Modeling for Surface Stimulation of the Median Nerve

Phantom Limb Pain, a pain or discomfort in the missing limb, is experienced by 50-80% of amputees. We hypothesize that by eliciting a “real” sensation in the phantom limb; phantom limb pain may be reduced or eliminated. Previous studies with single electrodes placed at the elbow produced sensation in a portion of the hand and the area increased with increasing stimulation. The objective of this study was to develop a computer model of surface stimulation that can be used to predict the effect of electrode location, size, and configuration on activation of the median nerve. A three-dimensional finite element model of the elbow was created using ANSYS Maxwell. The model was based on the anatomy of the arm and included electrical properties of the tissues including bone, muscles, blood vessels, tendons, and nerves. To validate the model, two 15x30 mm electrodes were applied to the skin of the model to match experimental conditions. Simulations were run on the model by applying a voltage across the electrodes. MATLAB and NEURON were then used to determine whether the axon fired for a given set of parameters. In order to improve the accuracy of the model, differences between motor and sensory axons were investigated. Ion channels were added to the nodes and internodes of the axons, and parameters were varied in order to distinguish motor and sensory axons. The thresholds at different axon diameters were found for the two types of axons to determine which type was activated first. The results were then compared to experimental results. Future work will include modeling an array of electrodes and developing an algorithm to control the stimulation to obtain partial activation of the nerve