When a dentist refers to crowning a tooth he means rebuilding that tooth to the shape and size that it should be in order to adequately serve its purpose by means of a prefabricated restoration shaped correctly inside to fit over the natural tooth following reduction, and on the outside to restore the correct size and shape.
When crowning back teeth, function is generally more important than appearance since these teeth tend not to show, and thus a precious metal crown, usually gold alloy, can often be the best option here. In the case of a front tooth, however, it is expected that a crown is constructed to closely resemble a natural tooth and this can be done using porcelain material, either on its own or with a metal sub-structure to make it stronger.
The need for a crown arises when a tooth is quite severely damaged by trauma or decay, and there is then insufficient tooth structure left to adequately hold a filling of some sort. The essence of a filling is that it fits into a tooth, whilst a crown fits over it, hence the name. In some cases with a badly damaged tooth it might be possible to rebuild the tooth with filling material but either the prognosis would be poor because of the heavy load of chewing which might break it down, or else, in the case of an anterior tooth, the result would be unacceptable aesthetically. The dentist then would tend to prescribe a crown as being the best option.
A crown is almost always made in a dental laboratory by skilled technicians and the whole process involves a number of steps. It is usual for the dentist to take impressions of the patient’s teeth which are cast into models at the laboratory, and then the crown or crowns are made on these. Depending on the type of crown there are different methods involved which may involve making a cast precious metal framework or building up an artificial tooth with porcelain powders and paste, and then baking the ceramic to form a lifelike artificial tooth.
Another reason to opt for a crown can be in the case of a tooth that has undergone root-canal therapy. This is because such a tooth is no longer alive and is therefore cut off from any further nutrient supplies from the bloodstream and its inherent strength is therefore considerably reduced. A tooth depends on its blood supply to maintain its strength, much like a plant does, and also its natural, vital appearance. A front tooth may darken and discolour as a result of pulp death and subsequent root-treatment, so a crown can not only improve a tooth’s strength but its appearance too, and is very often considered essential in order to retain that tooth.
In order to place a crown on a tooth it is necessary to remove a fair amount of tooth tissue all round in order to form a central cylindrical shape or core so that there is sufficient space for the crown material. The crown is then cemented on to that core with a special cement of which there are various types. Because the provision of a crown necessitates destroying quite a bit of the natural tooth the dentist needs to be convinced that the advantages outweigh the disadvantages. This is usually fairly clear cut, but on occasions can be something of a dilemma demanding careful technical analysis, and different dentists may have different opinions depending on their training and clinical judgement priorities.
In practice a crown usually takes about two weeks in all because of the time required in the laboratory in order to go through several stages in construction. In the meantime the patient is provided with a temporary crown that is usually made from acrylic or composite and protects the underlying tooth, in addition to making it look reasonably like a normal tooth. At the fitting stage the temporary is removed, and, once the accuracy of fit of the new crown and appearance is established, it is fixed in place using dental cement. The final result is an artificial tooth on top of the natural tooth, with hopefully its root and pulp (“nerve”), intact, and that will serve just as well as the original tooth before it was damaged.
Patients often ask how long a crown will last, and this can be a difficult question to answer precisely because there are so many variables. A crown made from gold alloy on a back tooth is inherently strong and stable and will last a lifetime, and even beyond! An all-porcelain crown is fairly strong but can in time become weakened by the stresses and strains of biting and chewing, and is not a flexible material that gives like gold. Internal stresses may thus cause cracks or even a fracture to occur leading to the need to replace the crown. Having said that, porcelain crowns, and porcelain fused to metal crowns, are generally long lived, and a life of five to fifteen years is considered usual. The main factor that could prejudice the life of a crowned tooth in reality is the same as for a natural virgin tooth, and that is loss of adequate support due to gum disease. Loss of proper gum attachment leads to gradual loosening of a tooth whether it has a crown on it or not, so it is imperative that a person should maintain adequate oral hygiene to prevent plaque build-up around the base of the teeth since this is the prime cause of gum disease. The other danger to the life of a crown is from dental decay getting in underneath and this has to be prevented by making the crown fit as closely as possible at its margins.
The type of crown your dentist recommends depends on the circumstances, and there are technical aspects to consider, and often also aesthetic ones. With regard to technicalities it is necessary to provide adequate thickness of porcelain for strength where the crown is made from porcelain alone, without any metal backing. In such cases it is necessary to reduce the tooth so that it allows for adequate porcelain thickness, but in some cases this may be considered too much of a risk to the tooth pulp. There is also always a greater risk of fracture with all porcelain crowns than porcelain fused to metal ones, and therefore one needs to take into account the amount of load a patient is likely to put on them. If they have a tendency to grind their teeth then this could be an indication for metal-backed crowns.
Crowns made without metal are generally aesthetically superior, looking more lifelike because of the degree of translucency achievable which is not possible with metal-backed crowns. Ceramic crowns may be bonded with luting cement that gives a better bond than that available for metal crowns and this is yet another factor to be taken into consideration.