An inlay replaces the central part of the chewing surfaces, in between the cusps or raised points of the tooth. An onlay is used for larger cavities where decay has destroyed part of the chewing surface and one or more of the cusps.

An inlay or onlay is a type of filling that is used to repair larger cavities in back teeth. The reason we need to use an inlay or onlay is to restore strength and structure to a tooth, and where a normal white composite filling would not be strong enough.