Inlays and onlays are pre-formed in a lab with before being bonded to the tooth. Depending on the amount of tooth structure that has been lost due to wear or decay, your dentist will help you decide which is most appropriate for you.
An inlay fits within the grooves that are within the cusps of your teeth. An onlay, the larger of the two, fits within the grooves but also covers the cusps thus covering more of the tooth’s surface. An onlay is used when the damage is more extensive and the restoration covers the entire chewing surface including one or more tooth cusps.
The procedure for inlays and onlays is the same as for crowns. The first visit involves preparation of the tooth to remove the decayed or damaged portion. This procedure is conservative in comparison to a crown, because having sufficient tooth structure allows the dentist to only remove what would be necessary to restore your tooth.