amalgam Flashcards
amalgam - valuable restorative material or cause for concern?
Mercury is known to be slightly toxic to humans – does this make amalgam a health risk?
- Can’t be proven as a health risk
Lots of dental materials do not undergo appropriate clinical trials
2 main types of amalgam
- traditional
- copper enriched
what makes amalgam
alloy formed by the reaction of:
- mercury (liquid)
- silver, tin, copper & other metals (powder)
Alloy is traditionally several metals. Amalgam is several metals and non-metal mercury (which is very dense)
use of amalgam
widely used
- England & Wales: 22 million restorations.
- USA: 160 million restorations
how long has amalgam been in use?
long history (first 659 AD China)
2 ways to classify amalgam
- composition
- particle size and shape
how to classify amalgam on composition?
- traditional
- copper enriched
most of today’s amalgams are zinc free
(silver, tin, copper, mercury)
constituents in amalgam powder
- silver, tin
- copper
- zinc (sometimes)
- mercury
silver, tin function in amalgam powder
- intermetallic compound Ag3Sn
Gamma phase, reacts with Hg liquid to form amalgam
copper function in amalgam powder
- increases strength & hardness
- ensures longevity in oral cavity
zinc function in amalgam powder
- scavenger during production - preferentially oxidises & slag formed / removed (sacrifice for other materials)
- some zinc free
mercury function in amalgam powder
(few materials)
“pre-amalgamated” alloys - react faster small in powder before adding more in liquid
liquid constituents of amalgam
mercury (50% by weight)
function of liquid mercury in amalgam
- triple distilled (very pure)
- reacts with other metals
2 particle types of amalgam
- lathe cut
- spherical, spheroidal
lathe cut amalgam particles
- coarse, medium, fine (fine shaving/shard, irregular shape)
- formed by filing ingots
spherical, spheroidal amalgam particles
- range of particle sizes
- formed by spraying molten metal into inert atmosphere form globules
- easier to control
setting reaction
Ag3Sn + Hg –> Ag3Sn + Ag2Hg3 + Sn2Hg3
gamma –> gamma (unreacted) + gamma1 + gamma2
what makes the amalgam matrix?
Ag2Hg3 + Sn2Hg3
gamma1 + gamma2
what do gamma1 phase do?
BINDS/encapsulates UNREACTED AgSn and gamma2 particles
set structure of different phases
gamma: good strength & corrosion resistance
- lathe cut
- intermetallic compound
- has strength and corrosion resistant
gamma1 good corrosion resistance
- Soup which encapsulates
- Gamma1 phase holds gamma and gamma2 phases together
- Good corrosion resistance but not as strong as gamma
Gamma2 weak and poor corrosion resistance
- Spherical
- Weakest and likely to erode
voids decrease strength & increase corrosion
gamma strength and corrosion resistance
good strength & corrosion resistance
- lathe cut
- intermetallic compound
- has strength and corrosion resistant
gamma 1 strength and corrosion resistance
good corrosion resistance
- Soup which encapsulates
- Gamma1 phase holds gamma and gamma2 phases together
- Good corrosion resistance but not as strong as gamma
gamma2 strength and corrosion resistance
weak and poor corrosion resistance
- Spherical
- Weakest and likely to erode
tensile strength of amalgam
has the mean value of all the stages of amalgam components in the reaction phases
traditional amalgam setting dimensional changes
- initial contraction - solution of alloy particles in Hg
- expansion - gamma1 crystallisation (ends up expanded by 0.4%)
modern amalgam setting dimensional changes
- small contraction – closer to original dimension
expansion / contraction is <0.2%, so little clinical sign
solid solution of Hg in Ag3Sn
why now zinc free amalgam?
interaction of unreacted zinc with saliva/blood -
Zn + H2O –> ZnO + H2
bubbles of H2 formed within amalgam
- pressure build up causes expansion
- downward pressure cause pulpal pain
- upward - restoration sitting proud of surface
amalgam properties are dependent on…. (2)
dependent on handling factors - proportioning & trituration (Membrane that separates the powder and liquid broken on shaking) - condensation - carving & polishing
dependent on cavity design
mechanical strength of amalgam
generally consider compressive but others important
- early (1hr) - Traditional materials, poor(ish)
- late (> 24hrs) - OK (develops strength)
abrasion resistance of amalgam
high,
- suitable for posterior teeth
- too high for deciduous
no undulations in surface due to poor abrasion resistance
factors decreasing amalgam strength (5)
undermixing
too high Hg content after condensation
too low condensation pressure
- not enough pressure when packing into cavity
slow rate of packing
- increments do not bond
corrosion
amalgam creep
When a material is repeatedly stressed for long periods at low stress levels (ie stress below elastic limit e.g. chewing) it may flow, resulting in permanent deformation
(amalgam, alloys, waxes, plastics)
why does amalgam creep?
visco-elastic
- high in traditional amalgams
what does amalgam creep effect?
