C04    Discuss why differences in the geometry of amalgam alloy particles influence the alloy/mercury ratio, the plasticity of an amalgam mix, and the condensation pressure required to produce a high quality restoration.

The geometry of particles used in different dental materials determines the total surface area of all particles. The size of the surface area determines the amount of liquid that is needed to wet all the particles. The total surface area increases per gram alloy powder as the particle size decreases. This is easily realized by looking at a cube that has 6 surfaces each with a surface area of 1 surface unit (s.u.).  If this cube is split into two halves, two new surfaces are formed and the total surface area changes from 6 surface units (s.u.) to 8 s.u.

The smallest surface area any body of a predetermined volume can assume is the surface area of a sphere. If we assume that the cube had a volume of 1 volume unit and a surface of 6 s.u., the surface area for a sphere of the same volume would be 4.836 s.u. Thus, the spherical particles require less liquid to be completely wet than the cubic  particle.

From the above we can conclude that spherical dental amalgam alloy particles need less mercury than lathe-cut particles. Lathe-cut and admixed amalgam alloys are mixed with 50-52wt% mercury, while spherical amalgam alloys need only 42-45 wt% Hg.

In reality dental amalgam alloy particles are either spherical or irregular (see above). The irregular particles are made by milling a cast Ag₃Sn rod in a lathe. The chips made with the lathe are made smaller by placing them in a rotating drum containing hard metal or ceramic spheres. Depending on the time and speed the metal chips are milled in the drum, different particle sizes can be produced. The spherical particles, on the other hand, are made by spraying the molten Ag₃Sn into an atmosphere consisting of a noble gas. During this process the liquid drops solidify and form the small metal spheres that are used in the spherical dental amalgam. 

The plasticity of an amalgam mixture is determined by particle shape, particle size and amount mercury. Spherical particles produce the lowest "internal friction" and thus result in an amalgam with high plasticity. Smaller particles result in lower "internal friction" and higher plasticity than courser particles. More mercury also increases plasticity.

To optimize the properties of a dental amalgam, it is important to maximize the density of the original phases (the dental amalgam alloy powder) in the final amalgam. This is achieved by condensing the material. During condensation, particles move closer to each others, and excess mercury comes to the surface. Excess mercury can be removed, which increases the density of the original particles in the amalgam placed in the prepared cavity. At the same time, porosity content decreases. The reduced amount of mercury and porosity content explains why condensation improves the quality of an amalgam.

The use of modern amalgams and modern triturators has resulted in amalgams with lower initial mercury levels. With these amalgams, the key objective with amalgam condensation is to optimize amalgam adaptation to the cavity wall and minimize the porosity content. With these amalgams, removal of excess mercury is not as crucial as it used to be because of the low initial mercury level. The introduction of spherical amalgams has made it even less important to use high condensation pressures because spherical amalgams are very plastic after they have been just triturated, and the particles are well packed already from the beginning of the condensation process. In other words, spherical amalgams do not require the same high condensation pressure as the lathe-cut amalgams.