Amalgam Restorations: Indications and Technique Explained

Dental amalgam is made by mixing a powdered metal alloy (mostly silver, tin and copper) with liquid mercury, which binds the powder into a strong mass that hardens in the tooth. Used for over 150 years, it is the classic silver-coloured filling for back teeth.

Amalgam still matters: it tolerates slight moisture, withstands heavy chewing, lasts 15 years or more, and costs far less than alternatives. Every dentist must know its indications, technique and safe handling — even as tooth-coloured materials take over front teeth.

Key Concepts

• Alloy composition: modern high-copper alloys contain 12–30 percent copper, eliminating the weak, corrosion-prone gamma-2 (tin-mercury) phase and greatly improving durability.
• Setting phases: unreacted alloy particles (gamma) locked in a silver-mercury matrix (gamma-1) — the interlocked structure behind amalgam's compressive strength.
• Trituration: mechanical mixing of alloy and mercury in a capsule, producing a workable mass in seconds.
• Condensation: packing amalgam in increments with firm pressure, squeezing out excess mercury and adapting it to every wall.
• Carving and burnishing: shaping chewing anatomy before hardening, then smoothing the surface against corrosion and plaque.
• Mercury hygiene: pre-proportioned capsules, no skin contact, good ventilation and amalgam separators — the rules making amalgam use acceptable.

How It Works

The cavity is prepared with 90-degree cavosurface margins, a dovetail for retention and adequate depth. A metal matrix band with a wooden wedge rebuilds the missing wall. After trituration, amalgam is placed in small increments, each condensed firmly — small condensers into corners first, then larger ones. Before hardening, the dentist carves the anatomy, burnishes, checks the bite and adjusts high spots. Polishing waits about 24 hours, since early polishing weakens the maturing surface.

Worked Example

Alloy-to-mercury ratio. Capsules mix at about 1:1 by weight: 500 mg of alloy powder + 500 mg of mercury = 1000 mg of mix, with mercury fraction 500 ÷ 1000 = 0.50 (50 percent). A large molar filling needing two capsules therefore contains 2 × 500 = 1000 mg (1 g) of mercury bound into the set filling. Once set, mercury is locked into the gamma and gamma-1 phases — intact amalgam releases only traces, while drilling one out without suction releases far more.

Common Misconceptions

• Fillings poison patients with mercury. In set amalgam mercury is chemically bound and exposure sits far below safety thresholds; the Minamata Convention phases amalgam down for environmental reasons.
• Amalgam expands and cracks teeth. Properly condensed high-copper amalgam does not expand destructively; slight marginal corrosion products actually help seal it.
• Amalgam is obsolete and untaught. It remains in every curriculum for its durability under load and its role in public-health dentistry.

Real-World Applications

• Large load-bearing molar fillings where strength matters more than colour.
• Patients with poor moisture control, where amalgam's tolerance beats technique-sensitive alternatives.
• Low-resource settings, where low cost and long life stretch limited budgets.

Amalgam earned its 150-year career by surviving in mouths. Its indications are narrower now and mercury hygiene is non-negotiable, but for a big back-tooth cavity in a heavy chewer it remains one of dentistry's most durable answers.