Amalgam

Question 1

Why do we add copper to amalgam

Answer 1

Increases the final strength

-Reduce the amount of Y2 (So less creep and less ditches in the amalgam)

Question 2

Why do we add zinc to amalgam

Answer 2

Reduce oxidation

Question 3

4 stages in amalgam setting reaction

Answer 3

1. Initial dissolution
   - silver and tin dissolve in mercury and we get Ag, Sn an Hg
2. formation of Y1
   -silver reacts with mercury to form Ag2Hg3grains which sticks at alloy particles
3. 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

Question 4

What is the evolution of strength in amalgam

Answer 4

At 15 mins- amalgam is weak
At 1 hour- Compressive strength of 50MpA
At 24 hours- Compressive strength is 300Mpa

Question 5

What does Creep (plastic deformation)cause in amalgam

Answer 5

Creep– protrusion–unsupported edges–fracture of edges and ditching

Question 6

what causes creep in amalgam

Answer 6

High Y2 content (Sn7Hg)

Question 7

What happens to amaglam in a wet enviroment

Answer 7

-as it is metallic, it corrodes

Question 8

How do we avoid corrosion

Answer 8

-Smoothening and polishing the amalgam

Question 9

How can corrosion stop microleakage

Answer 9

-It occurs at tooth and amalgam interface, the corrosion forms a seal and stops the microleakage

Question 10

How do the 3 dimensional changes occur in amalgam

Answer 10

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

Question 11

What is the thermal diffusivity in Amalgam

Answer 11

as it is metal, if placed closed to pulp it is dangerous as it has HIGH thermal diffusivity, so use a liner

Question 12

What does the thermal expansion mismatch in amalgam and dentine lead to?

Answer 12

There is micro-leakage and decay in dentine around amalgam

Question 13

Why is it important to reduce the amount of Hg in final restoration

Answer 13

The mercury can reduce the compressive strength of the restoration

Question 14

How do we reduce the amount of Hg ??

Answer 14

By condensing the material

Question 15

Why else is condensation of amalgam crucial

Answer 15

-Avoid Voids
-Marginal integrity
-

Question 16

Why do we overfill the cavity?

Answer 16

-As it is rich in Hg and so this is cut away.

Question 17

The 4 limitations of amalgam?

Answer 17

• Non aesthetic
• No adhesive bond to tooth sturcture
• Need retentive cavity design (cutting off tooth structure)
• High thermal diffusivity

Question 18

To make the initial silver tin compound (Ag3Sn), we end up needing zinc why?

Answer 18

As we need a reducing atmosphere but the metals oxidise.

-So we need zinc which acts as an oxygen scavenger.

Question 19

What shape are lathe cut particles

what shape are spherical cut particles

Answer 19

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.

Question 20

benefits of lathe over spherical

Answer 20

-Less overhangs and strong proximal contacts

Question 21

Benefits of spherical over lathe

Answer 21

• Less mercury
• Less condensation force
• Easier to carve and burnish

Question 22

3 benefits of mixed?

Answer 22

• Less mercury
• Better packing efficiency
• Increased performance

Question 23

Under trituration

Over trituration

Answer 23

Under= amalgam is crumbly as there is not adequate Y1
-Over= Excess contraction

Question 24

Why do we need correct trituration?

Answer 24

-To get adequate amalgamation of material and get a plastic mix for packing the material