TABLE OF CONTENTS Precious metals, of which gold is the most important, are rarely if ever used as biomedical materials, because of their relatively poor mechanical properties. On the other hand, the chemical inertness has proved desirable, notably in dentistry, and here precious metal alloys are widely used. Gold, in particular, been used in dentistry, its relatively poor mechanical properties being overcome by using it the form of an alloy, typically with copper.174 Indeed, the gold-copper system has been widely studied, and is the basis of the majority of gold-based alloys used in clinical dentistry.175 The major use of these alloys is in casting components such as dental crowns, which are retained on the prepared remnant of a tooth with the aid of so-called luting cements, and which function as replacement structures for lost or damaged teeth.176 As such, they have to withstand the twin assaults of the moist, corrosive environment of the mouth, and the cyclical compressive forces of biting and chewing.
For many years, these alloys were classified on the basis of their progressively decreasing gold content as Type I (highest gold content, greatest ductility) through to Type IV (lowest gold content, least ductility). More recently, this classification has been superseded by one which is more inclusive, in that it does not rely for definitions on the gold content, and can therefore be readily applied to alloys containing other noble metals, such as platinum or palladium. Current definitions are:
High-noble: Containing at least 60 wt%...
