1
Q
Dental amalgam is an <i>alloy</i>. It is formed by the reaction of what materials?
A
- Mercury (liquid)
- Silver, tin, copper and other metals (powder)
2
Q
What is an alloy?
A
- A material made from the mixing of 2 or more metals
3
Q
What are the 2 different ways in which you can classify amalgam?
A
- By composition
- By particle size and shape
4
Q
When classifying amalgams by composition. What are the 2 different possible compositions of amalgam?
A
- Traditional
- Copper enriched
5
Q
Why do we sometimes classify amalgams into particle shape and size?
A
- As this affects the handling and final properties
6
Q
What is the composition of amalgam?
A
- Ag (silver) = 70%
- Sn (Tin) = 25%
- Cu (Copper) = 3%
- Zn (Zinc) = 1%
- Hg (Mercury) = 1%
7
Q
What are the functions of silver and tin the amalgam?
A
- This is am intermetallic compound which reacts with Hg liquid to form amalgam - this is the gamma phase
8
Q
What is the function of copper in the amalgam?
A
- Increases strength and hardness (ensures that the material lasts longer)
9
Q
What is the function of zinc in the amalgam?
A
- Scavenger during production - preferentially oxidise s and slag formed/removed - some zinc free
(Zn oxidises to prevent the other materials from oxidising)
10
Q
What is special about ‘pre-amalgamated’ alloys?
A
- They react faster
11
Q
What makes the liquid Hg in amalgam very pure?
A
- It is triple distilled
12
Q
What is the function of liquid Hg in amalgam?
A
- Reacts with other metals
13
Q
What is a lathe cut particle and how is it formed?
A
- Coarse, medium, fine
- Like a shaving (creates all sorts of fragments of irregular shape)
- Formed by filing ingots
14
Q
What is an ingot?
A
- A loose term applied to any mass of material specifically cast from a molten state of known composition, weight, shape and physical dimension
15
Q
What is a spherical/spheroidal particle and how is it formed?
A
- Range of particle sizes
- Formed by spraying molten metal into the inert atmosphere and so they produce globules as they hit the surface underneath
16
Q
What is the setting reaction of amalgam?
A
- Ag3Sn + Hg -> Ag3Sn + Ag2Hg3 + Sn7Hg9
- Powder + liquid -> unreacted particles + amalgam matrix
17
Q
What properties does gamma in the set structure of amalgam have? (2 points)
A
- Good strength
- Good corrosion resistance
18
Q
What property does gamma 1 in the set structure of amalgam have?
A
- Good corrosion resistance
19
Q
What properties does gamma 2 in the set structure of amalgam have?
A
Weak and poor corrosion resistance
20
Q
What do voids in the set structure of amalgam cause?
A
- Decreases strength and increases corrosion
21
Q
What is the mean tensile strength of the gamma component of amalgam?
A
170MPa
22
Q
What is the mean tensile strength of the gamma-1 component of amalgam?
A
30MPa
23
Q
What is the mean tensile strength of the gamma-2 component of amalgam?
A
20MPa
24
Q
What is the mean tensile strength of amalgam?
A
60MPa
25
In relation to expansion and contraction during setting dimensional changes of amalgam, what happens in traditional amalgam?
- Initial contraction - solution of alloy particles in Hg
| - Expansion - Gamma crystallisation
26
In relation to expansion and contraction during setting dimensional changes of amalgam, what happens in modern amalgam?
- Small contraction
| - Solid solution of Hg in Ag3Sn
27
Why do we use zinc free amalgams for restoration material?
- Zinc interacts with saliva and water to produce zinc oxide and hydrogen
- The bubbles of hydrogen formed within amalgam will do the following:
= pressure build p causing expansion
= Downward pressure causing pulpal pain
= Upward - restoration sitting proud of surface
28
What handling factors is amalgam dependent on? (3 points)
- Proportioning & trituration
- Condensation
- Carving & polishing
29
Is amalgam dependent on cavity design?
- Yes
30
Will amalgam be affected by corrosion?
- Yes
31
What is the compressive strength of traditional amalgams like after 1) an hour and 2) 24 hours?
- One hour = poor(ish)
| - 24 hours = ok
32
What is the abrasion resistance of amalgam (hard material that resists the surface layer being removed)?
- High, suitable for posterior teeth
| - Too high for deciduous
33
What factors have an effect on decreasing the strength of amalgam? (5 points)
- Undermixing
- Too high Hg content after conden sation
- Too low condensation pressure
- Slow rate of packing (so increments do not bond)
- Corrosion
34
Why can creep be a problem with traditional amalgams?
- Affects the marginal integrity as the material changes shape when it undergoes creep
- NB marginal integrity also d epends on:
Cavity design
Corrosion
35
What is meant by creep?
- When a material is repeatedly stressed for long periods at low stress levels i.e. stress is below the elastic limit, it may flow, resulting in permanent deformation
36
What are 4 examples of materials that can be affected by creep?
- Amalgam
- Alloys
- Waxes
- Plastics
37
What is biocompatibility?
- The quality of not having toxic or injurious effects on biological systems
38
What is the concern over the biocompatibility of mercury?
