amalgam Flashcards
what is dental amalgam
an alloy formed by the reaction of
> mercury (liquid)
and
> silver, tin, copper and other metals (powder)
what is an alloy
consists of 2 or more metals
what are the 2 ways to classify amalgam
> composition
> particle size and shape
classify amalgam in terms of composition
> traditional
> copper enriched
what does particle size and shape of amalgam affect
affects handling and final properties
what is the the average composition of amalgam
- 70% Ag
- 25% Sn
- 3% Cu
- 1% Zn (a lot of today’s amalgams don’t have zinc and are zinc free - traditional amalgams have a small %)
- 1% Hg
what is included in the powder constituent of amalgam
> silver, tin
copper
zinc
Hg in powder
what is the function of silver, tin
intermetallic compound Ag3Sn
gamma phase - reacts with mercury (Hg) to form amalgam
what is the function of copper
increases strength and hardness
added to improve material
what is the function of zinc
scavenger during production
preferentially oxidises and slag formed / removed
some amalgams are zinc free
what is the function of mercury in powder
only a few materials have this
pre-amalgated alloys so it will react faster
[silver, tin and copper then mix with amalgam grounded down]
what is included in the liquid constituent of amalgam
mercury (Hg)
what is the function of mercury in liquid
reacts with other materials
it is triple distilled = very pure
name 2 particle types
> lathe cut
> spherical, spheroidal
what are lathe cut particles
> coarse, medium, fine
formed by filing ingots
creates lots of fragments / shards / shavings
what are spherical, spheroidal particles
> range of particle sizes
formed by spraying molten metal into inert atmosphere
easier to control
form globules on surface underneath
what is the setting reaction of amalgam
Ag3Sn + Hg = Ag3Sn + Ag2Hg3 + Sn7Hg9
gamma = gamma + gamma 1 + gamma 2
powder + liquid = unreacted particles + amalgam matrix
what is present in the set structure of amalgam
> gamma
gamma 1
gamma 2
what is gamma
intermetallic silver tin (Ag3Sn)
gives good strength and corrosion resistnace
what is gamma 1
Ag2Hg3
gives good corrosion resistance
its like a soup which encapsulates the rest
what is gamma 2
Sn7Hg9
it is weaker and has poor corrosion resistance
spherical particles
what do voids do
decrease strength
increase corrosion
what is the tensile strength of the different amalgam components
> gamma = 170
gamma 1 = 30
gamma 2 = 20
= amalgam = 60
what is the setting dimensional changes in traditional amalgam
initial contraction - solution of alloy particles in mercury
then expansion - gamma 1 crystallisation
what is the setting dimensional changes in modern amalgam
small contraction
solid solution of mercury in Ag3Sn
advantage = doesn’t expand
(note: expansion / contraction is <0.2% so little clinical sign)
what happens with zinc when it interacts with saliva / blood
Zn + H2O = ZnO + H2
bubbles of H2 formed within amalgam
> pressure build up causes expansion
> downward pressure causes pulpal pain
> upward pressure means the restoration will sit proud of the surface
forcing material to expand = not a desirable effect hence why there is zinc free materials
what is amalgam properties dependent on
> dependent on handling factors - proportioning and trituration - condensation - carving and polishing > dependent on cavity design > variations in products > affected by corrosion
name mechanical properties of amalgam
> strength
abrasion resistance
creep
biocompatibility
what is the abrasion resistance like for amalgam
high
suitable for posterior teeth
too high for deciduous teeth
what is the problem with creep
affects marginal integrity
changes shape
problem in tradiitonal materials
(depends on cavity design and corrosion)
what is the biocompatibility of amalgam
concern over mercury toxicity
what factors decrease strength
undermixing
too high mercury content after condensation
too low condensation pressure
slow rate of packing - increments don’t bond
(don’t want undulations on surface)
corrosion
what is creep
When a material is repeatedly stressed for long periods at low stress levels (ie stress below the elastic limit / will not cause the tooth to fracture) it may flow, resulting in permanent deformation [will change shape]
