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A crown is a type of dental restoration which completely caps or encircles a tooth or dental implant. Crowns are often needed when a large cavity threatens the ongoing health of a tooth.[1] They are typically bonded to the tooth using a dental cement. Crowns can be made from many materials, which are usually fabricated using indirect methods. Crowns are often used to improve the strength or appearance of teeth.
Porcelain-fused-to-metal crowns
Porcelain-fused-to-metal dental crowns (PFMs) have a metal shell on which is fused a veneer of porcelain in a high heat oven. The metal provides strong compression and tensile strength, and the porcelain gives the crown a white tooth-like appearance, suitable for front teeth restorations. These crowns are often made with a partial veneer that covers only the aspects of the crown that are visible. The remaining surfaces of the crown are bare metal. A variety of metal alloys containing precious metals and base metals can be used. The porcelain can be color matched to the adjacent teeth.
Zirconia
Zirconia is a very hard ceramic that is used as a strong base material in some full ceramic restorations. The zirconia used in dentistry iszirconium oxide which has been stabilized with the addition of yttrium oxide. The full name of zirconia used in dentistry is yttria-stabilized zirconia or YSZ.
The zirconia substructure of a crown is usually designed on a digital representation of the patients mouth, which is captured with a 3d digital scan of the patient, impression or model. The substructure is then milled from a block of zirconia in a soft pre-sintered state. Once milled the pre-sintered zirconia is sintered in a furnace, where it shrinks by 20% and reaches it full strength of approximately 850MPa.
On the core structure a dental technician can layer an aesthetic ceramic to create the final colour and shape of the tooth. Sometimes a ‘monolithic’ crown can be made exclusively of the zirconia ceramic with no aesthetic ceramic layered on top. These crowns tend to be dense in appearance with a high value and lack translucency and fluorescence.
Source: http://en.wikipedia.org/wiki/Crown_(dentistry)
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