Amalgam

Question 1

Main types of materials used for direct filling materials

Answer 1

• Composites
• Polyalkenoates (GIC)
• Amalgam

Question 2

Definition of amalgam

Answer 2

• Mixture, or blending, of mercury with another metal or alloy
• Not possible to have a mercury free amalgam
• Every metal can dissolve in mercury (apart from Iron) at room temperature

Question 3

Why mercury

Answer 3

-Every metal can dissolve in mercury (apart from Iron) at room temperature

Question 4

Classic metals in a dental amalgam

Answer 4

• Based on the system
• Silver-Mercury-Tin
• Other metals are added to this system to modify its properties
• Copper increases its final strength
• Zinc reduces oxidation

Question 5

Conventional Amalgam constituents

Answer 5

Based on a powder/liquid phase

• Liquid phase is simply triple distilled Hg
• Powder is an alloy based on the intermetallic compound Ag3Sn
• Known as the gamma phase

Question 6

Approximate % by weight and functions of metals in conventional dental amalgam

Answer 6

Silver: Minimum 65%. In the gamma phase

Tin: Max 29%. In the gamma phase

Copper: 6% max. Strength/hardness

Zinc: 2% max. Manufacturing

Mercury 3% max. Preamalgamation

Question 7

Manufacture of the alloy in the powder phase of conventional amalgam
Problem with it and how it overcome

Answer 7

-Melt components at high temperature in a reducing atmosphere to produce Ag3Sn

• Silver, copper and tin oxidise easily
• Use zinc as an oxygen scavenger
• Then remove the zinc oxide
• Final alloy always contains zinc

or

• Melt the alloy in an inert (oxygen-free atmosphere)
• Zinc free alloy (less expansion)

-Final alloy must be used in a powder form
-After the alloy is melted as a homogenous liquid, there are 2 possibilities:
Lathe cut- cooled down/mechanically grinded
Spherical- atomisation in an inert atmosphere

Question 8

What do you do after you melt the metal components together

Answer 8

-Either lathe cut or spherical

Question 9

Lathe cut process of the alloy manufacture

Answer 9

• Cast into an ingot and heated at 420 degrees celcius
• Cylindrical shaped alloy cut on a lathe
• Power generated by further ball milling
• Produces irregular size particles
• Particles are stressed and elongated
• Homogenised at 100 degrees celcius for 1 hour

Question 10

Spherical process of the alloy manufacture

Answer 10

• Melt is sprayed into an inert atmosphere
• Surface tension and low viscosity generate small spherical (or spheroidal) particles
• Solidifies into consistent sized particles

Question 11

Different forms of particle morphology in the alloy powder

Answer 11

Lathe-Cut
Spherical
Mixed

Many alloy powder are formulated by mixing particles

1) Increases packing efficiency
2) Reduces Hg needed
3) Increases performance

Question 12

Differences between lathe-cut and spherical alloys

Answer 12

Lathe-Cut requires more mercury
Spherical requires less

Lathe cut requires more condensation force into the cavity
Spherical requires less

Lathe cut requires smaller condenser point (smaller amalgam plug required)
Spherical requires a larger point

Less easy to carve and burnish lathe cut
Easier to carve and burning as smooth surfaces in spherical

Less overhands and strong proximal contacts in lathe cut
Overhangs and weak proximal contacts in spherical

Question 13

How is the setting reaction of amalgam initiation and what is this equation
Name all the phases

Answer 13

• Initiated by vigorous mechanical mixing
• Trituration of the powder and liquid

Ag3Sn + Hg –> Ag3Sn + Ag2Hg3 + Sn7Hg

Ag3Sn= Gamma
Ag2Hg3 Gamma 1
Sn7Hg= Gamma 2

Question 14

Phases involved in amalgam

Answer 14

Ag3Sn= Gamma
Ag2Hg3 Gamma 1
Sn7Hg= Gamma 2

Question 15

Setting reaction of amalgam and explanation

Answer 15

Initial Dissolution
-Outer surface of tin/silver particles dissolve in the triple distilled liquid mercury
Ag3Sn + Hg > Ag + Sn + Hg

Formation of Gamma 1

• Silver reacts quickly to form Ag2Hg3 grains
• Sticks preferentially along the alloy particles
• Ag + Hg > Ag2Hg3
• Gamma 1 gets dispersed into the matrix

Formation of Gamma 2

• Tin reacts slowly to form Gamma 2 which is randomly distributed inside the Gamma 1 matrix
• Sn and Hg> Sn7Hg

Set Amalgam
-Reaction is completely set when the Gamma 1 and Gamma 2 phases have formed a solid matrix and no mercury is left to dissolve Gamma

Question 16

Settiing reaction of amalgam and equations of each stage

Answer 16

Initial Dissolution:
Ag3Sn + Hg > Ag + Sn +Hg

Formation of Gamma 1
Ag + Hg > Ag2Hg3

Formation of Gamma 2
Sn + Hg > Sn7Hg

Question 17

Electron Microscope Image of set amalgam

Answer 17

-Core of unreacted gamma particles surrounded by a matrix of Gamma 1 (high concentration) and Gamma 2 (low concentration)

Question 18

Relative strength of the different phases

Answer 18

Assuming the amalgam has been correctly mixed, tensile strength:

Gamma > Amalgam > Gamma 1 > Gamma 2

Gamma 2 is the weakest phase and reducing it increases the strength of the restoration

Question 19

Evolution of strength with time

Answer 19

Amalgam develops slowly (>24 hours)
Amalgam remains very weak at the time the patient leaves the surgery
Compressive strength of 50MPa after 1 hour compared to 300MPa after 24 hours

