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.