In this vignette are shown data derived from an acute animal experiment, meaning the self-sizing electrode was a snug fitting cuff and there was no encapsulation tissue between the target tissues and the cuff. The experiment was designed to give insight into the relative selectivity of the monopolar and tripolar electrode configurations. Something to keep in mind when comparing these two configurations is that the monopolar configuration will require four lead wires between the stimulator and the electrode whereas the tripolar electrode will require at least eight leads in the cable between the stimulator and the electrode; more leads in the cable means a stiffer cable and more connectors at each end of the cable, generally speaking, attributes that are not desirable for a device in clinical use.
This is a busy vignette, so let’s take a moment to go over some basics. In the experimental setup there is a multi-contact self-sizing electrode placed around the sciatic nerve, and single contact electrodes placed on the four major branches of the sciatic nerve serving muscle that act the ankle joint. Torque about the ankle joint is measured in three planes, two are shown in this plot. At the termination of the experiment the position of the electrode contacts, relative to the fascicles in the sciatic nerve, was determined. The graphed data are plotted after the termination of the experiment.
Stimuli, in increasing magnitude, were applied to the tibial branch and the resulting torque measured and plotted. Similarly the resulting torque was plotted for stimuli applied to the common peroneal branch, the medial gastrocnemius branch, and and the branch serving the lateral gastrocnemius and soleus.
Moving to the multi-contact self-sizing spiral cuff electrode, stimuli of increasing magnitude were applied to the 0˚ contact. First the motor neurons in the tibial fascicle are recruited, followed by motor neurons in the medial gastric fascicle, then the lateral gastric/soleus fascicle and finally the motor neurons in the common peroneal fascicle.
Next, Stimuli of increasing magnitude were applied to the 90˚ contact, recruiting first motor neurons of the medial gastrocnemius fascicle, then the motor neurons in the lateral gastrocnemius/soleus fascicle, and finally the motor neurons of the tibial fascicle. Had the stimulus magnitude been increased even further, motor neurons in the common peroneal fascicle would have been recruited. There is a lot of animation activity happening in these last two sequences so you may want to rerun this section several times.
I find it interesting to note how the excitation current is so readily constrained to a fascicle from the initial invasion through the full recruitment of all motor neurons in that fascicle. This is consistent with the findings of Chintalacharuvu in her modeling studies where the current scattering barriers, e.g. perineurium, work favorably to facilitate selective activation.
Tarler, M.D. and J.T. Mortimer, “Comparison of Joint Torque Evoked With Monopolar and Tripolar-Cuff Electrodes, IEEE Trans. on Rehabilitation Engineering, Vol 11, pp227-235. 2003.