clinical aspects of amalgam Flashcards
Components and chemistry ◦ Properties ◦ Relating properties to clinical use ◦ Compare with other materials ◦ Safety
what is amalgam
an alloy of mercury with another metal or other metals
what is dental amalgam
consists of silver mercury and tin with some other metals to modify the properties
what is the chemical equation of amalgam
Ag + Sn —> Ag3Sn
what is the shape of amalgam
lathe cut
spherical
what is amalgam currently stored in
in a capsule and the mercury and the amalgam alloy are separated by a membrane but is ruptured before being placed in the amalgamator
what are the three possible stages of the dissolved metals
Ag+Sn—->Ag3Sn known as the gamma phase
Ag+ Hg—-> Ag2Hg3 known as the gamma phase 1
Hg+ Sn —->Sn7Hg known as the gamma phase 2
what is gamma 2 responsible for
corrosion
creep
decrease in strength
what does the addition of copper
AgCu + Sn7Hg ( gamma 2) = Cu6Sn5 +Ag2Hg3 ( gamma 1)
reduces the gamma 2 phase
what is the typical modern amalgam powder composed of
silver
tin
copper
zinc
what is the % by weight of silver
65 min
what is the % by weight of tin
29 max
what is the % by weight of copper
6-13 max
what is the % by weight of zinc
2 max
what is the powder:liquid ratio
Ideally final amalgam restoration should contain <50% mercury
how much mercury should be added to make the mixture movable
more than 50% to make it movable
what happens if there less than 50% mercury in the filling
it makes the amalgam very dry
what happens when the amalgam is packed
the mercury is drawn to the top and the excess is scraped away during carving
what do we use to pack the amalgam
the condenser
what layer lies on top of the restoration
the mercury rich layer which we can then scrap away
what are the properties of spherical amalgam
softer more flowable
what are the properties of lathe cut amalgam
less flowable,
good for building up large amounts of missing tooth structure
what do we typically use
ADMIXED
advantages of amalgam
good compressive strength good wear resistance easy to use kind to opposing teeth chemical set cheap radiopaque
disadvantages of amalgam
do not stick to teeth
weak in thin sections minimum is 2mm depth
thermal conductor
unaesthetic
what type of reaction can we get with amalgam fillings
lichenoid reactions
localised allergic reaction- red, white blotchy where the amalgam is located
how do we treat lichenoid reactions on the buccal surface
remove the amalgam filling and can replace with GIC/composite
what do we need to consider with amalgam fillings
they can act as a Plaque retention factor if there are overhangs
when do we use amalgam
class 1 and class 2 restorations
where heavy occlusal force is there
where aesthetics are not important
building up teeth before the tooth is crowned
how do we prepare teeth for an amalgam restoration
undercut
no unsupported enamel- wider at the base than the top
cavo surface angle should be greater than 90
the amalgam margin angle should be less than 70
how do we create an undercut
if we use the pear shaped bur in the HS and drill across the longitudinal axis of the tooth it should naturally create a undercut
what is retention
the prevention of the amalgam cavity being pulled out by vertical forces
macro mechanical
what is resistance
the prevention of the amalgam cavity being moved by lateral forces
what is the amalgam margin angle
where the restoration meets the prepared surface
what do we do if were missing a total cusp
we can use a pin but we DO NOT use them any longer
what are the disadvantages of a pin
could damage the pulp
if too close to the enamel it can cause the tooth cusp to fracture
what can we use instead of pins
pits and grooves- cut directly into the tooth to allow for the amalgam to macromechanically bond
what is bonding
the active substances (MDP or 4-META) form a hybrid layer with dentine and chemical bond to amalgam
what are the disadvantages of polishing
the heat generated from polishing might damage the pulp
will polishing improve the life expectancy
NO
in the 1998 study what is the median age at replacement for amalgam
15 years
in the 1998 study what is the median age at replacement for composite
8 years
how long does it take for amalgam to fully set
24 hours
how many amalgams were studied in the 2001 study
1800
how many composites were studied in the 2001 study
1500
in the 2001 study what was the average life span for amalgam
12 years
in the 2001 study what was the average life span for composite
5 years
what is the risk of secondary caries in the composite group
3.5 x
is mercury vapour safe
highly toxic
how is mercury vapour taken in
ingested
released when chewing or brushing
describe the toxicity of amalgam
When mercury enters the environment it can be converted by bacteria into methyl mercury (MeHg) in fresh and salt water
Methyl mercury is highly poisonous
which governments have banned mercury
sweden and germany
what do we need to do with mercury in the clinical skills lab
Report mercury spills
Put empty capsules and waste amalgam in the receptacles provided
Clean the heads into the designated sink( dirty sink) (This has a separator on it)
