DENTAL AMALGAMS-

  • Bad for the environment not because bad for pt’s

  • amalagam= a mixture of 2 or more metals one of which is mercury

  • dental amalgam= mercury + silver/tin alloy (amalgam alloy). Amalgam reaction results in hard shiny silver grey restorative.

  • encapsulated away like GIC. Seal is broken and activated. Trituration is process of mixing. needs to be a shiny mass if crumbly and not shiny then throw away!

COMPONENTS-

  • inter metallic compound.

  • usually a mixture of admixed lathe and spherical particles

  • silver tin and copper important component's

ROLE OF COMPONENTS-

  • silver/tin/copper alloy

  • intermetallic phase referred as gamma phase which readily amamlgmises with Hg

  • tin

    • too much expansion = pain, fracture

    • too much contraction = marginal gap

    • NEED MINIMAL CHANGE

  • copper

    • increases strength

    • reduces tarnish and corrosion

    • decreases creep, marginal deterioration

  • zinc

    • manufacturer - reduces oxidation reaction keeping alloy clean and reduces corrosion but can be accelerated with moisture contamination

    • restoration - lowers rate of marginal fracture (delays corrosion), moisture control causes delayed expansion, surface blistering, internal corrosion


HIGH Cu SINGLE PHASE AMALGAMS;

  • Terninary allow mixed with mercury

  • modifies setting reaction

    • weak phase not formed

    • Sn has greater affinity for Cu than Hg

    • Cu combines with Sn to form ‘eta’ phase

  • modern amalgams have high copper content (1960s)

  • copper forms with tin then tin forms with mercury

GENERAL SETTING REACTION;

  • vigarous mixing

  • in an amalgamator

  • Hg particles diffuse into outer layerof Ag-Sn-CU particles

  • particle size decreases

  • discrete phases are formed

KEY MATERIAL CHARACTERISTICS-

  • Ease of manipulation mix in amalgamator, pick up with amalgam carrier, insert and pack into cavity with force. quicker than resin based composites. less physical change eg light curing causing stress and gaps

  • acceptable marginal adaptation - packed in instead of gaps by curing and shrinkage

  • technique insensitive - bulk placement, less issue with bonding system

  • self-sealing?- minimise leakage and bacterial invasion

  • biocompatibility? no harm if handled correctly

  • mechanical properties/wear resistance - metal alloy

  • low cost


MERCURY CONTENT-

  • 40-50% by weight composition dependent

  • residual Hg (mostly tied up)

    • NOT free elemental Hg

    • trituration consumes all elemental mercury(mixing so less released)

    • ideal range is about 44-48%. if 55% which gives lower strength, higher corrosion (at surface), higher creep

      • poor condensation techniques

      • delay between trituration and condensation (apply force to condense material into cavity with condenser if not done properly all excess mercury may not be gone)- eg aspirator, any excess will come to surface.

    • highest residual mercury at surface and margins.


MECHANICAL PROPERTIES- STRENGTH:

  • not going to reach full strength until after 24 hours

  • maybe half of it by half an hour (eg occlusal surface recommend to pt not eat anything for an hour)

  • inter metallic alloy= set amalgam exihibits brittle failure

    • weak in thin sections

    • unsupported edges fracture easily

      • (amalgam placed with remaining supported tooth structure)


PHYSICAL PROPERTIES- CREEP:

  • can be issue if not places properly

  • creep is a time dependent response of a solid to stress and force

  • if material close to room temp or mouth temp will tend to flow - CREEP INCREASES AS MELTING POINT REACHES

    • Phase containing increased mercury results in increased creep

  • consequences-

    • amalgam flows over marginal edge

    • thin sections fracture easily

    • marginal deterioration ‘ditching’

  • more common in older pre 1960s amalgam before copper

  • high copper = 1%… chances increase if high residual mercury left in there.


MICROLEAKAGE;

  • more gaps created

  • key factors;

    • 2-20 micrometer gap

    • poor condensation technique

    • corrosion by-products (can fill gap)

    • difference in TEC (thermal expansion co-efficent aka thermal properties of materials) -restorative material may expand or retract to temp change differently to tooth tissue. occurring too much will create gaps at interface, more expansion creates force at interface.

    • MORE ISSUE IN RESIN COMPOSITES!!!


OPERATOR VARIABLES;

  • Proportioning

  • titration= coat each particle with Hg, ensure thorough amalgamation (mix), use recommended times

  • condensing= process of reducing volume of material. Create marginal adaptation eg into line angles, eliminate voids, remove excess Hg eg aspirate, spherical alloys difficult to condense.

  • carving/polishing = naturally shiny after 6months. increases hardness and decreases porosity. Why not polish? creates aerosols don’t want to create airborne particles of a amalgam where at surface residual mercury content high.


TRENDS IN USE OF AMALAGAM;

  • easy to use

  • adequate mechanical properties

  • cost-effective

  • usually outlast resin based composites (but technique good)


  • use of amalgam is declining

  • aesthetic demand

  • alleged health effect

  • environmental concern

  • different rates internationally