Methods: Electrical impedance of different concentrations of lidocaine hydrochloride was measured at a fixed temperature using a bipolar platinum electrode. The relation between concentration and conductance of the solution was examined. Two chambers were used; one was filled with extrapure water and the other 2 or 50% of lidocaine hydrochloride. Six premolars were extracted for the orthodontic treatment. The fresh tooth crowns were transversely cut at enamel/cementum junction and fixed between the two chambers with two O-rings. Lidocaine hydrochloride was put in the enamel side chamber and extrapure water was in the dentin side. Simulated hydrostatic pulp tissue pressure was applied to extrapure water. Change in the concentrations of lidocaine hydrochloride was measured every 2 minutes with a platinum recording electrode positioned in the center of the extrapure water. Two platinum plates parallel to each other were set at both ends of the chambers for stimulation using amplified AC sine. Passive diffusion without iontophoresis was used as control. After measuring electrical impedance, we examined the enamel surface of sample tooth crowns using scanning electron microscopy.
Results: One tooth crown that had enamel cracks and showed prominently higher conductance was excluded from the experimental samples. Electrical conductance (G, mho) correlated closely to the concentration (x, mmol/L) of lidocaine hydrochloride (G=2.16x2+0.0289x+0.000376, r2=0.999). Lidocaine hydrochloride passed through enamel and dentin increased with time against the dentinal fluid flow caused by simulated pulpal tissue pressure.
Conclusions: Diffusion of lidocaine hydrochloride through enamel and dentin increased in quantity using AC iontophoresis.