Onlay Preparation

Onlay Preparation
Onlay preparations are a type of indirect restoration used to restore teeth
that have significant loss of structure but still retain enough healthy tooth
structure to support a restoration. Onlays are designed to cover one or more
cusps of a tooth and are often used when a full crown is not necessary.

1. Definition of Onlay
A. Onlay

An onlay is a restoration that is
fabricated using an indirect procedure, covering one or more cusps of a
tooth. It is designed to restore the tooth's function and aesthetics while
preserving as much healthy tooth structure as possible.


2. Indications for Onlay Preparation

Extensive Caries: When a tooth has significant decay
that cannot be effectively treated with a filling but does not require a
full crown.
Fractured Teeth: For teeth that have fractured cusps or
significant structural loss.
Strengthening: To reinforce a tooth that has been
weakened by previous restorations or caries.


3. Onlay Preparation Procedure
A. Initial Assessment

Clinical Examination: Assess the extent of caries or
damage to determine if an onlay is appropriate.
Radiographic Evaluation: Use X-rays to evaluate the
tooth structure and surrounding tissues.

B. Tooth Preparation


Burs Used:

Commonly used burs include No. 169 L for initial cavity preparation
and No. 271 for refining the preparation.



Cavity Preparation:

Occlusal Entry: The initial occlusal entry should
be approximately 1.5 mm deep.
Divergence of Walls: All cavity walls should
diverge occlusally by 2-5 degrees:
2 degrees: For short vertical walls.
5 degrees: For long vertical walls.





Proximal Box Preparation:

The proximal box margins should clear adjacent teeth by 0.2-0.5 mm,
with 0.5 ± 0.2 mm being ideal.



C. Bevels and Flares


Facial and Lingual Flares:

Primary and secondary flares should be created on the facial and
lingual proximal walls to form the walls in two planes.
The secondary flare widens the proximal box, allowing for better
access and cleaning.



Gingival Bevels:

Should be 0.5-1 mm wide and blend with the secondary flare,
resulting in a marginal metal angle of 30 degrees.



Occlusal Bevels:

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




4. Dimensions for Onlay Preparation
A. Depth of Preparation

Occlusal Depth: Approximately 1.5 mm to ensure adequate
thickness of the restorative material.
Proximal Box Depth: Should be sufficient to accommodate
the onlay while maintaining the integrity of the tooth structure.

B. Marginal Angles

Facial and Lingual Margins: Should be prepared with a
30-degree angle for burnishability and strength.
Enamel Margins: Ideally, the enamel margins should be
blunted to a 140-degree angle to enhance strength.

C. Cusp Reduction

Cusp Coverage: Cusp reduction is indicated when more
than 1/2 of a cusp is involved, and mandatory when 2/3 or more is involved.
Uniform Metal Thickness: The reduction must provide for
a uniform metal thickness of approximately 1.5 mm 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

Definition: A bevel on the margins of the reduced cusp,
extending beyond any occlusal contact with opposing teeth, resulting in a
marginal metal angle of 30 degrees.


5. Considerations for Onlay Preparation

Retention and Resistance: The preparation should be
designed to maximize retention and resistance form, which may include the
use of proximal retentive grooves and collar features.
Aesthetic Considerations: The preparation should
account for the esthetic requirements, especially in anterior teeth or
visible areas.
Material Selection: The choice of material (e.g., gold,
porcelain, composite) will influence the preparation design and dimensions.