Why is Zinc Not Added to Dental Amalgam?
Zinc as such does not participate in the amalgamation process or in the setting reaction of silver amalgam alloy with mercury during and after the completion of the restoration process.
It is used during the silver alloy manufacturing process to scavenge the impurities that are produced during the production process. It acts as a scavenging material. After its function is finished, it remains in the silver alloy powder without any function.
Zinc does not produce any effect on set silver amalgam filling. But when the isolation is poor and the dentist does not take proper care to prevent moisture/saliva contamination to mixed dental amalgam mass, water reacts with zinc slowly, typically after 3-5 days, and starts creating hydrogen gas. This gas starts causing internal pressure thus expansion of the set mass and the filling tends to extrude from the confines of the prepared cavity. The expansion may reach more than 400 micrometer/cm (4%) and is known as delayed expansion.
Therefore, the manufacturers now have adopted newer techniques for scavenging purpose and produce silver powders without adding zinc. These alloys are known as zinc free alloys.
At the time of pandemic of the COVID-19, the prevention of delayed expansion becomes of paramount importance. Also, maintenance of the chain of sterilization and prevention of cross infection of coronavirus should be taken into the consideration while doing silver fillings. At the time of breakout of COVID-19 where the stalks are high, use of rubber dam should be encouraged along with the use of high-volume suction while restoration process is underway. These techniques improve the patient satisfaction, boost confidence in dentist and definitely prevent the cross contamination of COVID-19.
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