L11 Dental Silver Amalgams 1 Flashcards
What is amalgam?
A mixture of 2 or more metals one of which is mercury
What is a dental silver amalgam made of?
Mercury and silver/tin alloy
What does amalgamation reaction result in?
Hard shiny silver-grey restoration
Do amalgams last longer than composites and why?
Much easier to set than resin composites thus last much longer.
Is amalgam toxic and how is this prevented?
Presented in a capsule to prevent from exposure to liquid mercury (as the vapour is toxic). Once set and in place there is no evidence to suggest that it releases any toxic vapours.
What does pressing the capsule do?
Breaks the seal between the powder and liquid mercury
The amalgamated oscillates at a high frequency for a set number of seconds, what does this do?
Every particle of inter metallic alloy is coated by mercury.
What should the amalgam in the capsule look like?
Should not be crumbly or dull - this should not be used. Should be shiny and liquid.
Why is amalgam still around in the UK?
It is a cost-effective material. Mercury is liquid at room temperature so can create a plastic mass at room temperature and mould into a cavity chairside.
Why is dental silver amalgam being phased down?
Less conservative
Environmental concern
Cosmetic reasons
What is the minimata convention?
People getting very ill from mercury contamination
What are the EU regulations on mercury (heavily regulated and restricted)?
Cannot be used in:
- pregnant or breastfeeding women
- adolescents - children under 15
- deciduous teeth
(With exceptions if it is deemed strictly necessary)
What is the intermetallic compound (alloy)?
Ag3Sn
What is the intermetallic phase referred to as?
Intermetallic phase (silver/tin alloy) is referred to as the gamma phase which readily amalgamates with Hg
What is the expansion of tin and what does it do?
<25% expansion
Expansion means it fills the gaps, so less likely of infection occurring and failure of the material
What is the contraction of tin?
> 29%
What does tin do for the amalgam?
Tin controls the physical change of the material when it is in the cavity and when it is setting. Do not want too much shrinkage to cause stress at the interface and cause gaps (not much of an issue in amalgam because of the rate of set, stress is dissipated over time)
What does stress create?
Stress might cause micro cracks and pressure on cusps and enamel, or gaps might cause secondary infection.
What does copper do in amalgam?
Dispersion strengthening agent
Increases strength
Reduces tarnish, corrosion
Decreases creep, marginal deterioration (improving mechanical properties)
Zinc is not in all amalgams, but when it is what does it do?
Lowers rate of marginal fracture (delays corrosion)
Scavenges oxygen, keeping the alloy clean during production
What might moisture contaminate Zn-containing alloys cause?
Delayed expansion
Surface blistering
Internal corrosion
What is the general setting reaction for dental amalgam?
y + Hg —> y + y1 + y2
where gamma y is the intermetallic compound
gamma1 y1 is the silver mercury phase
gamma2 y2 is the tin mercury phase
What is an intermetallic compound?
2 or more metals combine with a discrete composition
How does the setting reaction work?
Vigorous mixing - Hg diffuses into outer layer of Ag3Sn particles
Particle size decreases
Solubility of Ag in Hg is much less than Sn in Hg, so Ag precipitates out first as gamma1 occupying a significant proportion of the microstructure followed by Sn as gamma2
Properties of the phases
y - hard and strong
y1 - brittle, midrange strength
y2 - soft, low strength, high flow and corrosion
Why are the compounds with mercury weaker?
Due to the electro negativity and softness of mercury
What is the limitation of the conventional amalgam?
Y2 phase is soft, prone to creep and prone to corrosion
Target is to reduce or eliminate y2
What is the target to bettering conventional amalgam?
Reduce or eliminate the y2 phase
What happens in the development of modern amalgams?
Dispersed phase: admix of Ag-Cu and Ag-Sn; Cu more uniformly distributed; physical properties improved
Spherical particles introduced, affecting setting and handling properties
What happens in the high copper dispersed phase amalgam?
Copper content increased modifies the setting reaction
The weak y2 phase is then removed. Sn has a greater affinity for Cu than Hg, Cu combines with Sn to form ‘eta’ phase
What is the setting reaction for high copper dispersed phase?
1: y + Hg —> y + y1 + y2
2: y2 + AgCu2 —> ‘eta’ phase + y1 (+y)
eta phase = Cu6Sn5
Compare the microstructure of conventional amalgam alloy with dispersed phase amalgam alloy
Pg62 microstructure image
What is the high Cu single phase amalgam?
Instead of having 2 binary alloys, combine them all together to create a terniary alloy. No formation and removal of y2, just formation of the ‘eta’ phase
What is the setting reaction for the high Cu single phase amalgam?
Ag-Sn-Cu + Hg —> y + y1 + eta phase
(Terniary spherical alloy)
Microstructure image on pg 62
What does the copper content need to be in modern amalgams?
At least 12%
What are the advantages of copper in amalgam?
Reach full strength more quickly
Easier to condense
Easier to carve and polish
Lower mercury content
Higher strength
More resistant to corrosion
