Clinical aspects of amalgam

Question 1

Define amalgam

Answer 1

an alloy of silver, mercury and tin with other metals added to change the properties

Question 2

What makes the amalgam paste?

Answer 2

alloy powder + liquid mercury -> mouldable paste for few mins

Question 3

Describe the amalgam capsules

Answer 3

sealed by law

contains liquid mercury in one end and alloy powder in the other

prevented from mixing by thin membrane that gets broken when plunger is pressed

mixed at high speed in amalgamator

Question 4

What are the 2 shapes of the allow powder particles?

Answer 4

spherical

lathe-cut

Question 5

Describe spherical particles

Answer 5

Made by spraying molten metal into a fine mist.

Softer, more flowable – easier to pack into cavity.

Question 6

Describe lathe-cut particles

Answer 6

Made by creating an ingot of alloy and grinding it down into a powder.

Less flowable, good for building up large amounts of missing tooth structure.

Question 7

What happens when the alloy powder dissolves in liquid mercury?

Answer 7

mercury dissolves powder

powder mainly contains silver and tin

silver and tin goes out into solution and ends up with solution containing mercury, tin and silver

Question 8

What are the 3 different phases that occur when metals in amalgam dissolve?

Answer 8

gamma phase - silver-tin Ag-Sn

gamma-1 phase - silver-mercury Ag2Hg3

gamma-2 phase - tin-mercury Sn8Hg

Question 9

Describe gamma phase Ag3Sn

Answer 9

the strongest phase

provides high mechanical durability

resistant to wear and fracture

Question 10

Describe gamma-1 phase Ag2Hg3

Answer 10

moderate strength, weaker than gamma phase

plays a role in binding amalgam together

Question 11

Describe gamma-2 phase Sn8Hg

Answer 11

unwanted product

poor mechanical properties

causes…
- corrosion
- creep
- decreased strength

main cause of amalgam failure

Question 12

What metal reduces gamma-2 phase?

Answer 12

copper

they can either reduce or eliminate this phase, improving longevity

Question 13

Define creep

Answer 13

The slow deformation of a dental material when exposed to long-term stress, such as chewing forces.

Question 14

What are the causes of creep?

Answer 14

weak structure due to presence of gamma-2 phase

continuous pressure from biting and chewing

Question 15

What are the effects of creep?

Answer 15

restoration gradually distorts

can lead to fractures and failure over time

weakens the margins of filling, allowing bacteria to enter

Question 16

What is corrosion?

Answer 16

a chemical reaction where the metal in amalgam degrades over time.

Question 17

What are the causes of corrosion?

Answer 17

saliva (electrochemical reactions)

bacteria and acids in the mouth

contact with other metals in the mouth (eg gold crowns)

Question 18

What are the negative effects of corrosion?

Answer 18

weakens amalgam, leading to failure

rough surfaces that trap plaque and bacteria

secondary caries

gamma-2 phase is highly prone to corrosion

Question 19

What is the positive effect of corrosion?

Answer 19

corrosion products fill gaps between amalgam and dentine

this prevents micro leakage, stopping bacteria from entering

helps reduce sensitivity by blocking tiny openings

Question 20

What is the typical composition by % wt of a modern-day amalgam?

Answer 20

silver - 65

tin - 29

copper - 6-13

zinc - 2

Question 21

Why is zinc put in amalgam?

Answer 21

prevents silver from oxidising during powder manufacture

acts as oxygen scavenger

no direct benefit to material

not necessary if manufactured in vacuum

Question 22

What can zinc cause during setting?

Answer 22

excessive expansion

Question 23

What are the issues if the mercury content is too low?

Answer 23

too dry and unworkable

if <50% mercury

Question 24

What happens if the mercury content is over 50%?

Answer 24

needed for workability

but weakens amalgam over time

Question 25

Describe the mercury removal process

Answer 25

excess mercury is removed during packing during condensation, mercury rises to surface excess mercury is scraped away during carving

Question 26

How does the amalgam mixture differ from before being placed to after the restoration?

Answer 26

fresh amalgam has >50% mercury then the mercury rich layer is removed leaving overall restoration with <50% mercury

Question 27

What are the advantages of amalgam?

Answer 27

compressive strength wear resistance kind to opposing teeth easy to use chemical set cheap radiopaque

Question 28

What are the disadvantages of amalgam?

Answer 28

non-adhesive weak in thin sections thermal conductor not aesthetic occasional lichenoid-type reactions

Question 29

What happens if the amalgam restorations expands or contracts?

Answer 29

leakage on side of restoration stress on tooth

Question 30

What are the clinical indications for amalgam?

Answer 30

larger cavities in posterior teeth areas with heavy occlusal forces aesthetics are not a priority high caries risk posterior restorations extending subgingivally core build-up before crowning a tooth

Question 31

In what direction does retention prevent amalgam from being pulled out?

Answer 31

vertically uses undercuts

Question 32

In what direction does resistance prevent amalgam from being pulled out?

Answer 32

horizontal prevents dislodgement under occlusal load so dovetails are added

Question 33

What effect does undercuts have on the cavity?

Answer 33

wider at base narrower at top

Question 34

What is the cavosurface angle?

Answer 34

angle between the surface of the tooth and the walls of the cavity needs to be above 90 degrees

Question 35

What bonding systems have been developed for amalgam?

Answer 35

adhesive systems like composite active substances : MDP or 4-META - hybrid layer with dentine which creates chemical bond to amalgam - eliminates need for pins in amalgam restoration alternative : wet, uncured RMGIC placed in cavity base, amalgam then placed on top

Question 36

What are the advantages of polishing?

Answer 36

increases resistance to corrosion improves aesthetics reduces plaque retention enhance patient comfort

Question 37

What are the limitations of polishing?

Answer 37

does not improve restoration lifespan excessive heat during polishing may harm the pulp

Question 38

What precautions should we have when polishing?

Answer 38

do not over polish ensure excess material is removed carefully

Question 39

Where is there a risk of mercury vapour release?

Answer 39

during mixing, placement, and removal of amalgam released from fillings during chewing and toothbrushing

Question 40

What is the impact of mercury on the environment?

Answer 40

can be converted by bacteria into methyl mercury in water (MeHg) highly toxic to humans and wildlife

Question 41

What does the minimata convention say?

Answer 41

ban on mercury containing products and phase down by 2030 proposal to stop teaching by 2015 lack of agreement on protocol between dental schools composite is expected to become the primary material for posterior restorations