Amalgam Flashcards
what is dental amalgam?
an alloy formed by the reaction of
- mercury - liquid
- silver, tin, copper and other metals - powder
amalgam - classifications
composition
- traditional
- copper enriched
particle size and shape
- affects handling and final properties
amalgam composition - percentages
70% silver
25% tin
3% copper
1% zinc
1% mercury
function of amalgam powder constituents (50%) by weight
silver, tin
- reacts with Hg liquid to form amalgam
copper
- increases strength and hardness
zinc
- scavenger during production
- preferentially oxidises
Hg in powder (few materials)
- pre-amalgamated alloys - react faster
Function of Hg liquid
triple distilled - very pure
reacts with other metals
Outline the amalgam setting structure
some regions are gamma, gamma 1 and gamma 2 phase
gamma phase
- good strength and corrosion resistance
gamma 1 phase
- good corrosion resistance
- holds gamma and gamma 2 phases together
gamma 2 phase
- weak and poor corrosion resistance
Effect of voids on amalgam
decreased strength and increased corrosion
tensile strength of amalgam components (MPa)
gamma = 170 MPa
Gamma 1 = 30
Gamma 2 = 20
amalgam = 60
Amalgam setting reaction
amalgam senting dimensional changes
modern
- small contraction
- solid solution of Hg in Ag3Sn
traditional
- initial contraction
- expansion due to gamma 1 crystallisation
Zinc reactions in amalgam
interacts with saliva and blood
bubbled of H2 formed within amalgam
- pressure build up leading to expansion
- downward pressure = pulpal pain
- upward expansion = restoration sitting proud of surface
amalgam properties depend on…
dependent on handling factors
- proportioning and trituration
- condensation
- carving and polishing
dependent on cavity design
affected by corrosion
some variations between products
Amalgam particle types
lathe cut
- coarse, medium, fine
- formed by filling ingots
spherical, spheroidal
- range of particle sized
- formed by spraying molten metal into inert atmosphere
advantages of spherical amalgam particles
less Hg required
higher tensile strength
higher early compressive strength
less sensitive to condensation
easier to carve
Amalgam mechanical properties
good strength
high abrasion resistance
- suitable for posterior teeth
- too high for deciduous teeth
prone to creep
factors decreasing amalgam strength
undermining
too high Hg content after condensation
too low condensation pressure
slow rate of packing
- increments do not bond
corrosion
What is creep?
when a material experiences low level stress (below elastic limit) which are applied repeatedly
- over a prolonged period of time it may flow, resulting in permanent deformation
dental materials prone to creep
amalgam
alloys
waxes
plastics
amalgam biocompatibility
concern about mercury toxicity
- mercury released from dental amalgams - quantities very small
- do not cause verifiable effects on human beings
- supportive data incomplete
amalgam thermal properties
thermal expansion = 3x tooth
thermal conductivity - high
- liner or varnish may be required in deep cavities
high thermal diffusivity
- when a transient thermal stimulus e.g, tea is applied , temperature of amalgam rises quickly
Amalgam - bonding
does not bond to tooth
needs mechanical retention
- some suggest bonding systems based on 4-META to get bond to tooth - not widely accepted
amalgam handling/viscosity
handling
- mixing, working, setting times - ok, varies between types
viscosity
- packed into cavity
- user friendly
amalgam - corrosion properties
gamma 2 most electronegative
weakens material particularly at margins
- however corrosion products may contribute to sealing margins
how to reduce amalgam corrosion
copper enriched amalgam
polishing margins
