Dental Crown Provision
The technical aspects of making a good crown are primarily that the fit is accurate, and that the crown blends properly into the general arrangement of teeth in the mouth. This means that a crown needs to replicate the features of an ideal natural tooth in having adequate contact points or areas with its neighbouring teeth to stop food packing in between, and in fitting in with the occlusion (the way the teeth meet together) without causing interference. A crown that is too high in the bite, even fractionally, can cause discomfort or even pain on biting, or can result in a deviation of the jaws as the patient subconsciously tries to avoid the prematurity.
The dentist tries to achieve these aims by taking full and accurate records that include jaw relationships, plus impressions of the teeth which are as accurate as possible.
Impressions are usually done by using a rubbery or putty-like material inside a mouth tray made of metal or plastic. The rubber material is sometimes syringed in as a very runny consistency which allows it to record fine detail, or applied as putty and then a second impression taken with a low-viscosity material inside. The material sets within a few minutes to give an impression of the teeth and this is sent to the laboratory for the construction of the crown or crowns. The impression material is fortunately fairly stable but it is best to have the models cast from impressions as soon as possible to avoid errors in fit from shrinkage or distortion.
In the laboratory the impressions are cast using a type of plaster that sets hard to form a solid model of the teeth on which the crown can be constructed. The precise process varies depending on what type of crown is being made. If it’s a porcelain fused to metal one then a thin shell has to be fabricated first from a wax pattern that is converted to gold or precious metal alloy by burning out the wax and pouring in molten metal, and then spinning in a centrifuge. The gold or other metal framework is removed from the mould and trimmed as necessary. The porcelain is then added to the sub-structure as a separate process.
In the case of all-porcelain crowns it is a matter of mixing appropriate powders of various types of porcelain, and then shaping and firing in a furnace to produce the final tooth-like result. There is considerable skill involved in being a good ceramist, similar in a way to that required of a glass-blower, but with added artistic skills necessary to use colours in a way that simulates the properties of natural teeth. One thing that a porcelain crown can achieve that a fused to metal crown can’t is a degree of translucence, where some light passes through as well as being reflected. This property is especially evident in the teeth of young persons, and can be an over-riding factor in providing a good aesthetic result with a crown. The crown obviously needs to match in colour, size, and shape to the surrounding teeth, and it is essential that the technician is given as much information as possible in order to make this possible.
A gold crown follows the same basic procedure as previously described, and the finished crown is returned to the dentist for fitting.
A completely different method of making porcelain crowns has been introduced which involves computer measurements in three dimensions of a patient’s tooth and then the precise cutting out and shaping of a crown from a solid block of porcelain in a “green” state. This method is referred to as CAD/CAM and achieves amazing accuracy of fit but requires considerable outlay on the part of the dentist for the expensive complex equipment required. The other major advantage however, is that usually the crown can be made and fitted on the same day, avoiding the need for delay and for temporary restoration. The traditional method of constructing a crown works just as well provided each step in the process is carried out in a careful, meticulous manner in order to avoid errors creeping in.