Dental amalgam: a practical guide

Amalgam safety

Dental amalgam is a combination of metallic particles (predominantly silver and tin) and liquid elemental mercury. The resultant multi-phase alloy contains approximately 50% mercury and forms a solid at room temperature.³

Multiple international authorities recognize that dental amalgam is an effective restorative material for the general population, with low risk of adverse health effects.^4,5,6

The main health concern regarding dental amalgam relates to the risk of release into the environment where aquatic micro-organisms can convert elemental mercury to organic compounds. These may then enter the human food chain, for example by eating fish and other marine species contaminated with organic methylmercury, which is the most toxic and bio-accumulative form of mercury.⁴

In this way, dental amalgam can indirectly contribute to a human health risk from mercury. Globally, it is estimated that approximately two-thirds of the mercury content in dental amalgam is eventually released into the atmosphere, soil, surface and groundwater.⁴ Amalgam can therefore contribute to environmental pollution via the following routes:⁴

Since children, infants and developing fetuses have increased susceptibility to the toxic effects of mercury compounds, restrictions on the use of amalgam have been recently introduced as precautionary measures to avoid even the theoretical risk of harm.^4,5 By targeting children and pregnant and breastfeeding women, it is hoped that this will create future generations of amalgam-free patients.⁴

Although amalgam is a durable and highly effective restorative material with proven safety, from time to time, patients and/or dental team members may raise concerns, which may be addressed by referring to a wide range of evidence-based guidance documents, statements from which are summarized in Table 4.^4,5,6

Minamata

The Minamata convention was held in Japan in 2013 and resulted in a global treaty designed to protect the environment from mercury pollution, and reduce the risk to human health by limiting the trade and supply of mercury-containing products.⁷ The Minamata treaty was signed by 128 countries and came into force in August 2017. Table 5 lists the legally binding restrictions introduced in the UK in 2018–2019.^4,5,6

The new legislation includes the slightly confusing exception that amalgam may be used in the prohibited patient groups ‘where it is deemed strictly necessary by the dental practitioner based on the specific medical needs of the patient’. Supplemental guidance statements have been published to assist clinical decision making and are summarized as follows:

Where an individual clinician chooses to use dental amalgam in the best interests of the patient, the decision must be justified and communicated to the patient so that they can provide valid consent, which should be recorded along with the justification in the patient's clinical record.⁴ Examples of exceptional circumstances include:

In 2010 it was estimated that the European Union accounts for 20–30% of the global demand for mercury.⁴ Prior to the Minamata Convention and EU regulations, several European countries had already phased down or phased out the use of dental amalgam. For example, Sweden and Norway completely banned its use in 2009 and 2011, respectively, and Finland, Denmark and the Netherlands have phased down amalgam usage to 1–5% of restorations.⁴

The British Dental Association emphasize that both the Minamata treaty and related EU and UK regulations are purely for environmental protection and do not reflect any concerns about adverse effects of amalgam on human health and that the phase down of amalgam should occur in recognition that:⁶

Amalgam alternatives

As amalgam is phased down because of its mercury content, it is important to consider that alternative materials are far from inert.

Resin composite is the natural successor to dental amalgam. Although posterior composites have been demonstrated to have lower survival rates than amalgam restorations, contemporary resin composites are capable of providing high-quality, adhesively bonded, aesthetic restorations that last for decades.¹¹ They are especially effective in low caries risk patients and in clinical situations where moisture control can be optimized.⁴

Case selection

The clinical indications for amalgam restorations have decreased, largely due to improvements in composite materials and adhesive technology. Many practices now reserve amalgam for patients with financial limitations and/or low aesthetic demands, and for clinical situations where greater strength is required or where moisture control or other technical difficulties preclude the use of composite materials.

While the selection of restorative materials should always be delayed until cavity preparation is complete, in the author's opinion, alongside the legal restrictions listed above, there are now virtually no clinical indications for the use of amalgam in the treatment of primary carious lesions, for the restoration of any occlusal cavity, or for the restoration of any class of cavity in maxillary and mandibular premolars.

Pre-operative assessment

A patient presented with a fractured disto-lingual cusp on a previously restored (and repaired) mandibular right first permanent molar (Figure 2). Pre-operative assessment included:

Clinical photographs with articulating paper marks may be used as a reminder of the pre-operative occlusal scheme to help inform carving procedures.

