Amalgam 1 Flashcards
Amalgams are set by … and is a reaction of …
- amalgamation
- metals with mercury
Amalgam has been used in dentistry for …, used since …
- 1000
- 1830s
Amalgam requires the mixing of …
- 2 or more metals including mercury
- powder (alloy) and liquid (mercury)
For dental amalgam, what is mixed?
- alloys have Ag, Sn, Cu (Zn, Hg)
- mercury is pure/triple distilled
… and … are the major components of alloy
How are these controlled?
- Ag and Sn
- concentration controlled to form Ag2Sn (gamma) phase
- intermetallic compound
- limit corrosion
Role of Cu in alloy
increase strength of set material
Too much copper can lead to …
excessive expansion
Zinc has what role in an alloy?
- scavenger
- reduces oxidation during alloy casting
Some products have mercury in the alloy itself - what is this called?
pre-amalgamated
Why is standardising alloy composition important?
- different products have different alloys
- leads to inconsistent performance
How to control alloy composition
- ISO standards defined concentration ranges - more consistency
- on weight percentage at least 65 silver, max 29 tin, max 6 copper, max 2 zinc
ISO specification pre 1986
- termed traditional amalgams
- high incidence of premature clinical features
Give the 3 steps of the setting reaction of amalgam
- powder and mercury is mixed
- step 1 is dissolution of small particles of alloy, diffusion of mercury into surface of larger particles
- stage 2 is a reaction to form gamma 1 and 2
- stage 3 is crystallisation of gamma 1 and 2
Set amalgam has a … structure
This consists of what?
- composite
- particles of unreacted alloy (Ag3Sn gamma phase)
- matrix of reaction products and mercury (Ag2Hg3 and Sn7Hg gamma 2 phase)
Alloy particles are responsible for … in set amalgam
explain
- strength
- need to control mercury concentration for mixing
- add enough mercury to make plastic mass but not too much which causes too much gamma 1 and 2 production
In MPa, tensile stregnth of phase
- gamma
- gamma 1
- gamma 2
- amalgam ?
- 170
- 30
- 20
- 60
Mechanical properties of set amalgam
- similar to enamel and dentine
- good compromise to withstand oral forces
Why can there be premature failures of set amalgam?
- low tensile strength
- margins susceptible to breakdown (called ditching)
How to increase tensile strength in set amalgam?
- beware of thin sections in bending
- need proper cavity design
In the past, was ditching common? What was the solution?
- common
- dentists would revise restoration
- but would facilitate secondary caries
- early signs of mechanical failure
- revised restorations are larger and fail faster
2 theories for causing ditching
- creep model
- corrosion model
Explain the Creep model of marginal breakdown
- creep is slow deformation under stress
- stress is often below yield stress, deformation is plastic/permanent (dynamic or static)
- model proposed amalgam is slowly deformed in the cavity, amalgam would extend below cavity margins and amalgam above margins will break under tension
Testing of components showed … is the main culprit for creep marginal breakdown
gamma 2 phase
Explain the corrosion model of marginal breakdown
- corrosion occurs when theres 2 or more dissimilar metals in contact, electrolytes which saliva is, amalgam contains multiple metals
- corrosion results in discolouration, mercury release, weaken thr material, metallic taste, delayed expansion but can seal margins
Gamma 2 phase forms what?
anode
The Gamma 2 phase is the … and … phase
This is when … and … happens
- softest. weakest
- creep and corrosion
How have modern amalgams reduced the gamma 2 phase to reduce creep and corrosion?
- increased copper
- increases final strength
- two phases form e and n phase which is slower than gamma 1 and 2
- too much can lead to expansion but good otherwise
How to achieve increased copper in amalgam?
- admixed amalgams
- two alloy powders - traditional and a 72% silver/28% copper - eutectic
How is the setting reaction different in increased copper amalgam?
- gamma 1 and 2 form as normal
- more copper means gamma 2 becomes unstable
- e and n form over time
How have we developed to have no gamma 2 phase at all?
- contain precious metals like palladium
- even lower creep and corrosion
Alloy powder is made by 2 methods. Name them
- lathe cut
- atomisation (termed spherical alloys)
Explain lathe cut made powder
- alloy cast into cylinders (ingots)
- lathe used to cut ingots into flakes - called grains
- different grain sizes possible
Explain atomisation of alloy powder
- termed spherical alloys
- molten alloys sprayed into inert atmosphere
- solidifies into spheres
What are admixtures?
combination of lathe and spherical particles
Explain process of dimensional changes in setting amalgam
- initial contraction (diffusion/dissolution stage)
- expansion (crystalisation stage)
- small overall change due to expansion and contraction balance (0.1-0.2%)
Excess dimensional change on setting causes what?
- protrusion or cracking of teeth
Effect of water contamination on zinc
- delayed expansion
- combines to make zinc oxide and hydrogen
- corrosion too
Role of zinc in amalgam
- oxides easily form on molten alloys - these are weak and reduce quality
- zinc oxidises faster than silver, tin and copper so acts as a scavenger for oxygen
- zinc oxide is scraped of the molten alloy
- atomised powders formed without oxygen - don’t need zinc
