The document discusses the principles of tooth preparation for dental restorations. It is divided into three categories: biologic considerations which focus on tissue health, mechanical considerations related to restoration integrity and durability, and esthetic considerations affecting patient appearance. Key points include the importance of conserving tooth structure, placing margins for easy finishing and hygiene, designing taper and contours for adequate retention and resistance, and selecting materials and techniques to meet esthetic needs. Proper tooth preparation is essential for successful long-term restoration outcomes.

1. BIOMECHANICAL PRINCIPLES
OF TOOTH PREPARATION
DR. AAMIR GODIL
DEPARTMENT OF PROSTHODONTICS
M.A.R.D.C.

2. PRINCIPLES OF TOOTH PREPARATION
The principles of tooth preparation may be
divided into three broad categories:
1. Biologic considerations:
which affect the health of the oral tissues
2. Mechanical considerations:
which affect the integrity and durability of
the restoration
3. Esthetic considerations:
which affect the appearance of the patient
Contemporary fixed prosthodontics. Stephen Rosenstiel, Martin Land, Junhei Fujimoto- 5th edition (2015)

3. BIOLOGIC CONSIDERATIONS

4. BIOLOGIC CONSIDERATIONS
Prevention of
damage
• Adjacent teeth
• Soft tissue
• Pulp (injury:
thermal, chemical,
bacterial)
Conservation of tooth structure
• Partial coverage
• Minimal taper
• Occlusal reduction mimicking
anatomy
• Retain maximal tooth structure
(conservative axial reduction,
orthodontic repositioning)
• Select conservative margin
geometry
• Avoid unnecessary apical extension
of preparation
Avoidance of over-
contouring
• Insufficient axial
reduction:
overcontoured
crowns
• Insufficient
occlusal reduction:
poor form
Margin placement
and adaptation
• Supragingival vs
subgingival
margins
• Smoothly flushed
• Acceptable
marginal gap for
coping is 10 µm
(Byrne et al) and
50 µm (Belser et al)
• Margin geometry
Harmonious occlusion
• Adequate occlusal
reduction
• Consider intentional
endodontics to
accommodate occlusal
preparation for
compensation of supra-
erupted antagonist tooth
• Diagnostic mock-ups and
preparation for planning
Preventing tooth
fracture
• Inlay restorations are
more susceptible to
fracture: occlusal forces
have wedging effect
between opposing walls
of preparation
• Onlay shows lesser
fracture
• Complete (full coverage)
crown provides greatest
protection

5. MARGINS
Supragingival margin
• Easily finished
• Easy hygiene maintenance
• Ease of recording
impression
• Ease of evaluation
Subgingival margin
• Indicated in cases with deep caries and short crown
heights where crown lengthening is contraindicated
• Proximal areas extending apical to level of gingival
crest
• When additional retention and resistance is needed
• Esthetic zone
• To control root sensitivity by application of bonding
agent
Ease of
preparation
Distinct boundary
for wax pattern
fabrication
Ease of
identification
Sufficient bulk of
material
Conservation of
tooth structure

6. MARGIN GEOMETRY
Chamfer Shoulder Sloped Shoulder
Knife EdgeFeather Edge
Radial Shoulder
Shoulder with Bevel

7. MARGIN GEOMETRY
C: Tilting toward the tooth results in an excessive
convergence angle of the preparation.
B: Tilting away from the tooth creates an undercut.
Opposing axial preparation walls diverge in an
occlusal direction.
A: Chamfer margin is formed as the negative
image of a round-ended tapered diamond.

8. MARGIN GEOMETRY

9. MECHANICAL CONSIDERATIONS

10. MECHANICAL CONSIDERATIONS
Retention form
•Magnitude of dislodging forces
(sticky food)
•Geometry of tooth preparation:
•Taper, angle of convergence,
undercut
•Surface area
•Stress concentration
•Type of preparation
•Roughness of surface being
cemented
•Luting agent: type and film
thickness
Resistance form
•Magnitude of dislodging forces
(masticatory load)
•Geometry of tooth preparation
•Axial wall taper
•50 – 220 taper (Shillingburg)
•Height- width ratio 4:10 or more
•Rounded axial line angles: ↓
resistance
•Provide pinholes, grooves, boxes
•Ideal groove height: 4 mm
•Physical properties of luting agent
Preventing deformation
•Strength of material
•Alloy selection
•Adequate tooth reduction
•Margin design
•Grooves and ledges

11. TAPER
• Taper is defined as the convergence of two opposite-facing external walls of a crown preparation as
viewed in a given plane (e.g., taper between a mesial wall and a distal wall, or between a buccal
wall and a lingual wall of a crown preparation).
• The extension of those planes forms an angle described as the angle of convergence.
• The relationship between the degree of axial wall taper and the magnitude of retention was first
demonstrated experimentally by Jørgensen (1955)
• The recommended convergence between opposing walls is 6 degrees, which has been shown to optimize
retention for zinc phosphate cement

12. TAPER
The tapered rotary instrument is moved through a cylindrical path as
the tooth is prepared, and the taper of the instrument should produce
the desired axial wall taper on the completed preparation

13. TAPER
• Resistance is a function of the relationship between axial wall taper, preparation
diameter, and preparation height. It decreases as taper or diameter increases or as
preparation height is reduced.
• Preparation taper of 5 to 22 degrees has been suggested as being within a clinically
acceptable range.

14. HEIGHT OF PREPARATION

15. ESTHETIC CONSIDERATIONS

16. ESTHETIC CONSIDERATIONS
All ceramic vs ceramo-
metal restorations
Metal ceramic
restorations: reduction
and margin placement
Partial coverage
restorations

17. THANK YOU