Cavity preparation

The majority of amalgams placed worldwide are for the replacement of existing amalgam restorations.^9,12 The well-established principles of cavity preparation for amalgam are summarized in Table 6 and illustrated in Figure 3. ^1,2,3,10

As amalgam is not intrinsically adhesive and is weak in thin section, it may be necessary to sacrifice additional healthy tooth tissue to optimize the restoration's retention form (to occlusal forces) and resistance form (to lateral forces). Amalgam should be carefully cut into sections using copious water spray and high-volume aspiration in a well-ventilated room, as excess heat increases the release of mercury vapour, which can be inhaled and absorbed in the lungs. Rubber dam isolation may be used to further reduce this risk and that of inadvertent ingestion of amalgam debris.⁸

Specialized metal-cutting burs (eg tungsten carbide, Tri Hawk, Ontario, Canada) are advised for the easy sectioning of failed amalgam restorations. Protective interproximal wedges are recommended to reduce the risk of iatrogenic damage to adjacent teeth (eg FenderWedges, Directa, Upplands Väsby, Sweden).

Retention and resistance form

While rough, prepared cavity surfaces will help retain well-condensed amalgams, it may be necessary to enhance retention and resistance form by a variety of methods, which may be used individually or in combination (Table 7, Figure 4).

Retentive dentine pins were introduced over a century ago.² Their use is now considered by many to be historical because they are associated with a range of well-documented complications, such as crack initiation, pulpal or periodontal ligament perforation and incomplete seating (Figure 5).¹⁵

Isolation

While a rubber dam is the optimal method of moisture control, it is rarely used for restorative procedures,¹¹ and in some clinical situations equivalent restorative success has been demonstrated without its use.¹¹ A rubber dam can be challenging to secure around badly broken-down teeth, access to deep cavities can be limited without splitting the dam and occlusal assessment cannot be carried out until the dam is removed. However, rubber dam was used in this clinical case as:

As with class II resin composite restorations, the rubber dam retainer should be placed on a distal tooth, and in this example a space between dam holes was needed to account for a missing tooth, to reduce dam tension (Figure 6).

Matrix systems

Matrix and wedge techniques are very important factors governing the success of amalgam restorations and have the following aims:²

Traditional matrix systems (eg Siqveland and Tofflemire, introduced in 1937 and 1946, respectively) present a range of disadvantages including the tendency to impart open or light contact points, which are placed too far occlusally, have flat proximal contour and may be associated with suboptimal marginal contour/seals.¹¹

A superior matrix system that is considered indispensable by users is the AutoMatrix (DentsplySirona.USA), which confers the advantages listed in Table 8 and is demonstrated in Figure 7.

Wedges are needed to create a tight cervical seal that facilitates maximum condensation pressure. Wedging also improves soft tissue retraction and haemostasis and provides tooth separation that effectively negates the thickness of the matrix band. Wedges should be firmly inserted from the side that results in the best cervical seal.^2,11 Plastic FlexiWedges (Optident. Ilkley, Yorkshire, UK) are recommended as their concave gingival contour optimizes seating over the interdental papilla and reduces the risk of matrix deformation. Plastic lugs facilitate placement and removal.¹¹

Alloy selection

Dental amalgam is an alloy of mercury (approximately 50%) with other metals, such as tin, silver and copper. Alloy particles may be irregular (lathe cut), spherical or admix (combination).³ The author recommends Tytin (Kerr, Bioggio, Switzerland), a fast-setting spherical alloy with 42.5% initial mercury content.¹

Bonded amalgams

While traditional linings, varnishes and bases under amalgams may now be considered obsolete,¹⁶ adhesive bonding has been suggested as a method of enhancing resistance and retention form. The potential advantages of amalgam bonding are the subject of debate, but a range of studies has demonstrated its success over the years, which may be improved by combination with other cavity modifications.^2,14 The benefits reported by amalgam bonding proponents include:

The following clinical stages of the technique for amalgam bonding are illustrated in Figure 8:

In the author's experience, it is not necessary to coat the inside of the matrix with any separation medium. The use of a material such as petroleum jelly is occasionally cited, but as the resin cement will not stick to the shiny matrix, this is an unnecessary stage.

Trituration

Mixing time is highly influential on adaptation to cavity walls and floor and, therefore, on marginal seal and post-operative sensitivity.¹

It is reported that most clinical teams tend to over-mix amalgam, resulting in material that has poorer handling properties, will have reduced working time due to temperature increase, will exhibit greater expansion on setting and reduced restoration strength.^1,2,3 An under-mixed mass of amalgam will also be weaker and will display increased porosity, corrosion and surface roughness.^2,3

As all mixing machines vary, calibration is recommended for individual alloys using a test mix. Testing should begin at a reduced trituration time (25–50%) and/or by using a lower mixing-speed setting, where available.¹

An ideal amalgam mix should not be difficult to remove from the capsule, should feel warm, but not hot to touch and, if dropped from approximately 30 cm, should stay in one slightly flattened piece with a wet surface gloss. An under-mixed mass will crumble.^1,3

