Clinical Aspects Of Dental Amalgam

Question 1

What is amalgam

Answer 1

Any alloy of mercury with another metal or other metals

Question 2

What is dental amalgam

Answer 2

An alloy of silver, mercury and tin with some other metals added to modify the properties

Question 3

Alloy powder particle shape?

Answer 3

Spherical - made by spraying molten metal onto fine mist
- softer, more flowable

Lathe-cut - made by creating an ingot of alloy and grinding down to a powder
- less flowable, good for building up large amounts of missing tooth structure

Question 4

What happens when the alloy powder dissolves in the liquid mercury?

Answer 4

Mercury surround all of the alloy powder particles and it dissolves them

The powder particles already contain silver and tin and as it dissolves the silver and tin goes out into solution creating a paste contains mercury, tin and silver

Crystallisation process then occurs - the metal ions combine with each other to form new compounds

Question 5

What compounds can be formed from the crystallisation process?

Answer 5

Ag + Sn —> Ag3Sn (gamma)
- main component that gives an amalgam restoration its strength

Ag + Hg —> Ag2Hg3 (gamma 1)

Sn + Hg —> Sn7Hg (gamma 2)

Question 6

Which compound is unwanted?

Answer 6

Gamma 2

Responsible for..
- corrosion
- creep (restoration gradually changes shape under load)
- decreased strength

Question 7

What is creep?

What can it cause?

Answer 7

deformation of the filling under load

secondary caries

Question 8

How to reduce the amount of gamma 2?

Answer 8

The addition of copper
Conversation gamma 2 into a copper compound

= a mixed amalgam

Question 9

properties of zinc and why it is added to dental amalgam?

Answer 9

• no benefit to material
• added to prevent silver from oxidising during manufacture of the powder
• acts as a scavenger for oxygen
• not needed if powder can be manufactured in a vacuum
• its presence can cause a lager expansion of the material as it sets

Question 10

When setting, how does the amalgam restoration act without zinc?

Answer 10

0.04% contraction (composite 3%)
Then after a few hours, expansion

More or less same as when initially places no contraction or expansion

Question 11

When setting, how does the amalgam restoration act with zinc?

Answer 11

Zinc obtaining amalgam + moisture = Expansion
= fracture

Therefore least amount of zinc possible

Question 12

Ideal ratio for dental amalgam?

Answer 12

Alloy powder : Liquid mercury

Slightly more alloy powder then liquid mercury = strong restoration

If less than 50% mercury to powder = dry and crumbly, hard to pack

Question 13

Properties of amalgam

Answer 13

• good compressive strength
• good wear resistance
• kind to opposing teeth
• easy to use
• chemical set (no need for light curing and will cure properly)
• cheap
• radio-opaque (good so can see well on radiograph and differentiate from secondary caries)

Question 14

Not so good things about amalgam?

Answer 14

• non adhesive
• weak in thin sections
• thermal conductor
• not aesthetic
• occasional lichenoid-type reaction (some patients have a skin reaction)
• can discolour tooth

Question 15

properties of amalgam - non invasive

Why is this bad?

Answer 15

It can easily come out of the cavity once set so we have to create an undercut

Question 16

properties of amalgam - thermal conduction

Answer 16

• heat and cold sensitivity
• may conduct to pulp

Question 17

properties of amalgam - thermal expansion
Why bad?

Answer 17

• cold drink = filling contracts - may cause leakage
• puts stress on tooth

Question 18

How can Amalgam can cause a plaque retentive factor if not handled properly?

Answer 18

• difficult to remove overhangs
• difficult to use any instruments sub-gingivally without causing damage to adjacent tooth/gums

Question 19

Clinical use - indications for amalgam use?

Answer 19

• larger cavities affecting posterior teeth
• where heavy occlusal forces are encountered
• where aesthetics are not important
• patients with a high caries rate (less likely to cause secondary caries)
• posterior restorations that extend subgingivally (its less affected by moisture)
• building up broken down teeth prior to crowning

Question 20

Cavity design - what do we need to keep in mind?

Answer 20

Retention - prevention of being pulled out of cavity vertically

Resistance - Prevention of being dislodged under load

Question 21

Retention?

Answer 21

Retention is macro mechanical - created by an undercut cavity design

Undercuts are sufficient for occlusal and small approximately

Question 22

Cavity design?

Answer 22

Remove infected dentine, leave affected (stained but firm) dentine and may have to remove some sound dentine to create the undercut if not already created

No unsupported enamel prisms

Question 23

How is resistance achieved for mesial/distal sideways restorations?

Answer 23

We need to stop the restoration from moving sideways

• create a dovetail design (jigsaw piece shape) which prevents the restoration from falling out sideways
• ensure the base towards the edge of the tooth is not angled it needs to be flat - base of box needs to be in a flat direction

Question 24

Cavity surface angle

Answer 24

Cavity surface angle of 90 degrees

Width of a cavity interproximally should be slightly wider than the contact area

Question 25

Under cut is only relevant if you have 2 opposing walls

If a tooth has lost a whole cusp, it is importable to create an undercut

How is this overcome?

Answer 25

Older technique - putting pins into the tooth which act as a wall and stops the amalgam from falling out
This was a risky procedure as if they came into contact with the dentine leading to death of the tooth or cause a fracture

Newer technique - grooves in dentine surface with small round bur

Question 26

Bonding in amalgam

Adhesive systems similar to those used with composite have been developed

Answer 26

The active substances (MDP or 4-META) form a hybrid layer with dentine and a chemical bond to amalgam

So we don’t need retentive factors

Question 27

Alternatively how else can we use bonding in amalgam?

Answer 27

Wet uncured RMGIC placed in base of cavity can have amalgam packed directly on top

RMGIC can set in the dark and can chemically bond to the tooth

Amalgam also bond to the wet uncured RMGIC.. si it bond to the tooth and bonds the amalgam itself

Question 28

Advantages to polishing amalgam?

Answer 28

Makes it more resistant to corrosion
Less plaque retentive
More aesthetically pleasing

Question 29

Disadvantages of polishing?

Answer 29

• polishing will not improve the life expectancy of the restoration.
• the heat generated may be harmful to the pulp

Question 30

Is amalgam safe

Answer 30

• 50% mercury - toxic
• mercury vapour is highly toxic
• mercury vapour is ingested whenever amalgams are mixed, placed or removed
• mercury vapour is released from amalgam filling during chewing and tooth brushing

Question 31

Environmental disadvantages of mercury

Answer 31

When mercury enters the environment it can be converted by bacteria into methyl mercury in fresh and salt water

Methyl mercury is highly poisonous

Question 32

Environmental plans regarding amalgam

Answer 32

Minimata convention
- ban on mercury containing products and ‘phase down’ of amalgam by 2020
- call for a cease to teaching amalgam by 2015
- little agreement over protocols between schools
- composite will become the most widely used material for posterior tooth restorations

Question 33

Evidence of risk from dental amalgam

Answer 33

• no evidence of risk from mercury in amalgam to patients or dental staff
• problem when mercury enters environment

Question 34

Article 10 of EU regulation 2017 on mercury

Answer 34

• from 2019 onwards, dental amalgam shall only be used in pre-dosed encapsulated form
• shouldn’t be used in the dental treatment of deciduous teeth in children under 15 and of pregnant women except when deemed strictly necessary
• preferably phase down by 2030
• requirement for dental facilities to be equipped with an amalgam separator