Computer-based finite element modeling of insulated Tuohy needles used in regional anesthesia.

BACKGROUND: Differences in needle design may impact nerve localization. This study evaluates the electrical properties of two insulated Tuohy needles using computational finite element modeling. METHODS: Three-dimensional geometric computer-based models were created representing two 18-gauge, insulated Tuohy needles: (1) with an exposed metal tip and (2) with an insulated tip. The models were projected in simulated human tissue. Using finite element methodology, distributions of current-density were calculated. Voltages in the modeled medium were calculated, and activation patterns of a model nerve fiber around the tip of each needle were estimated using the activating function. RESULTS: Maximum current density on the exposed-tip needle occurred along the edge of the distal tip; the distal edge was 1.7 times larger than the side edges and 3.5 times larger than the proximal edge. Conversely, maximum current density occurred along the proximal edge of the insulated-tip Tuohy opening; the proximal edge was 1.9 times larger than the side edges of the opening and 3.5 times larger than the distal edge of the opening. Voltages generated by the exposed-tip needle were larger and had a wider spatial distribution than that of the insulated-tip needle, which restricted to the area immediately adjacent to the opening. Different changes in threshold were predicted to excite a nerve fiber as the needles were rotated or advanced toward the modeled nerve. CONCLUSIONS: The needles displayed different asymmetric distributions of current density and positional effects on threshold. If this analysis is validated clinically, it may prove useful in testing stimulating needles before clinical application.

Antidromic propagation of action potentials in branched axons: implications for the mechanisms of action of deep brain stimulation.

Electrical stimulation of the central nervous system creates both orthodromically propagating action potentials, by stimulation of local cells and passing axons, and antidromically propagating action potentials, by stimulation of presynaptic axons and terminals. Our aim was to understand how antidromic action potentials navigate through complex arborizations, such as those of thalamic and basal ganglia afferents-sites of electrical activation during deep brain stimulation. We developed computational models to study the propagation of antidromic action potentials past the bifurcation in branched axons. In both unmyelinated and myelinated branched axons, when the diameters of each axon branch remained under a specific threshold (set by the antidromic geometric ratio), antidromic propagation occurred robustly; action potentials traveled both antidromically into the primary segment as well as "re-orthodromically" into the terminal secondary segment. Propagation occurred across a broad range of stimulation frequencies, axon segment geometries, and concentrations of extracellular potassium, but was strongly dependent on the geometry of the node of Ranvier at the axonal bifurcation. Thus, antidromic activation of axon terminals can, through axon collaterals, lead to widespread activation or inhibition of targets remote from the site of stimulation. These effects should be included when interpreting the results of functional imaging or evoked potential studies on the mechanisms of action of DBS.