Electrical nerve location: numerical and electrophoretic comparison of insulated vs uninsulated needles.

We compared the electrical characteristics of insulated and uninsulated needles in two models that simulate use of a stimulator for nerve localization. With a digital computer, we solved for and graphed the contours of constant electric field strength, defining regions of simulated tissue in which a nerve would become depolarized for a particular stimulation current. We found that with an uninsulated needle, these regions extend proximally along the needle shaft with their widest dimension located slightly shallow to the tip, but with insulated needles, the regions are almost circular and are centered slightly deep to the needle tip. We confirmed these findings by electrophoresis of bromphenol blue dye in polyacrylamide gel. We also found that the necessary stimulator current is much more dependent on the depth of needle insertion with uninsulated needles than with insulated needles. We conclude that the electrical characteristics of insulated needles are more favorable for successful nerve block.