L9: Crown Preparation and Technology – Principles of Tooth Preparations

Question 1

Which are the types of crown restorations?

Answer 1

• All-ceramic crown (Feldspathic, Lithium disilicate)
• Partial coverage crown (veneers, inlays, onlays).
• Full cast gold crown.
• Porcelain fused to metal (PFM) crown.
• Zirconia (ZrO2) crown (monolithic).

Question 2

Which are the principles of tooth preparations?

Answer 2

• Preservation of tooth structure.
• Retention and resistance.
• Structural durability.
• Marginal integrity.
• Preservation of the periodontium.

Question 3

What does preservation of the tooth structure include?

Answer 3

• Not just replace lost tooth structure but also preserve the remaining one.
• Respect integrity of the adjacent teeth.
• Tooth preparations must account for the geometry of the pulp chamber (radiographs prior preparation).

Question 4

How is pulp protection achieved during tooth preparation?

Answer 4

• Always use water spray with rotary instruments and burs.
• Change cutting burs often. Do not put excessive force while preparing.

Question 5

Retention:

Answer 5

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

Question 6

Resistance:

Answer 6

Resistance prevents dislodgment of the restoration by forces directed in an apical direction and prevents any movement of the restoration under occlusal forces.

Question 7

Taper:
How does the taper of prepared tooth affect the retention and resistance of restoration?

Answer 7

The more tapered the prepared teeth are, the less retention the restorations have.

Question 8

Convergence angle:

Answer 8

• Ideal convergence angle: within the range of 2-5°. - Not enough flow of cement.
• A taper of 6° has been proposed.
• Good retention and resistance with enough flow of the cement.
• Minimum convergence angle: 10-20°.
• Only resistance form was evaluated. Clinically acceptable.

Question 9

Surface area:

How does surface area affect retention?

Answer 9

The greater the surface area of the preparation, the greater the retention.

Question 10

Length:
How does the length of tooth preparation affect retention?

Answer 10

Longer preparations have more surface area and therefore more retentive.

Question 11

Grooves:
How do grooves contribute to a tooth preparation?

Answer 11

• Resistance to displacement for a short-walled preparation on a large tooth, can be improved by placing grooves in the axials walls.

Question 12

Path of insertion:

What is path of insertion?

Answer 12

An imaginary line along which the restoration will be placed onto or removed from the preparations.

• It is of special importance for abutment teeth of fixed partial dentures.

Question 13

functional/ non-functional cusp bevel:

Answer 13

Question 14

Structural durability-Axial reduction:

Answer 14

Question 15

Marginal integrity:

What is a margin in fixed prosthodontics?

Answer 15

• The outer edge of a crown, inlay, onlay, or other restoration
• A boundary surface of a tooth preparation also known as the finish line.

Question 16

Which site of tooth preparation is potential for recurrent caries?

Answer 16

The junction between a restoration and the tooth is always a potential site for recurrent caries.

Question 17

How is the risk for recurrent caries or periodontal disease lowered?

Answer 17

The more precisely the restoration is adapted to the tooth, the lower is the risk for recurrent caries or periodontal disease.

Question 18

How should margins be in prepared teeth?

Answer 18

Preparations must have smooth and even margins. Avoid rough, irregular, or “stepped” junctions between tooth and restoration.

Question 19

Margin designs:

Answer 19

Question 20

Marginal reduction:

Answer 20

Question 21

How is preservation of periodontium achieved?

Answer 21

• Proper retraction to protect soft tissues, cheeks and tongue.
• When possible, place the margins in areas that ease hygiene (at or above gingival line).
• Marginal integrity (smooth crown margins).

Question 22

Biologic width:

Violation of biologic width

Answer 22

• The finish line of the restoration should not be placed any closer than 2 mm form the bone crest (violation of biologic width).
• Can result to gingival inflammation, loss of alveolar crest height and periodontal pockets.