cosmetic dentistry_ beauty & science International Edition

will catastrophically fail after minor flexure. Thus, these materials are strong in compression but weak in tension. Contrast that with metals: metals are non-brit- tle (display elastic behaviour) and ductile (display plastic behaviour). This is because of the nature of the interatomic bonding, which is called metallic bonds; a cloud of shared electrons that can easily move when energy is applied defines these bonds. This is what makes most metals excellent con- ductors. Ceramicscanbeverytranslucenttoveryopaque. In general, the more glassy the microstructure (i.e. non-crystalline), the more translucent; and the more crystalline, the more opaque. Many other factors contribute to translucency, for example, particle size, particle density, refractive index and porosity to name a few. _Different types of ceramics used in dentistry The term “ceramic” technically refers to a crystalline material. Porcelain is a mixture of glass and crystal components. A non-crystalline- containing material is simply a glass. However, dentistry typically refers to all three basic materi- als as dental ceramics. How ceramics are classified can be very confusing. Ceramics can be classified by their microstructure, (i.e. amount and type of crystalline phase and glass composition). They can also be classified by processing technique (powder/liquid, pressed or machined) and by their clinical application. We will give a classification based on the microstructure of ceramics, with the inclusion of how the ceramics are processed and the effect of this on durability, to help the reader better understand the ceramics available in dentistry. _Microstructural classification At a microstructural level, we can define ce- ramics by the nature of their composition of glass– crystalline ratio. There can be infinite variability in the microstructures of materials but they can be broken down into four basic compositional cate- gories with a few subgroups: _Category 1: glass-based systems (mainly silica); _Category 2: glass-based systems (mainly silica) with fillers, usually crystalline (typically leucite or a different high-fusing glass); _Category 3: crystalline-based systems with glass fillers (mainly alumina); and _Category 4: polycrystalline solids (alumina and zirconia). Figs. 6a & b_A pressed ceramic restoration. Fig. 7_A scanning electron micrograph of the microstructure of a lithium disilicate glass-ceramic. Acid etching reveals the fine crystal structure. review _ ceramics I I 27cosmeticdentistry 1_2014 Material Inlays,onlays,veneers Anterior crowns Posterior crowns Anterior bridges Posterior bridges Translucency Leucite/feldspar- YES YES NO NO NO 1 based pressable Lithium disilicate YES YES YES NO NO 2 Alumina YES YES YES NO NO 3 VITA In-CeramALUMINA YES YES YES YES NO 3 VITA In-Ceram SPINELL YES YES NO NO NO 1 VITA In-Ceram ZIRCONIA YES YES YES YES YES 4 Pure zirconia YES YES YES YES YES 3 VITABLOCS Mark II YES YES YES NO NO 1 Table 1_A clinical use selection guide. Fig. 6b Fig. 7Fig. 6a CDE0114_26-34_McLaren 11.06.14 14:06 Seite 2