Amalgam

Question 1

what is dental amalgam?

Answer 1

an alloy formed by the reaction of
- mercury - liquid
- silver, tin, copper and other metals - powder

Question 2

amalgam - classifications

Answer 2

composition
- traditional
- copper enriched
particle size and shape
- affects handling and final properties

Question 3

amalgam composition - percentages

Answer 3

70% silver
25% tin
3% copper
1% zinc
1% mercury

Question 4

function of amalgam powder constituents (50%) by weight

Answer 4

silver, tin
- reacts with Hg liquid to form amalgam
copper
- increases strength and hardness
zinc
- scavenger during production
- preferentially oxidises
Hg in powder (few materials)
- pre-amalgamated alloys - react faster

Question 5

Function of Hg liquid

Answer 5

triple distilled - very pure
reacts with other metals

Question 6

Outline the amalgam setting structure

Answer 6

some regions are gamma, gamma 1 and gamma 2 phase
gamma phase
- good strength and corrosion resistance
gamma 1 phase
- good corrosion resistance
- holds gamma and gamma 2 phases together
gamma 2 phase
- weak and poor corrosion resistance

Question 7

Effect of voids on amalgam

Answer 7

decreased strength and increased corrosion

Question 8

tensile strength of amalgam components (MPa)

Answer 8

gamma = 170 MPa
Gamma 1 = 30
Gamma 2 = 20
amalgam = 60

Question 9

amalgam senting dimensional changes

Answer 9

modern
- small contraction
- solid solution of Hg in Ag3Sn
traditional
- initial contraction
- expansion due to gamma 1 crystallisation

Question 10

Zinc reactions in amalgam

Answer 10

interacts with saliva and blood
bubbled of H2 formed within amalgam
- pressure build up leading to expansion
- downward pressure = pulpal pain
- upward expansion = restoration sitting proud of surface

Question 11

amalgam properties depend on…

Answer 11

dependent on handling factors
- proportioning and trituration
- condensation
- carving and polishing
dependent on cavity design
affected by corrosion
some variations between products

Question 12

Amalgam particle types

Answer 12

lathe cut
- coarse, medium, fine
- formed by filling ingots
spherical, spheroidal
- range of particle sized
- formed by spraying molten metal into inert atmosphere

Question 13

advantages of spherical amalgam particles

Answer 13

less Hg required
higher tensile strength
higher early compressive strength
less sensitive to condensation
easier to carve

Question 14

Amalgam mechanical properties

Answer 14

good strength
high abrasion resistance
- suitable for posterior teeth
- too high for deciduous teeth
prone to creep

Question 15

factors decreasing amalgam strength

Answer 15

undermining
too high Hg content after condensation
too low condensation pressure
slow rate of packing
- increments do not bond
corrosion

Question 16

What is creep?

Answer 16

when a material experiences low level stress (below elastic limit) which are applied repeatedly
- over a prolonged period of time it may flow, resulting in permanent deformation

Question 17

dental materials prone to creep

Answer 17

amalgam
alloys
waxes
plastics

Question 18

amalgam biocompatibility

Answer 18

concern about mercury toxicity
- mercury released from dental amalgams - quantities very small
- do not cause verifiable effects on human beings
- supportive data incomplete

Question 19

amalgam thermal properties

Answer 19

thermal expansion = 3x tooth
thermal conductivity - high
- liner or varnish may be required in deep cavities
high thermal diffusivity
- when a transient thermal stimulus e.g, tea is applied , temperature of amalgam rises quickly

Question 20

Amalgam - bonding

Answer 20

does not bond to tooth
needs mechanical retention
- some suggest bonding systems based on 4-META to get bond to tooth - not widely accepted

Question 21

amalgam handling/viscosity

Answer 21

handling
- mixing, working, setting times - ok, varies between types
viscosity
- packed into cavity
- user friendly

Question 22

amalgam - corrosion properties

Answer 22

gamma 2 most electronegative
weakens material particularly at margins
- however corrosion products may contribute to sealing margins

Question 23

how to reduce amalgam corrosion

Answer 23

copper enriched amalgam
polishing margins

Question 24

other amalgam properties

Answer 24

poor aesthetics
radiopaque
not cariostatic
smooth surface if polished well
- may deteriorate over time

Question 25

benefits of copper enriched amalgam

Answer 25

higher early strength
less creep
higher corrosion resistance
increased durability of margins

Question 26

amalgam, advantages

Answer 26

strong
hard
durable
radiopaque
user friendly

Question 27

amalgam disadvantages

Answer 27

corrosion
leakage
- does not bond
poor aesthetics
mercury
- perceived toxicity
- environmental impact

Question 28

amalgam posterior failure rate after 8 years

Answer 28

5.8%