Inlay Preparation

Inlay preparations are a common restorative procedure in dentistry, particularly for Class II restorations.

1. Definitions

A. Inlay

• An inlay is a restoration that is fabricated using an indirect procedure. It involves one or more tooth surfaces and may cap one or more cusps but does not cover all cusps.

2. Class II Inlay (Cast Metal) Preparation Procedure

A. Burs Used

• Recommended Burs:
  • No. 271: For initial cavity preparation.
  • No. 169 L: For refining the cavity shape and creating the proximal box.

B. Initial Cavity Preparation

• Similar to Class II Amalgam: The initial cavity preparation is performed similarly to that for Class II amalgam restorations, with the following differences:
  • Occlusal Entry Cut Depth: The initial occlusal entry should be approximately 1.5 mm deep.
  • Cavity Margins Divergence: All cavity margins must diverge occlusally by 2-5 degrees:
    • 2 degrees: When the vertical walls of the cavity are short.
    • 5 degrees: When the vertical walls are long.
  • Proximal Box Margins: The proximal box margins should clear the adjacent tooth by 0.2-0.5 mm, with 0.5 ± 0.2 mm being ideal.

C. Preparation of Bevels and Flares

• Primary and Secondary Flares:
  • Flares are created on the facial and lingual proximal walls, forming the walls in two planes.
  • The secondary flare widens the proximal box, which initially had a clearance of 0.5 mm from the adjacent tooth. This results in:
    • Marginal Metal in Embrasure Area: Placing the marginal metal in the embrasure area allows for better self-cleansing and easier access for cleaning and polishing without excessive dentin removal.
    • Marginal Metal Angle: A 40-degree angle, which is easily burnishable and strong.
    • Enamel Margin Angle: A 140-degree angle, which blunts the enamel margin and increases its strength.
  • Note: Secondary flares are omitted on the mesiofacial proximal walls of maxillary premolars and first molars for esthetic reasons.

D. Gingival Bevels

• Width: Gingival bevels should be 0.5-1 mm wide and blend with the secondary flare, resulting in a marginal metal angle of 30 degrees.
• Purpose:
  • Removal of weak enamel.
  • Creation of a burnishable 30-degree marginal metal.
  • Production of a lap sliding fit at the gingival margin.

E. Occlusal Bevels

• Location: Present on the cavosurface margins of the cavity on the occlusal surface.
• Width: Approximately 1/4th the depth of the respective wall, resulting in a marginal metal angle of 40 degrees.

3. Capping Cusps

A. Indications

• Cusp Involvement: Capping cusps is indicated when more than 1/2 of a cusp is involved and is mandatory when 2/3 or more is involved.

B. Advantages

• Weak Enamel Removal: Helps in removing weak enamel.
• Cavity Margin Location: Moves the cavity margin away from occlusal areas subjected to heavy forces.
• Visualization of Caries: Aids in visualizing the extent of caries, increasing convenience during preparation.

C. Cusp Reduction

• Uniform Metal Thickness: Cusp reduction must provide for a uniform 1.5 mm metal thickness over the reduced cusps.
• Facial Cusp Reduction: For maxillary premolars and first molars, the reduction of the facial cusp should be 0.75-1 mm for esthetic reasons.

D. Reverse Bevel (Counter Bevel)

• Definition: A bevel given on the margins of the reduced cusp.
• Width: Varies to extend beyond any occlusal contact with opposing teeth, resulting in a marginal metal angle of 30 degrees.

E. Retention Considerations

• Retention Form: Cusp reduction decreases the retention form due to reduced vertical wall height. Therefore, proximal retentive grooves are usually recommended.
• Collar and Skirt Features: These features can enhance retention and resistance form.