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Dental alloy(aka. Gamma phase Ag3Sn)
- Alloy being a combination of 2 metals
o
Metals are in its solid rather than ionic form
o 65% Ag; 35% Sn
- Not pliable (means easily bent/flexible)
- Needs Hg to help pack it into the tooth
o Hg converts y to y1and y2which increases the pliability of
amalgam
o y it self is too porous/ brittle
-
Not perfectly soluble
-
Not homogenous (sections of just Ag and sections of just Sn
within Ag3Sn)
o If an oxide layer on the surface of Ag3Sn becomes too thick
prevents binding of
Hg (passivated layer)
Passivated layer: Prevents the metal to corrode any further
2 types of cut of Ag3Sn:
-
Lathe cutproperties are varied, not as effective for
amalgamation
o Shuave Spirals that are produced by puttingAg3Sn on a lathe
machine These
spirals are later crunched up to smaller pieces
-
Spherical cutparticles are regular spherically sized so reaction
with Hg is more reliable
and it is more readily condensed into a cavity
Amalgam Steps1.
Trituration (mixing)
2. Condensation (packing)
3. Carving
4.
Burnishing5.
Polishing
TriturationOverall Reaction:
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*y1 and y2 are inferior in strength, corrosion resistance and
creep to y phase limit y1 an y2 by
limiting the amount of Hg
Trituration Process:
- Add Hg to gamma phase
o Initial contraction occurs (volume of Hg and gamma mixture is
less than the total of
the initial volumes of the two mixtures)o This is as y acts as a
sponge absorbing the Hg
- Shake mixture to break up the alloy removes oxide layer and
increases surface area for
Hg to react with gamma
o Increase SA as you break up the alloy Break off gamma
particles off the big
unreacted gamma particle (breaking off anita)
o Hg reacts with Ag rich area to form gamma 1
o Hg reacts with Sn rich area to form gamma 2
o Continued shaking ensures that you remove the newly form y1
and y2 particles
Short Trituration Time: Stop shaking after a short period of
time
- y1 and y2 dont fall off the y particle (no new surface area at
y for Hg to react diffusion of
Hg into y particles are suppressed)- Gamma 1 and 2 crystallise
upon formation (nucleation) and results in expansion
o Nucleation = Forming the new phase of y1 and y2 from y results
in expansion
*Crystallisation: Forming solid crystalsy1 and y2 gets
BIGGER
*Too much expansion = Bad = Cracking of tooth
*Also not enough y1 and y2 forms Amalgam not very packable as y
itself is brittle/hard/not pliable
Long Trituration Time: Stop shaking only after a long time
- For a long time; gamma 1 and 2 are continually removed from
the surface of gamma and a
new surface is available for reaction with Hg
o More gamma 1 and 2 are formed resulting in a smaller gamma
core
o This results in a greater contraction
*Too much contraction = Bad = Leakage around margins of tooth
and restoration
*Too much y1 and y2 = Inferior to y
AIM of trituration
- Need to achieve a balance between the contraction and
expansion:
o Dont want : Formation of gamma 1 and 2 with no gamma remaining
(completed
reaction)
o We want:
Some gamma 1 and 2 formation to keep the superior properties of
gamma
in the product
Limit formation of gamma 2 (mainly) as gamma 2 has weaker
properties
Easy to handle in oral cavity
Cannot become too porous (dont want toomuch product) so that
it
withstands forces
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But it needs to still be pliable enough to allow packing into
oral
cavity
Must be able to set in a wet environment (within saliva)
*Time of trituration hence is the most important factor
Limiting the formation of gamma 2- Reduce trituration time and
increase condensation force
o Reduce trituration less surface area available (and less
contact time) for Hg to
react with gammacan only be reduced to a certain extent as
amalgamation
MUST occur
o Increase condensationpushes excess Hg to the surface and
removes it less Hg
is present to react with gamma to form gamma 2 after amalgam is
set in mouth
(reduces post-setting expansion)
- Add a dispersed alloy (gamma alloy also containing other
metals eg. Ag and Cu)
o Ag reacts with Hg to form more gamma 1
o
Cu competes with Hg in gamma 2 to form a beta phase (Cu and Sn)-
Mechanical properties are improved BUT more galvanic corrosion
occurs as more metals are
presentneed a balance between the two
*Y2 properties are inferior to gamma and gamma 1
