Requirements of finish line
As smooth as possible and are fully exposed to
cleansing.
Can be finished by the dentist and kept clean by
the patient.
Finish lines must be placed so that they can be
duplicated by the impression
Should have sufficient strength to withstand the
forces of mastication

40. “Decay does not occur at margins as long as
they are covered by reasonably healthy gum
tissue.”
G V Black (1981)
Considered as “caries-free zone”
Subgingival sulcus
Subgingival margin

41.  Subgingival margin placement: no
longer acceptable.
 major etiologic factor in
periodontitis.
 The deeper the restoration margin
resides in the gingival sulcus, the
greater the inflammatory response.
 No difference between subgingival and
supragingival margins in a 3-year clinical
study, recommended that placement be
supragingival whenever possible. (Richter
WA et al;1973)
Jameson LM, Malone WF. Crown contours and gingival response. J Prosthet Dent 1982;47:620– 624.
Richter WA, Ueno H. Relationship of crown margin placement to gingival inflammation. J Prosthet Dent 1973;30:156–161.

42. Supragingival
margin
Equigingival
margin
Subgingival
margin
Placed in non-esthetic
area
At the crest of the
marginal gingiva
Below the gingiva
Least impact on
periodontium
More impact on the
periodontium
Greatest biologic risk
More plaque retentive May violate biologic
width
Based on location..

43.  Dentist can miss marginal defects as great as 120 μm
when the margins are subgingival. (Christensen GJ;
1966)
 Magnification devices improved the precision of
tooth preparation under simulated clinical conditions.
(M Eichenberger et al; 2018)
Christensen GJ. Marginal fit of gold inlay castings. J Prosthet Dent 1966;16:297–305.
Eichenberger M, Biner N, Amato M, Lussi A, Perrin P. Effect of magnification on the precision of tooth preparation in dentistry. Operative
dentistry. 2018 Sep;43(5):501-7.
Margin location is not as crucial when placed by a highly skilled dentist in the
mouth of a motivated, cooperative patient
Precision of margin
Subgingival margins are unavoidable:
Subgingival caries,
Extensions of previous restorations,
Trauma,
Esthetics.

44. How to place subgingival margin?
 Prep
 Probe
 Pack
 Precision
Probing depth: 1.5 mm
Depth of margin: 0.5-
0.7 mm
Top cord removal,
prior taking
impression

45. Maghrabi AA, Ayad MF, Garcia‐Godoy F. Relationship of Margin Design for Fiber‐Reinforced Composite Crowns to Compressive Fracture Resistance. Journal of Prosthodontics:
Implant, Esthetic and Reconstructive Dentistry. 2011 Jul;20(5):355-60.
Digital versus
conventional
impression
3-dimensional measurement
2-dimensional measurement

46. Biological consideration
 Water cooling
 Age of the patient
 Type of preparation
Cavity preparation also produces
temperature changes, with an increase of
20°C in temperature during dry cavity
preparation 1 mm from the pulp and a
30°C increase 0.5 mm from the pulp.

48. References
• Maghrabi AA, Ayad MF, Garcia‐Godoy F. Relationship of Margin Design for Fiber‐Reinforced
Composite Crowns to Compressive Fracture Resistance. Journal of Prosthodontics: Implant, Esthetic
and Reconstructive Dentistry. 2011 Jul;20(5):355-60.
• Christensen GJ. Marginal fit of gold inlay castings. J Prosthet Dent 1966;16:297–305.
• Eichenberger M, Biner N, Amato M, Lussi A, Perrin P. Effect of magnification on the precision of
tooth preparation in dentistry. Operative dentistry. 2018 Sep;43(5):501-7.
• Jameson LM, Malone WF. Crown contours and gingival response. J Prosthet Dent 1982;47:620–
624.
• Richter WA, Ueno H. Relationship of crown margin placement to gingival inflammation. J Prosthet
Dent 1973;30:156–161.
• Maghrabi AA, Ayad MF, Garcia‐Godoy F. Relationship of Margin Design for Fiber‐Reinforced
Composite Crowns to Compressive Fracture Resistance. Journal of Prosthodontics: Implant, Esthetic
and Reconstructive Dentistry. 2011 Jul;20(5):355-60.
• Riccitiello F, Amato M, Leone R, Spagnuolo G, Sorrentino R. In vitro evaluation of the marginal fit
and internal adaptation of zirconia and lithium disilicate single crowns: micro-CT comparison
between different manufacturing procedures. The open dentistry journal. 2018;12:160.

49. THANK YOU!