Filling, inlay or onlay: what is the difference?

How the three differ

A filling is soft material placed directly into the tooth and hardened there. An inlay or onlay is made outside the mouth as one piece of ceramic or composite, from an impression or scan of the tooth, and then bonded in place. An inlay sits between the cusps, the raised points on the chewing surface. An onlay also covers one or more cusps.

Cavities where a filling fits

In small and medium cavities a filling is a quick, conservative and adequate solution, finished in one visit. As the cavity grows, it becomes harder for the dentist to build the right contact with the neighbouring tooth and the shape of the chewing surface inside the mouth. Large composite fillings can wear more over time, and because the material shrinks slightly as it sets, the fit at the edges becomes more delicate.

When inlays and onlays come in

An inlay or onlay is considered when the cavity is wide but most of the tooth walls are still sound. Because the piece is made outside the mouth, the contact points and chewing surface are shaped with more control. If a cusp has been weakened, an onlay covers and protects it. In this way the all-round reduction needed for a full crown is often avoided.

The process and its limits

Inlays and onlays usually take two visits: at the first the tooth is prepared and an impression or scan is taken, and at the second the piece is bonded. Where it can be made on site, this can be shorter. Enough enamel is needed for the piece to hold firmly. If very little tooth remains or the tooth is cracked, a crown is the sounder choice. Which route fits is decided with an examination and an X-ray.

This content is for general information only and does not replace an examination.