cosmetic dentistry_ beauty & science International Edition

I review _ ceramics (c)Partiallystabilisedzirconia:Porous Zirconia frameworks milled from porous blocks are fabricated in a similar fashion to those milled from alumina blocks. There are a variety of meth- ods to press the powder into a mould. Uniaxial pressing involves pressing from one direction, biaxial pressing involves pressing from two equal and opposite directions, and isostatic pressing involves uniform pressing in all directions. There are advantages and disadvantages to all methods but the desired result is the same: to produce a ho- mogeneous block that shrinks uniformly. As is the case with the alumina block, the milled zirconia framework shrinks about 25 % after a 4–6 hour cycle at around 1,300–1,500 °C. The particle size is about 0.1–0.5 µ. (d)Partiallystabilisedzirconia: Hotisostaticpressingblocks Fully dense zirconia is produced by a method called hot isostatic pressing. The zirconia powder may be pre-pressed into a block or the powder itself may be packed into a flexible mould. Either the block or mould is then vacuum sealed in an airtight rubber or plastic bag and placed into a fluid-filled chamber. Pressure is then applied to the fluid and this pressure is transmitted evenly all around the zirconia. Heat is applied to the chamber, which sinters the zirconia to full density (Fig. 18). Zirconia blocks produced in this manner may achieve flexural strength values of about 1,200–1,400 MPa. However, it requires extended milling to produce the framework and the higher strength value does not generally justify the loss in productivity. The accuracy may be improved versus the porous block method and may be pre- ferred for large frameworks that span the arch. 3.2Additive 3.2.1Electrodeposition VITA In-Ceram powder dispersions used in the slip-casting technique have been applied to elec- trodepositionsystems,whichapplyacurrentacross the dispersion and deposit the powder particles automatically on the surface of a conductive die. This approach is efficient for single units, but be- comes cumbersome and potentially unreliable for multi-unit frameworks. _Discussion and summary Ceramics can be classified in many ways. Two classification systems were given to aid the reader in understanding the types of ceramics available for dental use. The processing technique has a very large impact on strength and thus clinical perform- ance, and should be one of the primary considera- tions in choosing a material. There are many clinical aspects that are im- portant for success with all-ceramic materials that are not as critical with metal-based restorations that cannot be covered here (e.g. preparation de- sign, management of stresses, and cementation techniques). The reader is advised that significant knowledge and training in these areas are requisite for success with all-ceramic materials._ Editorial note: A complete list of references is available fromthepublisher. Fig. 18_A diagram of hot isostatic pressing. 34 I cosmeticdentistry 1_2014 Prof.Edward A.McLaren,DDS,MDC, is the founder and director of UCLA postgraduate aesthetic dentistry,and Director of the UCLA Center for Esthetic Dentistry in LosAngeles in California in the US. Prof.Russell Giordano,DMD,CAGS,DMSc, is an associate professor in Materials Science and Engineering at the Boston University College of Engineering in Massachusetts in the US. HereceivedhisDMDfromtheHarvardSchoolofDental Medicine in the US.Presently,he has several private and federally funded projects.His major research foci are ceramics and ceramic matrix composites. cosmeticdentistry_about the authors Fig. 18 CDE0114_26-34_McLaren 11.06.14 14:07 Seite 9