Dental Amalgam Flashcards
(66 cards)
1
Q
What is amalgam?
A
an alloy of any mercury with any other metals
2
Q
What is dental amalgam?
A
an alloy of dental mercury (highly purified) with a dental powder alloy formed of silver, tin, copper and sometimes zinc
3
Q
Dental amalgams are classified according to particle shape into…
A
- irregular
- spherical
4
Q
Dental amalgams are classified according to composition into…
A
- low copper (conventional)
- high copper
5
Q
High copper composition amalgam is classified into…
A
- admixed
- unicompositional
6
Q
What are the properties of silver?
A
- increased strength
- decreased creep
- increased expansion
- increases reactivity with mercury as well as tarnish and corrosion resistance
- decreases setting time
- 65% min.
7
Q
What are the properties of tin?
A
- decreased strength
- increased creep
- decreased expansion
- decreases reactivity with mercury
- increases setting time
- 29% max.
8
Q
What are the properties of copper?
A
- increased strength
- decreased creep
- increased expansion
- increases tarnish and corrosion resistance
- 6%max.
9
Q
What are the properties of zinc?
A
- it acts as scavenger (deoxidiser) during alloy manufacturing
- however it causes delayed excessive expansion of the filling if contaminated with moisture during trituration or during condensation
10
Q
How is dental amalgam prepared?
A
- hand mixing using pestle and mortar
- mechanical mixing using amalgamator with pre-portioning capsules
11
Q
How to produce an irregular (lathe cut) alloy?
A
- ingot production: Ag, Cu, Sn & Zn are melted and poured into a mold to form the ingot then the ingot is cooled slowly to allow the formation of the greatest amount of Ag3Sn intermetallic compound (γ-phase). However, the formed ingot is non-homogenous
- homogenisation: to eliminate coring (non homogeneity of the ingot) and hence increasing corrosion resistance, the ingot is heated at 400 degrees for 24 hours to uniformly distribute Ag3Sn (gamma-phase)
- powder production: the ingot is placed in a lathe or by ball milling machine. These methods of cutting induce internal stresses in the alloy particles
- aging or annealing: the freshly cut alloy powder are highly reactive and set rapidly on mixing with mercury due to the stresses induced during cutting. Stresses are removed rapidly by heating the powder at 100 degrees for 1 hour or slowly by storing it at room temperature for several months
12
Q
How to produce spherical powder?
A
Ag, Cu, Sn, & Zn are melted and then sprayed into either an inert gas (argon) where the droplets solidify into spheres or water to produce spheroidal particles
13
Q
What are the advantages of spherical and spheroidal powder?
A
- better wettability by less mercury/alloy ratio
- superior mechanical properties
- easier condensation
14
Q
Why should dental mercury be triple distilled?
A
to be very pure otherwise the surface contaminates and will interfere with the setting reaction (delay it)
15
Q
What is the amalgamation reaction of low copper alloys?
A
mixing of the Ag3Sn alloys (γ-phase) with mercury causes the outer layer of the alloy particles to react with the mercury forming two new phases
16
Q
What is the microstructure of low copper alloys?
A
it consists of unreacted Ag3Sn (gamma) phase as cores that are surrounded by gamma 1 + gamma 2 phases as a matrix
17
Q
Why is it beneficial to eliminate gamma 2 phase?
A
it is the weakest, most corrodible and liable to creep so its elimination leads to an amalgam restoration with superior properties
18
Q
What is the surface reaction of low copper alloys?
A
Ag3Sn + Hg –> Ag2Hg3 + Sn8Hg + Ag3Sn
19
Q
Which phase has the highest corrosion resistance and strongest?
A
gamma phase
20
Q
What is the oldest restoration?
A
Amalgam
21
Q
How much % is low amalgam?
A
less than 6%
22
Q
What is the problem of ingot production?
A
cored structure that has core
23
Q
How to eliminate coring?
A
homogenisation reaction
24
Q
What is gamma?
A
Ag3Sn
25
What is gamma 1?
Ag2Hg3
26
What is gamma 2?
Sn8Hg
27
What does the amalgamation reaction of low copper alloys consist of?
- wetting
- diffusion
- surface reaction
28
What characterises high copper amalgam alloys and how?
absence of gamma 2 phase as tin preferentially reacts with copper rather than with mercury
29
What is admixed amalgam alloy composed of?
- 2/3 conventional lathe-cut low copper alloy (Ag3Sn)
- 1/3 eutectic spherical alloy (AgCu)
30
What is the amalgamation reactions of high copper amalgam alloys?
- initial reaction
Ag3Sn (gamma) + AgCu + Hg --> unreacted Ag3Sn + unreacted AgCu + Ag2Hg3 (gamma 1) + Sn8Hg (gamma 2)
- solid state reaction
Sn8Hg (gamma 2) + AgCu --> Cu6Sn5 (eta phase) + Ag2Hg3 (gamma 1) + unreacted AgCu
31
What is the difference between the microstructure of high and low copper amalgam alloys?
