Casting Metals, Solders, and Wrought Metal Alloys
Historically, the most widely used material in restorative and corrective dentistry has been metal. Because a pure metal may not possess the physical and mechanical properties desired for a restoration, it may be combined with one or more other metals to form an alloy with the properties desired. Metal alloys generally have high strength and consequently make durable dental restorations. They melt at high temperatures, conduct temperature and electricity, can be polished to a high shine (luster), and have varying degrees of ductility (ability to be pulled or drawn into a wire). Gold is ductile and the margins of gold restorations can be burnished for better adaptation to the tooth preparation margins. As esthetic nonmetal materials improve in their physical properties, they are slowly replacing metals in some applications
The dental health care worker is in contact with and involved in the manipulation of metal dental materials in various ways every day. Grinding dust from certain metal alloys can present the dental health care worker with health hazards, so personal protective equipment must be used. It is essential that the dental assistant or hygienist have an understanding of the characteristics of various metal materials in order to correctly manipulate and care for them and to be able to answer questions by patients relative to a particular material that will be used for their treatment.
Casting Alloys
All-Metal Castings
In the early 1900s, W.H. Taggart developed a technique for making dental restorations from metal that was melted and cast into a mold, using the lost wax technique (see Chapter 16 [Gypsum and Wax Products]). Pure metals are seldom used in dentistry, because they lack properties that make them useful in the oral cavity. Therefore, they are usually combined with other metals in portions that achieve desirable physical and mechanical properties. This combination of two or more metals is called an alloy. Alloys used with the lost wax technique are called dental casting alloys. Unlike amalgam, restorations made from these alloys are not placed directly into the preparation but are made outside the mouth by an indirect technique, and then are cemented in place (see Chapter 6[Composites, Glass Ionomers, and Compomers] and Chapter 14 [Dental Cement]). Cast metal restorations can be classified as intracoronal (preparation is made within the crown of the tooth) or extracoronal (preparation is made primarily on the outside of the crown of the tooth). An inlay is an intracoronal restoration, whereas an onlay has both intracoronal and extracoronal components in that it has an inlay preparation and also covers the outer surface of one or more cusps (Figure 11-1). Other extracoronal cast restorations include partial coverage (¾ and ⅞ crowns) and full coverage crowns (Figure 11-2). Cast metal alloys can also be used to make fixed partial dentures (bridges) and removable partial dentures for replacement of missing teeth.
The American Dental Association (ADA) classifies dental casting alloys according to their noble metal content and divides them into three categories: