Amalgam Flashcards
What is amalgam
an alloy formed by the reaction of mercury (liquid) and silver, tin, copper & other metals (powder)
how can amalgam be classified
composition
particle shape and size
what are the two amalgams with differing compositions
traditional
copper enriched
what does the particle shape and size determine
handling and final properties
it was originally applied to traditional but now also to copper enriched
what is the composition of the powder
zinc
silver, tin
copper (copper enriched)
mercury (sometimes)
what is the function of zinc
Most amalgams nowadays don’t have zinc in them, they used to have a small amount in them and its purpose was to act as a scavenger as it was preferentially oxidized during the manufacturing process which prevented the other metals in the alloy being oxidized
what is the function of silver, tin
silver is the main component, present in combination with tin as the intermetallic compound Ag3Sn known commonly as the γ-phase. It reacts with mercury liquid to form amalgam
what is the function of copper
present to increase the strength and hardness of the amalgam
what is the function of mercury in the powder
mercury is sometimes added to provide a more rapid reaction in what is referred to as pre-amalgamation
what is in the liquid
mercury (50% by weight)
what is the function of mercury
it is triple distilled (very pure) and reacts with other metals. The mercury has to be very pure otherwise a surface layer of contaminants is formed that interferes with the setting reaction.
what do the particle types apply to
The alloy is used in the form of a powder and the size and shape of the particles in this powder are critical to the handling characteristics and the final properties of the restoration.
what are the two types of particles
lathe cut
spherical
how are lathe cut particles produced
by machining a solid ingot of the alloy on a lathe. The chippings that are produced are graded and those in the right size range are used in the powder to be amalgamated with mercury.
what are the different types of lathe cut particles
The alloy is available as coarse, medium or fine grained powder and each will handle slightly differently.
what should be done to lathe cut amalgam and why
The individual chippings will become highly stressed during the machining, and this makes their surface very reactive to mercury. A consequence of this is that the setting reaction is far too rapid unless heat treatment (which relieves internal stresses) is applied. The heat treatment is usually carried out by placing the powder in boiling water
what are spherical particles
the various ingredients of the allow are melted together and then sprayed into an inert atmosphere, where the droplets solidify as small, spherical pellets of various sizes. This method of manufacture has the advantage that no further machining processes are required and that the composition of the alloy can be readily altered.
how is the setting reaction for amalgam initiated
vigorous mixing of the two ingredients. This mixing causes the other layer of the alloy particles to dissolve into the mercury, forming two new phases which are solid at room temperature
what is the setting reaction for amalgam
Ag3Sn + Hg > Ag3Sn + Ag2Hg3 +Sn7Hg
y + mercury > y + y1 + y2
powder + liquid > unreacted alloy + amalgam matrix
what is the set structure for amalgam
Not all of the alloy particles dissolve in mercury but rather a considerable amount remains so that the final structure is one of a core of gamma held together by a matrix of predominantly y1 which is interspersed with y2
what is the function of gamma
provides good strength and corrosion resistance
what is the function of gamma 1
provides good corrosion resistance
what is the function of gamma 2
y2 is weak and poor corrosion resistance
what do voids do
voids decrease strength and increase corrosion
what has the strongest tensile strength from the amalgam components
gamma
what amalgam component has the lowest tensile strength
gamma 2
describe the setting dimensional changes in traditional amalgam
there is initial contraction (0.2%) as there is a solution of alloy particles in the mercury then there is expansion due to the crystallization of y1 (0.4%)
describe the setting dimensional changes in modern amalgam
there is a small contraction (0.2%) and the end result is a solid solution of mercury in Ag3Sn
why was zinc removed from amalgam
if some of the zinc is unreacted then it may react with water in saliva/blood to produce bubbles of hydrogen
what can hydrogen bubbles forming in the amalgam result in
- Pressure build up which causes expansion
- Downward pressure causing pulpal pain
- The material being forced to expand over the surface of the tooth
what are the amalgam properties dependent on
handling factors
cavity design
what are the different handling factors that the properties of amalgam are dependent on
- Proportioning and trituration
- Condensation – the final mercury content of amalgam is dependent on the quality of condensation technique more than anything else, with a properly condensed amalgam having a mercury content of 50%
- Carving and polishing
describe the compressive strength of amalgam
The early compressive strength of amalgam (first hour) of traditional materials is poor however after 24 hours it becomes okay. The strength depends on the proportion of y2 formed and if its proportion can be minimized then this will result in a stronger amalgam
describe the abrasion resistance of amalgam
high making it suitable for posterior teeth but too high for deciduous
describe the factors that decrease strength in amalgam
- Undermixing
- Too high a Hg content after condensation (composition of y1 and y2 is strongly dependent on the amount of mercury in the final composition – y2 is not strong)
- Too low condensation pressure
- Slow rate of packing meaning 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
what does creep affect
marginal integrity
what materials does creep affect
amalgams, alloys, waxes and plastic
describe biocompatibility of amalgam
there are concerns about mercury toxicity. The NHS concluded that although mercury is released, the quantities are small and do not cause verifiable effects on human beings. While the current evidence supports the concept that existing dental restorative materials are safe, it must be recognized that the supporting data are incomplete.
