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Amalgamation

The process of mixing liquid mercury with one or more metals or alloys to form an amalgam.

Creep

The <b>time-dependent</b> strain or deformation that is produced by a stress. The creep process can cause an amalgam restoration to extend out of the cavity preparation, thereby increasing its susceptibility to marginal breakdown.

composition of amaglam

silver → wt% 65
tin → wt% 29
copper → wt% < 6

γ₂ amalgam is:

Sn₇₈Hg

•The tin-mercury phase lacks corrosion resistance and is the weakest component of the dental amalgam.
•However, amalgams of tin-rich alloys display less expansion than silver-rich alloys.

γ₁ phase is:

Ag₂Hg₃

γ phase

Ag₃Sn

an ↑ copper content_______ and _______ the silver-tin alloy.

Hardens and strengthen

copper can be ↑ at the expense of Ag up to 4-5 wt%

The chief function of zinc in amalgam alloys is:

The chief function of zinc in amalgam alloys is that of a <b>deoxidizer</b>.

drawback - if moisture is present, it can cause delayed expension.

% of zinc in zinc containing amalgam:

~ 1% by wt.

in an Ag-Sn alloy, rapid quenching of the alloy ingot results in the maximum amount of _____ phase retained, whereas slow cooling results in the formation of the maximum amount of the _____ phase.

in an Ag-Sn alloy, rapid quenching of the alloy ingot results in the maximum amount of β phase retained, whereas slow cooling results in the formation of the maximum amount of the γ phase.

Why is amalgam treated with acid during production/

it makes it more reactive

The average particle sizes of modern powders range between:

15 and 35 μm

As the size of particles ↓ the amount of mercury needed to form an acceptable amalgam _______.

increases

How is having smaller particle size help the restoration.

it will give you a smoother surface upon carving and polishing and therefore increases <b>early strength</b> of the restoration.

(T/F) the bulk of the finished restoration is composed of particles of the original alloy surrounded by reaction products.

True

Spherical alloys require less mercury than typical lathe-cut alloys. Why?

because spherical alloy powder particles have a smaller surface area per volume than do the lathe-cut alloy particles. Amalgams with a low mercury content generally have better properties.

The alloy is usually mixed with mercury in about a _:_ ratio

1:1

The more unconsumed Ag-Sn particles that are retained in the final structure, the ______ the amalgam.

a. stronger
b. weaker

a. stronger

The hardness of γ₂ is approximately ____% of the hardness of γ₁

10%

how does ↑ of copper increases the strength of amalgam?

The silver-copper particles as well as the Ag-Sn particles probably act as <b>strong fillers</b> in amalgam, thereby strengthening the amalgam matrix.

(T/F) Admixed alloy powders usually contain 30 wt% to 55 wt% spherical high-copper powder.

True

The total copper content in admixed alloys ranges from approximately ____________.

9 wt% to 20 wt%

When is amalgam considered high copper?

If all γ₂ phase is eliminated during hardening reaction, then the amalgam is said to be high copper.

ε phase

Cu₃Sn

η phase

Cu₆Sn₅

Single composition alloy

Ag-Sn-Cu ALLOY PARTICLES + Hg → γ₁ + η + UNCONSUMED ALLOY PARTICLES

60 wt% silver, 27 wt% tin, and 13 wt% copper.

(T/F) Nevertheless, in most single-composition amalgams, little or no γ₂ forms.

True

According to ADA, the expansion of amalgam can not be more than ___

20 μm @ 37°C

How is condesation of amalgam helps prevent expansion?

Effective condensation allows residual mercury to move to the top and be removed. if sufficient mercury is present in the mix when the measurement of dimensional change begins, expansion will be observed. Otherwise, contraction will occur. Therefore manipulation that results in less mercury in the mix, such as lower mercury/alloy ratios and higher condensation pressures, favors contraction.

how is the size of particles related to expansion of amalgam.

Smaller particles use up more mercury faster and therefore there will be less mercury available and thus ↓ expansion.

delayed expansion

Happens when Zinc containing amalgams react w/ water to produce a hydrogen gas. The hydrogen gas causes the amalgam restoration to expand, seeming to push out the preparation.

<b>note</b>: It should be noted that the contamination must occur during trituration or condensation.

Most common type of defect in amalgam restoration

marginal defects

compressive strength of a satisfactory amalgam probably should be at least ____ megapascals (MPa)

310

Which type of amalgam has the highest <b>compressive</b> strength?

a. Low copper
b. admix
c. single composition

c. single composition

The effect of trituration on strength depends on

1. type of amalgam alloy
2. time of trituration*
3. speed of amalgamator

* both over- and undertrituration ↓ the strength

(T/F) excess mercury promotes formation of the γ₂ phase in a high-copper amalgam.

True

how long does it take for an amalgam restoration to reach its 70% compressive strength?

about 8 hrs

Which one has lower creep rate?

a. low copper amalgam
b. high copper amalgam

b. high copper amalgam

(T/F) The ability to seal against microleakage is shared by both the low-copper amalgams and the newer high-copper amalgams. However, the accumulation of corrosion products is slower for the high-copper alloys.

True

How can corrosion of an amalgam restoration lead to both positive and negative outcomes?

The build-up of corrosion products gradually seals the space between the restoration and tooth structure, making dental amalgam a self-sealing restoration.

The most common corrosion products found with traditional amalgam alloys are:

oxides and chlorides of tin

Which type of amalgam has the best <b>survival rate</b>

high copper with zinc

What steps can be taken by a dentist to extend the survival time of amalgam restorations?

...

(T/F) The operator variable is a major factor affecting the clinical lifetime of the restoration.

T

What is the ideal mercury alloy/ratio for an admixed amalgam and a spherical particle amalgam?

The mercury content of the finished restoration should be comparable to that of the original mercury/alloy ratio, usually about 50 wt%, with lesser amounts (∼42 wt%) being used with spherical alloys.

What's one potential disadvantage of using spherical alloy to lathe-cut?

The potential disadvantages of a spherical alloy compared with an admixed alloy (lathe-cut and spherical particles) are the tendency for overhangs in proximal areas and weak proximal contacts.

Top 4 common causes for amalgam restoration failure:

<b>(1) secondary caries</b>
(2) marginal fracture
(3) bulk fracture
(4) tooth fracture

(T/F) However, there is also evidence that in controlled clinical trials zinc-containing alloys have better marginal integrity and longer survival times than similar zinc-free alloys

true

What variables affect marginal breakdown of amalgam restorations? Which of these factors are under the control of the dentist?

• Improver cavity preparation and finishing
• Excess mercury
• Creep (mostly important for low-copper amalgam)

advantage of amalgam compared to composite material:

» It is far less technique-sensitive,
» more durable
» less costly.
» Less time involved for placement.
» excellent wear resistance
» abrasiveness against enamel is negligible, and corrosion tends to seal itself against leakage and bacterial invasion.

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