Inlays and onlays are called indirect restorations, because the restoration is fabricated outside of the mouth, instead of being built in layers directly into the teeth like a filling. The dentist takes impressions of the teeth, then the restoration is shaped to match the part of the tooth it will replace. The restoration is attached with dental cement.
An indirect restoration is an inlay if it covers part of a tooth, without extending to a cusp (pointed tip of tooth). It is an onlay if it extends to replace a tooth’s cusp. Inlays and onlays are an alternative to fillings and also offer structural reinforcement. If a tooth’s original structure is too compromised to support a filling, these indirect restorations are an excellent alternative.