Amalgam
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why is there concern amalgam has health risks
role of mercury in disease causation with its systemic distribution and accumulation in the body
what is amalgam
an alloy formed by the reaction of: • mercury (liquid) • silver, tin, copper and other metals (powder)
when was the ‘first amalagm war’
1840s (american society of dental surgeons banned use of amalgam)
when was the second amalagm war
1970s-1990s mercury fear - health and environment
how is amalgam classified
by composition and by particle shape and size
what are the different types of composition of amaglam
• tradtional - copper enriched
what are the different types of particle shape and size of amalgam
• originally applied to traditional but now also to copper enriched • affects handling and final properties
what is amalgam composed of
• Ag (70%) • Sn (25%) • Cu (3%) • Zn (most today don’t have zinc) (1%) • Hg (1%)
what is the function of silver, tin in the powder
intermetallic compound (Ag3Sn) - gamma phase, reacts with Hg liquid to form amalgam
what is the function of copper in the powder
increases strength and hardness
what is/ was the function of zinc in powder
it was there to oxidise rather than have the other metals oxidise (sacrifical lamb)
what is the function of Hg in powder
“pre-amalgamated” alloys react faster
what proportion of weight of amalgam is powder and liquid
powder: 50% liquid: 50%
what is the function of Hg in liquid
• triple distilled (very pure) - reacts with other metals
what are the different particle types in amalgam
Lathe cut • coarse, medium, fine • formed by filling ingots Sperical, spheroidal • range of particle sizes • formed by spraying molten metal into inert atmosphere
Describe the setting reaction of amalgam
silver + tin –> unreacted silver/tin + tin/mercury
describe the set structure of amalgam
gamma (silver/tin) - good strength and corrosion resistance gamma 1 (silver/mercury) - good corrosion resistance gamma 2 (tin/mercury) - weak and poor corrosion resistance
what do voids do to the set structure
decrease strength and increase corrosion
what is the tensile strength of gamma
170MPa
what is the tensile strength of gamma 1
30MPa
what is the tensile strength of gamma 2
20MPa
What is the mean strength of amalgam
60MPa
how do setting dimensional changes differ between traditional and modern amalgams
Traditional (big changes in expansion) • intial contraction (solution of alloy particles in Hg) • expansion (gamma 1 crystallisation Modern (expansion/contraction is <0.2% so little clinical sign) • small contraction • solid solution of Hg in Ag3Sn
why do we now have zinc free materials
It can react with saliva to get zinc oxide and hydrogen, so bubbles of hydrogen formed within amalgam • pressure build up causes expansion • downward pressure cause pulpal pain • upward - restoration sitting proud of surface