17-01-2013, 10:22 AM
Metals in dentistry
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General information
Alloy: a mixture of two or more metals
Pure metals are rarely used in dentistry because they are weaker than they are when mixed with other metals.
All metal casting
Classification of cast restorations:
Intra-coronal (e.g. inlay)
Extra-coronal (e.g. crown)
Cast metal alloys can be used for bridges, partial dentures
Gold alloys (Au)
Most corrosion resistant
Pure gold is 24 karat, 100%, or 1000 fine (percentage * 10)
Pure gold is too soft, so, gold alloys were mostly used
Gold alloys classified:
Hardness (resistance to penetration)
Malleability (ability to be shaped by tapping)
Ductility (ability to be elongated)
Platinum is not used much because:
Too expensive
High melting point
Difficult to mix with gold
Palladium is used more widely because:
Good corrosion resistance
Increases hardness of alloy
Silver is precious but not noble because it corrodes.
Crystal formation
Alloys start to form crystals as they cool down after being poured into molds.
Small crystals produce better qualities than larger ones
Some alloys such as gold maybe reheated (annealing) to improve properties
Reheating base metal alloys is not recommended.
Porcelain bonded alloys
High noble
Noble
Base metal
Composition is slightly modified to make them more compatible with porcelain. How?
Blended and mixed to withstand high temperature when porcelain is fired (850-1350 °C)
Small amounts of indium and tin are added to form oxides on metal surface to which porcelain is bonded
Silver and copper is not used to avoid green staining of porcelain
Porcelain bonding alloy
When PFM restorations are constructed, layers of porcelain are fired in an oven on the metal base to cover the metals’ dark color
Body and incisal porcelain are added in layers to simulate enamel and dentine colors and translucency.
Porcelain and metal should have compatible rates of thermal expansion or porcelain will crack.
Biocompatibility
Noble metals are more biocompatible than base metals because they corrode less (corrosion products can cause allergy):
Nickel is associated with allergy (9-12% of population), especially in women
Seen on free gingival tissue in contact with metal
Mostly more sever with fixed prosthesis
Skin response may occur