Chapter 6 Dental amalgam
An amalgam is formed when an alloy of two or more metals is mixed with mercury. This reaction is called amalgamation. Dental amalgam is the product of the amalgamation between mercury and an alloy containing silver, tin, often copper, and sometimes other elements combined in varying amounts.
Dental amalgam has been used to treat teeth for many centuries. Substantial research during this time and particularly in the past 40 years has resulted in refinements of the constituents, culminating in the alloys which are used today. All this work has ensured the consistent handling, properties and clinical performance of the material. It is indeed a testimony to the material’s many attributes that despite the huge advances in dental materials science, dental amalgam is still widely used today.
When to Use Dental Amalgam
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).
Fig. 6.2 Tooth 25 core restored with amalgam. Note the contrast between the amalgam core colour and tooth, facilitating identification of the margin of the core and of the preparation placed on tooth tissue.
Composition of Dental Amalgam
The mercury used for dental amalgam is produced by distillation. It is distilled three times (like Irish whiskey!) to remove any impurities. This is important as contamination leads to inferior physical properties and adversely affects the setting characteristics of the amalgam. Contaminated mercury has a dull surface. Most dental amalgam products used in dentistry today are presented in encapsulated form (see p. 62). This means the dentist should have less concerns about the purity of the mercury as this is packaged in the manufacturing plant.
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.
|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.
The role of zinc
Zinc was traditionally included in the alloy because it acts as an oxygen scavenger during the alloy’s initial production. It preferentially reacts with any oxygen present to produce clean castings of the ingot. Alloys containing more than 0.01% zinc are called zinc-containing alloys and those containing less than this are termed zinc-free alloys.
If a zinc-containing dental amalgam is contaminated with water from saliva during the condensation process, it produces hydrogen gas, which becomes incorporated within the setting amalgam mass and leads to excessive delayed expansion. This is clearly undesirable and may cause pain and even tooth fracture. Good moisture control is therefore highly desirable when placing dental amalgam restorations (as with any other restorative material) and, in particular, those alloys containing zinc. Most modern alloys are now manufactured in an inert atmosphere so the need for zinc has been obviated.
The gamma (γ)2 phase
The γ2 (tin–mercury, see Figure 6.3) phase is the most chemically and electrically active component of the set amalgam. Its physical properties are inferior than the γ (silver–tin) and γ1 (silver–mercury) phases and its presence in conventional amalgam alloys results in:
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 high copper amalgam alloy
• Dispersed phase alloys – in these alloys, the copper is dispersed throughout the alloy and has the effect of increasing its strength. The alloy particles consist of either silver–tin or silver–copper. This means that the total silver content may be kept the same as in conventional alloys.
• Ternary alloys – ternary means a compound containing three elements, usually silver, tin and copper in this case. Ternary alloys are sometimes called unicompositional alloys as they have all three components in one particle.
Many manufacturers reduce the silver and tin content when increasing the copper content. A small amount of zinc is sometimes added to high copper amalgam to improve its clinical performance, in particular by decreasing marginal breakdown. Table 6.2 gives the general composition of the three types of alloy.
Implications of the composition of the alloy for corrosion
The susceptibility of the γ2 phase to corrosion offers a clinical advantage. The corrosion products fill the microgap (marginal gap) at the tooth–amalgam interface, which helps to decrease microleakage. One drawback of the high copper amalgam is that the microgap persists due to lack of the γ2 phase, and so these alloys have been associated with greater microleakage. Clinically, this may manifest as increased postoperative sensitivity with a risk of recurrent caries. It has been suggested that an intermediate bonding agent should be used in combination with these alloys to seal the dentine at the microgap (see p. 61).
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).
Lathe cut (irregular) alloys
These alloys are formed by grinding an ingot of the alloy, which produces irregular particles of varying size ranges. The different sizes of particle require different handling and their reactivity with mercury also varies. A blend of various particle sizes is used by the manufacturer to control the properties and working and setting times of the amalgam. Sometimes, after the machining process, the particles are too reactive to be used. If they are used in this state, the setting reaction would be too rapid (especially for the smaller particles as their surface area to volume ratio is higher). During commercial production, the alloys are placed in boiling water to reduce their reactivity.
These alloys are made by heating the alloy components to a molten liquid and then spraying this liquid into an inert atmosphere, usually argon. The particles coalesce as they fall, forming solidified spheres.
The clinical ramifications of the alloy types are reflected in the different handling properties between them. Traditionally, a moderate amount of force was required to pack the amalgam in the cavity. Dentists prefer this handling characteristic and so some manufacturers have attempted to mimic this property with the newer spherical alloys by sandblasting the particles. The clinician can exert greater condensation pressure when using an admixed alloy than with other, untreated, spherical alloys.
Properties of Dental Amalgam
The dental amalgam must be strong enough to withstand forces during function, and ideally develop early strength so that it is not damaged before it is fully set. The material (as with other brittle materials) is strongest in compression and weakest in tension. The properties exhibited by the final set material depend on the structure of its various phases, their proportions and their individual strengths.
Thermal diffusivity and thermal expansion
As dental amalgam is metallic, it transmits heat readily (the material has a high thermal diffusivity, 9.6 mm2/s). Its coefficient of thermal expansion (22–28×10−6/°C) is greater than that of the surrounding tooth. During thermal cycling, significant expansion and contraction may lead to:
When dental amalgam is setting, a dimensional change occurs. Initially the material contracts slightly as the mercury diffuses into the outer surface of the alloy particles and reacts with the silver and tin portions. The material then expands as the amalgam is setting because the (γ1) crystals expand by growth. From a clinical perspective, the combination of these two phenomena should not lead to any significant expansion or contraction:
Other factors that can affect dimensional change of the amalgam are the size and shape of the particles and the type of alloy used. As previously mentioned, zinc-containing alloys expand significantly if contaminated with water at placement. The quality of the clinical condensation also affects dimensional change, so thorough condensation is important.
Inexperienced clinicians are strongly advised to use a slow-set alloy until they have gained sufficient experience, because failure to condense the material quickly will result in an inferior restoration. If a crown preparation is to be carried out at the same appointment as placing an amalgam core, a fast-setting amalgam should be used.
• Attention should also be paid to the cavo-surface angles (see Figure 6.7) so that neither weak amalgam nor weak unsupported tooth is left at the margin.