Aesthetic Ceramics for Dental Restorations

解 説 1．IntroductionDental caries, severe periodontitis, and severe tooth loss are a global burden, affecting 3.9 billion people of the global population of 7.5 billion 1) .For example, in Australia with 24 million people, there are over 19 million decayed teeth with 11 million additional decayed teeth each year, costing approximately AU$2 billion/year for dental care 2) .Dental patients demand aesthetic, biocompatible and chemically inert ceramic restorations and aging societies will drive this demand even higher 3) .A solution to this worldwide oral health challenge requires a global effort.Ceramic restorations can be traced to 1792 when the processing of porcelain teeth was patented 4) .Ceramics have been used as main aesthetic dental restorative materials for over a century 5) .Particularly, over the last thirty years, computer-aided digital manufacturing of dental crowns and bridges has accelerated the development of many new materials and processing techniques [6][7][8][9][10][11] .Today there is a wide range of dental ceramic or ceramic-like restorative materials available for clinical applications 7,12) .It is difficult for clinicians to make decisions on materials selection in restorative processes, many of which were made based on advertising claims, other than on the basis of a scientific understanding of characteristics of the materials.Due to a large number of dental ceramic products and rapidly evolving new materials, there are many classification systems.From a materials science point of view, materials should be classified according to their compositions and microstructures, because these determine the properties and functional behavior of the materials.Thus, dental ceramic and ceramic-like materials can be scientifically classified as glass ceramics, polycrystalline ceramics and resin matrix ceramics 12) .This paper reviews current aesthetic dental ceramics and their reliability with respect to their microstructures and mechanical properties to provide insights into rational materials selection to dental clinicians. 2．Dental ceramics 1 Glass ceramicsGlass ceramics are ceramic materials containing a glass phase.Mica, feldspar, leucite, lithium disilicate, and alumina glass ceramics are commonly used in dentistry.Mica glass ceramics contain mica platelets which are internally nucleated and crystallized from the fluorine-containing base glass to form tetrisilicic fluormica (KMg 2.5 Si 4 O 10 F 2 ) 13) .In this process, heating temperatures ranging from 1000˚C to 1360 ˚C control mica platelet diameters of 1.1 µm to 10 µm 9,13) .A representative of these materials is Dicor (Dentply), in which mica platelets have a diameter of approximately 2 µm 13) .The mechanical properties of the material are: Vickers hardness H = 3.5 GPa, Young s modulus E = 68 GPa, fracture toughness K IC = 1.5 MPa.m 1/2 , and flexural strength σ = 70 MPa 14,15) .Due to the