describe thermal expansion of amalgam
we want amalgam’s thermal expansion to match that of the tooth and it is 3 times that of the tooth. There is potential that as amalgam warms up, it expands more than dental tissue and this may make a gap for bacteria which results in a risk.
describe thermal conductivity of amalgam
the thermal conductivity of amalgam is high meaning there is the potential for heat to reach the dental pulp therefore we require a liner to act as an insulator to prevent the heat from going all the way through amalgam and reaching the dental pulp
describe bond to tooth systems of amalgam
amalgam does not bond to tooth and instead requires mechanical retention.
describe aesthetics of amalgam
poor
is amalgam radiopaque
yes
is amalgam anticariogenic
no
is amalgam a smooth surface
yes, if polished well, may deteriorate over time
describe setting shrinkage of amalgam
modern materials tend to have a net overall shrinkage
describe mixing, working times, setting times of amalgam
okay but varies between types
describe viscosity of amalgam
packed (condensed) into cavity making it user friendly
why was corrosion often cited as an advantage
corrosion products help produce a good marginal seal
why is corrosion not an advantage in amalgam
crevice corrosion, caused by the formation of oxidation cell in the marginal gap can cause rapid deterioration in the properties of amalgam.
what phase is the corrosion process especially associated with
y2 phase
why is the corrosion process especially associated with y2 phase
The y2 phase is considerably more electronegative than the y and y1 phases. This means that in the presence of an electrolytic solution the y2 phase will act as an anode of the oxidation cell and will gradually dissolve.
This weakens the material particularly at the margins
how is the corrosion process reduced
It is reduced by copper enriched amalgam, polishing the margins and avoiding galvanic cells
what is the advantages of spherical particles
- Less mercury is required
- Higher tensile strength
- Higher early compressive strength
- Less sensitive to condensation
- Easier to carve
what are the names for copper enriched alloys
Can be called non-gamma2 amalgams or high copper amalgams
what are the 2 types of copper enriched amalgams
dispersion modified
single composition
describe dispersion modified copper amalgams
Originally a spherical allow (silver and copper) with a higher copper content was added to the conventional lathe cut allow
It was originally thought that spherical particles would act as a strengthening agent but increased copper content gave beneficial modifications to the setting reaction
what is the second reaction in dispersion modified copper amalgams
Y2 + AgCu Cu6Sn5 + y1 (takes several days)
how can Cu6Sn5 be seen
as a halo around the spheres
why is the second reaction in dispersion modified amalgams so beneficial
It is beneficial as we get rid of the gamma 2 phase
what is simple composition amalgams
Powder containing silver, tin, copper
Copper content of 12-30%
Particle types are spherical or lathe cut
what is the setting reaction for single composition
Ag-Sn-Cu + Hg Ag-Sn-Cu + y1 + Cu6Sn5
what are the benefits of copper enriched amalgams
- Higher early strength
- Less creep
- Higher corrosion resistance
- Increased durability of margins
what is the thermal expansion coefficient for amalgam
Coefficient for enamel is 11 and for dentine is 8
For amalgam it is 22-28 so almost double that of enamel/dentine
Has a better range than composite
GIC has the best thermal expansion coefficient
what are the advantages of amalgam
strong
user friendly
what are the disadvantages of amalgam
corrosion
leakage - does not bond
poor aesthetic
mercury (perceived toxicity, environmental impact)
what is the amalgam used in GDHS
permite
what is permit
Permite is non gamma 2, spherical & lathe cut
It has a significant compressive strength 1 hour after placement of 260 MPa
It has a high compressive strength of 500 MPa which is greater than other brands
The least microleakage compared to other brands – 0.04 ml/min
describe mechanical properties of amalgam
higher compressive strength and tensile strength than enamel
less rigid than enamel
what is the failure rate for amalgam after 8 years
5.8 - lower than composite
