The Different Types of Crowns

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It is fairly commonplace for a dentist to say “I think this tooth needs a crown”, and usually this is because the tooth is too far gone to adequately restore any other way. Modern tooth coloured fillings based on composite resins are extremely good, but in the case of a tooth that is very broken down a large composite filling may not be appropriate. It may not look adequately satisfactory, and it does not lend much extra strength to the tooth so there is likely to be failure sooner ratherthan later because of the forces on the tooth during chewing. Thus instead of trying to put a filling in a tooth to restore it, the alternative is to save the inner core of the tooth which contains the vital elements, nerve tissue and blood supply, and to cover that over with a hard protective material that can be built up into the shape of the original tooth, and this is what a crown aims to do.Ideally a crown should not only function like a natural tooth but look like one too, but unfortunately there are technical constraints with materials that sometimes make it difficult to meet both aims adequately, and a certain degree of compromise may be necessary.Thus on a back tooth gold is often used for a full crown rather than either porcelain which might not be strong enough, or porcelain fused to gold because this might necessitate removal of too much tooth tissue in order to make room for the crown materials of both gold and porcelain.

Gold has traditionally been used for dental restorations for many years since it is impervious to the oral environment and doesn’t rust. It is actually a soft metal which in fact is helpful because it gives a little with the forces of biting and flexes slightly, whereas porcelain is rigid and if too much force is applied, instead of giving slightly, would fracture. Gold is more aesthetic in most people’s eyes than silver, copper, or aluminium, and in fact has sometimes been purposely used on teeth to announce to the world that the owner is a wealthy person, or perhaps in certain modern circles that the owner is a rap-master, and showing gold on the front teeth as crowns or “grills” is considered very “bling” by some.

From a technical point of view gold can be cast quite accurately to provide a close fitting crown at the margins which is all important to prevent bacterial ingress underneath, and can even be swaged or coaxed slightly by burnishing when fitting to give an even better marginal fit. A gold crown does not need to be of great thickness to be adequately strong which means that not too much tooth tissue needs to be removed in order to place it. All these features add up to gold being the best functional all-round material for a crown, but the one thing that stands against this is that whilst some people like to show gold in the mouth, most do not, and prefer their teeth to look like natural teeth but as white as possible.

Thus for crowns that are on show it is necessary to use porcelain in some form or another, or porcelain fused to metal in a way that the metal doesn’t show or is confined to the back surface of the crown.

Variations on the Porcelain Crown

Dental porcelain has been made since the 1980s and is a ceramic material made from several inorganic materials including: silica (SiO2), alumina (Al2O3), feldspar (K2O-Al2O3-6SiO2), and other non-metallic oxides. The materials are powdered and mixed with water to form a mouldable paste, which, after shaping is baked (sintered) at temperatures between 850-1300 degrees Centigrade. Then another firing is required to create the glaze to give a nice, shiny surface.

Some variations on the type of ceramic crown come about as a result of different formulations of the basic materials. Zirconia is a very hard ceramic that is used as a strong base material in some types of porcelain restorations and is a form of zirconium oxide.

Leucite, is a mineral composed of potassium and aluminium tectosilicate, K(AlSi2O6), and is used in a type of crown known as the “Empress” which has proved to have excellent physical properties and can be made to match well to natural teeth.

An alternative method of constructing porcelain crowns has arisen since the advent of computers, and this method called CAD/CAM dentistry involves a digital scan, either directly of the patient’s tooth in the mouth or a model made from an impression, and the precise three dimensional measurements are used to enable the cutting and milling of a crown from a solid block of porcelain in a soft state.  Once shaped the result is sintered at high temperature and can then be treated to coloured glazes to produce a lifelike result.

The making of porcelain fused to metal crowns involves several stages. The dental technician first makes a shell of metal that will fit over the prepared tooth and provides a strong sub-structure. A veneering of porcelain is then fused to this metal (in a high-heat oven), giving the crown a tooth coloured appearance. Porcelain fused to metal crowns usually use a gold alloy in order to avoid any problems from saliva acting on the metal, and also gold backing porcelain results in a more pleasing colour than other metals. Porcelain fused to gold crowns can be a good choice for either front or back teeth when the patient has a heavy bite and the provision of porcelain alone might not be adequately strong. They are often prescribed since they look good, and give both dentist and patient peace of mind knowing that they will last well.

A slight disadvantage with these types of crowns is that the metal can cause a fine dark line to be visible sometimes right at the edge where the overlying porcelain is very thin, and this may necessitate the dentist placing that margin just beneath the gum line so that it does not show. In such cases it is imperative to keep the margins free of plaque, but by brushing in a way that doesn’t cause wear of the gum itself. Excessive horizontal brushing can abrade the thin gum tissue whilst plaque accumulation from not brushing will cause gum loosening or shrinkage. Either eventuality can result in the crown margin becoming exposed and spoiling the overall visual effect.

A dentist has to consider both form and function when deciding on the best type of crown in any given circumstance, but you can be sure that there is a solution to your problem when it comes to the need for a crown.