- high copper amalgam alloys consist of eta phase that replaces gamma 2 found in low copper amalgam alloys
- AgCu is there as unreacted core with Ag3Sn (gamma) which are held together by gamma 1
32
What makes the high copper amalgam alloy better than low copper amalgam alloy?
- higher compressive strength
- more rapid set to full strength
- reduction in creep
- reduced susceptibility to corrosion
33
What does the powder of uni-compositional alloy consist of?
- Ag
- Sn
- Cu (13-30%)
34
What does the powder of uni-compositional alloy consist of AFTER MANUFACTURING?
- Ag3Sn (gamma)
- spherical Cu2Sn (epsilon phase)
35
What is the reaction of uni-compositional alloy?
Ag3Sn (gamma) + Cu3Sn (epsilon) +Hg --> unreacted Ag3Sn + Ag2Hg3 (gamma 1) + Cu6Sn5 (eta)
36
Are there voids in all types of amalgam alloys?
yes
37
38
What is the strength of amalgam?
- brittle
- high compressive strength
- the less the gamma 2 production, the more strength the amalgam has
39
What type of amalgam has the highest and lowest compressive strengths?
- irregular lathe-cut low copper amalgam alloys have the lowest compressive strength (300)
- low copper amalgam alloy spherical particles (350)
- admixed high copper amalgam (400)
- uni-compositional amalgam has the greatest compressive strength (500)
40
Which is the most expensive amalgam alloy?
uni-compositional high copper amalgam alloy
41
What is the minimum compressive strength of amalgam according to the ADA specification?
80Mn/m2 after 1 hour as this reduces the possibility of fracture by premature contact before final strength is reached
42
How can resistance of amalgam to fracture be obtained?
- roundation of all line angles in the prepared cavity to prevent stress concentration
- amalgam should be placed in thick thickness to resist bending stresses (increase depth and width at isthmus portion to increase bulk of amalgam
- patients should be informed not to chew on the new restoration for several hours because the amalgamation reaction continues for many hours and completed after several hours
43
Why shouldn't amalgam contract on setting?
because it prevents formation of a gap between filling and cavity wall which would've resulted in marginal leakage which is followed by recurrent caries
44
Why is it bad for amalgam to expand?
- it causes the filling to protrude from the cavity which is usually followed by fracture (ditching)
- pressure on pulp (causes pain)
45
How does amalgam contract?
due to diffusion of mercury into the alloy particles
46
How does amalgam expand?
due to formation of intermetallic compounds (gamma 1 + gamma 2)
47
What is the net result of dimensional changes of amalgam?
contraction
48
When can huge expansion occur?
- high Hg/alloy ratio (increase mercury)
- excessive delayed expansion (Zn + H2O --> ZnO + H2 gas)
49
What are the consequences of excessive delayed expansion (400 microns/cm)?
- roughness and corrosion
- protrusion (ditching)
- marginal fracture
50
What is the average dimensional change that is accepted by ADA specification?
dimensional changes between 5 minutes and 24 hours must fall within 20 micron/cm (a value which is not detectable clinically)
51
What causes the severe excessive expansion?
hydrogen gas during trituration or condensation
52
What is creep?
time-dependant plastic deformation under static load below PL after complete setting
53
Why does creep occur in amalgam?
due to its viscoelasticity
54
What is the maximum creep specified by ADA specification?
3% 7 days after cylinder preparation when a static load of 36 mega newton/m2 is placed on it for 4 hours at 37 degrees
55
Which is the most common class that c creep occurs in?
class 1 amalgam
56
How is creep calculated?
change in length of cylinder 4 hours after load application
57
What significantly influences creep?
presence of gamma 2
58
Which type of amalgam has the highest and lowest creep percentage?
- low copper amalgam alloy lathe cut has 2% (highest)
- low copper amalgam alloy spherical has 1%
- admixed high copper amalgam alloy has 0.3%
- uni-compositional high copper amalgam alloy has 0.1% (lowest)
59
What are the disadvantages of amalgam?
- bad aesthetic
- low tensile strength (brittle)
- viscoelastic (creep)
- tarnish and corrosion
60
What are the advantages of amalgam?
- direct (no impression) with rapid and easy manipulation
- very high compressive strength
- excellent wear resistance
- insoluble in oral fluids
- excellent adaptability to cavity walls (due to minimal dimensional changes)
- self sealing ability
- minimal post-operative sensitivity
- median survival time of 10-15 years
- very economic
- no scientific research revealed any dangerous health hazard effect
61
Why does corrosion occur in all types of amalgam?
they are multi-phases (heterogenous structure)
62
Why is corrosion more significant in low copper amalgam?
attack is centred around tin-mercury (gamma 2) phases as they are the most susceptible phases to corrosion and are easily oxidised in oral environment
63
Which phase is the most corrodible in high copper amalgam alloy?
eta phase
64
Which is more corrosion-resistant type of amalgam?
high copper amalgam alloys
65
What do corroded amalgams generally have?
darkened surface
66
What is the cause of the darkened surface?
tarnish of silver as it contacts sulfides in foods (especially eggs)
