Sometimes a simple filling is not always the best course of action. When over 50% of a tooth's biting surface is damaged by trauma or decay your dentist may feel that either an inlay or an onlay is the best from of treatment.

Inlays and onlays are made of porcelain, gold, or a composite resin that is cast in sections that are bonded to the damaged area of a tooth. An inlay, which is similar to a filling, lies inside the cusp tips of the tooth. An onlay is more extensive than an inlay and covers one or more cusps of the damaged tooth. 

Until recently inlays and onlays have been made of gold, but advances in dental technology have made porcelain an attractive alternative.