cosmetic dentistry_ beauty & science International Edition

review _ ceramics I amounts of other metal oxides. Partially stabilised zirconia is one of the materials that allow produc- tion of reliable multi-unit all-ceramic restorations for high-stress areas, such as the posterior re- gion of the mouth. Zirconia (or zirconium dioxide, chemical formula ZrO2) may exist in several crystal types(phases),dependingupontheadditionofmi- nor components, such as calcia (or calcium oxide, chemical formula CaO), magnesia (or magnesium oxide, chemical formula MgO), yttria (or yttrium oxide, chemical formula Y2O3), and ceria (or cerium(iv) oxide, chemical formula CeO2). Specific phases are said be stabilised at room temperature by the minor components. Typically for dental applications, about 3 wt% of yttria is added to the pure zirconia (Figs. 13, 14a & b). Zirconiahasuniquephysicalcharacteristicsthat makeittwiceasstrongandtoughasalumina-based ceramics. Values for flexural strength for this ma- terial range from about 900 to 1,100 MPa.30, 31 It is important to note that there is no direct correlation betweenflexuralstrength(modulusofrupture)and clinical performance. Another important physical property is fracture toughness, which has been reported to lie between 8 and 10 MPa m1/2 for zirco- nia.30 This is significantly higher than any previous dental ceramic. Fracture toughness is a measure of a material’s ability to resist crack propagation. Zirconia has the apparent physical properties to be used for multi-unit anterior and posterior fixed partial dentures. Clinical reports on zirconia have not demonstrated problems with zirconia frame- works.32–34 The problems have been associated with chipping and cracking of porcelain. Using a slow- cooling protocol at the glaze bake to equalise the heat dissipation from the zirconia and porcelain increased the fracture resistance of the porcelain by 20 %. Zirconia may be in the form of porous or dense blocks that are then milled to form the frame- works, or recently full contour single-unit restora- tions. Most are fabricated from a porous block, milled oversize by about 25 % and sintered to full density in about a 4–6 hour cycle. An alternate approach involves milling a fully dense block. However, owing to the nature of zirconia, this approach requires about 2 hours of milling time per unit, whereas milling of the porous block re- quires only about 30–45 minutes for a three-unit bridge. Within categories 2 and 3, there can be great variation of composition and there are several commercial materials in these groups. Glass-based systems (categories 1 and 2) are etchable and thus easily bondable. Crystalline-based systems (cate- gories3and4)arenotetchableandthusmuchmore difficult to bond. Categories 1–3 can exist in a pow- deredformthatisthenfabricatedusingawetbrush technique, or they can be preprocessed into a block form that can be pressed or machined. As a rule, powder/liquid systems have much lower strength than pre-manufactured blocks do owing to a much larger amount of bubbles and flaws in the finished restoration. _Classification based on processing technique A more user-friendly and simplistic way to classify the ceramics used in dentistry is by how they are processed. It is important to note that all materials can be processed by various tech- niques but in general for dentistry they can be classified as: _powder/liquid glass-based systems; _machinable or pressable blocks of glass-based systems; and _CAD/CAM or slurry, die-processed, mostly crys- talline (alumina or zirconia) systems. It is an important classification method, as there appears to be a greater correlation with clinical success (and thus failure) due to processing technique. Even though a material may have the same chemistry and microstructure, the processing methodologyusedtoproducearestorationmayim- prove or decrease the final properties and clinical success. Specifically, machined blocks of materials have performed better than powder/liquid versions of the same material. Fig. 13_A scanning electron micrograph of the microstructure of a zirconia ceramic. Figs. 14a & b_A zirconia crown. I 31cosmeticdentistry 1_2014 Fig. 14a Fig. 14bFig. 13 CDE0114_26-34_McLaren 11.06.14 14:07 Seite 6