Dental Alloys Properties

Basic Material Properties

• An alloy is a mixture of two or more metals or non-metals (elements).
• Dental alloys typically contain at least four metals and often six or eight different metals, making them metallurgically complex.
• Alloy compositions are diverse and have evolved over the past 20 years, with many types of alloys emerging that were unknown just a few years ago.
• Most dental alloys were previously based on gold, but today's alloys may be based on silver, gold, palladium, nickel, cobalt, or titanium.

Alloy Composition

• The composition of dental alloys can be expressed in two ways: weight percentage (wt.%) or atomic percentage (at.%).
• Weight percentage is the most common way to describe an alloy's composition, but atomic percentage is better for predicting the number of atoms available to be released.
• The wt.% and at.% of an alloy may be substantially different from one another.

Analysis of Dental Alloys

• Dental alloys are classified as medical devices, and their use must be documented by the dentist and laboratory technician.
• Information about an alloy's composition is available in the literature or from manufacturers.
• Metal shavings can be taken from intraoral restorations and analyzed using scanning microscopy and energy-dispersive x-ray (EDX) analysis to determine the alloy's composition.

Corrosion and Release of Elements

• Corrosion is the primarily electrochemical process of alloys involving the ionization of elements that are released into the environment.
• Elements that are initially uncharged lose electrons and become positively charged ions as they are released into solution.
• Corrosion is a chemical property of an alloy that influences other properties, including esthetics and strength.
• The release of elements from an alloy is necessary for an alloy to have adverse biological effects such as toxicity, allergy, or mutagenicity.
• Factors that influence the corrosion of dental alloys include:
  • Composition of the alloy (particularly at the surface)
  • Phase structure of the alloy
  • Surface structure (roughness, presence of oxides)
  • Crevices, pits
  • Thermal treatment/history
  • Combinations of alloys (gold coating, soldering)
  • Time in service

Factors Influencing Element Release

• The composition of an alloy is a fundamental factor that influences element release.
• Certain elements, such as copper, zinc, and nickel, have higher tendencies to be released than elements such as gold and palladium.
• The lability of an element may be substantially modified by other elements around it.
• The phase structure of the alloy and thermal treatment can also influence element release.

Clinical Practice Advice

• Gold surface coating of nickel-based or cobalt-based alloys should be discouraged because the combination of the alloys and their permanent contact may enhance corrosion rather than retard it.
• Only those solders or alloy combinations should be used that have a low tendency for corrosion.

Nonimplanted Dental Alloys

• Elements released from nonimplanted dental alloys may or may not gain access into the body.
• There is some evidence that elements from dental crowns and other alloys gain access to local gingival tissues or the oral mucosa.
• However, there is little evidence to demonstrate that elements released from nonamalgam dental alloys contribute significantly to the systemic body burden of elements.