Inlays and onlays are used when direct restorations (composite and amalgam) are less than adequate. Because a series of laboratory procedures is involved, they require two visits, thus the term indirect. The first visit is for preparation of the tooth and the second for placement of the final restoration.
Inlays are restorations that are placed into the tooth and are cemented or bonded in place. They are limited to teeth that have enough well supported tooth structure remaining to support the restoration.
Onlays are similar to inlays but also cover or replace parts of the tooth that are weak or missing.
Both inlays and onlays require precise tooth preparation followed by an impression of the prepared tooth. A temporary filling is placed to protect the tooth until your second visit. In the laboratory the impression is used to create a plaster model and a restoration is made. At a subsequent visit, the temporary filling is removed, the inlay or onlay is checked for accurate fit and bite and is then cemented or bonded to the prepared tooth. The materials used are either high density heat cured plastics, porcelain or gold. The result is a more durable, more protective option to traditional direct restorations (composite or amalgam).
Traditional fillings fill the voids left by decay but can do little to make up for the resulting loss in strength. As an alternative, since inlays and onlays are bonded directly onto the tooth using special high-strength resins, they can actually increase the strength of a tooth by up to 75 percent. As a result, they can last from 10 to 30 years. In some cases where the damage to the tooth is not extensive enough to merit an entire crown, onlays can provide a very good alternative.