6: Dental amalgam

Chapter 6 Dental amalgam

When to Use Dental Amalgam

When placed correctly, dental amalgam has sufficient strength to withstand the high loads generated during mastication. It is therefore chiefly indicated for:

Dental amalgam is used mainly in the posterior sextants of the mouth because of its unaesthetic appearance (Figure 6.1). However, as amalgam is not tooth coloured, the core material and the tooth tissue can be easily identified, ensuring that the margins of the crown are placed onto tooth tissue and not the core material (Figure 6.2).

Composition of Dental Amalgam

The basic amalgamation reaction is that of mercury reacting with the surface of the alloy particles.

The alloy

Prior to 1986, all alloys, whatever their composition, were referred to as conventional alloys. However, work in the preceding decade evaluating the differing elemental compositions of the alloys increased the understanding of the structure and properties of the dental amalgam formed. These newer alloys produced dental amalgams which exhibited superior clinical performance.

Conventional alloys

The typical constituents of commercially available conventional alloys, their functions and clinical influence are shown in Table 6.1.

Table 6.1 Constituents of dental amalgam alloy

Element Function
Silver Main constituent of alloy, combines with tin
Tin Combines with silver
Copper Increases mechanical properties, decreases creep, increases corrosion resistance, decreases the amount of the γ2 phase formation
Zinc Acts as a scavenger of oxygen
Mercury Sometimes added to increase the rate of reaction (pre-amalgamation)
Indium, palladium, selenium, platinum, gold All increase corrosion resistance and improve certain mechanical properties of the final product. Decrease creep

Amalgamation reaction of a conventional alloy

The mercury reacts with the outer layer (3–5 μm) of the silver–tin alloy particle in a reaction shown in Figure 6.3A. This means the bulk of the particle is left unreacted. These unreacted cores sit within a matrix of the silver–mercury and tin–mercury phases. The structure of the set material is shown in Figure 6.3B.

High copper amalgam alloys (Figure 6.5)

More recently, attempts have been made to reduce or even eliminate the γ2 phase by increasing the copper content in the alloy to above 13%. This modification of the setting reaction has resulted in some important beneficial changes in the properties of the amalgam:

Setting reaction of high copper amalgam

The setting reaction of these alloys is the same as the reaction for conventional alloys (see Figure 6.3A). After the formation of the γ2 phase, there is a reaction between this and the silver–copper component, leading to the formation of a copper–tin phase and γ1. Although silver is present in the alloy particles, there is a preferential reaction between the copper and tin, thus forming a silver–tin–copper complex. The result is that little or no γ2 is left in the final amalgam.

Types of alloy

Currently, three types of dental amalgam alloys are available: lathe cut, spherical and admixed (Figure 6.6). Their handling characteristics are all very different and it is important that they are manipulated correctly when used in the clinic for optimal performance of the set product (see pp. 63–65).

Properties of Dental Amalgam

Jan 31, 2015 | Posted by in Dental Materials | Comments Off on 6: Dental amalgam
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