Amalgam

• Composites • Polyalkenoates (GIC) • Amalgam

• Mixture, or blending, of mercury with another metal or alloy • Not possible to have a mercury free amalgam • Every metal can dissolve in mercury (apart from Iron) at room temperature

-Every metal can dissolve in mercury (apart from Iron) at room temperature

• Based on the system • Silver-Mercury-Tin • Other metals are added to this system to modify its properties • Copper increases its final strength • Zinc reduces oxidation

Based on a powder/liquid phase • Liquid phase is simply triple distilled Hg • Powder is an alloy based on the intermetallic compound Ag3Sn • Known as the gamma phase

Silver: Minimum 65%. In the gamma phase Tin: Max 29%. In the gamma phase Copper: 6% max. Strength/hardness Zinc: 2% max. Manufacturing Mercury 3% max. Preamalgamation

-Melt components at high temperature in a reducing atmosphere to produce Ag3Sn • Silver, copper and tin oxidise easily • Use zinc as an oxygen scavenger • Then remove the zinc oxide • Final alloy always contains zinc or • Melt the alloy in an inert (oxygen-free atmosphere) • Zinc free alloy (less expansion) -Final alloy must be used in a powder form -After the alloy is melted as a homogenous liquid, there are 2 possibilities: Lathe cut- cooled down/mechanically grinded Spherical- atomisation in an inert atmosphere

-Either lathe cut or spherical

• Cast into an ingot and heated at 420 degrees celcius • Cylindrical shaped alloy cut on a lathe • Power generated by further ball milling • Produces irregular size particles • Particles are stressed and elongated • Homogenised at 100 degrees celcius for 1 hour

• Melt is sprayed into an inert atmosphere • Surface tension and low viscosity generate small spherical (or spheroidal) particles • Solidifies into consistent sized particles

Lathe-Cut Spherical Mixed Many alloy powder are formulated by mixing particles 1) Increases packing efficiency 2) Reduces Hg needed 3) Increases performance

Lathe-Cut requires more mercury Spherical requires less Lathe cut requires more condensation force into the cavity Spherical requires less Lathe cut requires smaller condenser point (smaller amalgam plug required) Spherical requires a larger point Less easy to carve and burnish lathe cut Easier to carve and burning as smooth surfaces in spherical Less overhands and strong proximal contacts in lathe cut Overhangs and weak proximal contacts in spherical

• Initiated by vigorous mechanical mixing • Trituration of the powder and liquid Ag3Sn + Hg –> Ag3Sn + Ag2Hg3 + Sn7Hg Ag3Sn= Gamma Ag2Hg3 Gamma 1 Sn7Hg= Gamma 2

Ag3Sn= Gamma Ag2Hg3 Gamma 1 Sn7Hg= Gamma 2

Initial Dissolution -Outer surface of tin/silver particles dissolve in the triple distilled liquid mercury Ag3Sn + Hg > Ag + Sn + Hg Formation of Gamma 1 • Silver reacts quickly to form Ag2Hg3 grains • Sticks preferentially along the alloy particles • Ag + Hg > Ag2Hg3 • Gamma 1 gets dispersed into the matrix Formation of Gamma 2 • Tin reacts slowly to form Gamma 2 which is randomly distributed inside the Gamma 1 matrix • Sn and Hg> Sn7Hg Set Amalgam -Reaction is completely set when the Gamma 1 and Gamma 2 phases have formed a solid matrix and no mercury is left to dissolve Gamma

Initial Dissolution: Ag3Sn + Hg > Ag + Sn +Hg Formation of Gamma 1 Ag + Hg > Ag2Hg3 Formation of Gamma 2 Sn + Hg > Sn7Hg

-Core of unreacted gamma particles surrounded by a matrix of Gamma 1 (high concentration) and Gamma 2 (low concentration)

Assuming the amalgam has been correctly mixed, tensile strength: Gamma > Amalgam > Gamma 1 > Gamma 2 Gamma 2 is the weakest phase and reducing it increases the strength of the restoration

Amalgam develops slowly (>24 hours) Amalgam remains very weak at the time the patient leaves the surgery Compressive strength of 50MPa after 1 hour compared to 300MPa after 24 hours

• Relatively good replacement for the natural tooth substance • Hardness of amalgam is lower than that of enamel • May lead to surface facet formtion

-Done in order from first to third largest Modulus of elasticity: Enamel 1, Dentine 3, Amalgam 2 Compressive strength: Enamel 3, Dentine 2, Amalgam 1 Tensile strength after 7 days: Enamel 3, Dentine 2, Amalgam 1 Vikers Hardness Enamel 1 Dentine 3 Amalgam 2

• First contraction due to dissolution of gamma phase into the mercury • Crystallisation of gamma 1 and gamma 2 leading to expansion of the amalgam • Zinc can cause dramatic expansion: zn + H20 > ZnO + H2 Contraction leads to marginal gaps Expansion results in protrusions or even tooth cracks

-Metallic material -High thermal diffusivity -Need to protect the base of large cavity to avoid harmful affect on the pulp (pulp capping?) Thermal expansion mismatch can cause microleakage -Occurrence of decay in the dentine surrounding the amalgam -Amalgams thermal diffusivity and thermal coefficient of expansion is much higher than dentines

• Multiphase metallic material sitting in a wet environment • Inevitable corrosion • Gamma 2 is more elctronegative than Gamma and Gamma 1 • Acts as an anode and dissolves, releasing free mercury • Corrosion can be reduced by polishing the restoration to a smooth surface • Beneficial advantage: Corrosion occurs more at the amalgam/tooth interface forming a seal which prevents microleakages