phase stabilized zirconia (zirconium dioxide, ZrO2). Previous attempts to utilize high-performance ceramics such as zirconium-oxide were thwarted by the fact that this material could not be processed using the traditional methods used in dentistry. Because of its high strength and comparatively much higher fracture toughness, sintered zirconium oxide can be used in posterior crowns and bridges, implant abutments, and root dowel pins. Lithium disilicate (used in the latest Chairside Economical Restoration of Esthetic Ceramics CEREC product) also has the fracture resistance needed for use on molars. Some all-ceramic restorations, such as porcelain-fused-to-alumina set the standard for high aesthetics in dentistry because they are strong and their color and translucency mimic natural tooth enamel. Cast metals and porcelain-on-metal were the standard material for crowns and bridges for long time. The full ceramic restorations are now the major choice of patients and are of commonly applied by dentists. Comparison Composites and amalgam are used mainly for direct restoration. Composites can be made of color matching the tooth, and the surface can be polished after the filling procedure has been completed. Amalgam fillings expand with age, possibly cracking the tooth and requiring repair and filling replacement, but chance of leakage of filling is less. Composite fillings shrink with age and may pull away from the tooth allowing leakage. If leakage is not noticed early, recurrent decay may occur. A 2003 study showed that fillings have a finite lifespan: an average of 12.8 years for amalgam and 7.8 years for composite resins. Fillings fail because of changes in the filling, tooth or the bond between them. Secondary cavity formation can also affect the structural integrity the original filling. Fillings are recommended for small to medium-sized restorations. Inlays and onlays are more expensive indirect restoration alternative to direct fillings. They are supposed to be more durable, but long-term studies did not always detect a significantly lower failure rate of ceramic or composite inlays compared to composite direct fillings. Porcelain, cobalt-chrome, and gold are used for indirect restorations like crowns and partial coverage crowns (onlays). Traditional porcelains are brittle and are not always recommended for molar restorations. Some hard porcelains cause excessive wear on opposing teeth. Experimental The US National Institute of Dental Research and international organizations as well as commercial suppliers conduct research on new materials. In 2010, researchers reported that they were able to stimulate mineralization of an enamel-like layer of fluorapatite in vivo.