Question 20

Amalgam strength compared to dental tissues

Answer 20

• Relatively good replacement for the natural tooth substance
• Hardness of amalgam is lower than that of enamel
• May lead to surface facet formtion

Question 21

Modulus of elasticity, compressive strength, tensile strength at 7 days and vikers hardness for enamel, dentine and amalgam

Answer 21

-Done in order from first to third largest

Modulus of elasticity: Enamel 1, Dentine 3, Amalgam 2
Compressive strength: Enamel 3, Dentine 2, Amalgam 1
Tensile strength after 7 days: Enamel 3, Dentine 2, Amalgam 1
Vikers Hardness Enamel 1 Dentine 3 Amalgam 2

Question 22

Dimensional changes of amalgam from trituration and effects

Answer 22

• First contraction due to dissolution of gamma phase into the mercury
• Crystallisation of gamma 1 and gamma 2 leading to expansion of the amalgam
• Zinc can cause dramatic expansion: zn + H20 > ZnO + H2

Contraction leads to marginal gaps
Expansion results in protrusions or even tooth cracks

Question 23

Thermal Properties of Amalgam

Answer 23

-Metallic material
-High thermal diffusivity
-Need to protect the base of large cavity to avoid harmful affect on the pulp (pulp capping?)
Thermal expansion mismatch can cause microleakage
-Occurrence of decay in the dentine surrounding the amalgam
-Amalgams thermal diffusivity and thermal coefficient of expansion is much higher than dentines

Question 24

Corrosion of Amalgam

Answer 24

• Multiphase metallic material sitting in a wet environment
• Inevitable corrosion
• Gamma 2 is more elctronegative than Gamma and Gamma 1
• Acts as an anode and dissolves, releasing free mercury
• Corrosion can be reduced by polishing the restoration to a smooth surface
• Beneficial advantage: Corrosion occurs more at the amalgam/tooth interface forming a seal which prevents microleakages

Question 25

Creep of amalgam

Answer 25

Increase in strain after a constantly applied stress is placed on a material -Plastic deformation - Flow, or deformation, due to applied load over a long period of time and does not recover - The higher the content of gamma 2, the higher the creep - Creep causes protrusion, leading to weak, unsupported edges - Fractures at the edges - Potential ditching

Question 26

How to make the amalgam stronger

Answer 26

- Get rid of the gamma 2 phase - Eliminate gamma 2 achieved by using a high copper concentration - Gamma 2 shows increased strength with reduced creep and corrosion -Copper is integrated in the powder blend: dispersion-modified copper enriched alloy

Question 27

How to make a dispersed phase in amalgam

Answer 27

- Combination of 2 parts of conventional lathe cut gamma (with <6% copper) and 1 part of a Ag-Cu eutectic (72% and 28% copper) - Typical overall copper content is 12% Hg + Gamma + Cu > Gamma + Gamma 1 + Cu6Sn5 + Cu3Sn ``` n= Cu6Sn5 E= Cu3Sn ```

Question 28

Dispersed phsse compared to a conventional amalgam

Answer 28

- Higher compressive strength - Lower creep - Lower corrosion - Faster achievement of final strength

Question 29

Importance of proportioning in amalgams

Answer 29

- Correct Alloy:Hg ratio is necessary to obtain the optimum properties - If the mix is dry, it will not be possible to condense it - If the mix is slightly wet, the excess Hg can be removed by the correct condensing technique 5: 8 for hand mixing 10: 8 for mechanical mixing (particle dependent)

Question 30

Effect of mercury concentration

Answer 30

- Important to reduce the Hg concentration - Final concentration should be around 40-45% - Excess Hg dramatically reduces the restorations strength

Question 31

Clinical Application of amalgam restoration steps

Answer 31

1) Cavity Prep and pulp protection 2) Matrix and wedge application 3) Alloy selection and proportioning 4) Trituritation 5) Condensing 6) Carving and Burning 7) Wedge and Matrix Removal 8) Finishing and polishing

Question 32

Trituration importance and over/under

Answer 32

- Essential to ensure both adequate amalgamation and the production of a plastic mix suitable for packing - Correct trituration time depends both on the alloy and the mixing system (5-20s) - Undertrituration does not allow an adequte formation of the gamma and n phases - Results in a crumbly mix - OVer tritutration results in excessive contraction

Question 33

Condensation objectives and process of

Answer 33

- Remove excess Hg - Ensure there are no voids in the restoration - Ensure marginal integrity - Must be carried out immediately after mixing - Condense small increments - Overfill the cavity because the surface layers tend to be richer in Hg - Spherical amalgams require a lower condensation pressure than lathe cut ones

Question 34

How long can you carve/finish for Carving and finishing for lathe cut or spherical Process of

Answer 34

- Carving of the restoration possible for 2-3 minutes, dependent on the amalgam - Spherical restorations produce a better surface than lathe cut ones - Avoid too much pressure-friction heating can cause the release of Hg - Smooth surface minimised the probability of corrosion

Question 35

Limitations of amalgam

Answer 35

``` No adhesion to tooth substance Retentive cavity design required leading to loss of sound tooth substance Poor aesthetics Galvanic effects High thermal diffusivity ```

Question 36

Mercury issues

Answer 36

- No adverse effects according to evidence - Pollution - Avoid spillage - High levels of mercury are associated with adverse effects in the brain and kidneys

Question 37

Reactions to amalgam

Answer 37

- Allergic reaction to mercury rare | - Lichenoid reactions

Question 38

What should u tell a patient worried about mercury from amalgam

Answer 38

- Very low levels - Less than total amount patient is exposed to in their daily environment or from what they eat - Removing amalgams can result in unnecessary loss of healthy parts of the tooth and can release more mercury - Only replace is worn, broken down or decay

Question 39

Read about the brexit affect on the slides

Answer 39

Yeah ok