to maintain adaptation of the band to the tooth surface cervically and it separates the teeth slightly. Once the wedge is in place the matrix can be loosened slightly to facilitate burnishing against the adjacent tooth.

21.6 Manipulative variables

The manipulation of amalgam involves the following sequence of events.

(1)Proportioning and dispensing;

(2)Trituration;

(3)Condensation;

(4)Carving;

(5)Polishing.

The way in which each of these operations is carried out has an effect on the properties of the final restoration.

Proportioning and dispensing: Alloy/mercury ratios vary between 5 : 8 and 10 : 8. Those mixes containing greater quantities of mercury are ‘wetter’ and are generally used with hand mixing. Those mixes containing smaller quantities of mercury are ‘drier’ and are generally used with mechanical mixing. For any given alloy/mercury ratio, the nature of the mix may vary depending upon the size and shape of the alloy particles. Spherical particle alloys, for example, require less mercury to produce a workable mix.

For optimum properties the final set amalgam should contain less than 50% mercury. Those materials used at alloy/mercury ratios at or approaching 5 : 8 require the removal of excess mercury following trituration and during condensation.

The optimal final mercury content ranges from an average of 45% for lathe-cut materials to an average of 40% for spherical materials. Hence, the amount of mercury required to produce a workable plastic mass of material is generally greater than that required to produce optimal properties in the set material. If too much mercury is present in the final set amalgam it is likely that too much of the relatively hard γ phase will be converted into relatively weak and soft γ2 phase and that a considerable amount of mercury will remain unreacted. If, on the other hand, an attempt is made to use too little mercury there may not be enough to wet the surface of the alloy particles and produce sufficient matrix phase material to

bind the unreacted γ cores together. This would result in porosity being present in the set material.

Various methods of dispensation are available. The most accurate method is to weigh the mercury and alloy components using a balance. This method is rarely used however, and both are commonly proportioned using volume dispensers.

The simplest type of volume dispenser consists of a glass bottle with a plastic, screw-top cap. The cap has a spring-loaded plunger which releases a known volume of either mercury or alloy when depressed. This method of dispensation is relatively accurate and reproducible for mercury but less so for the powdered alloys since the amount of alloy released depends on the way in which the particles are packed together in the container.

An alternative method of dispensation for the alloy is preproportioned as a powder in a small sachet or envelope or as a tablet in which the powder particles are compressed together. Mixing involves the use of either the contents of one envelope or one tablet with a given volume of mercury.

Perhaps the most commonly used method of dispensation involves the use of semi-automatic dispensers which also carry out the mixing or trituration. These devices typically have two hoppers. One is filled with alloy, the other with mercury. The alloy/mercury ratio can be set by the operator and the required amount of each component is released into a mixing chamber on the throw of a switch or the press of a button.

Another convenient method of dispensation involves the use of encapsulated materials. Each capsule contains both alloy and mercury in proportions which have been determined by the manufacturer. The two components are initially separated by an impermeable membrane which is readily shattered using a purpose-built capsule press or on starting to vibrate the capsule in a mechanical mixer. The capsules are similar to those for some dental cements (Fig. 24.2) and are mixed using devices such as that shown in Fig. 24.3. Capsules which do not require the use of a press are called self-activating capsules.

Trituration: The mixing or trituration of amalgam may be carried out by hand, using a mortar and pestle, or in an electrically powered machine which vibrates a capsule containing the mercury and alloy.

For hand trituration, a glass mortar and pestle with roughened surfaces are normally used. A low alloy/mercury ratio (around 5 : 8) is often required to produce a workable mix and care must be taken not to use excessive pressure during trituration in order to prevent splintering of alloy particles which may change the character of the mix.

The trituration time may have an effect on the properties of the final set amalgam. Some products require at least 40 seconds trituration in order to achieve full ‘wetting’ of alloy particles by mercury and optimal properties in the amalgam. Following trituration it is necessary to reduce the mercury content of the mix before condensing. This is normally done by placing the amalgam into a strip of gauze or chamois leather and squeezing to express excess mercury which appears as droplets on the outside.

