Fig. 4. Effect of rotating each needle about its long axis on the relative threshold for activation of a model nerve fiber lying at different positions (labeled 1–4) and radii (1, 2, or 4 mm) from the needle. (  A ) Geometry of finite element model of the needle inserted into the tissue and positions of model nerve fiber, which ran perpendicular to the plane of the page. (  B ) Relative differences in excitability as the exposed-tip needle was rotated 360 degrees, quantified as the differences between the peak values of the activating function ([  f ^max–  f ^min]/[  f ^max+  f ^min]) for the modeled nerve fiber in the four different radial positions at three different distances between the needle and the fiber (radius). (  C ) Relative differences in excitability as the insulated-tip needle was rotated 360 degrees.