Inlays and onlays are indirect restorations, because the restoration is fabricated outside of the mouth, instead of being built in layers directly into the teeth like a filling. They still perform the same function as a filling, which is to replace tooth structure that has been damaged by a cavity, fracture, etc.
The dentist removes the affected part of the tooth, and takes an impression of the teeth which is sent to a dental lab. A restoration is made which is shaped to match the part of the tooth it will replace. The restoration is then glued in with dental cement.
An indirect restoration is called an inlay if it replaces part of a tooth without extending to the tip of the tooth (which is called a cusp). If it extends to replace a cusp, it is called an onlay.
Inlays and onlays can be more effective than regular 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 basic filling, these indirect restorations can provide fantastic, long-lasting, beautiful results.