These are restorations that are best described as laboratory constructed fillings. They are made of either gold or ceramic. They are cemented into the tooth and form a very strong bond with the tooth surface.
They are used in larger cavities or when a tooth is heavily broken down. They have been largely superseded by modern composite materials and thus not used as often as they used to be. Often, one sees 30-year-old Gold inlays looking as good as they did the day they were originally fitted!
Dental inlays and onlays are the same kind of restoration, but they cover different proportions of the tooth. A dental inlay fills the space in between the cusps, or rounded edges, at the centre of the tooth’s surface. The dental onlay works like an inlay but covers one or more cusps of the tooth.