Fixed partial dentures

1. Biomechanical Principles of
Tooth Preparation in FPD
Guided by :
Dr. S. R. Godbole
Dr. Md. Abid Zahir Hussain
Presented by : Dipti Rane

2. Introduction

3. Tooth Preparation
The process of removal of diseased &/or healthy
enamel and dentin & cementum to shape a tooth to
receive a restoration. (G.P.T-8)
The mechanical treatment of dental disease or injury
to hard tissues that restores a tooth to original form.
Tylman’s 8th ed

4. Objectives of tooth preparation
Reduction of the tooth in miniature to
provide retainer support.
Preservation of healthy tooth structure to secure
resistance form.
Provision for acceptable finish lines.
Performing pragmatic axial tooth reduction to
encourage favorable tissue response.
Tylman’s 8th ed. Pg no 113.

5. Mechanical
Retention form
Resistance form
Deformation
Biologic
Conservation of tooth structure
Avoid overcontouring
Supragingival margin
Harmonious occlusion
Protection against
fracture
Esthetic
Minimum display of metal
Max thickness of porcelain
Porcelain occlusal surface
Subgingival margin
Rosenstiel 4th ed pg no 209

6. Biologic Considerations

7. Prevention of damage to adjacent teeth
Matrix band

8. Proximal enamel lip
Tooth is wider at the
contact area than the
CEJ by 1.5mm.

9. Prevention of soft tissue damage
Mouth mirror
Cotton rolls Rubber dam
Aspirator tip Flanged saliva ejector

10. Protecting gingival margin
Placing retraction cord
before tooth preparation

11. Prevention of damage to pulp
Causes of pulpal injury
Temperature
Chemical irritation
Bacterial action

12. Temperature
Do not use excessive pressure -use feather light
touch with burs on the tooth
Higher rotational speeds – above 1,00,000 rpm
vibrations least perceptible to pulp
Cutting instruments
Sharper the bur – less the heat generated

13. Coolants
• Air spray- dessication of dentinal tubules
• Air water spray- prevents heat build up
prevents clogging
improves visibility &
cutting efficiency
Heat should not be raised > 46 degC
Sturdevant’s Operative Dentistry

14. Chemical action
Dental materials – resins, luting agents
Zinc oxide eugenol – obtundent action on pulp
Zinc phosphate – antimicrobial properties
(PH 3.5)
Glass ionomer – releases fluoride
Cavity varnish- forms a barrier
Dentin desensitizers – seal & desensitize dentin
after tooth preparation

15. Bacterial action
All carious dentin should be removed
Indirect pulp capping not recommended
Preoperative radiographs and pulp vitality tests
essential.

16. Conservation of tooth structure
Use of partial coverage restorations rather than
complete coverage restorations.
Preparation of teeth with the minimum practical
convergence angle (taper) between axial walls

17. Taper
The convergence of two opposing external
walls of a tooth preparation as viewed in a
given plane. (GPT-8)
Excessive taper results in
considerable loss of
tooth structure.

18. Occlusal surface reduction should follow the
anatomic planes
Selection of a conservative margin compatible with
the other principles of tooth preparation
Avoidance of unnecessary apical extension of
the preparation

19. Preparation of axial surfaces such that maximum
thickness of tooth structure is retained.

20. Axial reduction
Crown should duplicate contours and profile of
the original tooth.
Avoid overcontouring.

21. Margin Placement
Whenever possible, the margin of the preparation
should be supragingival
Subgingival margins often
lead to the periodontal disease

22. Supragingival Margin: coronal to the gingival
crest.
Equi gingival margin at the gingival crest
Subgingival Margin apical to the free gingival
margin.
JPD 1990; 64: 636-42.

23. Advantages of supragingival margins
They can be easily finished.
They are more easily kept clean.
Impressions are more easily made .
Less potential to soft tissue damage.
Restorations can be easily evaluated at recall
appointments.

