5- Dental Amalgam Flashcards
What is amalgam?
An alloy of mercury with another metal
Why do we use amalgam?
- inexpensive material
- easy to use
- been used for over 100 years
- resin-free [so less allergies than composite]
What are the advantages of using amalgam?
- inexpensive
- rigid
- strong
- wear resistance
- durable
- easy to manipulate
What are the disadvantages of using amalgam?
- bad aesthetics
- mercury hazard
- MARGINAL DITCHING : [CREEP AND CORROSION RISK]
- non-bonding (doesn’t rely on adhesive so more tooth tissue is removed)
What are 2 different types of amalgam cut you can have?
- Irregular (lathe-cut)
2. Spherical (atomised)
What are the 2 different sizes of amalgam cuts?
- Fine cut [35 nanometers]
2. Micro cut [25 nanometers]
What do the properties of amalgam depend on?
Alloy composition i.e material
What do the handling characteristics of amalgam depend on?
Shape and size of the alloy particle
Narrow head condenser?
Irregular alloy particle
Wide head condenser?
Spherical alloy particle
On average how much silver is in amalgam?
70
Difference between conventional and blended amalgam?
Blended contains much higher copper count (13)
What is the primary reaction of amalgam?
Reaction of alloy with mercury
State primary reaction equation?
Ag3Sn + Hg –> Ag2Hg3 + Sn7Hg + Ag3Sn
What is gamma 1 phase?
Silver mercury [Ag2Hg3]
What is gamma 2 phase?
Tin mercury [Sn7Hg]
What is the unreacted core i.e gamma phase?
Ag3Sn
Which phase do we try to minimise?
Phase 2 (it is corrosive, weak, low strength)
Properties of Ag3Sn (Gamma)
- hard
- strong
Properties of Ag2Hg3 (gamma 1)
- brittle phase
- intermediate strength
- small crystals
- low melting temperature
Properties of Sn7Hg (Gamma 2)
- Soft-phase
- weak low strength
- high flow
- corrosion
- long, blade-like crystals
State secondary reaction equation
Sn7Hg + Ag-Cu –> Cu6Sn5 + Ag2Hg3
What happens if excess mercury is present in the secondary reaction?
Then not all of gamma 2 phase is removed
Things to think about amalgam?
- conductivity
- strength magnitude
- risk of creep?
- corrosion
- tarnish (corrosion of top layer only)
- marginal integrity and microleakage
- toxicology and biocompatibility
Colour of amalgam?
lustrous, shiny white
Radiopacity of amalgam?
> 2mm, aluminium
Coefficient of thermal expansion of amalgam?
25ppm/degree C
Electrical conductivity of amalgam?
High
Thermal conductivity of amalgam?
High
What is corrosion?
metal reacting with oxygen to form an oxide degrading the surface
What is creep?
when you apply a force how much does it tilt (could affect marginal seal leading to leakage and recurrent caries)
Equation when corroded?
Sn8Hg (+ h20 from saliva) –> Hg + SnO2 + Sn(OH)6
Harms of corrosion?
releases free mercury
How can we prevent corrosion?
Using high-grade Cu amalgam, eutectic spheres make the material safer as well as stronger
What is the difference between using high-grade Cu amalgam and low grade in secondary reaction?
Low grade leaves lots of gamma phase 2 whereas high grade there is barely any G2 only G1 which is much stronger and safer
What is blended amalgam made of?
2 parts conventional alloy + 1 part Ag-Cu eutectic spheres
Why should you use cavity varnish when applying amalgam?
improves marginal seal
why should you have moisture control when applying amalgam?
zinc reacts with water releasing hydrogen ions that cause delayed expansion
Why should you carve the amalgam restoration?
- removes the mercury rich surface layer
- rebuilds the anatomy of the tooth
Why should you polish the amalgam restoration?
- gives lustrous surface
- better corrosion resistance
How is mercury toxicity possible?
- Direct contact with mercury
- removing old restorations
- mercury hypersensitivity
Why has there been a decline in the use of amalgam?
- Environmental concerns
2. Aesthetic issues
Environmental risks of using amalgam?
- Recycling/disposal of amalgam
- Mercury Spillage
- Residual mercury left in capsules
