Dental Alloys

Alloys in Dentistry

• An alloy is a mixture of two or more metals or non-metals (elements) and is metallurgically complex.
• Dental alloys usually contain at least four metals, and often six or eight different metals.
• Alloy compositions are diverse, and more than 25 elements in the periodic table are used in today's dental alloys.

Understanding Alloy Composition

• Alloy composition 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 better predicts the number of atoms available to be released and affect vital tissues.
• The weight percentage and atomic percentage of an alloy may be substantially different from one another.

Phases in Alloys

• Phases are areas within an alloy that have essentially the same composition.
• Single-phase alloys have a similar composition throughout their structure.
• Multiple-phase alloys have areas that differ in composition, which can affect their corrosion properties and biocompatibility.

Corrosion and Biocompatibility

• The corrosion of an alloy is critical to its biocompatibility because the release of elements from the alloy is necessary for an alloy to have adverse biological effects.
• The biological response to an alloy depends on the biological effects of released elements, the quantities released, and the duration of tissue exposure.
• Factors that influence corrosion 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
  • Time in service

Elemental Release and Corrosion

• Elemental release and corrosion occur from all alloys, regardless of type or composition.
• The amount of corrosion and elemental release may vary dramatically among alloys.
• Elements like copper, zinc, and nickel have higher tendencies to be released than elements like gold and palladium.
• The lability of an element may be substantially modified by other elements around it.
• Phase structure and surface characteristics can influence element release.

Surface Characteristics and Oral Environment

• Surface roughness tends to increase elemental release because rough surfaces have high surface areas that expose more atoms to the external environment.
• The oral environment near the alloy, such as reduced pH, can increase corrosion.
• Corrosion is particularly high in crevices, gaps, and pits, and in the local environment of the gingival sulcus.

Thermal Treatment and Alloy Combinations

• Thermal treatment, such as firing of a ceramic alloy, may cause an alteration of the structure within the alloy.
• Metal oxides generated during the firing process can increase the attachment between ceramic and alloy, but may also promote elemental release and increase toxicity.
• The juxtaposition of different alloys may increase corrosion when they are in permanent contact.

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.