Amalgam

Alloy formed by the reaction of Mercury (liquid) Sliver, tin, copper and other metals (powder)

Sliver Tin

Lathe cut -coarse, medium, fine -formed by filing ingots Spherical, spheroidal -range of particle sizes -formed by spraying molten metal into inert atmosphere

Good strength and corrosion resistance Made out of sliver and tin

Good corrosive resistance silver and mercury stronger than gamma 2 but weaker than gamma

Weak and poor corrosive resistance, tin and mercury (copper is added to counteract it)

Interaction of zinc with water in saliva/blood forms hydrogen bubbles and creates a pressure build up causing expansion Downward pressure causes pulpal pain

increases strength & hardness eliminates gamma 2 phase

trituration

• less Hg required • higher tensile strength • higher early compressive strength • better packing and density

• Strength – generally consider compressive but others important – early (1hr) - Traditional materials, poor(ish) – late (> 24hrs) - OK • Abrasion Resistance – high, suitable for posterior teeth – too high for deciduous • Creep – amalgam visco-elastic – high in traditional materials – affects marginal integrity NB also marginal integrity depends on: * cavity design

• undermixing • too high Hg content after condensation • too low condensation pressure • slow rate of packing – increments do not bond • corrosion

When a material experiences low-level stress levels (ie below elastic limit stress) which are applied repeatedly over a prolonged time period, it may change shape, resulting in PERMANENT DEFORMATION

as the amalgam changes shape and the edges of the cavity protrude, the margins become vulnerable to fracture

concern about mercury toxicity “Although mercury is released from dental amalgams the quantities released are very small and do not cause verifiable effects on human beings. While the current evidence supports the concept that existing dental restorative materials are safe, it must be recognised that the supporting data are incomplete.”

• Thermal expansion - x 3 of tooth • Thermal Conductivity – high, may need to use liner / varnish in deep cavities • Thermal diffusivity – high, so when a transient thermal stimuli is applied (eg drinking tea) amalgam’s temperature rises quickly

does not bond to tooth - needs mechanical retention

condensed and packed into cavity

gamma 2 component most electronegative- weakens material particularly at margins * reduce by – copper enriched, polishing margins – avoiding galvanic cells

• Aesthetics – poor • Radiopaque - yes • Anticariogenic - no • Smooth surface - yes, if polished well, may deteriorate over time • Setting shrinkage - modern materials tend to have net overall shrinkage - see earlier

• Dispersion modified – original type • Single composition types – introduced to increase uptake by profession

• originally Ag-Cu spheres + conventional lathe cut alloy to eliminate gamma 2 in two step setting reaction • takes several days as two step setting reaction

• powder Ag-Sn-Cu, copper is already incorporated in the alloy • can be spherical or lathe cut • one step

• Higher early strength • Less creep • Higher corrosion resistance • Increased durability of margins