14.2 Amalgam & Metal Restorations
Key Takeaways
- Dental amalgam is a metallic alloy formed by reacting a powder of alloy (mainly silver–tin, with copper and zinc) with liquid mercury; the setting reaction forms the γ₂-eliminating high-copper amalgam matrix (Cu₆Sn₅ η-phase traps the tin).
- High-copper amalgams (≥6% copper) eliminate the γ₂ (Sn₇Hg) phase, giving higher strength and corrosion resistance than low-copper amalgams.
- Amalgam phases: γ (Ag₃Sn, the alloy powder), γ₁ (Ag₂Hg₃, matrix), γ₂ (Sn₇Hg, the weak corrosion-prone phase).
- Mechanical retention (undercuts, locks, grooves) is required because amalgam does not bond to tooth — cavity design follows principles of retention and resistance form.
- Modern amalgams are capped condensable spherical or admixed alloys; in Great Britain the assimilated Regulation (EU) 2017/852 bars amalgam for under-15s and pregnant or breastfeeding women unless strictly necessary, while the EU ban on general use (Regulation (EU) 2024/1849, from 1 January 2025) does not apply in Great Britain and is derogated in Northern Ireland until 31 December 2034.
What Is Dental Amalgam?
Dental amalgam is formed by triturating (mixing) a powder alloy with liquid mercury to produce a plastic mass that is condensed into the cavity and hardens by a setting reaction. The powder is principally silver–tin (the γ phase, Ag₃Sn), with copper and small amounts of zinc.
The Setting Reaction and Phases
In the low-copper amalgam reaction:
- Ag₃Sn (γ, powder) + Hg → Ag₂Hg₃ (γ₁, matrix) + Sn₇Hg (γ₂, weak, corrosion-prone) + unreacted γ.
The γ₂ phase (Sn₇Hg) is the weakest and most corrosion-prone phase; it is responsible for the lower strength and corrosion of low-copper amalgams.
High-copper amalgams (≥6% copper, now standard) react so that tin reacts with copper instead of mercury, forming Cu₆Sn₅ (η-phase) and Cu₃Sn (ε-phase). This eliminates the γ₂ phase, giving greater strength and corrosion resistance.
| Phase | Formula | Origin | Property |
|---|---|---|---|
| γ (gamma) | Ag₃Sn | Alloy powder | Strong, the unreacted core |
| γ₁ (gamma-1) | Ag₂Hg₃ | Matrix | Hard, forms the body |
| γ₂ (gamma-2) | Sn₇Hg | Low-Cu only | Weak, corrodes — eliminated in high-Cu |
| η (eta) | Cu₆Sn₅ | High-Cu reaction | Traps tin, hard, corrosion-resistant |
Alloy Types and Particle Shape
- Lathe-cut — filings cut from an ingot; irregular, give a 'scratchy' condensation feel.
- Spherical — atomised droplets; condense easily, higher early strength, less mercury (≤43%).
- Admixed (blended) — mix of lathe-cut and spherical; combines strength and handling; the common modern choice.
Spherical alloys need lower mercury content and condense readily; admixed alloys pack better against matrix bands and are preferred for pins.
Mechanical Properties and Corrosion
| Property | Amalgam | Note |
|---|---|---|
| Compressive strength | ~300–400 MPa at 24 h | High — suited to load-bearing Class I/II |
| Tensile strength | Low (~50 MPa) | Brittle; needs bulk and mechanical retention |
| Creep | Low in high-Cu | Creep correlates with marginal fracture |
| Corrosion | γ₂ phase corrodes (low-Cu); high-Cu resists | Corrosion products can seal margins but weaken the restoration |
Marginal breakdown (Ditching) results from creep and corrosion at the margin; high-copper, low-creep alloys reduce it.
Cavity Design — Retention and Resistance Form
Because amalgam does not bond to tooth, it needs mechanical retention. Classical cavity design (Black's principles, modified for modern practice) provides:
- Retention form — features that resist dislodgement: undercut walls, dovetails, pins, grooves, locks, and the isthmus width.
- Resistance form — features that resist fracture under load: adequate bulk (≥1 mm at the isthmus, often 1.5 mm), flat floors, rounded angles (stress concentrators cause fracture), and a 90° cavosurface margin (butt joint).
Specific Design Points
| Class | Retention/resistance features |
|---|---|
| Class I | Depth 1.5–2 mm, flat floor, walls slightly divergent occlusally; isthmus not too wide |
| Class II | Proximal box with gingival floor in dentine (~1 mm axial), gingival wall at 90° to axial wall, isthmus 1.5 mm wide |
| Class V | Undercut at occlusal and gingival walls; cavity shallow to avoid pulp |
Sharp internal angles concentrate stress and cause fracture; rounded internal line angles are a hallmark of amalgam design.
Handling and Clinical Technique
- Mercury content — modern capsules are pre-proportioned (~42–43% mercury in the final mix for spherical). Trituration time affects consistency and properties.
- Condensation — condense each increment with sufficient pressure to adapt to walls and express mercury-rich matrix to the surface; under-condensation weakens the restoration.
- Carving — carve to anatomy after initial set; do not over-carve margins (thins them → fracture). Overhangs must be removed.
- Polishing — final polish 24 h later reduces corrosion and plaque retention; over-polishing generates heat and mercury vapour.
The Mercury Question and Regulation
The Minamata Convention on Mercury (agreed 2013, in force 2017) commits parties to phasing down dental amalgam. In Great Britain, the assimilated Regulation (EU) 2017/852 still applies and prohibits amalgam for:
- Children under 15 (deciduous teeth, with limited exceptions).
- Pregnant or breastfeeding women, except where strictly medically necessary.
Dental teams must use amalgam separators (from 2019) and follow mercury hygiene. The EU ban extends to a full prohibition on amalgam use from 1 January 2026 (amended 2024), with limited derogations. UK practice follows this direction; the MHRA and BDA advise reducing use and using alternatives where clinically appropriate.
Mercury Hygiene
- Pre-capsulated alloy only (no bulk mercury handling).
- Rubber dam, high-volume evacuation, amalgam separator.
- Avoid polishing with dry stones; collect and store scrap amalgam for disposal.
Alternatives
- Direct composites — bonded, aesthetic, mercury-free; technique-sensitive.
- Glass ionomer / resin-modified GIC — chemically bond and release fluoride; for low-stress and liners.
- Cast gold / ceramic inlays/onlays — high strength/wear resistance, indirect.
In a high-copper dental amalgam, which phase is responsible for eliminating the corrosion-prone γ₂ (Sn₇Hg) phase?
Which cavity design feature most directly provides resistance form for a Class II amalgam restoration against fracture under occlusal load?
Under the mercury regulation that still applies in Great Britain, for which patient group is dental amalgam prohibited except where the dentist judges it strictly necessary?