An inlay is usually a filling consisting of a solid substance like gold, porcelain or, less often, a cured composite resin, which is fitted into a cavity in the tooth and cemented into place. This technique involves fabricating the restoration outside of the mouth using the dental impressions of the prepared tooth, rather than placing a soft filling into the prepared tooth before the material sets hard.
An onlay is the same as an inlay, except that it incorporates a replacement for a tooth cusp by covering the area where the missing cusp would be. Crowns are a form of onlays - just one that completely cover the whole tooth surface.
Inlays and onlays are of two kinds - direct and indirect.
Your dentist will remove the decay and shape the tooth. He or she will put a soft material called composite resin on the tooth. The composite resin is molded to fit the tooth. The inlay is removed and hardened in a special oven. Then, your dentist will cement the hardened inlay to your tooth. After the inlay is cemented, it will be polished. Your dentist may have to shape it slightly to adjust the way your teeth come together. There is also new technology that allows the dentist to have a ceramic inlay or onlay made in the office and cemented on at the same visit.
Your dentist will remove the decay. He or she then will make an impression of the prepared tooth and neighboring teeth. The tooth will get a temporary filling. The impression will be sent to a laboratory, which will make the inlay or onlay. At your second visit, your dentist will take out the temporary filling and clean and dry the tooth. He or she will then cement the inlay or onlay to the tooth. Then it will be polished. Your dentist may have to shape it slightly to adjust the way your teeth come together. Some dentists have a machine that allows the dentist to take a picture (instead of an impression) of the prepared tooth. Then a computer guides the machine in making a ceramic inlay, onlay or crown. This process can take less than one hour. Your inlay or onlay can then be cemented into place at the same visit