Once mixed, amalgam should be immediately transferred to the cavity using a specialized amalgam carrier. For larger cavities, whole or half increments of amalgam may be placed using tweezers, with great care if rubber dam isolation is not used.¹

Amalgam packing

Packing/condensing amalgam is a critical stage that aims to:^1,2

Condensation is reported to be an underdone stage, but may be improved using the following techniques (Figure 9):^1,2

Pre-carve burnishing

This is described as gentle rubbing directed from the amalgam towards the tooth. Burnishing may be considered an extension of the packing process that improves marginal adaptation, creates denser marginal amalgam and brings further mercury to the surface to be carved away.^1,2 Burnishing helps to reveal the cavity outline form and initiates the anatomical shaping process. Specialized amalgam burnishers are recommended (eg LM 31-32, LM Instruments, Finland) (Figure 10). Acorn burnishers may also be useful for smaller amalgam restorations.¹⁷

Amalgam carving

The reduction in teaching and practice with amalgam at undergraduate level increases the risk of accurate amalgam carving becoming a ‘lost art’.^9,18 This is unfortunate because precisely carved amalgams offer a range of advantages:

A variety of amalgam carving instruments may be used. For fast, accurate carving the author recommends a Frahm's carver (various suppliers) and refinements may then be made with a sharp half-Hollenback (various suppliers) and discoid-cleoid carvers (eg LM 731-732, LM Instruments, Finland). Mastering efficient anatomical amalgam carving has been demonstrated to be improved by detailed understanding of tooth anatomy, and by use of a systematic carving sequence (Table 9, Figure 11).^2,9,17,18

Amalgam carving (and composite shaping) skills have been shown to be significantly improved by repeated practice using wax carving simulation exercises.^9,18 By using these techniques, it is possible to learn how to quickly achieve anatomically shaped restorations that need minimal occlusal adjustment following dam removal, reducing the risk of catastrophic fractures during occlusal assessment.

Burnishing/smoothing

Gentle post-carve burnishing is also recommended to smooth restorations and maximize marginal adaptation.^1,2 Light pressure should be used to reduce the risk of marginal fracture. A PKT3 instrument (Peter K Thomas; various suppliers) is a useful instrument for burnishing axial amalgam margins and refining fissure patterns.

A dry cotton wool roll may be used to smooth the amalgam surface. When using strong, fast-setting amalgams (eg Tytin, Kerr, Bioggio, Switzerland), gentle smoothing may be carried out using a thick mix of pumice and glycerine, lightly applied with a rubber cup (Figure 13)

Polishing

Although polishing amalgams is considered a clinically unnecessary stage, polished amalgams will feel smooth, collect less plaque, be easier to clean and undergo less surface corrosion (Figure 14).^1,2

Once completely set (>48 hours) amalgams may be polished using pumice and glycerine on a polishing brush, followed by zinc oxide and white spirit on a rubber cup. Brown followed by green silicone polishing tips (Shofu, Japan) may also be used intermittently, with slow rotation and water coolant as dry polishing generates heat, which can release more mercury vapour than any other clinical stage, and may increase the risk of discomfort and long-term surface degeneration.^1,2 Even without this stage, amalgams often take on a polished appearance within 6 months of function.

Monitoring and repair

Optimizing cavity preparation, placement and carving of amalgams will result in restorations that perform as well or better than any other restorative material (Figure 15).^1,2

Over time, amalgams demonstrating signs of wear and tear are a very common finding. A wide range of publications has highlighted the challenges in accurately assessing existing restorations, especially when seeing a patient for the first time.^1,2,12 It is widely recognized that a significant number of amalgam restorations are unnecessarily replaced following an incorrect diagnosis of failure,¹⁰ as the difference between true secondary caries and a non-carious crevice at the restoration interface is difficult or impossible to detect clinically.¹² Explicit use of a standardized, universal criteria to decide whether to replace an existing restoration is still needed.¹²

It has been further demonstrated that accurate carving and/or re-polishing may reduce the risk of unnecessary restoration replacement.¹ Set amalgams may be recontoured using fine, multi-bladed tungsten carbide finishing burs and aged amalgams may be re-polished to refine margins and remove tarnish.

Although common practice, the routine replacement of restorations demonstrating early signs of failure has been shown to be highly flawed.¹ Furthermore, when failure does occur, restorations are often amenable to localized repair, increasing the functional longevity of the tooth/restoration complex.

Contemporary restorative management should favour repair over complete replacement. Amalgam may be used very successfully to repair existing amalgams. Retentive features (eg dovetail locks) may be prepared to compliment the chemical bond between new and old amalgam, which may be up to 60% as strong as a solid amalgam specimen (Figure 16).²