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Onlay – Dental Restoration Explained

An onlay is a dental restoration that covers one or more cusps of a tooth. It is used when a filling is insufficient but a full crown is not yet necessary.

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Things worth knowing about "Onlay"

An onlay is a dental restoration that covers one or more cusps of a tooth. It is used when a filling is insufficient but a full crown is not yet necessary.

What Is an Onlay?

An onlay is an indirect dental restoration fabricated outside the mouth – in a dental laboratory – and then bonded onto the affected tooth. Unlike a conventional filling (inlay), which remains entirely within the tooth structure, an onlay additionally covers one or more cusps (the raised portions of the chewing surface). It therefore occupies an intermediate position between an inlay and a full dental crown.

Indications – When Is an Onlay Used?

An onlay is recommended when:

the tooth defect is too large for a conventional filling, yet sufficient healthy tooth structure remains to avoid a full crown,
one or more cusps are damaged or at risk and need to be protected by the restoration,
the tooth requires stabilisation following root canal treatment,
heavily worn or fractured posterior teeth need to be restored.

Materials

Onlays can be fabricated from several materials:

Ceramic (full ceramic / zirconia): Aesthetically superior, tooth-coloured material; highly biocompatible and durable.
Composite resin: Also tooth-coloured; slightly less wear-resistant than ceramic but more cost-effective.
Gold or metal alloys: Extremely durable and precise fit; less aesthetic due to the metallic colour.

Fabrication and Treatment Process

Treatment with an onlay typically takes place over two appointments:

First appointment: The dentist removes decayed or damaged tissue and prepares the tooth. An impression (or digital scan) is taken and sent to the laboratory. A temporary onlay protects the tooth in the interim.
Second appointment: The finished onlay is tried in, adjusted if necessary, and permanently cemented or bonded in place.

Modern CAD/CAM systems (computer-aided design and manufacturing) allow some practices to fabricate and place an onlay in a single visit.

Advantages and Disadvantages

Advantages

More natural tooth structure is preserved compared with a full crown
High precision fit due to laboratory fabrication
Long-lasting and stable restoration
Aesthetically pleasing (ceramic or composite options)

Disadvantages

Higher cost than a direct filling
Usually requires two treatment appointments
Some healthy tooth structure must be removed during preparation

Distinction: Inlay, Onlay, and Overlay

These three terms describe different extents of indirect restorations:

Inlay: Lies entirely within the cusps without covering them.
Onlay: Additionally covers one or more cusps.
Overlay: Covers the entire occlusal surface without fully encasing the tooth sides (an intermediate step before a full crown).

References

Shillingburg HT et al. – Fundamentals of Fixed Prosthodontics. 4th ed. Quintessence Publishing, 2012.
Federlin M et al. – Partial ceramic crowns: influence of ceramic thickness, preparation design and luting material on fracture resistance and marginal integrity in vitro. Oper Dent. 2007;32(3):251-260.
Magne P, Belser U – Bonded Porcelain Restorations in the Anterior Dentition: A Biomimetic Approach. Quintessence Publishing, 2002.

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