affects marginal integrity - does it maintain good contact with surrounding dental material
NB also marginal integrity depends on:
- cavity design
- corrosion
biocompatibility of amalgam
Concern about mercury toxicity
- Never enough data – so far it is safe
NIH Technology Assessment Conference, 1992 - Effects and Side Effects of Dental Restorative Materials
- “Although mercury is released from dental amalgams the quantities released are very 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 recognised that the supporting data are incomplete.”
thermal expansion of amalgam
3 times of tooth tissue
- create gap - risk
thermal conductivity of amalgam
high, may need to use liner / varnish in deep cavities as heat/cold can reach pulp potentially
- liner acts as an insulator to prevent heat getting to pulp
amalgam bond to tooth/compatible with bonding systems
does not bond to tooth
- needs mechanical retention
(walls splay outwards)
- some suggest using new bonding systems based on 4-META to get bond to tooth not yet widely accepted
mixing, working and setting times of amalgam
OK - reasonable
- varies between types
viscosity of amalgam
packed (condensed) into cavity
- “user friendly”
aesthetics of amalgam
poor
- used on posterior teeth only
is amalgam radiopaque?
Yes
- Able to investigate on X-rays for gaps, parts broken off etc
is amalgam anticariogenic?
no
does amalgam have a smooth suface?
Yes, if polished well, may deteriorate over time
- Not a plaque trap if smooth
does amalgam have setting shrinkage?
modern materials tend to have net overall shrinkage
corrosion of amalgam
- gamma2 most electronegative (weakest)
weakens material particularly at margins
- corrosion products may contribute to sealing margins (!)
reduce by
- copper enriched, polishing margins
- avoiding galvanic cells
advantages of spherical amalgam particles (5)
- less Hg required
- higher tensile strength
- higher early compressive strength
- less sensitive to condensation
- easier to carve
other names for copper enriched alloy amalgams
- non-gamma2
- higher copper
how much copper is in copper enriched amalgams?
> 6%
2 types of copper enriched amalgams
Dispersion modified
- original type
Single composition types
- introduced to increase uptake by profession
dispersion modified amalgam
originally Ag-Cu spheres + conventional lathe cut alloy
- tried to see if makes stronger - as traditional has weak gamma 2 phase (corrosion likely)
- (now some single composition dispersed alloys - spheres & lathe cut particle same composition)
originally thought spherical particles would act as strengthening agent, but increased copper content gave beneficial modifications to setting reaction
- 1. as conventional material
(gamma + Hg –> gamma + gamma1 + gamma2)
- Add silver copper reacts with gamma 2 phase
(2. gamma2 + Ag-Cu –>Cu6Sn5 + gamma1 (takes several days))
Converted to copper tin and some gamma 1
No gamma 2 and silver copper particles added have a halo of copper tin
single composition copper enriched formulations
Incorporate copper into silver tin instead of silver copper particles
- So silver, tin and copper in powder added to liquid mercury
powder Ag - Sn - Cu
- copper 12-30%
particle types: spherical; lathe cut
setting reaction
- Ag-Sn-Cu + Hg –> Ag-Sn-Cu + g1 + Cu6Sn5
No gamma2 phase
beneficial property of setting reaction for copper enriched amalgams
no gamma 2 phase
benefits of copper enriched amalgams (4)
- Higher early strength (first hour; greater than traditional amalgam)
- Less creep (most common cause of amalgam failure)
- Higher corrosion resistance
- Increased durability of margins
which amalgam type has the least amount of creep/
copper enriched single formulation
over traditional lathe and spherical and copper enriched dispersion
constituents of copper enriched dispersion amalgam
Ag-Cu spheres + conventional lathe cut amalgam
constituents of copper enriched single formulation amalgam
silver, tin and copper in powder added to liquid mercury
which amalgam type is strongest?
copper enriched single formulation
- Stronger at an early stage and after a week
- better at resisting fracture in first few days of placement – practical for eating perspective
thermal expansion coefficient of amalgam
2-3 times greater expansion/contraction than tooth tissues
2 advantages of amalgam use
- strong
- user friendly
4 disadvantages of amalgam use
- corrosion
- leakage - does not bond (due to creep)
- poor aesthetics
mercury: - perceived toxicity
- environmental impact
advantage of encapsulated amalgam
Hg hygiene
what amalgam is used in GDH?
permite
- Non-gamma2, spherical and lathe cut
Stronger compared to other brands
- More likely to survive for longer time
- Resisting more forces than other brands
lower microleakage
amalgam compressive strength compared to enamel and composite
Twice compressive strength of enamel (500 Vs 250 MPa) and stronger than composite (300MPa)
elastic modulus of amalgam compared to enamel and composite
More rigid than composite (100 Vs 90 KHN) but not as good as enamel (350)
what material has the lowest posterior failure rate after 8 years)
amalgam (5.8%)
