porcelain-fused crowns. By contrast, when porcelain is fused to glass-infiltrated alumina, crowns are very natural-looking and very strong, though not as strong as monolithic zirconia crowns. Zirconia crowns are said to be less abrasive to opposing teeth than metal-ceramic crowns. Other crown material properties to be considered are thermal conductivity and radiolucency. Stability/looseness of fit on the prepared tooth and cement gap at the margin are sometimes related to materials selection, though these crown properties are also commonly related to system and fabricating procedures. Lithium-disilicate Another monolithic material, lithium disilicate, produces extremely translucent leucite-reinforced crowns that often appear to be too gray in the mouth. To overcome this, the light shade polyvalent colorants take on a distinctly unnatural, bright white appearance. Metal-ceramic crowns (P-F-M Crown) These are a hybrid of metal and ceramic crowns. The metal part is normally made of a base metal alloy (termed bonding alloy). The properties of the metal alloy chosen should match and complement that of the ceramic to be bonded otherwise problems like delamination or fracturing of the ceramic can occur. To obtain an aesthetic finish which is able to be functional with normal mastication activity, a minimal thickness of ceramic and metallic material is required, which should be planned for during tooth preparation stage. Ceramic bonds to the metal framework by three methods: Compression fit (via ceramic shrinkage on firing) Micro-mechanical retention (via surface irregularities) Chemical union (via oxide formation) Tissue control and gingival retraction Gingival retraction refers to the displacement of the free gingivae. For crowns with margins which are supragingival, there is no need for gingival retraction, provided there is good moisture control. For crown preparations which have subgingival margins, tissue control is necessary at the preparation stage and impression stage to ensure visibility, good moisture control and ensure enough bulk of impression material can be placed to accurately record the marginal areas. Options available are gingival retraction cord, Magic Foam cord, and ExpaSyl. Another method to expose the margins of a subgingival preparation is to use electrosurgery or crown lengthening surgery. Tooth preparation The design of a preparation for a tooth to accept a crown follows five basic principles: Retention and resistance Preservation of tooth structure Structural durability Marginal integrity Preservation of the periodontium Aesthetics can also play a role in planning the design. Retention and resistance As there are currently no biologically compatible cements which are able to hold