Trituration by hand is not extensively practised in developed countries nowadays. Mechanical mixing is far more widely used. There are three levels of sophistication which may be employed. Following proportioning, the mercury and alloy may be placed in a capsule which is vibrated on a purpose-built machine, often referred to as an amalgamator. Alternatively, mechanical mixing in a semi-automatic machine, which also proportions mercury and alloy, is possible. The use of encapsulated, preproportioned materials is probably the most convenient, although also the most expensive option. For all three options, trituration times of 5–20 seconds are normal. The trituration time will vary according to the nature of the alloy and the alloy : mercury ratio. For uncapsulated products the mercury and alloy are separated by a diaphragm which commonly has to be broken mechanically before the material can be triturated. Manufacturer’s instructions should be followed at all times for length of trituration.

The advantages of mechanical trituration are as follows.

(1)A uniform and reproducible mix is produced.

(2)A shorter trituration time can be used.

(3)A greater alloy/mercury ration can be used.

This negates the requirement to express excess mercury before condensing. Encapsulated materials have the extra advantage that they are proportioned by the manufacturer.

Another potential advantage of using encapsulated materials is that they may help to reduce the

risk of atmospheric mercury contamination. In order for this potential advantage to be realized it is essential that the capsules do not release mercury during trituration. If the capsule is not properly sealed, considerable amounts of mercury may escape as the temperature within the capsule increases during mechanical mixing. A further precaution is recommended when trituration is complete and the capsule is opened. The contents remain warm at this stage and the capsule should be opened away from the face in well-ventilated conditions.

Condensation: Following trituration, the material is packed or condensed into the prepared cavity. A variety of methods have been suggested to condense amalgam including ultrasonic vibration and mechanical condensing tools. The mechanical tools apply quite high loads with a reasonably large amplitude of movement of the condensing tool. As a consequence they may be associated with damage to teeth, notably cuspal fracture during condensation.

Ultrasonic condensers tend to produce local heating of the amalgam with detrimental effects both in terms of mercury vapour release and modification in the setting reaction of the material.

The most widely used method of condensation is with a hand instrument called an amalgam condenser. These are flat-ended and come in a variety of styles. The shape and size of the condenser should be chosen with the size of the cavity in mind. The condenser must be able to fit within the cavity outline and should be able to get reasonably close to the peripheral margin of the restoration. This can be a problem with boxes on the surface of a tooth as a large round condenser will not pack the amalgam well into the box walls close to the matrix. It is often better to use a smaller diameter round condenser or an ovoid instrument to facilitate this first stage of packing. The amalgam is packed in increments, each increment being equivalent to the volume of material which can be carried in an amalgam ‘gun’. This is the device used to transfer the material from the mixing vessel to the prepared cavity. During condensation, a fluid, mercury-rich layer is formed on the surface of each incremental layer. The cavity is overfilled and the mercury-rich layer carved away from the surface. This effectively reduces the mercury content of the filling thus improving its mechanical properties.

The technique chosen for condensation must ensure the following.

(1)Adequate adaptation of the material to all parts of the cavity base and walls.

(2)Good bonding between the incremental layers of amalgam.

(3)Optimal mechanical properties in the set amalgam by minimizing porosity and achieving a final mercury content of 44–48%.

There should be a minimal time delay between trituration and condensation. If condensation is commenced too late, the amalgam will have achieved a certain degree of set and adaptation, bonding of increments and final mechanical properties are all adversely affected,

There is good correlation between the quality of an amalgam restoration and the energy expended by the operator who condenses it. For lathe-cut amalgam alloys best results are achieved by using a high condensing force at a rapid condensing frequency and continuing to condense until the amalgam feels hard and a mercury-rich layer has been formed at the surface. Amalgams made from spherical alloy particles have very different condensation characteristics to their lathecut counter parts. They require lower condensation pressures to achieve the same degree of homogeneity and physical strength. Indeed, there is a risk when condensing spherical alloys of using too great a condensation pressure. If this occurs the alloy particles roll over each other and are displaced by the condenser rather than being condensed into an homogenous mass. Recently some manufacturers have started to sand blast the spherical alloy particles. This has the effect of giving the material a similar handling characteristic to lathe-cut alloys but still retaining their ease of condensing with low pressure.

