C09 Discuss and demonstrate why the creep properties of amalgams are important from a clinical standpoint.
Creep of an amalgam means that under load the amalgam deforms over time. If a Class II amalgam is deformed over time, the facial and lingual cusps may be displaced in facial and lingual directions, which can result in cusp fractures. This may explain why the lingual cusps are often the cusps that fracture in the lower jaw, while the facial cusps are those that fracture in the upper jaw, at least as long as we are dealing with normal occlusion.
Low creep values have also been used to distinguish high-copper amalgams from low-copper amalgams. The discovery that there was a correlation between low creep values and low marginal fracture failures was originally misinterpreted. This misinterpretation goes back to a clinical study comparing two low-copper amalgams with the first available high copper amalgams (see figure below). According to this study, Material C had the lowest marginal fracture frequency and the lowest flow and creep values. This material had also the lowest compressive strength, lowest "slow" compressive strength, average tensile and transverse strength, and highest transverse deflection value. In other words, most mechanical strength properties, except for creep and flow correlated poorly with clinical performance (low margin fracture frequency). This study explains why there was a believe during the late 60th that there was a direct relationship between low creep values and low marginal failure rates.
However, today we know that the key reason for this correlation was that the high-copper amalgams did not contain the gamma-2 phase (tin-mercury). This was not known during the late 60th. It was the corrosion of the gamma-2 phase that resulted in weakened margins and marginal fractures. In the above study, only one high-copper amalgam was included. By comparing high-copper amalgams only, one would find that there is no correlation between low creep values and low marginal fracture frequency values.
