cosmetic dentistry_ beauty & science International Edition

I review _ ceramics 1.Powder/liquid 1.1Conventional These are typically veneer materials, which may be all glass or a mixture of glass and crystal com- ponents.Theseincludeveneersforall-ceramicand metal frameworks, and may also be used alone as anterior veneer restorations. Typically, these materials are mixed by hand with deionised water or a special modelling liquid supplied by the man- ufacturer. They are built up by hand and vibrated (condensed) to remove water and air. These are firedinavacuumtohelpremoveremainingairand improve the density and aesthetics of the veneer. Since these restorations are made by hand, there are often voids present in the fired material. This is inherenttotheprocessandmaybeworseorbetter depending upon environmental conditions, the skill of the technician, and the firing cycle. Often, one sees bubbles remaining in the hand-layered veneer material. 1.2Slipcasting The original VITA In-Ceram and some partially stabilised zirconia blocks are fabricated based on slip casting of alumina or zirconia. The slip is a ho- mogenous dispersion of ceramic powder in water. The pH of the water is often adjusted to create a charge on the ceramic particles and the ceramic powder is coated with a polymer to cause the particles to be evenly suspended in the water. In the case of VITA In-Ceram, the slip is painted on a gypsum die with a brush to form the underlying core for the ceramic tooth. The water is removed via capillary action of the porous gypsum, which packs the particles into a rigid network (Fig. 15). The alumina core is then slightly sintered (0.2 % shrinkage)inafurnacetocreateaninterconnected porous network. The lanthanum glass powder is placed on the core, and the glass becomes molten and flows into the pores by capillary action to produce the interpenetrating network. The last step in the fabrication of the restoration involves application of aluminous porcelain to the core to produce the final form of the restoration. Other powder dispersions, such as those created with zirconia, may be poured into a gypsum mould that withdraws the water and leads to a homogeneous block of zirconia being formed. 2.Pressable Pressed ceramic restorations are fabricated using a method similar to injection moulding. Monochromatic porcelain or glass-ceramic ingots areheatedtoallowthematerialtoflowunderpres- sure into a mould formed using a conventional lost-wax technique. The restoration may be cast to its final contours and subsequently stained and glazedtoprovideanaestheticmatch.Alternatively, a coping may be moulded upon which porcelain is added to achieve the final shape and shade of the restoration. IPS Empress restorations and other materials with a similar leucite/glass structure are fabricated in this manner. The glass-ceramic IPS e.max is also fabricated this way. Pressables may be used for inlays, onlays, veneers and single- unit crowns. 3.CAD/CAM 3.1Subtractive(removalofexcessmaterialtofabri- catetherestoration,milling) 3.1.1Fullcontour Full contour restorations, such as inlays, onlays, crownsandveneers,maybefabricatedfromvarious blocks of materials. In general, these blocks are fabricated from starting powders that are mixed with a binder and then pressed into a mould or ex- truded like a sausage into a block form. The binder helps hold the powder together so that the shape is maintained after pressing or extrusion. The blocks are then transferred to a furnace to remove the binder and sintered to full density. As mentioned Fig. 15_Slip casting a VITA In-Ceram framework. Fig. 16_An electron micrograph of the microstructure of a hand-layered porcelain, a pressed crown, and a VITABLOCS Mark II CAD/CAM block. 32 I cosmeticdentistry 1_2014 Fig. 16Fig. 15 CDE0114_26-34_McLaren 11.06.14 14:07 Seite 7