There are other benefits to inlays and onlays in comparison to metal fillings:
Inlays and onlays are durable — they’re made from tough, hard-wearing materials which last up to 30 years.
They help to strengthen teeth by up to 75 percent, unlike traditional metal fillings which can actually reduce the strength of the teeth by up to 50 percent.
Inlays and onlays prolong tooth life and prevent the need for more dental treatment in the future.

Dental inlays and onlays are used when old fillings need to be removed or replaced. A dental inlay is similar to a filling and fits inside the cusp tips (top edges) of the tooth. A dental onlay is more extensive and extends over the cusps of the treated tooth.

During treatment the dentist removes the old fillings under local anesthesia and takes an impression of the tooth, which is sent to the dental laboratory. The new inlay or onlay is made from this mold in porcelain, gold or composite resin material. The inlay or onlay is then cemented into place at the next appointment. The inlay or onlay blends successfully with the treated tooth and the rest of the teeth to achieve a natural, uniform appearance.