Clinical Aspects Of Dental Amalgam Flashcards
What is amalgam
Any alloy of mercury with another metal or other metals
What is dental amalgam
An alloy of silver, mercury and tin with some other metals added to modify the properties
Alloy powder particle shape?
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
What happens when the alloy powder dissolves in the liquid mercury?
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
What compounds can be formed from the crystallisation process?
Ag + Sn —> Ag3Sn (gamma)
- main component that gives an amalgam restoration its strength
Ag + Hg —> Ag2Hg3 (gamma 1)
Sn + Hg —> Sn7Hg (gamma 2)
Which compound is unwanted?
Gamma 2
Responsible for..
- corrosion
- creep (restoration gradually changes shape under load)
- decreased strength
What is creep?
What can it cause?
deformation of the filling under load
secondary caries
How to reduce the amount of gamma 2?
The addition of copper
Conversation gamma 2 into a copper compound
= a mixed amalgam
properties of zinc and why it is added to dental amalgam?
- 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
When setting, how does the amalgam restoration act without zinc?
0.04% contraction (composite 3%)
Then after a few hours, expansion
More or less same as when initially places no contraction or expansion
When setting, how does the amalgam restoration act with zinc?
Zinc obtaining amalgam + moisture = Expansion
= fracture
Therefore least amount of zinc possible
Ideal ratio for dental amalgam?
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
Properties of amalgam
- 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)
Not so good things about amalgam?
- non adhesive
- weak in thin sections
- thermal conductor
- not aesthetic
- occasional lichenoid-type reaction (some patients have a skin reaction)
- can discolour tooth
properties of amalgam - non invasive
Why is this bad?
It can easily come out of the cavity once set so we have to create an undercut
properties of amalgam - thermal conduction
- heat and cold sensitivity
- may conduct to pulp
properties of amalgam - thermal expansion
Why bad?
- cold drink = filling contracts - may cause leakage
- puts stress on tooth
How can Amalgam can cause a plaque retentive factor if not handled properly?
- difficult to remove overhangs
- difficult to use any instruments sub-gingivally without causing damage to adjacent tooth/gums
Clinical use - indications for amalgam use?
- 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
Cavity design - what do we need to keep in mind?
Retention - prevention of being pulled out of cavity vertically
Resistance - Prevention of being dislodged under load
Retention?
Retention is macro mechanical - created by an undercut cavity design
Undercuts are sufficient for occlusal and small approximately
Cavity design?
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
How is resistance achieved for mesial/distal sideways restorations?
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
Cavity surface angle
Cavity surface angle of 90 degrees
Width of a cavity interproximally should be slightly wider than the contact area
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?
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
Bonding in amalgam
Adhesive systems similar to those used with composite have been developed
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
Alternatively how else can we use bonding in amalgam?
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
Advantages to polishing amalgam?
Makes it more resistant to corrosion
Less plaque retentive
More aesthetically pleasing
Disadvantages of polishing?
- polishing will not improve the life expectancy of the restoration.
- the heat generated may be harmful to the pulp
Is amalgam safe
- 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
Environmental disadvantages of mercury
When mercury enters the environment it can be converted by bacteria into methyl mercury in fresh and salt water
Methyl mercury is highly poisonous
Environmental plans regarding amalgam
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
Evidence of risk from dental amalgam
- no evidence of risk from mercury in amalgam to patients or dental staff
- problem when mercury enters environment
Article 10 of EU regulation 2017 on mercury
- 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
