Tooth Preparation Principles for Prosthodontics

Tooth Preparation Principles

• Finishing Line for Maxillary Partial Veneer Crowns:
  • Must create an acute angle with substantial metal.
  • Enamel must be protected by a bevel to prevent chipping.
  • There are narrow (0.3-0.5mm) and contra-bevel types. Contra-bevel is used when esthetics are less critical; narrow is used with deep bite.

Mechanical Considerations

• Tooth preparation design for fixed prosthodontics must follow mechanical principles to prevent dislodgment or distortion/fracture.
• Mechanical considerations fall into four categories:
  • Retention form
  • Resistance form
  • Preventing restoration deformation
  • A common path of insertion

Retention Form

• The quality of a preparation that prevents removal of the restoration along the insertion path.
• Forces like those caused by sticky food act on the restoration in the insertion direction.

Path of Insertion

• An imaginary line the restoration will follow to be placed or removed from the preparation.

Factors Affecting Retention Form

• Magnitude and direction of dislodging forces
• Geometry of the tooth preparation
• Roughness of the restoration fitting surface
• Cementing material
• Type of luting agent

Geometry of Tooth Preparation

• Most fixed prosthodontics depend on the preparation geometry, not cement, for retention.
• Some cements are non-adhesive; they increase friction between tooth and restoration.
• Ideal preparations have parallel walls.
• Undercuts prevent the restoration from seating.

Taper

• The angle created by the opposing walls of the preparation; a 6-degree angle is recommended for optimal retention.
• Taper can be achieved without tilting the cutting instrument.

Length

• A slight taper is desirable in complete crown preparation. A smaller taper limits withdrawal path, increasing retention.
• Larger taper or unlimited withdrawal path reduces retention.

Features for Retention

• Axial grooves; e.g., V-shaped, U-shaped, box-shaped.
• Grooves, pinholes, and ledges (in preparation).

Surface Area

• Crowns with long axial walls have better retention than those with short walls.
• Molar crowns are more retentive than premolar crowns, due to greater surface area.

Stress Concentration

• Stress is not uniform throughout the cement. Concentrated near the junction of axial and occlusal surfaces. Reducing sharp angles reduces stress.
• Cement strength is important, sharper angles place more stress.

Types of Preparations

• The different types have varying retentive values dependent on the surface area. Retention of complete crowns is approximately double that of partial coverage ones.

Roughness (Restoration Fitting Surface)

• Very smooth surfaces lead to failure through the cement-restoration interface.

Materials

• More reactive alloy materials can have better bonding with luting agents, enabling improved retention.
• Base metal alloys provide better retention than high-gold materials.

Luting Agents

• The type of luting agent used impacts the restoration's retention.
• Adhesive resin cements are more retentive than conventional cements.

Resistance Form

• Quality that prevents displacement of restorations during eccentric contact (oblique or horizontal forces).
• Mastication, and para-functional activity (e.g. chewing on pipes/bruxism), may induce large oblique forces that need to be withstood by the preparation/restoration.
• The axis of rotation associated with forces is at the gingival margin.

Tipping Path

• The path along which a restoration can be displaced due to occlusal forces.
• Resistance areas reside outside the tipping path and are critical to preventing displacement.

Geometrical factors affecting resistance form

• Importance of tooth preparation geometry in providing resistance to rotational displacement.

Occlusal Convergence

• A slight convergence between prepared axial surfaces is critical for good resistance.
• Excessive convergence causes reduced resistance area.

Occluso-gingival height

• Severe shortening of the preparation limits the resistance provided by tooth structure. Ideal occluso-gingival height should not exceed one-third of the total height.

Types of Preparations and Features

• Partial coverage restorations have less resistance than complete coverage crowns.

Axial Grooves

• Provide resistance to displacing forces.
  • V-shaped are used in anterior teeth.
  • U-shaped resist horizontal forces well.
  • Box-shaped provide high resistance to displacing forces.

Types of Restoration and Considerations

• All-ceramic restorations aim for natural aesthetics, with minimal tooth/structure loss to retain optimal health and functionality.
• Metal-ceramic restorations have a minimum reduction of 1.5 mm, and the porcelain thickness is critical for appropriate light reflection.

Partial Coverage Considerations

• The placement of proximal and facial margins impacts esthetic outcomes. A crucial goal is precisely positioning the margin to allow for optimal fit without showing metal, while maximizing retention for success and durability.

Path of Insertion for Restorations

• Multiple ways to guide the restoration (single or in a bridge).
• The path of insertion should parallel the neighbouring tooth contact. Avoiding undercuts is a prime consideration.

This quiz explores the essential principles of tooth preparation, specifically focusing on maxillary partial veneer crowns and mechanical considerations in fixed prosthodontics. It covers finishing lines, retention form, and the path of insertion critical for achieving optimal results in restorative procedures.