24. INDICATION OF SUBGINGIVAL
MARGIN
Caries, erosion, restoration extending subgingivally
Additional retention is desired
Short clinical crowns
Esthetics
Root sensitivity
Modification of axial contour is indicated

25. Feather edge
Advantage: Conservation of tooth structure
Disadvantage: Does not provide sufficient bulk
Indications: Not recommended

26. Chisel Edge
Advantage: Conservation of tooth structure.
Disadvantage: Location of margin is difficult
thus produces overcontouring
Indication: Occasionally on the tilted tooth,
Not acceptable

27. Bevel
Advantage: Removes unsupported enamel,
allows finishing of metal
Disadvantage: Extends preparation into sulcus
Indication: Facial margins of maxillary partial
coverage restoration, inlays, onlays

28. Chamfer
Advantages: Distinct margin, adequate bulk,
easier to control
Disadvantages: unsupported lip of enamel
Indications: Cast metal restorations, lingual
margin of metal ceramic crowns

29. Shoulder
Advantages: Bulk of material.
Disadvantages: Less conservative.
Indications: Facial margins of metal ceramic
crowns, all ceramic crowns

30. Sloped shoulder
Advantages: Bulk of material, reduce
unsupported enamel.
Disadvantages: Less conservative.
Indications: Facial margins of metal ceramic
crowns.

31. Shoulder with bevel
Advantages: Bulk of material, removes
unsupported enamel
Disadvantages: Less conservative, extends
preparation subgingivally.
Indications: Facial margin of metal ceramic
crown.

32. Margin adaptation
d = D sin m

33. Occlusal considerations
Careful analysis with preoperative radiographs
Diagnostic preparation and waxing procedure
Optimal occlusal clearance

34. MECHANICAL CONSIDERATIONS
Providing Retention form
Providing Resistance form
Preventing Deformation of the
restoration

35. Retention form
It is the ability of a preparation to prevent
removal of restoration along its path of
insertion.

36. Factors To be considered For Retention
1. Magnitude of the dislodging forces.
2. Geometry of the tooth preparation.
3. Roughness of the fitting surface of the
restoration.
4. Materials being cemented.
5. Type of the luting agent.
Rosenstiel pg no 226

37. 1. Magnitude & direction of dislodging
force
Forces that tend to remove a restoration are small
compared to those tend to seat it.
It depends on the stickiness of the food and the
surface area of the restoration being pulled

38. 2. Geometry of the Tooth Preparation
2 opposing vertical surfaces in same preparation
provide retention
Extracoronal restoration -Sleeve retention
Intracoronal restoration -Wedge retention

39. Taper
Jorgensen and Kaufman et al have demonstrated
that as retention decreases as taper is increased.
D.C.N.A 2001: 85 :363-376

40. A taper of 2.5 to 6.5° has been suggested
as optimum.
To produce 6º Angle of convergence, the
opposing axial wall should have an
inclination of 3°.
Shillingburg – 6°
Ward – 3-12°
Tylman – 2-5° per side

41. Evaluation of Taper
Correct method Incorrect method

42. Retention is improved by limiting the number of
paths of withdrawl.
Maximum retention is achieved
when there is only one path
Short, over tapered preparation
less retentive due to infinite number of paths.

43. Surface area
Longer preparation will have
more surface area & will be more
retentive.
Wider preparation with large
diameter will have greater
retention than a narrower
preparation of same length

44. Full veneer crown is more retentive than a partial
veneer.
To create more retentive forms grooves, pinholes
or boxes are substituted for missing axial walls.
The groove should be distinct
& perpendicular to the adjoining
axial surfaces

45. 3. Surface roughness
When the internal surface of a restoration is very
smooth, retentive failure occurs at the cement-
restoration interface.
If the restoration is roughened retention increases.
It is most effectively prepared by air-abrading the
fitting surface with 50 µm of alumina

46. 4. Type of restorative material
More reactive the alloy, more adhesion with luting
agents.
Base metal alloys > Gold alloys

47. 5. Type of luting agent
Adhesive resin cements are most retentive
followed by Glass Ionomer Cement, zinc
polycarboxylate & zinc phosphate and zinc oxide
eugenol
Film thickness of luting agent
< 25 µm Shillingburg

48. Resistance form
It is the ability of the preparation to
prevent dislodgement of the restoration by
forces directed in oblique, apical or
horizontal direction.

49. Occlusogingival length
Decreasing the length of preparation decreases the
resisting area.
Crown with long axial walls resist
strong tipping force.

50. Tooth width
Crown on the narrow tooth have greater resistance
to tipping than on the wider tooth.
Crown On the narrower tooth has shorter radius of
rotation resulting in lower tangent line & large
resisting area.

