Onlays and inlays: restoring a tooth without preparing it for a full crown

Key points

• An onlay replaces only the lost walls and cusps, not the whole crown portion of the tooth.
• Healthy tooth walls are left intact — there is no need to prepare the tooth down to a stump.
• The tooth can often be kept vital, without root canal treatment done purely to retain the restoration.
• A crown is held by preparation shape and cement; an onlay is bonded under an adhesive protocol.
• Suitable for teeth with large old fillings and thinned walls, and for teeth after root canal treatment.

How an onlay differs from a crown

A crown covers the whole tooth, and to fit one the tooth is prepared on all sides. An onlay — a ceramic or hybrid-composite restoration — replaces only the walls and cusps that have been lost, covering the weakened areas.

The clinician removes damaged enamel and dentine and leaves healthy tissue. The more of the patient’s own tissue is preserved, the stronger the foundation for the restoration.

Because there is no need to extend deep below the gum or to devitalise the tooth in order to retain the restoration, the tooth often stays vital.

What an adhesive protocol is

A conventional crown is retained mechanically: by the shape of the preparation and by cement.

An onlay is fixed differently — it is bonded to the tooth tissue. The join forms at a chemical and micromechanical level, and restoration and tooth then work as a single structure.

This is why surface preparation before bonding matters so much: the strength of the join depends on it.

Who it suits

Teeth with large old fillings where the walls have thinned and are at risk of fracture.

Teeth after root canal treatment, where the cusps need covering so they do not break under chewing load.

Patients with increased tooth wear, where the height of the bite has to be restored without aggressive preparation.

Whether an onlay suits a given situation is determined by the extent of damage, the condition of the tissue and the bite.

How load is distributed

The restoration covers weakened cusps and distributes chewing pressure closer to the way the tooth’s own enamel does.

For a tooth whose walls are already thin, this reduces the risk of a split — the scenario in which the tooth has to be removed.

Frequently asked questions

Is an onlay more reliable than a filling?

With significant destruction, as a rule yes: it is made outside the mouth, reproduces the anatomy more precisely and covers weakened cusps. For small defects a direct restoration is enough.

Will the tooth need root canal treatment?

Usually not. One advantage of the technique is that retaining the restoration does not require devitalising the tooth. The decision depends on the state of the pulp, which the clinician assesses.

How long does such a restoration last?

Longevity depends on the material, the quality of bonding, the bite and oral hygiene. A prognosis for a particular tooth is given after examination.

How do inlays, onlays and overlays differ?

They differ in how much is covered: a restoration within the contour of the cusps, one covering part of the cusps, and one covering the entire chewing surface. The choice depends on how much tissue has been lost.

Sources used to prepare this guide

Content updated: 2026-07-31

• Restorative dentistry materials and techniques — American Dental Association, Oral Health Topics