C07 Discuss the effect of moisture contamination on delayed expansion of amalgams and how such expansion can be avoided.
If an amalgam is contaminated with saliva during condensation, a poor amalgam will be produced. The reason is simply that saliva droplets trapped inside the amalgam convert to saliva layers during condensation, and these saliva layers prevent the different amalgams layer to adhere optimally. Shortly, the strength of the amalgam decreases.
If the amalgam contains zinc, another complication will occur too. When the saliva reacts with the zinc inside the restoration, hydrogen gas will be formed, and this gas will cause the amalgam to slowly expand over time. This phenomena is called delayed expansion, and delayed expansion can after weeks and months result in pain and cusp fractures.
H2O + Zn -> ZnO + H2
Saliva contacting the amalgam on the outer surface will not be able to generate a pressure inside the amalgam. Thus, if an amalgam is not contaminated with saliva during condensation, it will not suffer from delayed expansion if it is exposed to saliva or water immediately after condensation and carving have been completed.
Delayed expansion can be avoided by use of a dry field technique (e.g. by use of rubberdam or cotton rolls and an efficient suction system) or by use of a zinc-free amalgam alloy. Just remember that any saliva contamination will have a negative effect. Also, be aware that a zinc-containing amalgam results in better restorations as long as it is not contaminated with water or saliva during insertion and condensation.
