Casting Alloys, Solders, and Wrought Metal Alloys Chapter 10

Casting Alloys Dental casting alloys have been classified by the American Dental Association as high-noble alloys, noble alloys, and base-metal alloys. A noble alloy is one that does not corrode very readily in the oral environment. Gold is the most corrosion resistant of the noble metals.

Casting Alloys (cont’d) Gold alloy is classified by its gold content as karats, percentage, or fineness. Pure gold is 24 karat. Pure gold is not used in dentistry because it is too soft. Other metals are added to gold to make it stronger and thus suitable for dental restorations.

Casting Alloys (cont’d) Other noble metals include platinum and palladium. Platinum is not used much because of it expense, high melting point, and difficulty mixing with gold. Palladium is used widely because it has good corrosion resistance, increases hardness of the alloy, and is less expensive than gold.

Casting Alloys (cont’d) Base-metal dental casting alloys consist of less than 25% noble metals. The primary base metals used in casting alloys are copper, nickel, silver, zinc, tin, and titanium. Copper and silver are often added to gold alloys to increase their hardness. Zinc reduces oxidation.

Casting Alloys (cont’d) The stiffness of base alloys is twice as great as that of gold-based alloys, so it takes twice the stress to deform them. This is especially important for use in partial denture frames. Drawbacks of base metals include their higher casting temperatures and their potential biocompatibility problems.

Lost Wax Technique Everything starts at the lab with an impression, taken by the doctor, of the patient’s teeth. From the impression, a model is made. The lost wax technique basically consists of buildup of the unit in wax to an exact replica of the desired crown or substructure.

Porcelain Bonding Alloys Porcelain bonding alloys are essentially the same as other casting alloys and have similar physical properties. They are also classified as high-noble, noble, and base-metal alloys. Minor changes in their composition make them compatible with dental porcelains. They must withstand the high temperatures at which porcelain is fired.

Removable Prosthetic Casting Alloys Partial denture frameworks are made from base-metal alloys with minor quantities of noble elements. These base metals include nickel, titanium, chromium, aluminum, cobalt, vanadium, iron, beryllium, molybdenum, gallium, and carbon.

Removable Prosthetic Casting Alloys (cont’d) The most common base-metal alloys are chrome-cobalt and nickel-chrome. The chromium content gives these metals their corrosion resistance. These metals are very hard and are difficult to cast. Measurement must be precise, or they won’t fit.

Biocompatibility Noble metals are more compatible with oral tissues because they tend to corrode less than base metals Some of the products are responsible for allergic response Nickel has the highest incidence of allergic response Women have higher allergy rate than men, ratio of 10 to 1 Overall allergy rate to nickel for the general population is 9% to 12%

Solders Solders are alloys that are used to join metals together or to repair cast restoration. Solders used in crown and bridgework generally are gold-based alloys. Silver solders are used in orthodontics to secure wires and bands together.

Wrought Metal Alloys Wrought metal alloys are different from casting alloys in that they are formed after the metal is cast. Usually the metal is drawn or extruded through a die or is formed in a press to the desired shape. Wrought metal alloys have been changed mechanically into another form.

Wire Wire is a wrought metal that may be soft and easily shaped or may resist bending. Various degrees of resistance to bending can be created by annealing. Orthodontic wires are used most commonly in dentistry.

Orthodontic Wire Orthodontic wires are used most commonly in dentistry. Orthodontic wires are composed mostly of base metals. Orthodontic wires are also wrought metal alloys commonly made of stainless steel, cobalt-chrome-nickel, titanium, or an alloy of nickel and titanium, called Nitinol.

Bands and Brackets Orthodontic brackets and bands are bonded or cemented on the teeth; they retain the arch wire that the orthodontist has shaped. The arch wire is shaped into a form that will guide the teeth into their new position. Orthodontic bands are formed from a stainless steel alloy and are pre-formed or formed at chairside by the dentist. Stainless steel brackets and tubes are welded onto the bands or brackets for the purpose of attaching intraoral wires or extraoral headgear.

Endodontic Posts Posts are metal or nonmetal dowels or rods placed within the root canal space after a root canal treatment. The purpose of a post is to retain the core buildup over which the final restoration (crown) is placed. Posts can be classified as active or passive. Active posts engage the root canal surface with threads; passive posts are simply cemented into the canal space without actively engaging the canal walls. Posts can be custom made or preformed

Summary Metals play a significant role in restorative dentistry. Gold and gold alloys are the most prevalent, but non-noble metals have their specific purposes as well. Metals have low solubility, if any, and are biocompatible.