Galvanism
Write a short note on galvanism.
Answer:
The presence of metallic restorations in the mouth may cause a phenomenon known as galvanism or galvanic current. When dissimilar filings are present on opposing teeth, in the presence of saliva as an electrolyte, an electric cell is formed and this causes the production of galvanic current, for example, an amalgam filling is placed opposite to a gold crown or filling
The patient experiences pain during contact with the filings. Galvanism is prevented by planning for restorations or by the placement of an insulating base and applying varnish on cavity walls before placement of the restoration.
Following are the different examples:
- When two different restorations are in direct contact: An amalgam restoration (anode) is placed directly in contact with a gold restoration (cathode) leading to the short circuit, which causes sharp pain experienced by the patient. This can also cause the weakening of both the alloys; discoloration of both restorations, and presence of a metallic taste in the mouth.
- When two different restorations are not in direct contact: When two dissimilar restorations are not in direct contact, an electrical circuit associated with the difference in potential or electromotive force between the two restorations still exists. Here saliva forms electrolytes and the hard and soft tissues form an external circuit. The electrical resistance in this instance may be more than that present when the two restorations are in direct contact.
- When there are two different electrolytes: Current may be present even in a single, isolated metallic restoration. In this situation, the electrochemical cell is generated as a result of electrical potential differences created by two electrolytes, i.e. saliva and tissue fluids. Here in this situation tissue flids indicate the dentinal fluid, soft tissue fluid, or blood which completes the external circuit.
Heterogeneous Surface Composition:
It occurs inside the structure of restoration itself. Such compositions can cause galvanic corrosion.
- The difficulty arises when a metal surface has both anode and cathodes on the same surface.
- Metal ions go inside the solution or form corrosion products due to anodic reactions and other ions get reduced in cathodic reactions.
- Both cathodic and anodic reactions occur at randomly distributed sites on metal, at the anode metal dissolves, and at the cathode ions get discharged
- Various examples of heterogenous composition are:
- Solder joints present between dental alloys are prone to corrosion due to differences in the composition of the alloy and solder.
- When a eutectic alloy is immersed in electrolyte metallic grains with lower electrode potential gets attacked and corrosion occurs.
- In core structure, there is a difference in composition in alloy grains.
- So a part of the grain acts as an anode and others act as a cathode
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