what does creep do to the restoration
causes the amalgam to change shape and sit proud of the tooth surface
protrudes around the margins
margins are exposed so when forces are applied the amalgam will fracture
resulting in ditched margins
what is the thermal expansion of amalgam
3 times more than the tooth
expands and contracts more than the tooth so it can leaev gaps
what is the thermal conductivity of amalgam
high
may need to use a liner / varnish (insulator) in deep cavities to protect the pulp
is amalgam compatible with bonding systems
amalgam does not bond to the tooth
needs mechanical retention
cavity walls need to splay outwards
what is the handling of amalgam like
- mixing, working times and setting times are ok
- varies between types
what is the viscosity of amalgam like
packed (condensed) into cavity
user friendly
what is the aesthetics of amalgam
poor
dont use anteriorly
what is the radiopacity like of amalgam
yes radiopaque
will show if you have left gaps in the restoration
is amalgam anticariogenic
no
does amalgam give a smooth surface
yes if polished well
may deteriorate over time
what is the setting shrinkage of amalam
modern materials tend to have net overall shrinkage
how does corrosion happen in amalgam
gamma 2 more electronegative
weakens materials (particularly at margins)
corrosion products may contribute to sealing margins
reduce by
> copper enriched polishing margins
> avoiding galvanic cells
what are the advantages of spherical particles
- Less mercury required
- Higher tensile strength
- Higher early compressive strength
- Less sensitive to condensation
- Easier to carve
what are copper enriched alloys also called and why have they this name
also called non-gamma 2
high copper content (>6%)
what are the types of copper enriched
> dispersion modified
- original
> single composition types
- introduced to increase uptake by profession
what is dispersion modified copper enriched amalgam
> originally Ag-Cu spheres and conventional lathe cut alloy (now some single composition dispersed alloys - spheres and lathe cut particle same composition)
originally thought spherical particles would act as a strengthening agent
but increased copper content gave beneficial modifications to setting reaction
i dunno if this made sense lol love my notes x
what is the dispersion modified setting reaction
as conventional material
- gamma + Hg = gamma + gamma 1 + gamma 2
- gamma 2 + Ag-Cu = Cu6Sn5 + gamma 1
(takes several days but gets rid of gamma 2)
what does the dispersion modified setting reaction do to the silver copper
adds a copper tin halo to it
Cu6Sn5 goes around the Ag-Cu
what is the difference between single composition and dispersion modified
it goes straight to having no gamma 2 present instead of having the 2 separate reactions
what is in single composition formulations
powder - Ag-Sn-Cu
particle types = spherical and lathe cut
what is the setting reaction of the single composition formulation
Ag-Sn-Cu + Hg = Ag-Sn-Cu + gamma 1 + Cu6Sn5
what are the benefits of copper enriched amalgam
> higher early strength
less creep
higher corrosion resistance (will last longer)
increased durability of margins (more difficult to break down)
how is each amalgam material affect by creep
> traditional lathe cut = 6.3% = massive
traditional spherical = 1.1%
copper dispersion = 0.46%
copper single = 0.07%
what amalgam material has the best compressive strength
copper single
even after 1 day placed it is double that than any of the others
what is the thermal expansion coefficient of amalgam
22-28 ppm/degrees celsius
what are the advantages of amalgam
• Strong
• User friendly
○ Especially copper enriched
what are the disadvantages of amalgam
• Corrosion • Leakage ○ Does not bond ○ Potential to creep • Poor aesthetics • Mercury ○ Perceived toxicity ○ Environmental impact
what is the advantage of encapsulated amalgam
mercury hygiene
what is good about traditional amalgam
lasts a long time
average = 4-5 years but can last longer than 10 years
what amalgam does the dental hospital do
permite
has a high compressive strength
lower microleakage
