Influence of Electrical Potential on Ion Migration in Teeth 2. Quantitative Measurements of 1-131 Penetration by an Acid-Leaching Technique

¹ Dental Research Laboratory and Dental Clinic, Veterans Administration Hospital, Long Beach, Calif., and the Schools of Medicine and Dentistry, University of Southern California, Los Angeles, Calif.

Radioactive sodium iodide-I¹³¹ was allowed to penetrate wax-covered teeth through a window in the enamel, under the influence of a positive potential. An indication of the amount entering the tooth was obtained by determining the radioactivity levels in successive acid extracts. This was invariably greater under the influence of a positive potential than in the paired control; the mean ratio of activities was approximately 3.2. This ratio did not appear to depend upon voltage in the range between 1 and 12.5 volts, nor upon the current carried, over very wide limits, nor upon the duration of iontophoresis between 15 minutes and 24 hours. It is possible that the accelerating effect of potential was due to a depolarization of the negative surface charge normally present on enamel. This could be decided by more precise quantitative measurements of penetration at different voltages, by comparison of the effect of negative potential on a comparable cation, and by determining if a negative potential retards iodide penetration, as well as by connecting the enamel surface with the root by means of a good conductor during immersion of the enamel in isotope, using appropriate controls in each case.

Penetration towards the pulp was deeper when the tooth was positively charged, but the concentration gradients were not noticeably changed. An average of more than 90 per cent of the total extracted activity was removed by four rinses in distilled water, whether or not a potential had been applied. This proportion was usually higher for the controls than for the experimental samples.

The technique of acid extraction as used in preliminary experiments gave inconsistent results. Dissolution of tooth mineral proceeded uniformly towards the pulp in some teeth, while in other cases certain areas were eroded much faster than others. By modifying the technique so that the exposed enamel window was set off by grooves, the variability was reduced very significantly. While it is likely that the technique could be further refined, biologic variability would still limit the precision of the results. The rapidity and convenience of the method make it particularly useful for exploratory investigations.

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