Operative Dentistry MCQ Answer 14

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The Correct Answer: a) To reduce Hg release during mastication

Composition of amalgam:

Alloy type:	%Silver	%Tin	%Copper	%Zinc
Conventional low copper	65-70	21-29	2-4	0-2
High copper	40-60	26-30	6-28	0-2

Indium (5-15% wt.) is added to reduce the release of Hg during mastication, and during and after setting.

Zinc is a deoxidizer (oxygen scavenger) during manufacturing. It decreases brittleness. It may decrease corrosion and improve marginal integrity.

Higher copper alloys have high early strength, low creep, good corrosion resistance, and good resistance to marginal fracture. High copper alloys reduce gamma two phase (weaker).

Silver and tin are the major components of amalgam, combining with Mercury to produce an amalgam with varying working properties.

Palladium is added to enhance mechanical properties and corrosion resistance.

Chemistry of amalgam:

Mercury + Ag₃Sn (gamma phase) = Ag₃Sn (unreacted) + Ag₂Hg₃ (gamma one) +

Sn₇Hg ₈(gamma two)

Gamma one and two are inferior in properties than gamma phase.

Gamma two is weaker and more prone to corrosion than gamma phase.

In high copper alloys (>6% wt.), gamma two reacts with cilver-copper eutectic to eliminate the gamma two phase.

Sn₇Hg₈ (gamma two) + AgCu = Ag₂Hg₃ (gamma one) + Cu₆Sn₅

In high copper alloys the reaction is this:

Ag₃Sn (gamma phase) + Hg = Ag₃Sn (unreacted) + Cu₆Sn₅

Vaporization of Hg from Hg-in amalgams during setting and after abrasion. Ferracane,

Materials in Dentistry, Stannard, J.G. 1986 Denali Publishing

Dental Materials and Their Selection 2nd Ed., O’Brien WJ, 1997 page 189

Restorative Dental Materials 10th Ed., Craig, R.G. 1997 page 210