A NOTE ON DENTAL AMALGAM

DENTAL AMALGAM

General information

Dental amalgam has been the most widely used material for the restoration of posterior teeth due to:

High strength and durability

Ease of use

Good physical characteristics

However, its use has decreased over the past decade due to concerns regarding:

Esthetics

Mercury content

Alloy versus amalgam

Alloy: is a mixture of two or more metals.

Main components of amalgam alloy:

Silver

Tin

Copper

Zinc

Palladium or indium

When these components are mixed with mercury, the reaction that occurs is amalgamation and the resulting material is dental amalgam.

Composition

Weight (%)
Metals	Limits prior to 1986    (conventional alloy)	Current limits (Cu-rich alloy)
Silver	65 (min)	40 (min)
Tin	29 (max)	32 (max)
Copper	6 (max)	30 (max)
Zinc	2 (max)	2 (max)
Mercury	3 (max)	3 (max)

Classification

According to the shape of particles in the powder:

• Irregular: Formed by shaving particles from a block of the alloy by a lathe (lathe-cut alloy)
• Spherical: Formed by spraying molten alloy into an inert gas (spherical alloy)
• Admixed: Mixture of the two (admixed alloy). Alloy maybe made from different particle shapes to increase packing efficiency and reduce amount of Hg needed to obtain a workable mix.

According to composition:

Conventional alloys (6% copper content, refer to Table 1)

Copper enriched alloy:

• Single composition-copper enriched alloys
• Dispersion modified-copper enriched alloys: ratio is 2 (conventional alloy): 1 (silver-copper alloy), overall copper content 12%

Setting transformation

• Alloy powder + liquid mercury mixed  Packable amalgam in a cavity firming phase of the mix  1^st stage of firming amalgam can b carved.
• When amalgam reaches initial set, it cannot be carved, but still not fully reacted (brittle). Needs 24 hours to attain full strength.

• When the alloy is mixed with mercury, three phases occur:

Gamma phase (γ) silver-tin alloy phase, strongest with least corrosion

Gamma 1 phase (γ₁) consists of mercury reacting with silver, not as strong as γ.

Gamma 2 phase (γ₂) consists of mercury reacting with Tin. Weak phase and corrodes easily.