1
Q
Allot composition affects durability. There have been … controlled clinical trials.
On who?
A
- 14
- class 1 and 2, 5-15 year follow up, 3119 restorations
2
Q
Explain the clinical trials on durability of alloys
A
- divided into 4 groups
- zinc (free or containing)
- copper (conventional or high)
- failures reason assessed as related to restoration, process or patient
3
Q
Results of durability of alloy trials
A
- 481 failed restorations
- 77% failed due to restoration
- 80% due to fracture of amalgam
- conventional zinc-free has shortest
- high copper zinc-containing longest
4
Q
Mechanical properties of amalgam
A
- hard
- rigid
- brittle
- strong in compression, weak in tension
- low flexural strength
5
Q
How is strength developed in amalgam over time?
What to tell patient?
A
- slowly
- danger of early fracture so patients warned
- alloy powder side influences setting rate - smaller particles leads to faster strength development
6
Q
Key features of cavity design and placement of amalgam
A
- sufficient bulk
- avoidance of thin amalgam sections
- final mercury content of less than 50%
7
Q
What variables can be manipulated with amalgam?
A
- proportioning/mixing
- mercury to alloy ratio
- trituration - mixing by grinding
- mercury hygeine
8
Q
Why is proportions and mixing
manipulated?
A
- ease
- reproducibility
- safety
9
Q
How can the mercury alloy ratio be manipulated?
A
- enough mercury for fluid mix
- less than 50% mercury for good properties
10
Q
Why is trituration manipulated?
A
- ease
- reproducibility
- safety
11
Q
Why is mercury hygeine manipulated?
A
- to protect patient
- and dental personnel
12
Q
4 stages of manipulation of amalgam
A
- proportioning and mixing
- condensing
- carving
- polishing
13
Q
Difference between traditional and modern proportioning
A
- traditionally done by dentist or assistant
- now encapsulated products
14
Q
Final strength of amalgam depends on …
A
- keeping as much gamma as possible
- reducing mercury concentration to 40-45% in set material
- mercury to alloy ratio is important
15
Q
Explain traditional trituration
A
- done by hand
- mortar/pestle
- required a wet mix (8:5 mercury to alloy)
- took 40 seconds
- resulted in excess mercury - and weakness therefore
16
Q
What is trituration?
A
mixing by grinding
17
Q
How is trituration done now?
A
- an amalgamator
- requires less mercury 1:1 if possible
- mechanical for 8-15 secs
18
Q
Explain condensation stage
A
- transfer mixed amalgam to cavity
- build up increments
- condense with plugger/condenser
- condense while plastic soon after trituration - don’t delay
19
Q
What does condensation allow to happen?
A
- adapt to cavity form
- binds together increments
- reduces porosity
20
Q
Condensation pressure affects what?
A
final amalgam strength
21
Q
Condensation reduces …
How?
A
- final mercury content
- causes mercury to rise to the surface
22
Q
Effect of Lathe cut particles in condensation
A
- resist flow due to friction and interference
- require high force and high rate of application
- gives good feedback to dentist
23
Q
Effect of spherical particles in condensation
A
- easily flow passed each other
- requires lower forces and wider pluggers
24
Q
How do a mixture of particles affect condensation?
A
- faster setting times of small particles
- good feedback from larger particles
25
Explain carving of amalgam
- always over fill cavity in condensation
- remove mercury rich layer with carver (sharp steel or stellite)
26
Why don't we carve when it's too soft?
- drags out material
- drags are thin sections which are weak
27
Why don't we carve when it's too hard?
- can't carve it
- material is brittle and fractures
28
How does particle size influence carving?
- fine grain easier than coarse
- spherical amalgams easier to carve
29
Explain polishing process
- final stage
- ideally when setting is complete - next visit
- use abrasive suspended in fluid
- zinc oxide in alcohol, pumice in glycerine, chalk
30
Why do we polish amalgam?
- improve appearance of restoration
- prevent accumulation of food and micro-organisms
- improves corrosion resistance - corrosion occurs rapidly in crevices and polishing creates a 'Beilby layer' to form an amorphous layer with good corrosion resistance
31
4 features of the amalgam interface with tooth
- no adhesion
- possible leakage
- thermal diffusivity
- coefficient of thermal expansion
32
Why is there no amalgam adhesion at tooth interface?
- retention by cavity design of undercuts
- destroys sound tooth
- replacement destroys more tooth
33
Why is leakage possible at amalgam interface with tooth?
- corrosion products can seal margins - in vitro evidence
- bonding agents can be used - not widely though
34
How does amalgam interface have thermal diffusivity?
- pulp sensitive to temp change
- needs lining
35
How does amalgam interface with tooth have coefficient of thermal expansion?
- mismatch with tooth
- 'percolation' is possible
36
Where do concerns about amalgam come from?
- mercury content
- rival business claims
- public health scares
37
Sources of mercury
- propotioning amalgam - modern capsules
- trituration - modern amalgamators
- residual mercury in fillings - condensation
- corrosion - gamma 2 free amalgams now used
38
Where have bans on amalgam?
- Norway, Sweden and Denmark have bans on import, transport and use of mercury compounds
39
What did Jones find about amalgam?
- no conclusive evidence shows link between amalgam and neurological disorders and impaired kidney function
- same with animals
40
Potential environmental sources of mercury
- over 50% from natural sources
- 42% from fossil fuels burning
- daily intake from 10 amalgam surfaces in mouth - 2% of WHO levels
41
Explain the Minimata Tragedy
- 1952 Japanese chemical industry booms
- fish found floating in Minamata bay
- domestic cats behaving bizarrely - neurological disorders and cat suicides
- humans affected sporadically, no warning, death/paralysis/maiming
- linked to high mercury in fish, mainly shellfish
- 2955 contracted the disease and 1784 have died
42
Mercury ban in Norway
- fish industry major employer
- german u-boat of 65,000 of mercury on ocean floor
- acid-rain gave env mercury - similar to other places
- env concerns lead to attempt to reduce mercury use
- not based on amalgams
43
Sweden and Denmark mercury ban
- Sweden largest employers are automobile and battery manufacturers - large mercury use
- amalgam not based on health risk from amalgam
44
What is the Minimata Convention?
- global drive to reduce mercury use (not just amalgam)
- no amalgam use in pregnant women, under 16s, likely phase out of all amalgam use
Dental Materials
flashcards
Decks in class (17)
# Cards
