Porcelain inlays or onlays refer to indirect fillings made of porcelain that fit into or onto the affected tooth. Its purpose is to make the tooth stronger by holding it together while retaining a natural esthetically pleasing colour. They are used in instances when there is deficient tooth structure to support a filling but the tooth is still in a condition that does not require a crown.
Indirect fillings are similar to regular fillings with the exception being that they are permanently cemented by one's dentist and are made in a dental lab as opposed to being molded to shape in one's mouth. These fillings take two different forms: inlays which fit into the space remaining after the decay of an old filling or cavity has been removed and onlays (partial crowns) which sit on the tooth and builds up its shape. The advantages of indirect fillings are that they are more durable than fillings lasting between 10 and 30 years. Whereas fillings have been known to reduce the strength of a tooth by up to 50% inlays and onlays can actually increase a tooth's strength up to 75%. They are also able to be made from a variety of materials including porcelain gold and tooth-coloured composite resin.
The advantage that porcelain inlays or onlays have over gold inlays or onlays is its esthetic value. However it has the disadvantage of being more brittle and consequently more likely to break than indirect fillings made of gold material. Additionally porcelain has been known to cause accelerated wear of the opposing tooth when biting something that gold does not.
Receiving porcelain inlays or outlays requires two visits to a dental specialist. In the first visit the decay of an old filling or cavity is removed and an impression is taken. This impression is used by a dental laboratory to create the indirect filling and records the shape of the damaged tooth and the surrounding teeth. While the inlay or outlay is being made the patient is given a temporary filling to protect the damaged tooth. During the second visit this temporary filling is removed and the indirect filling is fitted. If it is an acceptable fit it is permanently cemented into place.