When cavities become too large, an ordinary filling is not strong enough to cope with the normal biting forces. The dentist will need to have stronger fillings made in a laboratory. An inlay is used for this purpose and as the name suggests it fits inside the tooth. An onlay covers the top of the tooth and is used for worn teeth, or where the remaining tooth needs strengthening to prevent it breaking away. Sometimes a combination of both is used. They are either cemented or bonded to the natural tooth.
They can be made of gold, non precious metals, porcelain or special plastics. The choice depends on the position in the mouth in which they are to be placed, the strength of the bite, whether the patient grinds their teeth, and to some extent the wish to have a tooth coloured filling.
There are two stages in fitting these devices: The first is the preparation of the cavity and the taking of an impression of the cavity and of the opposing teeth so the bite can be made correctly. A temporary filling is then fitted. The impression is sent to a laboratory and the filling is constructed The second is the fitting. The tooth may need to be numbed to prevent discomfort during the fitting. There could also be a need for some minor adjustments to the bite but that is all.
As there are two stages and some quite complex laboratory work they are more expensive than ordinary fillings but less than crowns. They last between ten and fifteen years, which is probably longer than ordinary fillings and about the same as crowns.