Zirconia is a very hard ceramic that is used as a strong base material in some full ceramic restorations. Zirconia is relatively new in dentistry and the published clinical data is correspondingly limited. The zirconia used in dentistry is zirconium 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 (core) is usually designed on a digital representation of the patient’s mouth, which is captured with a 3d digital scan of the patient, impression, or model. The core is then milled from a block of zirconia in a soft pre-sintered state. Once milled, the zirconia is sintered in a furnace where it shrinks by 20% and reaches its full strength of 850MPa to 1000MPa.
The zirconia core structure can be layered with tooth tissue-like feldspathic porcelain to create the final color and shape of the tooth. Because bond strength of layered porcelain fused to zirconia is not strong, “monolithic” zirconia crowns are often made entirely of the zirconia ceramic with no tooth tissue-like porcelain layered on top. Zirconia is the hardest known ceramic in industry and the strongest material used in dentistry. Monolithic zirconia crowns tend to be opaque in appearance with a high value and they lack translucency and fluorescence. For the sake of appearance, many dentists will not use monolithic crowns on anterior (front) teeth.
To a large extent, materials selection in dentistry determine the strength and appearance of a crown. Some monolithic zirconia materials produce the strongest crowns in dentistry (the registered strength for some zirconia crown materials is near 1000MPa.), but these crowns are not usually considered to be natural enough for teeth in the front of the mouth; though not as strong, some of the newer zirconia materials are of better appearance, but they are still not generally as good as porcelain fused crowns. When porcelain is fused to the zirconia core, these crowns are more natural than the monolithic zirconia crowns but they are not strong. By contrast, when porcelain is fused to glass infiltrated alumina, crowns are very natural-looking and very strong, though not as strong as monolithic zirconia crowns. Another monolithic material, lithium-disilicate, produces extremely translucent leucite-reinforced crowns that often appear to be too gray in the mouth, and to overcome this, the light shade polyvalent colorants take on a distinctly unnatural, bright white appearance. Other crown material properties to be considered are thermal conductivity and radiolucency. Stability/looseness of fit on the prepared tooth and cement gap at the margin are sometimes related to materials selection, though these crown properties are also commonly related to system and fabricating procedures.