Zirconium in dentistry

Ceramics are inorganic, nonmetallic materials widely used as restorative dental materials in different applications. Dental ceramics can be classified according to fusion temperature, fabrication methods, and crystalline content. Zirconia or zirconium dioxide has 96%-99% of crystalline content with no glassy phase, showing high-physicochemical properties, similar to titanium. Two types of zirconia are available in Dentistry: yttrium-stabilized tetragonal zirconia (Y-TZP) and zirconia-toughened alumina (ZTA). Yttrium oxide is added to zirconia in order to inhibit the propagation of a crack, which occur in dental restorations. The ZTA ceramics exhibits transformation toughening at high temperature, improving some physicochemical properties, such as fracture toughness, mechanical strength, and stability. The adhesive cementation has not a stablished protocol. Several mechanical and chemical treatments on zirconia surface are proposed for adhesion to zirconia in dental restorations. Phosphoric acid (30%-40%) gel is used to remove contaminants of bonding surface of prosthetic restoration. Aluminum oxide airborne-particle abrasion is recommended to increase the bonding area available for mechanical interlocking and surface wettability of zirconia ceramic, improving the bond between rougher zirconia and resin-based materials. Tribochemical silica coating is other mechanical treatment option that consists of airborne-particle abrasion of aluminum oxide particles modified by silica, promoting roughened surface that is coated with silica and receptive to silanization treatment. This procedure creates a chemical adhesion with long-term resin bonding to zirconia ceramics and water resistance. Others protocols, such as plasma technology, a thin layer of vitreous porcelain glaze, high-concentrated hydrofluoric acid (>40%), and coupling agents (zirconia primers, metal primers, and silanes) can be used on clinical treatments of zirconia surface. The chipping and delamination of veneering porcelain and the difficulty in adhesion are some disadvantages and limitations of zirconia restorations. Airborne-particle abrasion can cause damage to the material and jeopardizing the long-term clinical service of restorations. Zirconia is the high-strengthened ceramic available for dental application, allowing restorations that were only possible in metal. Following the indications, zirconia is a very useful and predictable material, with high clinical success rates.