C15 Define the goal of amalgam condensation.
The goal with the condensation is to compact the amalgam into the prepared cavity so the greatest possible density is attained, with sufficient mercury present to ensure complete continuity of the matrix phase between the remaining alloy particles. Another key objective is to optimize the adaptation between the amalgam and the cavity walls. If these goal are achieved, the strength of the amalgam is increased and the creep is decreased, and at the same time the restoration will seal the cut cavity.
Modern capsule amalgams seldom contain more than 50-52% mercury, making the mercury removal less important than it used to be with the 7:5 mercury/alloy powder amalgam mixtures that were commonly used to the mid 70th.
Before the cut cavity is filled with an amalgam, it must be prepared for the amalgam treatment. Such a preparation may include the addition of parapulpal pin (see above figure, distal-lingual region of the cut surface), placement of varnish, liner or a base, and placement of a matrix band.
The matrix band is a thin stainless steel band that is attached to a matrix band holder. With the latter it is possible to tighten the matrix band around the tooth. To stabilize and improve the seal of the band in the proximal regions, wooden wedges are also placed. The latter is done to reduce the risk for getting poor contacts and cervical overhangs (see below).
When the amalgam is inserted into the cavity and consdensed with the condenser (S), the material flows sideward (see below). The ability to flow depends on particle shape ("internal friction") and mercury level.
Because of the plastic flow of the amalgam during condensation, gaps (S) may open up behind narrow regions (see a below) when the condenser (C) is forced down into the plastic amalgam. A flexing matrix band can also contribute to gap formation (see b below).
During the amalgam condensation process, it is very important to make sure that the amalgam adapts well to cervical-proximal line-angles (see below). When a freshly placed amalgam is inspected after removal of a matrix band, it may look as the amalgam seals the cervical-proximal corner (a). However, in reality this corner may just contain mercury and as the amalgam sets the mercury is drawn into the amalgam when the setting reaction proceeds. Under such circumstances, an open defect forms in this corner (b), and recurrent caries will most likely develop in this region.
To avoid these problems, it is important to use small instruments in these regions (see A below). The same is also true regarding the condensation of narrow fissures (see B below). If a too wide condenser is used, material located to x in B below cannot be compacted. Instead, mercury accumulates in this region during the condensation process, which will result in a weaker amalgam margin in this region.
During the entire condensation process, it is important to make sure that every area of the cavity is well covered by the tip of the condenser (A and B below). At the end of the condensation process, the cavity should be overfilled with a ~1.0 mm thick layer of amalgam (C below) that later on can be burnished and cut away.
The figure shown below shows that it is easier to optimize the strength of a spherical amalgam with a rather low condensation pressure. The pressure unit being used, kp/mm2, can be converted to MPa. 1 kp/mm2 = 10MPa. The unit "kp" stands for "kilopond" which is equivalent to 9.81 N. The conclusion to be drawn is that a spherical amalgam is less technique sensitive (condensation pressure dependant) than a lathe-cut amalgam.