*rate of formation is directly dependent on the amount of Hg
present
Condensation-
Condensation: Pressure applied with a packing instrument when
placing the amalgam into a
cavity
- Increase condensation pressure will
o
Counteracts setting expansion
This is as the initial diffusion layer of Hg is disrupted and
more Hg diffusion
takes place
o Also packs the amalgam alloy particles closer together leaving
less room for Hg
Inhibits contraction from initial reaction? Sponge absorbing
Hg
Excess Hg moves to the surface and is removed (less likely to
react after it
sets) Prevent further formation of y1 and y2 (which tends to
counteract
expansion)
- Good condensation will remove as much of the mercury rich y1
and y2 phases
-
Inefficient condensation:
o Dont pack amalgam strong enoughincreased porosity which
can lead to 30% increase in creep
Restoration failureConsequences of Creep
-
Creep: Is the flow of setamalgam which causes
o Flattening of contact points between the amalgam restoration
and neighbouring
tooth
o Overhang Margins
o Biting on amalgam can cause amalgam to flow up/over to the
margins and can chip
off and cause leakageo Amalgam can protrude into tooth margins
which leads to marginal breakdown
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*Overhang Margins: An excess of dental filling material beyond
the cavity margin
Consequences of Corrosion?
- Corrosion: Amalgam undergoes chemical reaction with its
environment forming breakdown
products
- Corrosion reaction: y2+ O2Tin Salts and free Hg
o
From this reaction; a passivating film of oxides is formed
Insulate amalgam fromfurther reaction/corrosion?
o However, biting in amalgam removes the oxide layer and exposes
amalgam Draw
picture
Gamma 2 is most reactive and a small electrolytic cell is
created (galvanic
corrosion)
Gamma 2 is lost into the saliva as it corrodes (higher reduction
potential
gain electrons and will be released as tin salts and free
Hg)
o Zinc can be added to react with O2preferentially to reduce
corrosion (prevents
corrosion/oxidation of y2)
If it is contaminated it results in expansion
*Continued corrosion causes staining of dentine and stains
amalgam as a dark grey/blackTooth fracture
-
Cavity preparations with sharp internal line angles are usually
needed to increase
mechanical retention for amalgam
o There is very little bonding strength between amalgam and the
tooth
o However sharp line angles = act as stress raises are
contribute to tooth fracture
o COMPROMISE: use a slightly rounded cavity that also provides
sufficient mechanical
retention
-
Round off the acute cavosurface angles (unsupported enamel rods)
to decrease risk of
chipping/marginal breakdowns Cavosurface angles should be
90o
- Amalgam does not reinforce remaining cusps on teeth
o
Cusps are more likely to fracture/chip offCoefficient of thermal
expansion/contraction
- Large discrepancy between tooth (11.4) and amalgam (28)
o Expands and contracts very differently to tooth upon exposure
to heat
o Contraction = leakage
o Expansion = cracking of tooth
- Amalgam also has a high coefficient of thermal
conductivity
o Composite has similar thermal expansion/contraction as amalgam
but much lower
thermal conductivity heat transfer rate is low;
expands/contracts slower
-
Opening the interface between tooth and restoration Allows
bacterial ingress and
recurrent caries
Contamination of cavity
- Need to work in a dry environment Want amalgam to bond
directly to the tooth (nothing
between tooth and amalgam which can cause failure at margins of
restoration)
- Contamination with moisture (saliva) during packing can cause
expansion (along with short
trituration and poor condensation)
o This can cause cracking of mesial/distal margins of teeth with
Class 1 restorations
Amalgam failure
o Restoration is too thin relative to the surface
Cracks easily when biting on it
o Line angles in the cavity are too sharp
o Cavity preparation has poor mechanical retention
Amalgam restoration can fall out of cavity
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Gross Amalgam Fracture
- Caused by shallow preparations (shallow = not enough
strength), non-retentive cavity preps
or sharp line angles
Extra Points
*Can control size of Ag3Sn to minimize the amount of Hg required
e.g. smaller y particles will have alarger SA for reaction with Hg
(so can decrease amount of Hg required)
*Less seconday caries occur in amalgam compared to composite
*Last 20-30 years; Composite a few years
