C06    Describe the dimensional changes occurring with time during setting of dental amalgams. Identify the effects of alloy/mercury ratio, condensation pressure, particle size, and particle shape on the final dimensions.

Trituration Time

The above figure shows that an amalgam shrinks initially (during the first hour(s), then expands (up to 5-10 hours) and then slowly shrinks during the following 100 hours. The above figure also shows that a variable such as trituration time determines whether the amalgam will expand or shrink.

The above expansion-contraction curves were produced with a low-copper containing lathe-cut amalgam. During trituration, the particles were broken up more and more, and as that happened, the setting reaction proceeded faster and faster. This is important to remember, because as a rule of thumb: "The faster the amalgam sets, the more likely it will shrink rather than expand."

Effect of alloy/mercury ratio

More mercury increases expansion - as well as setting time.

Condensation pressure

As the condensation pressure increases, the tendency for shrinkage increases. Increased condensation pressure results in removal of more excess mercury and a faster setting reaction.

Particle size

As the particle size decreases, the total particle surface area increases. This increase results in a faster setting reaction, as long as no additional mercury is added. Amalgams made from smaller dental amalgam alloy particles shrink more as long as the mercury ratio is not high.

Particle shape

Spherical particles need less mercury because of the smaller total surface area. Thus, spherical amalgams often set faster than lathe-cut amalgams. Because of the lower mercury ratio, spherical amalgams often shrinks more than lathe-cut alloys.

The above figure shows that a microcut (smaller particles than the other amalgams shown in the figure) conventional amalgam shrinks more than anyone of the other amalgams. Regarding the two spherical amalgams (either conventional(c) or high-copper), they have almost the same amount of shrinkage, while the admixed (a) high-copper amalgam is the amalgam coming closest to a zero expansion-contraction value. The above figure may explain why some dentists claim that they have a higher frequency of post-operative sensitivity (cool drinks) for spherical high-copper amalgams than they have for admixed high-copper amalgams. It is however important to emphasize that it has not yet been possible to prove in clinical studies that there is a significant difference in sensitivity between the two amalgams.

To conclude. Factors that slows down the setting reaction (more mercury, less intense trituration and condensation, less reactive particles) result in amalgams that have a tendency to expand. Factors speeding up the setting reaction tends to result in amalgams that shrink.

ADA approved amalgams are allowed to expand or shrink  more than 20 microns/cm (0.2%) during setting.