Dental Filling Discomforts Illustrate Electrochemical Potential of Metals In modern dentistry the material most commonly used far filling decayed teeth is a composition known as dental amalgam. Its principal components are silver, tin and mercury. Dental amalgam fillings are generally inert and usually cause little concern for the patient. However, they can result in considerable pain or discomfort under circumstances in whieh they inadvertently make contact with a second, different metal inside the mouth. The prohlems thereby caused are of two distinct types; bath can he understood by considering the electrochemical behavior of the metals involved. Dental amalgam is made by addition of mercury t o a specially prepared alloy containing approximately 70% silver, 26% tin, and minor amounts of copper and zinc. The final mercury content is about 40%. The completed filling consists of three microscopic phases having stoichiometries approximately corresponding t o Ag2Hg3. AgsSn and Sn,Hg (where x = 7 to 9). Standard electrode potentials, E D ,for these phases are HgPIAgzHgs +0.85 V Sn2+/Ag,Sn -0.05 Sn2+/Sn,Hg -0.13 (Electrode potentials for intermetallic phases can be derived from the Gibbs free energies of formation of the phases and the electrode potentials of their component elements.) The first of the discomforts is known to anyone who has bitten a piece of aluminum foil, perhaps on an incompletely unwrapped piece of candy, in such a way that aluminum presses against a filling. The sharp pain which momentarily results is quite memorable. In effect, a voltaic cell has been created in the mouth. The aluminum ( E o = -1.66 V) is the anode and the filling is the cathode. Salivaand gum tissue fluid act as the electrolyte. When the two metals touch, a short circuit results and a feeble current flows. The nerve of the tooth is more than sensitive enough t o detect this current. Rapid polarization a t the electrodes causes the pain to he brief. In the writer's experience less active metals, such as nickel, iron, stainless steel, and copper, do not deliver a detectable sensation. A more sensitive individual, however, might be able to discern differences and, thus, construct an electrochemical series physiologically. The second type of problem results when a w r y passive metal touches a filling. Such is the ease when a dentist positions a gold (E' = +1.50 V) inlay or denture in such a way that it makes contact with an existing filling. The patient may experience a persistent metallic taste. A voltaic cell has again been created, but the filling is now anodic. It undergoes accelerated chemical dissolution by oxidation in a fashion similar to that of a sacrificial zinc bar used to protect iron or steel from corrosion. As would be predicted from the E Dvalues, the phase most prone to carrasion is Sn,Hg. Release of stannous ions into the mouth accounts far the unpleasant taste. In severe cases replacement of the filling with one of gold is necessary. Chicago State University Chicago, Illinois 60628
Richard S. Treptow
Volume 55. Number 3. March 1978 / 189
