The technology of this product is extremely advanced. the restorations are milled to a tolerance of 15 microns. Since it fits the tooth precisely, the cement line, which is the weak link in any ceramic system, is extremely thin, making for a very long-lasting restoration. It is virtually unbreakable and is used mainly for the molar area in the back of the mouth. Zirconia crowns are made from zirconium dioxide (zirconia) , a high-strength ceramic material which is used in other high-tech applications, such as in the aerospace industry. It is often referred to as "ceramic steel" because of its high tensile strength. It needs no metal framework. It is also extremely bio-compatible, virtually eliminating the risk of allergic reactions. Similar materials are used in hip replacements.