Crowns can be made from stainless steel, metallic alloys, porcelain, plastic or ceramics. A combination of metal and ceramic is also possible to maximise strength and simulate the appearance of natural teeth. Commonly used varieties include:
Stainless steel crowns
These are often prefabricated for the back teeth and has to be shaped by your dentist prior to fitting. It can be fitted within one visit, making it convenient and affordable. The crown shape is however generic, resulting in a less than ideal margin and contact with adjacent as well as opposing teeth. Stainless steel crowns are seldom used for front teeth due to their appearance. It is therefore more suitable as temporary restorations in the treatment of baby teeth in children.
Metal crowns are commonly fabricated from alloys of noble (e.g. gold, palladium, platinum and silver) and base metals (e.g. tin, copper, nickel, chromium). Full gold crowns are of a better quality when they are high in noble content (at least 60% noble metal, of which at least 40% must be gold). Metal crowns require less removal of tooth structure, cause less opposing tooth wear and is able to withstand chewing forces well. The metallic colour is the main drawback, making them a good choice for out-of-sight molars, especially in patients with a strong bite or grinding habits.
Porcelain fused to metal crowns
These are hybrid crowns made of a metal core and an outer layer of porcelain. The metal core provides strength while the porcelain mimics the colour, translucence and fluorescence of natural teeth. Porcelain fused to metal crowns attempts to seek the best combination of durability and appearance. Sometimes the metal beneath the porcelain can show through as a dark line, especially at the gum margins of teeth with receded gums.
Plastic crowns are fabricated using resin based acrylic. They are tooth coloured and can be made chair-side in a single visit, making it an affordable and convenient solution. They however tend to wear down faster, are prone to fractures and tend to discolour over time when compared to other tooth coloured crowns. Plastic crowns are more suitable as a temporary restoration.
Leucite reinforced crowns
Commonly referred to as an “Empress Crown”, leucite reinforced crowns consist of a core of pressure injected reinforced ceramics and an outer layer of porcelain. Leucite reinforced crowns have excellent tooth like characteristics and are more fracture resistant compared to traditional porcelain crowns.
Alumina reinforced crowns
Alumina reinforced crowns consist of an alumina core and an outer layer of porcelain. It has been in use for more than 25 years and is at present considered to be the aesthetic standard for tooth coloured crowns. Glass infiltrated alumina cores have recently been introduced to enhanced its bond strength to porcelain.
Yttria-stabilized zirconia oxide is a very hard ceramic material that has recently been introduced in dentistry. Monolithic zirconia crowns are made entirely of zirconia oxide. It comes in a variety of tooth-like shades and is suitable for back teeth due to its hardness. Zirconia cores can be layered with porcelain to further improve its colour, translucence and fluorescents although it does not form a very strong bond with porcelain, rendering it more susceptible to chip.