Dental Amalgam Restorations

Composites, GIC (Glass Ionomer Cements), and Amalgams

Mercury (Hg) is the only metal that is liquid at normal temperature and atmospheric pressure and can dissolve other metals at room temperature.

A dental amalgam is any mixture, or blending, of mercury with another metal or alloy.

Dental amalgam is primarily based on the Ag-Hg-Sn (silver, mercury, tin) system. Other metals like copper (increases strength) and zinc (reduces oxidation) can be added to modify its properties.

The liquid part is triple distilled Hg, and the powder is an alloy based on the intermetallic compound Ag3Sn, known as the gamma phase.

Melt components at high temperature in a reducing atmosphere to produce Ag3Sn, though silver, copper, and tin can oxidize easily.

A powder form.

Lathe cut: cooled down/mechanically grinded and Spherical: atomisation in an inert atmosphere.

Mixing particles increases packing efficiency, reduces the amount of Hg needed, and increases overall performance.

Vigorous mechanical mixing (trituration).

Initial dissolution: the outer surfaces of the silver/tin particles dissolve in the liquid mercury: Ag3Sn + Hg --> 3Ag + Sn + Hg

Formation of γ1: the silver reacts quickly to form Ag2Hg3 grains which stick preferentially along the alloy particles: 2Ag + 3Hg --> Ag2Hg3

Formation of γ2: the tin reacts slowly to form γ2 which is randomly distributed inside the γ1 matrix: 7Sn + Hg --> Sn7Hg

A set amalgam, the reaction is completely set when the γ1 and γ2 phases have formed a solid matrix and no mercury is left to dissolve γ.

Assuming that the amalgam has been correctly mixed, the tensile strength would be: γ > amalgam > γ1 > γ2. γ2 is the weakest phase so reducing it will increase the strength of the restoration

It takes up to 24 hours so it develops slowly and the amalgam remains weak at the time the patient leaves the surgery so it would be good practice to recall the patient after a week to refine the edges of the restoration and polish.

The mechanical properties of an amalgam restoration are similar to those of natural tooth substance.

New ISO standards require amalgam restoration material to not exceed contractions more than 0.1% and expansion by more than 0.2%.

High thermal diffusivity. This can result in thermal shock so you need to protect the base of the large cavity to avoid harmful effect on the pulp; Thermal expansion mismatch can cause microleakage which can increase occurrence of decay into dentine surrounding amalgam

Flow, or deformation, happens due to applied load over a long period of time and does not recover. The higher the content of γ2, the higher the creep. Creep causes protrusion --> weak unsupported edges--> fracture of the edges--> ditching

They were introduced in an attempt to eliminate the γ2 phase because a γ2-free amalgam shows increased strength with reduced corrosion and creep; There is a higher compressive strength and a faster achievement of final strength.

1. Cavity preparation & pulp protection; 2) Rubber dam, matrix and wedges application; 3) Amalgam trituration; 4) Amalgam application (small quantity); 5) Condensation; 6) Repeat 4 & 5 until cavity is filled (in slight excess); 7) Carving and burnishing; 8) Removing matrix; 9) Refine carving in particular at the edges of restoration; 10) Finishing & Polishing

Avoid leaving unsupported enamel; The lateral outer portion of the cavity can be used to create retentive structure which can help avoid creep and overhang; Create rounded shapes towards the centre of the cavity to avoid cracks from chewing forces; Consider support of residual enamel structure and proximity to pulp

It's to use capsules to ensure that the manufacturer's recommended ratio is used

To ensure both adequate amalgamation and the production of a plastic mix suitable for packing.

It depends on both the alloy and the mixing system (between 5 and 20 seconds); Under trituration does not allow adequate formation of γ1 and η phases and results in a crumbly mix; Over-trituration results in excessive contraction.

The objectives are to: Remove excess Hg; Ensure that there are no voids in the restoration; Ensure marginal integrity.

Must be carried out immediately after mixing; Condense small increments; Overfill the cavity because the surface layers tend to be richer in Hg; Spherical amalgams require a lower condensation pressure than lathe cut

Carving of the restoration is possible for up to 2-3 minutes depending on the amalgam; Spherical restorations produce a better surface than lathe cut; Avoid too much pressure- friction heating can cause the release of Hg; A smooth surface minimises the possibility of corrosion

No adhesion to tooth substance; Need retentive cavity design- loss of sound tooth substance; Poor aesthetics; Galvanic effects; High thermal diffusivity

Environmental: Hg is a polluting agent; Health: Mercury vapours are released in small amounts from dental amalgams . High levels of Hg vapours are associated with adverse effects in kidneys and brain FDA has reviewed scientific evidences and determined that amalgam restorations are safe for adults and children 6>

The Minimata Treaty advocates a phase-down of the use of dental amalgam, in line with the domestic circumstances of each country and in tandem with recommendations for prevention programmes and increased research into alternative materials. The regulation does not ban the use of amalgam.

Hg levels during removal of amalgams exceed governments' safety limit

Avoid removing good congruous restorations; Use rubber dam on patient to avoid ingestion and inhalation of Hg microparticles; Use appropriate PPE and control aerosol

Treatment of deciduous teeth; Children under 15; Pregnant or breastfeeding women.