Cast Metal

Dentists have been replacing the damaged crowns of molar teeth with gold alloys. These have been cast by the lost-wax process. In this process, a wax crown is invested in a wet silica-gypsum mixture. Once this has hardened, the wax is burned away and molten gold-copper-silver-palladium-platinum-zinc alloy is cast under pressure into the space left behind. Some of the alloys can be heat treated to form super-lattices and increase their strength. This makes them suitable for the construction of dental bridges, which replace a missing tooth either by cantilevering an artificial tooth from an adjacent tooth, or by suspending it between two such teeth. In either case the supporting teeth will have been cut down to accommodate a close fitting casting, which is cemented into place.

Bonded Restorations

Alloy porcelain combinations, known to the dentist as bonded restorations have been available. These porcelain covered metal castings combine the strength of a metallic superstructure with the aesthetic appearance of dental porcelain, creating the illusion that the restorations are real teeth. Alloys have been developed to which dental porcelains form durable retentive bonds, and many of these are now based on nickel-chromium. These metal frameworks are so rigid that they can be bonded via composites to the backs of acid etched teeth, thus eliminating the need for cutting down sound teeth, figure 1. Just as etching dental enamel creates retentive ‘chasms’, these nickel-chromium alloys can be electrolytically etched to produce features that allow the formation of mechanical bonds with resin-based composite cements.