Amalgam Flashcards
Why do we add copper to amalgam
Increases the final strength
-Reduce the amount of Y2 (So less creep and less ditches in the amalgam)
Why do we add zinc to amalgam
Reduce oxidation
4 stages in amalgam setting reaction
- Initial dissolution
- silver and tin dissolve in mercury and we get Ag, Sn an Hg - formation of Y1
-silver reacts with mercury to form Ag2Hg3grains which sticks at alloy particles - Formation of Y2
-Tin and mercury forms Y2
-y2 is randomly distributed in the y1 matrix
Sn7Hg
4- we get SET amalgam when there is no mercury left to dissolve silver and tin.
Y1+Y2 make solid matrix
What is the evolution of strength in amalgam
At 15 mins- amalgam is weak
At 1 hour- Compressive strength of 50MpA
At 24 hours- Compressive strength is 300Mpa
What does Creep (plastic deformation)cause in amalgam
Creep– protrusion–unsupported edges–fracture of edges and ditching
what causes creep in amalgam
High Y2 content (Sn7Hg)
What happens to amaglam in a wet enviroment
-as it is metallic, it corrodes
How do we avoid corrosion
-Smoothening and polishing the amalgam
How can corrosion stop microleakage
-It occurs at tooth and amalgam interface, the corrosion forms a seal and stops the microleakage
How do the 3 dimensional changes occur in amalgam
1-Dissolution of Y (Ag2Hg3) into mercury–> contraction
2-Crystallisation of Y1+Y2 –> expansion which causes protrusions and tooth cracks
3- Zinc causes expansion (ZnO)–> protrusions and crack
What is the thermal diffusivity in Amalgam
as it is metal, if placed closed to pulp it is dangerous as it has HIGH thermal diffusivity, so use a liner
What does the thermal expansion mismatch in amalgam and dentine lead to?
There is micro-leakage and decay in dentine around amalgam
Why is it important to reduce the amount of Hg in final restoration
The mercury can reduce the compressive strength of the restoration
How do we reduce the amount of Hg ??
By condensing the material
Why else is condensation of amalgam crucial
-Avoid Voids
-Marginal integrity
-
Why do we overfill the cavity?
-As it is rich in Hg and so this is cut away.
The 4 limitations of amalgam?
- Non aesthetic
- No adhesive bond to tooth sturcture
- Need retentive cavity design (cutting off tooth structure)
- High thermal diffusivity
To make the initial silver tin compound (Ag3Sn), we end up needing zinc why?
As we need a reducing atmosphere but the metals oxidise.
-So we need zinc which acts as an oxygen scavenger.
What shape are lathe cut particles
what shape are spherical cut particles
Lathe cut= Irregularly sized as particles are stressed and elongated as they are cut on a lathe
-Spherical= spherical as the alloy is sprayed into inert atmosphere. There is surface tension and low viscosity.
benefits of lathe over spherical
-Less overhangs and strong proximal contacts
Benefits of spherical over lathe
- Less mercury
- Less condensation force
- Easier to carve and burnish
3 benefits of mixed?
- Less mercury
- Better packing efficiency
- Increased performance
Under trituration
Over trituration
Under= amalgam is crumbly as there is not adequate Y1 -Over= Excess contraction
Why do we need correct trituration?
-To get adequate amalgamation of material and get a plastic mix for packing the material
