In this section, we shall discuss the composition, type and properties of the dental amalgams alloys used for restoration purpose in posterior teeth as per following format. The source of these notes, MCQs and explanation is Phillips’ Science of Dental Materials, 12th Ed and Sturtevant’s Art and Science of Operative Dentistry.
- Dental Amalgam alloys
- Clinical Manipulation of Amalgam for Restorations
- Properties of Amalgam
- Clinical Performance of Amalgam Restorations
- Safety of Amalgam Fillings
Amalgam—It is an alloy that contains mercury.
Amalgamation—It is the process of mixing liquid mercury with one or more metals or alloys to form an amalgam.
Creep—It is the deformation that is produced by a stress. The creep process can cause an amalgam restoration to extend out of the cavity preparation. Therefore, the creep increases a restoration’s susceptibility to marginal breakdown.
Delayed expansion—It is the slow expansion of a zinc-containing amalgam over a period of weeks to months. Delayed expansion is associated with the development of hydrogen gas, which is caused by the insertion of moisture in the plastic mass during its manipulation in a cavity preparation.
Dental amalgam—It is an alloy that is formed by reacting mercury with silver, copper, and tin. Dental amalgam may also contain palladium, zinc, and other elements to improve handling characteristics and clinical performance.
Dental amalgam alloy (alloy for dental amalgam)—It is an alloy of silver, copper, tin, and other elements that is manufactured in the form of powder particles or as a compressed pellet.
Marginal breakdown—It is the gradual fracture of the margin of a dental amalgam filling. It leads to the formation of gaps between the amalgam and the tooth.
Trituration—It is the mixing of amalgam alloy particles with mercury in a device called a triturator. The term is also used to describe the breaking of a solid to fine particles by grinding or friction.
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Dental Amalgam alloys
The silver amalgam alloy (dental amalgam alloy) is used to restore the original shape and size of a damaged tooth to restore its function. The typical composition of silver alloy in early days around 1980’s were as follows:
- Silver: 66.7% to 71.5% ,
- Tin: 24.3% to 27.6%
- Copper: 1.2% to 5.5%
- Zinc: 0% to 1.5%
- Mercury: 0% to 4.7%
After amalgamation, these alloys had mostly γ phase (Ag3Sn) with some ε phase (Cu3Sn).
Above mentioned dental amalgam alloy is classified as low copper alloy (conventional alloy). The other type is high copper alloy because the copper content which used to be not more than 5.5% earlier is now added as high as 13%. The major content of the alloys is silver and tin. In varying percentage, the Indium and Palladium are also added by some manufactures. The typical composition of few commercial amalgam alloys is shown below.
- Low copper lathe cut alloy- Ag: 70.3%, Sn: 25.9%, Cu: 2.8%, Zn: 0.9%
- Low copper spherical alloy- Ag: 72.0%, Sn: 25.0%, Cu: 3.0%, Zn: 0.0%
- High copper admixed alloy- Ag: 69.5%, Sn: 17.7%, Cu: 11.8%, Zn: 1.0%
- High copper spherical alloy- Ag: 61.0%, Sn: 26.0%, Cu: 13.0%, Zn: 0.0%
Silver-Tin alloys are brittle because they moderately blend with each other. Therefore, copper is added to substitute silver. This increases the hardness and strength of the alloy. The Zinc is added as deoxidizer. It removes the oxygen and prevents the formation of oxides of other elements at the time of melting the constituents during manufacturing. Zinc decreases the brittleness of set amalgam mass and increases the plasticity which is a needed property during condensation and carving of silver filling. As per ADA specification no 1, some mercury is also allowed in the amalgam alloy.
Lathe Cut Alloy
The particle shape does affect the chemical reaction and setting process after amalgamation and condensation of the dental alloy. Therefore, it is essential to know the effect of shape and size of particles and how does it affect the filling. In early days, the ingots of alloys were prepared and put to lathe machines to make alloys chips. Therefore, there were known as lathe cut alloys. The particle shape and size of these alloys were irregular. Therefore, their packing during condensation was less condensed resulting in less strength and hardness of the silver filling with porous set mass.
As the copper content increased in silver alloys, their grinding by lathe machine increasingly became difficult. Therefore, the manufactures started melting the constituent elements in a pot and throw through a fountain. This way, the liquid of prepared alloy was converted to small balls and by the time they fell in the containers with coolants, they had had became a solid sphere. This is the same process when a barber sprays water on your head to wet your hair before trimming it. The only difference is a barber’s water spray fall as water droplets on your head whereas the silver alloys fall as small spheres.
When two third of low copper lathe cut powder and one third of spherical eutectic alloy powder are mixed together, they are then termed as high copper ‘admixed alloy’ or ‘dispersion alloy’. In this type of alloy, the copper content ranges from 9% to 20% and are termed as third generation silver alloys. They contain high residual mercury content in a filling leading to more creep, delayed setting reaction, increased setting time, have less strength at the initial stage. They require high condensation pressure while filling the cavity preparation. Their finishing is hard and achieving smooth surface is difficult.
Single composition spherical silver alloy contains copper from 13% to 29%. Compared to low copper admixed alloy. They have less residual mercury in a filling causing less creep, fast setting reaction & decreased setting time, have more initial strength. They need less condensation pressure while filling the cavity. Their finishing is easy and achieving smooth surface is easy.
Amalgamation is the process of mixing the powder of silver alloy with mercury. It is done either in mortar & pestle manually or in mechanical amalgamator.
You should note that mixing time affects the physical properties of mix as well as the final physical properties of the set mass. It is always preferable to overmix rather undermix the alloy and mercury. Increasing the mixing time from even 6 to 8 seconds makes the plastic mix more smooth, increases working time for condensation and carving the dental amalgam while filling a cavity preparation.
Following topics will be activated in next batch
Clinical Manipulation of Amalgam for Restorations
Properties of Amalgam
Clinical Performance of Amalgam Restorations
Safety of Amalgam Fillings