51. Taper
Resisting area decreases as the taper increases.
In ideally tapered preparation, the resisting area
covers less than half the axial walls.

52. To provide adequate resistance minimum
occlusocervical dimension for
Incisors & premolars 3mm
Molars 4mm
With 10 -20 deg of taper
Occlusocervical/ incisocervical dimension
(DCNA 2004 :48 :359-385 )

53. Rotation around vertical axis
Partial veneer crown without grooves has little
resistance to rotation.
Grooves provide resistance
by blocking arc of rotation.

54. Path of insertion
It is an imaginary line along which the restoration
will be placed onto or removed from the
preparation

55. Evaluate preparation in mouth

56. The walls of a groove must be perpendicular to
rotating forces to resist displacement.
Grooves also help in limiting the path of
placement.

57. Grooves should be placed parallel to the long
axis of the tooth.
In anterior teeth it should be parallel to incisal
two thirds of the facial surface.

58. Occlusal Reduction
• For gold alloys- 1.5mm on functional cusps &
1mm on nonfunctional cusp.
• Metal ceramic crowns- 1.5 to 2mm on functional
cusps & 1 to 1.5mm on nonfunctional cusp.
• All ceramic- 2mm clearance.

59. Functional cusp bevel

60. Often tipped tooth are short of occlusal plane
& require less reduction than tooth in normal
occlusion.
Uniform reduction will produce excessive
occlusal clearance & shortened axial walls.

61. Axial Reduction
Adequate axial reduction creates space for bulk of
metal within normal contours of tooth.
Inadequate axial reduction results in thin, weak
walls or a restoration with bulky, plaque
promoting contours.

62. Preventing deformation of restoration
Alloy selection – Type III or IV gold alloys
Base metal alloys
Alloy thickness –
Base metals 0.3- 0.5 mm
Gold alloys 0.7 mm
Margin selection- Sufficient bulk for restoration

63. ESTHETIC CONSIDERATIONS
Proper case history & intraoral examination
Assessment during smiling, talking etc.
Patients esthetic requirements
Patient prefer restorations which looks more
natural
No display of metal

64. Colour , translucency
Harmonious transition from restoration to
tooth margin
Contour, shape
Masking of metal
Lip line

65. All ceramic crowns
An incisal/occlusal reduction 2mm
Facial and lingual reduction -1.2mm
Margin - shoulder

66. Porcelain laminate veneers
Facial reduction of 0.5 mm
Proximal reduction extended to gingival crest
leaving contact area intact.
Margin -heavy chamfer

67. Metal ceramic crown
• Facial Reduction - minimum 1.5 mm.
• Incisal reduction - 2 mm
• Lingual Reduction – 0.5 mm

68. Gingival porcelain
Porcelain labial margin

69. Summary
Form of prepared teeth & amount of tooth
structure removed are important contributors to
the mechanical, biological and esthetic success of
the overlying crown or FPD.
Therefore it is important to develop clinical
guidelines that can be used to optimize success in
fixed prosthodontics.

70. REFERENCES
1. Shillingburg HT. Fundamentals of fixed prosthodontics. 3rd ed
2. Rosenstiel SF. Contemporary fixed prosthodontics 4th ed
3. Tylman’s Theory and Practice of Fixed Prosthodontics. 8th ed
4. Parker MH. Resistance form in tooth preparation . D.C.N.A 2004,48:
387-396.
5. Goodacre CJ. Designing tooth preparations for optimal success.
D.C.N.A 2004,48: 359-385.
6. Gilboe DB. Fundamentals of extracoronal tooth preparation. J
prosthet Dent 2005;94:105-7.

71. 7. Parker MH. A technique to determine a desired preparation axial
inclination. J prosthet Dent 2003;90:401-5.
8. Donovan TE. Cervical margin design with contemporary esthetic
restoration. D.C.N.A 2004,48: 417-431
9. Shillingburg: Fundamentals of tooth preparations for cast metal and
porcelain restorations
10. Sturdevant’s Operative Dentistry. 4th ed
11. Summitt JB, Robbins JW. Fundamentals of operative dentistry 3rd ed
12. Shovelton DS, Kantorowicz GF. Inlays,Crowns & Bridges. A clinical
Handbook. 5th ed
13. Goodacre CJ. Tooth preparations for complete crowns. J prosthet
Dent 2001;85:363-76.

72. THANK YOU