Application of zinc-phosphate cements in combination with amalgams
Although modern dentistry offers a wide range of restorative materials, classic techniques still require the use of zinc-phosphate cements. Their main purpose when working with metal fillings is to create a reliable barrier between the dental tissues and the restorative material.
For example, when using copper amalgam, zinc-phosphate cement is used as a proper insulating base. This eliminates or significantly mitigates the two main disadvantages of amalgam fillings:
- High thermal conductivity, which can cause pulp irritation during temperature fluctuations.
- Discoloration of hard dental tissues to a dark shade due to the penetration of metal oxides.
Glass-ionomer cements (GIC)
In addition to zinc-phosphate materials, chemical-, dual-, and triple-cure glass-ionomer cements are widely used for luting and lining.
Examples of chemical-cure materials:
- Fuji I GC — used for luting prosthetic restorations. Available in a capsule system that requires a special dispenser (applicator gun) for activation and placement.
- GC Lining Cement — used for creating insulating bases. Supplied in a traditional powder-liquid format (hand-mixed with a plastic spatula on a mixing pad).
- GC Miracle Mix — another chemical-cure GIC option from GC.
Complex-cure materials: Among 3M ESPE products, Vitremer stands out as a triple-cure glass ionomer. Its setting is achieved through both a classic chemical reaction and light-curing.
Indications for Vitremer: filling of carious cavities (Classes I, II, III, and V), treatment of patients with poor oral hygiene, management of non-carious lesions, and filling of primary teeth.
Amalgams: classification and setting mechanism
Amalgam is an alloy of mercury with one or more metals (silver, tin, copper, zinc). The process of mixing them is called amalgamation, resulting in a plastic mass that subsequently hardens. Setting is driven by a chemical reaction between mercury and metals, forming intermetallic compounds.
Composition-based classification:
- Simple amalgams — contain 2 components.
- Complex amalgams — contain 3 or more components.
Based on the base metal, silver and copper amalgams are distinguished.
Characteristics of copper amalgam
Copper amalgam is a solution of copper in liquid mercury. It is supplied in the form of small pressed tablets (squares).
Composition:
- Mercury (59–66%) — solvent for the other metals.
- Copper (32–37%) — base metal. To ensure a better reaction with mercury, it is obtained by electrolytic precipitation from a copper sulfate solution.
- Zinc (2–4%) — reduces filling compressibility, improves mercury binding, and stabilizes color.
Material properties: Advantages include high plasticity, significant strength, and dimensional stability (the filling does not pull away from cavity margins). Disadvantages include lack of adhesion to cavity walls, dark discoloration of dental tissues, high thermal conductivity, slow setting, poor chemical resistance in the cervical area, and the ability to cause corrosion of gold crowns due to mercury release.
Preparation of amalgam for use
The preparation of copper amalgam requires strict adherence to temperature protocols and safety measures (working in a fume hood, using an enameled tray):
- 2–3 tablets are heated in a special spoon over a burner at 240-260 °C. The mercury expands during this process and breaks up the mass.
- After 5–10 seconds, droplets of mercury appear on the surface.
- The mass is transferred to a glass mortar (kept on the table, not held in hand) and vigorously triturated with a pestle for about 2 minutes until uniform.
- The finished mass is thoroughly washed with alkaline water (5–6 drops of ammonia water or 0.5 g of soda per cup of water) to remove oxides.
- Excess mercury is squeezed out through gauze or chamois leather to ensure high filling quality.
Caution: Overheated amalgam disintegrates and sets slowly, whereas underheated amalgam remains hard and cannot be triturated.
Briefly: answers to topical questions
What is the chemical composition of the powder and liquid of zinc-phosphate cements?
The powder of zinc-phosphate cements is a product of finely grinding a sintered mixture of oxides, consisting predominantly of zinc oxide with additions of silicon dioxide, magnesium oxide, calcium oxide, and aluminum oxide. The liquid is a syrupy aqueous solution of phosphoric acid containing modifiers in the form of zinc, aluminum, and magnesium oxide hydrates for partial acid neutralization.
- Zinc oxide (ZnO) — accounts for 75–90% of the powder composition, providing adhesiveness and plasticity.
- Silicon dioxide (SiO2) — accounts for 0.05–5%, imparting transparency and gloss.
- Magnesium oxide (MgO) — accounts for 5–13%, increasing strength and plasticity.
- Phosphoric acid (H3PO4) — accounts for 38–44% in the aqueous liquid solution.
Class and exam questions
Why is a zinc phosphate cement base placed under an amalgam?
It serves as an insulating base that prevents the hard dental tissues from staining dark and protects the pulp from temperature fluctuations caused by the high thermal conductivity of metal.
Why is zinc added to copper amalgam?
Zinc (2–4% in composition) is necessary to reduce filling compressibility, increase color stability, and improve mercury binding.
How to properly wash the finished amalgam mass?
The mass must be washed with alkaline water by adding 5–6 drops of ammonia water or 0.5 g of soda to one cup of water. This washes away metal oxides from the mass.
What happens if the temperature protocol is violated during amalgam heating?
If overheated, the mass will easily crumble into pieces, and its setting time will increase significantly. If underheated, the tablets will remain hard and impossible to triturate with a pestle.
What else to study on this topic
- Application features of Vitremer glass-ionomer cement in non-carious lesions
- Comparison of silver and copper amalgam
- Intermetallic compounds: chemical reactions during the setting of complex amalgams
- Protocol for working with capsule systems (using Fuji I GC as an example)
- Influence of the hygiene index on the choice of restorative material
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