17.1 Dental Amalgam: Metallurgy and UK Regulation

Dental Amalgam: Metallurgy, Setting Reactions & Matrix Systems

Dental amalgam is an alloy created by triturating liquid elemental mercury with solid alloy powder particles containing silver, tin, copper, and zinc.

                                    Dental Amalgam Setting Reactions
                                                   │
         ┌─────────────────────────────────────────┴─────────────────────────────────────────┐
         ▼                                                                                   ▼
Traditional Low-Copper Amalgam                                              Modern High-Copper Amalgam
• γ (Ag₃Sn) + Hg                                                            • Powder contains 12% to 30% Copper
• Products:                                                                 • Eliminates the γ₂ (Sn₇₋₈Hg) phase!
  - γ₁ (Ag₂Hg₃) - Matrix phase                                              • Cu reacts with Sn: forms η phase (Cu₆Sn₅)
  - γ₂ (Sn₇₋₈Hg) - WEAKEST, HIGH CORROSION & CREEP                          • Superior compressive strength, low creep

Elemental Composition & Functions

• Silver (Ag, 40% to 70%): Increases compressive strength, increases setting expansion, and enhances resistance to tarnish.
• Tin (Sn, 15% to 30%): Facilitates amalgamation with mercury, retards setting time, and causes setting contraction.
• Copper (Cu, 12% to 30% in high-copper alloys): Dramatically increases compressive and tensile strength, hardness, and corrosion resistance.
• Zinc (Zn, 0% to 2%): Acts as a chemical deoxidizer during manufacturing, scavenging oxygen to prevent alloy oxidation.

[!CAUTION] Clinical Trap — Zinc Delayed Expansion: If a zinc-containing amalgam is contaminated with moisture (saliva, blood, or sweat) during condensation, an electrolytic reaction occurs: $\mathrm{Zn + H_2O \longrightarrow ZnO + H_2 \uparrow}$ The liberated hydrogen gas becomes trapped within the amalgam mass, developing severe internal gas pressures exceeding 14–16 MPa. This causes delayed expansion (occurring 3 to 10 days post-operatively), manifesting as restoration extrusion beyond cavity margins, severe post-operative pulpal pain, and cuspal fracture. Non-zinc alloys or absolute moisture control with rubber dam eliminates this risk.

Elimination of the Corrosive Gamma-2 ($\gamma_2$) Phase

1. Traditional Low-Copper Setting Reaction:

$\mathrm{Ag_3Sn\,(\gamma) + Hg \longrightarrow Ag_3Sn\,(\gamma) + Ag_2Hg_3\,(\gamma_1) + Sn_{7-8}Hg\,(\gamma_2)}$

• The $\gamma_2$ ($Sn_{7-8}Hg$) phase is the weakest, softest, and most chemically unstable metallurgical component. It undergoes extensive electrochemical corrosion in oral fluids, releasing free mercury and tin salts, and exhibits severe creep (slow, permanent plastic deformation under masticatory loads), resulting in unsupported restoration margins, ditching, and catastrophic secondary caries.

1. Modern High-Copper Setting Reaction (>12% Cu):
   • High-copper alloys (admixed spherical/lathe-cut or single-composition spherical particles) contain high levels of copper that preferentially bind tin, completely eliminating the $\gamma_2$ phase:

$\mathrm{Sn_{7-8}Hg\,(\gamma_2) + Ag\text{-}Cu \longrightarrow Cu_6Sn_5\,(\eta) + Ag_2Hg_3\,(\gamma_1)}$

• The newly formed $\eta$ phase ($Cu_6Sn_5$) is hard, strong, and highly corrosion-resistant. High-copper amalgams show vastly superior marginal integrity, negligible creep (<0.1%), and exceptional clinical longevity.

Condensation, Carving & Matrix Systems

• Condensation: Incremental placement using smooth or serrated amalgam pluggers with firm axial and lateral pressure ($30\text{ to }40\text{ N}$) to adapt the alloy to cavity line angles, eliminate voids, and force excess mercury-rich matrix ($\gamma_1$) to the cavosurface margin for carving removal.
• Burnishing & Carving: Pre-carve burnishing densifies margins; anatomical carving with hollenback or discoid-cleoid carvers reproduces occlusal morphology without overcarving (which leaves thin, weak feather-edges);
• Matrix Systems:
  • Tofflemire Matrix: Circumferential steel band; available in universal straight or contra-angle designs (for lingual positioning). Best suited for Class II two-surface restorations.
  • Siqveland Matrix: Cylindrical, adjustable circumferential matrix system that allows tapering to accommodate cervical tooth convergence. Paired with anatomical wooden wedges placed from the wider embrasure (typically lingual) to compress the band against the gingival seat, achieve separation for tight proximal contacts, and prevent gingival amalgam overhangs.

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The Minamata Convention on Mercury: UK & EU Regulatory Mandates

The United Nations Environment Programme (UNEP) Minamata Convention on Mercury (ratified in the UK and enforced via EU Regulation 2017/852, retained in UK Law) aims to protect the global environment from anthropogenic mercury releases.

                         UK Legal Restrictions on Dental Amalgam
                                            │
       ┌────────────────────────────────────┼────────────────────────────────────┐
       ▼                                    ▼                                    ▼
Deciduous Teeth                     Children Under 15                    Pregnant & Breastfeeding
• ABSOLUTELY PROHIBITED             • ABSOLUTELY PROHIBITED              • ABSOLUTELY PROHIBITED
• Use GIC, RMGIC, or Composite      • Use GIC, RMGIC, or Composite       • Use GIC, RMGIC, or Composite
• Only exception: strict medical    • Only exception: strict medical     • Only exception: strict medical
  necessity justified in notes        necessity justified in notes         necessity justified in notes

Mandatory UK Clinical & Environmental Rules

1. High-Risk Demographic Bans: Dental amalgam shall not be used in:
   • The dental treatment of deciduous (primary) teeth;
   • Children under 15 years of age;
   • Pregnant or breastfeeding women.
   • Sole Legal Exception: Amalgam may be used only when deemed strictly necessary by the dental practitioner based on the specific medical needs of the patient (which must be comprehensively documented in the clinical notes).
2. Environmental Clinical Engineering:
   • Dental facilities utilizing amalgam or removing existing amalgam restorations must be equipped with amalgam separators installed in the suction line, achieving a minimum 95% efficiency in capturing amalgam particles.
   • The use of free bulk liquid mercury is prohibited; only pre-dosed, hermetically sealed encapsulated amalgam capsules are legally permitted.
   • Complete phase-out timelines are actively advancing across the UK NHS and European dental frameworks.

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Amalgam in Contemporary UK Practice

Candidates should be able to state what the current restrictions actually are rather than describing amalgam as banned. Use in deciduous teeth, in children under 15 and in pregnant or breastfeeding women is prohibited except where the clinician deems it strictly necessary on grounds of specific medical need, and that judgement must be recorded. Amalgam separators are mandatory, encapsulated amalgam must be used, and waste amalgam, including extracted teeth containing amalgam, is hazardous waste requiring segregated disposal.