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 onlay if it extends to replace a tooth’s cusp. They are more effective than fillings in reinforcing teeth to withstand biting forces, and more resistant to further decay. If a tooth’s original structure is too compromised to support a filling, these indirect restorations can provide fantastic, long-lasting, beautiful results.