- Concern over mercury toxicity
39
What is the thermal expansion of amalgam compared t o a tooth?
- Three times that of a tooth
- Ideally want thermal expansion of amalgam to be the same as enamel --> as amalgam warms up it expands more than the tooth tissue which may cause a gap
40
What is the thermal conductivity of amalgam?
- High, may need to use a liner / varnish in deep cavities
| - This is because the heat reaching the dental pulp may be an issue
41
Does amalgam bond to teeth?
- No, needs mechanical retention for restoration to stay
42
How is the handling of amalgam? (mixing, working times, setting times)
- It is reasonable
| - This varies between types
43
How is the viscosity of amalgam?
- Quite easy to pack (condense) into cavity - it is 'user friendly'
44
How are the aesthetic properties of amalgam?
- Poor
45
How are the radiopaque properties of amalgam?
- Amalgam is radiopaque
46
Does amalgam have anti-cariogenic properties?
- No
47
Does amalgam have smooth surface properties?
- Yes, if polished well
| - May deteriorate over time
48
What is the setting shrinkage property like for amalgam?
- Modern materials tend to have net overall shrinkage
49
Corriosion weakens materials. Where does this particularly happen on teeth?
- At margins
50
How can corrosion of amalgam be reduced? (3 points)
- Copper enriched, polishing margins
| - Avoiding galvanic cells
51
What are the advantages of spherical amalgam particles? (5 points)
- Less Hg required
- Higher tensile strength
- Higher early compressive strength
- Less sensitive to condensation
- Easier to carve
52
What is tensile strength?
- The max load a material can support without fracture when being stretched, divided by the original cross-sectional area if the material
53
What are 2 names that can be used to describe copper enriched alloys and what % of copper is in these?
- Non-gamma 2
- High copper
- Copper > 6%
54
What are the 2 types of copper enriched alloys?
- Dispersion modified (original)
| - Single composition types (introduces to increase uptake by profession)
55
Originally, what were dispersion modified copper enriched alloys made from?
- Ag-Cu spheres + conventional lathe cut alloy
56
What was original thought about dispersion modified copper enriched alloys?
- Originally thought the spherical particles would act as a strengthening agent BUT increased copper content gave beneficial modifications to the setting reaction
57
What is the dispersion modified setting reaction (for copper enriched amalgam)?
1. As conventional material:
Gamma + Hg -> Gamma + gamma 1 +gamma 2
2. Gamma 2 + Ag-Cu -> Cu6Sn5 + gamma 1
- If add copper, it will react with the gamma 2
- Produces copper tin and some gamma 1 so have no gamma 2 left
* note - after the reaction silver copper has a halo which is copper tin (Cu6Sn5)
58
In single composition formulations (of copper enriched amalgams) what is the copper percentage in the powder?
12%-30%
59
In single composition formulations (of copper enriched amalgams) what are the particle types present?
- Spherical
| - Lathe cut
60
What is the setting reaction for single composition formulations for copper enriched amalgams?
- Ag-Sn-Cu + Hg -> Ag-Sn-Cu + gamma 1 + Cu6Sn5
| notice no gamma 2
61
What are the benefits of copper enriched amalgams? (4 points)
- Higher early strength (developed within the first hour)
- Less creep
- Higher corrosion resistance (so will last longer)
- Increased durability of margins (margins more difficult to break down)
62
What is the creep % for traditional lathe cut amalgams?
- 6.3%
63
What is the creep % for traditional spherical amalgams?
1.1%
64
What is the creep % for Cu dispersion modified amalgams?
0.46%
65
What is the creep % for Cu single composition amalgams?
- 0.07%
66
In which type of amalgam is compressive strength after 1 day the highest?
- Cu single composition formulas
67
In which type of amalgam is compressive strength after 1 day the lowest?
- Lathe cut amalgams
68
In which type of amalgam is compressive strength after 7 days the highest?
- Cu single composition formulations
69
In which type of amalgam is compressive strength after 1 day the lowest?
- Lathe cut amalgams
70
What is the thermal expansion coefficient of amalgam?
- 22-28 ppm/Celsius
| Enamel = 11, so 2-3x enamel
71
What are the advantages of using amalgam? (2 points)
- Strong
| - User friendly (especially if use copper enriched)
72
What are the disadvantages of using amalgam? (4 points)
- Creep
- Leakage - does not bond
- Poor aesthetics
- Mercury:
* perceived toxicity
* Environmental impact
73
Why should amalgam be encapsulated?
- Hg hygiene
74
What is the life expectancy of traditional amalgams?
- Lifetime in excess of 10 years but average lifetime 4-5 years
75
What is the compressive strength of PERMITE after 24 hours?
- 500MPa
| so much more likely to survive for a longer period of time and resist larger forces than other amalgams
76
What is the tensile strength of amalgam (Permite)?
- 60MPa
77
What is the elastic modulus of amalgam (Permite)?
- 30GPa
78
What is the hardness of amalgam (Permite)?
100 (VHN)
79
What is the microleakage of amalgam (Permite)?
0. 04 ml/min
| - Much better than many other amalgams
80
What is the failure rate of amalgam like compared to other restorative materials?
- Low compared to other materials
BDS2 - DMS
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