Amalgam Flashcards
why is there concern amalgam has health risks
role of mercury in disease causation with its systemic distribution and accumulation in the body
what is amalgam
an alloy formed by the reaction of:
- mercury (liquid)
- silver, tin, copper and other metals (powder)
when was the ‘first amalagm war’
1840s (american society of dental surgeons banned use of amalgam)
when was the second amalagm war
1970s-1990s
mercury fear - health and environment
how is amalgam classified
by composition and by particle shape and size
what are the different types of composition of amaglam
- tradtional
- copper enriched
what are the different types of particle shape and size of amalgam
- originally applied to traditional but now also to copper enriched
- affects handling and final properties
what is amalgam composed of
- Ag (70%)
- Sn (25%)
- Cu (3%)
- Zn (most today don’t have zinc) (1%)
- Hg (1%)
what is the function of silver, tin in the powder
intermetallic compound (Ag3Sn) - gamma phase, reacts with Hg liquid to form amalgam
what is the function of copper in the powder
increases strength and hardness
what is/ was the function of zinc in powder
it was there to oxidise rather than have the other metals oxidise (sacrifical lamb)
what is the function of Hg in powder
“pre-amalgamated” alloys react faster
what proportion of weight of amalgam is powder and liquid
powder: 50%
liquid: 50%
what is the function of Hg in liquid
- triple distilled (very pure)
- reacts with other metals
what are the different particle types in amalgam
Lathe cut
- coarse, medium, fine
- formed by filling ingots
Sperical, spheroidal
- range of particle sizes
- formed by spraying molten metal into inert atmosphere
Describe the setting reaction of amalgam
silver + tin –> unreacted silver/tin + tin/mercury
describe the set structure of amalgam
gamma (silver/tin)
- good strength and corrosion resistance
gamma 1 (silver/mercury) - good corrosion resistance
gamma 2 (tin/mercury) - weak and poor corrosion resistance
what do voids do to the set structure
decrease strength and increase corrosion
what is the tensile strength of gamma
170MPa
what is the tensile strength of gamma 1
30MPa
what is the tensile strength of gamma 2
20MPa
What is the mean strength of amalgam
60MPa
how do setting dimensional changes differ between traditional and modern amalgams
Traditional (big changes in expansion)
- intial contraction (solution of alloy particles in Hg)
- expansion (gamma 1 crystallisation
Modern (expansion/contraction is <0.2% so little clinical sign)
- small contraction
- solid solution of Hg in Ag3Sn
why do we now have zinc free materials
It can react with saliva to get zinc oxide and hydrogen, so bubbles of hydrogen formed within amalgam
- pressure build up causes expansion
- downward pressure cause pulpal pain
- upward - restoration sitting proud of surface
what do properties of amalgam depend upon
- handling factors
- cavity design
- products
- corrosion
What is the strength like of amalgam
early: fairly poor
later (>24hrs): OK
what is the abrasion resistance like of amalgam
- high, suitable for posterior teeth
- too high for deciduous
what factors decrease the strength of amalgam
- undermixing
- too high Hg content after condenstaion
- too low condensation pressure
- slow rate of packing (increments do not bond)
- corrosion
What is ‘creep’
when a material is repeatedly stressed for long periods at low stress levels (i.e. stress is below elastic limit) it may flow, resulting in permanent deformation
why is creep a problem for amalgam
- amalgam is visco-elastic
- high in traditional materials
- affects marginal integrity
n. b. also marginal integrity depends on cavity design and corrosion
what materials are affected by creep
amalgam
alloys
waxes
plastics
how does creep affect amalgam
- amalgam fits cavity, force is low magnitude force and is applied frequently
- amalgam changes shape, sitting proud of tooth surface
- amalgam vulnerable to fracture at margins (ditched margins)
what is the biocompatibility of amalgam like
so far it’s safe to use but concern about mercury toxicity
what is the thermal expansion of amalgam like
x3 of tooth
what is the thermal conductivity of amalgam like
high, may need to use liner/ varnish in deep cavities
does amalgam bond to tooth
no
how do we get amalgam to stay in place
needs mechanical retention
what are the handling properties of amalgam like
mixing, working and setting times are OK, varies between types
what is the viscosity properties of amalgam like
packed (condensed) into cavity - user friendly
aesthetics of amalgam
poor
radiopaque?
yes
anticariogenic?
no
smooth surface?
yes, if polished well, may deteriorate over time
setting shrinkage?
modern materials tend to have net overall shrinkage
what part of the set structure is the most electronegative and why is that important
gamma 2
weakens material particularly at margins
corrosion
how do we reduce corrosive properties of amalgam
- copper enriched, polishing margins
- avoiding galvanic cells
what are advantages of spherical particles
- less Hg required
- higher tensile strength
- higher early compressive strength
- less sensitive to condensation
- easier to carve
what are copper enriched alloys also called?
non-gamma2
higher copper
copper enriched
what proportion of copper enriched alloys are copper
> 6%
what are the different types of copper enriched alloys
- dispersion modified (original type)
- single composition types (introduced to increase uptake by profession)
how many reactions happen in dispersion modified copper enriched alloys
2
what are dispersion modified copper enriched alloys made of
Ag-Cu spheres + conventional lathe cut alloy
What are the two reactions that happen in dispersion modified copper enriched alloys
- gamma + Hg –> gamm + gamma 1 + gamma 2
2. gamma 2 + Ag-Cu –> Cu6Sn5 + gamma 1
takes several days
how is Ag-Cu and Cu6Sn5 arranged in dispersion modified setting reaction
Ag-Cu as a sphere
Cu6Sn5 as halo around spheres
(also gamma 1 and gamma present)
in single composition formulations (Cu enriched) what is the powder composed of
Ag, Sn, Cu
- copper 12-30%
- particle types: spherical and lathe cut
in single composition formulations (Cu enriched) what is the setting reaction
Ag-Sn-Cu + Hg –> Ag-Sn-Cu + gamma 1 + Cu6Sn5
no gamma 2
what are the benefits of copper enriched alloys
- higher early strength
- less creep
- higher corrosion resistance
- increased durability of margins
out of trad-lathe, trad-spherical, Cu dispersion and Cu single, what are the % creep values
trad-lathe (6.3%)
trad-spherical (1.1%)
Cu dispersion (0.46%)
Cu single (0.07%)
how do the compressive strength values change between the 4 alloys between day 1 and day 7
lathe (45-302 MPa)
spherical (120-370)
CU dispersion (118-387)
Cu single (272-485)
what is the thermal expansion coefficient of amalgam compared to enamel and dentine
Amalgam (22-28ppm/oC)
enamel (11)
dentine (8)
advantages of amalgam
strong
user friendly
disadvantages of amalgam
- corrosion
- leakage (doesn’t bond)
- poor aesthetics
- mercury (pereived toxicity and environmental impact)
which amalgam should we use?
Encapsulated
- Hg hygiene
Trad alloys
- served well, lifetime in excess of 10 years
- but av lifetime 4-5 years (?)
Copper enriched
- some clinical trials showing material is superior
GDH&S
- we use permite
what are the properties of permite
- non-gamma 2, spherical and lathe cut
- huge compressive strength quickly (260MPa in 1hr, 500MPa in 24hrs)
- very low microleakage compared to other brands (0.04 ml/min)
- compressive strength(500)
- tensile strength (60)
- elastic modulus (30)
- hardness (100)
how does amalgam compare to other composites failure rate after 8 years
much better than all composites (lower failure rate)