Carving: The objectives of carving an amalgam restoration are to remove the mercury-rich layer on the amalgam surface and to rebuild the anatomy of the tooth, re-establishing contact with the opposing dentition. Obviously a knowledge of normal tooth anatomy is necessary for this purpose.

Soon after condensing the amalgam, the surface layer, which is rich in mercury, is carved away with a sharp instrument. Carving should be carried out when the material has reached a certain degree of set. If attempts are made to carve too soon there

is a danger of ‘dragging out’ significant amounts of material from the surface. If carving is delayed too long the material may become too hard to carve and there is a danger of chipping at the margins. It is useful to try to retain a mental picture of the extent of the cavity when carving. The amalgam needs to be cut back to the cavity margins. If this is not done, long fine-tapered extensions of amalgam will lie on top of the enamel surface. These will be poorly supported and will fracture rapidly if under occlusal load producing a positive margin (the amalgam stands proud of the tooth structure).

It will be necessary to check the pattern of occlusal contacts whilst carving a restoration. When the amalgam is still soft, rubbing the surface with cotton wool produces a matt finish, if the subject then gently taps their teeth together or moves them from side to side in contact, any areas of contact will show up as bright burnished spots and can be removed/reduced as required. If carving is not complete by the time that the amalgam has become hard, occlusal contacts need to be marked using thin articulating tape. High areas can be removed using steel instruments or ultimately a bur, usually a steel bur in a slow hand piece.

Spherical amalgams are easier to carve than lathe-cut materials and fine-grain products easier than coarse-grain.

Removal of the matrix: If a matrix has been used to help form a large amalgam restoration this should be removed during the carving process. The amalgam must be sufficiently set that removal of the band will not result in bulk failure of the restoration. Equally, the materials should be sufficiently soft that any marginal excesses can be removed easily once the band has been removed. It is important to check that excess amalgam has not been forced beyond the matrix band gingivaly during condensation. This would otherwise result in a marginal ledge formation which could lead to periodontal disease as a consequence of inadequate cleansibility.

Polishing: Polishing is carried out in order to achieve a lustrous surface having a more acceptable appearance and better corrosion resistance. The fillings should not be polished until the material has achieved a certain level of mechanical strength, otherwise there is a danger of fracture, particularly at the margins. The strength which

should be attained before polishing is commenced is not certain but many products require a delay of 24 hours between placing and polishing. It is claimed that some faster setting materials can be polished soon after placing.

Gross irregularities in the surface are reduced using multi-bladed steel burs in a slow hand piece. This stage should result in a smooth surface contour. Fine polishing to produce a lustre is then undertaken using graded abrasives, either flours of pumice followed by zinc or ceric oxides with water as a carrier or by using abrasive impregnated rubber points and wheels. Pastes need to be applied using a rubber cup or brush, but are less frequently used now that impregnated points are available. One beneficial effect of using a rubber carrier for the abrasive (either a cup or a point) is

to burnish the surface of the restoration, improving its marginal adaptation. Care is required when using impregnated rubber points as the amalgam can be heated significantly as a result of friction between the material and the rotating point. They should be used with intermittant contact pressures rather than continuous loods.

21.7 Suggested further reading

Eley, B.M. (1997) The future of dental amalgam: A review of the literature. Br. Dent. J. 182, 7 parts.

Horsted-Bindlsev, P. (2004) Amalgam toxicity – environmental and occupational hazards. J. Dent. 32, 359.

ISO 1559 Alloys for dental amalgam.

Jones, D.W. (1993) The enigma of amalgam in dentistry. J. Can. Dent. Assoc. 59, 155.