Dental Prosthodontics Mechanical Principles

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Questions and Answers

What is the recommended convergence angle between opposing walls for effective tooth preparation?

8 degrees

12 degrees

6 degrees (correct)

10 degrees

Which type of crown preparation generally provides the highest retention?

Premolar crown

Partial coverage restoration

Complete crown (correct)

Molar crown

How does increasing the height of axial walls influence retention in crowns?

It has no effect.

It decreases retention.

It increases retention. (correct)

It makes the preparation easier.

What effect do sharp occlusoaxial line angles have on retention?

They can precipitate retentive failure. Signup and view all the answers

What is a method to increase retention in excessively tapered tooth preparations?

Adding grooves Signup and view all the answers

Which material is most likely to result in better cement adhesion when used in dental restorations?

Amalgam Signup and view all the answers

How does the smoothness of the internal surface of a restoration impact retention?

It can cause retentive failure at the interface. Signup and view all the answers

Which type of metals generally have better retention in crown preparations?

Base metal alloys Signup and view all the answers

What is the primary purpose of retention form in tooth preparation?

To provide stability against dislodging forces Signup and view all the answers

Which factor does NOT affect the retention form according to the principles of tooth preparation?

Shape of the restoration material Signup and view all the answers

What is the significance of the geometry of the tooth preparation in retention?

It influences the friction between sliding metal parts Signup and view all the answers

What does the term 'taper' refer to in the context of tooth preparation?

The convergence of opposing external walls of a preparation Signup and view all the answers

Which of the following is a mechanical consideration for preventing deformation of the restoration?

Implementing a proper luting technique Signup and view all the answers

What is the relationship between dislodging forces and the retention of a cemented restoration?

Dislodging forces are smaller than those that tend to seat or tilt the prosthesis Signup and view all the answers

What happens to the retention form if the roughness of the restoration fitting surface is decreased?

It may lead to increased dislodging forces Signup and view all the answers

Which principle is essential for the stability and durability of fixed prosthodontics?

Mechanical principles Signup and view all the answers

What is the primary purpose of resistance form in dental restorations?

To oppose lateral dislodging forces Signup and view all the answers

Which factor does NOT affect the resistance form of a cemented restoration?

Color of the restoration Signup and view all the answers

What is the minimum alloy thickness recommended for functional cusps in a dental restoration?

1.5 mm Signup and view all the answers

Which of the following is NOT a factor affecting deformation in dental restorations?

Patient age Signup and view all the answers

What is the optimal minimum material thickness required for all-ceramic restorations to ensure esthetics?

1 to 1.2 mm Signup and view all the answers

What aspect of restoration design can prevent permanent deformation during function?

Strength of the restoration Signup and view all the answers

Which type of restoration is considered to offer the most pleasing esthetic results?

All-Ceramic Restorations Signup and view all the answers

What should margin design achieve in dental restorations?

Ensure enough room for bulk of metal Signup and view all the answers

Study Notes

Mechanical Considerations

The success of fixed prosthodontics relies on following mechanical principles which ensure the restoration's stability and durability
Mechanical considerations can be divided into three categories: providing retention form, providing resistance form, and preventing deformation of the restoration.

Retention

Retention form refers to the quality of a preparation that prevents the restoration from becoming dislodged by forces parallel to the path of placement.
Factors affecting retention form:
- Magnitude of the dislodging forces
- Geometry of the tooth preparation
- Roughness of the restoration fitting surface
- Materials being cemented
- Film thickness of the luting agent
Magnitude of the dislodging forces: Forces that tend to remove a cemented restoration along its path of placement are smaller compared to those that tend to seat or tilt it.
Geometry of the tooth preparation:
- Most fixed dental prostheses depend on the geometric form of the preparation rather than on adhesion for retention because most traditional cements are nonadhesive.
- Taper is defined as the convergence of two opposing external walls of a tooth preparation as viewed in a given plane. The recommended convergence between opposing walls is 6 degrees.
- The surface area of the restoration influences its retention: crowns with tall axial walls are more retentive than those with short axial walls, and molar crowns are more retentive than premolar crowns of similar taper.
- Retention form of an excessively tapered preparation can be increased by adding grooves.
- Sharp occlusoaxial line angles should be rounded to minimize stress concentration, which can precipitate retentive failure.
- Different types of preparations have different retentive values. The retention of a complete crown is more than double that of partial coverage restorations.
Roughness of the restoration fitting surface: When the internal surface of a restoration is very smooth, retentive failure occurs not through the cement but at the cement-restoration interface.
Materials being cemented: Retention is affected by both the type of casting alloy and any core or buildup material that is present on the axial walls of the crown preparation. Cement adheres better to amalgam than to composite resin or cast gold. The base metal alloys are better retained than are less reactive metals with high gold content.

Resistance

Resistance form refers to the restoration's ability to oppose lateral dislodging forces.
Factors affecting resistance form:
- Magnitude and direction of the dislodging forces
- Geometry of the tooth preparation
- Physical properties of the luting agent

Deformation

A restoration must have sufficient strength to prevent permanent deformation during function.
Factors affecting deformation:
- Alloy Selection: The choice of alloy significantly impacts the strength and resistance to deformation.
- Adequate Tooth Reduction: A minimum alloy thickness of about 1.5 mm over functional cusps (buccal in the mandible, lingual in the maxilla) is recommended. Nonfunctional cusps can be protected with less metal, typically 1 mm. This ensures adequate bulk for strength and prevents distortion.
- Margin Design: Proper margin design is critical for preventing distortion. The tooth reduction should provide sufficient room for bulk of metal at the margin, ensuring adequate strength and resistance to deformation during function.

Esthetic Considerations

Dental restorations should look as natural as possible.
Esthetic considerations include:
- All-Ceramic Restorations: Considered the most pleasing esthetic restorations. A minimal material thickness of approximately 1 to 1.2 mm is necessary to ensure optimal esthetics.
- Metal-Ceramic Restorations
- Partial-Coverage Restorations.

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Description

Explore the mechanical considerations vital for fixed prosthodontics. This quiz delves into retention, resistance forms, and factors influencing the stability and durability of dental restorations. Test your knowledge about the impact of geometry, materials, and dislodging forces.