Chapter 5

Dental Amalgam

Aim

Amalgam is one of the most established materials for the restoration of posterior teeth, but recently its use has been challenged because of health and environmental concerns. The aim of this chapter is to increase knowledge of dental amalgam, including its use with enamel and dentine adhesive systems in the bonded amalgam technique.

Outcome

Readers will gain insight into the indications, advantages and disadvantages of amalgam, including its safe use and disposal.

Introduction

In many countries, dental amalgam continues to be one of the most widely used dental materials for the restoration of posterior teeth. It has been successfully used for over 170 years, with favourable cost-effectiveness and clinical durability. Dental amalgam is, however, unaesthetic and lacks the ability to bond to remaining tooth tissues.

Indications

Amalgam is primarily indicated for the restoration of occlusal and occlusoproximal cavities in posterior teeth. It also continues to find application as a core build-up material prior to preparing a tooth for a crown. The use of dental amalgam is falling internationally, with the application of tooth-coloured alternatives increasing steadily.

Advantages

• cost-effective

• less technique-sensitive than composite resin

• high clinical survival rates (durability)

• fewer allergic or hypersensitivity reactions than composite resin

• strong evidence base.

Disadvantages

• unsightly restorations

• does not bond to tooth substance, requires mechanical retention and resistance features

• relatively low tensile strength, weak in thin (<2 mm) sections

• few, if any, applications in minimally interventive dentistry

• concerns regarding mercury as a potential toxin and environmental contaminant.

Composition

Dental amalgam is created when mercury is mixed, or triturated, with metal alloys. The composition of the metal alloys used has changed with time. Zinc-free and, in particular, high-copper alloys now predominate. The compositions of conventional (low-copper) and high-copper amalgam alloys are given in Table 5-1. In addition to the major elements, alloys may also contain small amounts of palladium, platinum, indium and gold.

Modern dental amalgam alloys may be classified as admixed high-copper alloys or single-composition high-copper alloys. As the alloy composition varies, so does the setting reaction. The metal alloy powder is triturated with liquid mercury in a sealed plastic capsule (Fig 5-1) using a mechanical triturator or amalgamator (Fig 5-2).

The mixing process produces a soft silver pellet of dental amalgam, which is condensed in increments into the cavity using a hand-held amalgam condenser. For condensation of the amalgam to be effective, amalgam condensers must be firmly applied to adapt the material and remove excess mercury. Given the need to firmly condense amalgam, matrices must be secure and well wedged if marginal excesses are to be avoided.

When triturated, mercury dissolves silver and tin from the metal alloy particles to form matrices of intermetallic compounds that bind the silver–tin particles (gamma phase) together into a solid set mass. The details of the setting reaction vary with the type of alloy used.

Amalgam Trituration

• Overtrituration produces a hot, sticky mix with decreased working and setting time and slightly increased setting contraction.

• Undertrituration produces a grainy, crumbly mix, which cannot be used.

Conventional (Low-copper) Alloys

On trituration, a conventional (low-copper) alloy with a silver–tin alloy (gamma phase) releases tin and silver, which react with mercury to form silver–mercury (gamma 1) and tin–mercury (gamma 2) phases (Figs 5-3a and 5-3b).

Admixed High-copper Alloys

When triturated, tin moves to the surface of the silver–copper alloy particles and reacts with the higher copper content to create a new copper–tin (eta phase), which surrounds the unreacted cores of the silver–copper particles. These particles are bound together with silver–mercury (gamma 1) phase (Figs 5-4a and 5-4b). Thus, there is almost no gamma 2 phase in these amalgams, which is weak and susceptible to corrosion.

The actions of each amalgam alloy constituent are summarised in Table 5-2.

High-copper Alloys

High-copper alloys are now the most popular type of amalgam used, as they give high />

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