Concentric needle recording characteristics related to depth of tissue penetration

This study investigates the influence of tissue penetration depth as it relates to a concentric needle electrode, particularly delineating regions where the cannula potential predominates over the core potential. The regions of cannula predominance is studied by means of a standard and 20 times enlarged physical model of an electromyographic concentric needle electrode in a homogeneous volume conductor by delineating the zero isopotential which partitions where the core potential predominates versus where the cannula potential predominates. Clinical studies in muscle tissue are used to test and confirm results from the enlarged physical model. At shallow electrode insertions equivalent to 4 mm, the concentric needle model records a net negative potential, which is a region where the cannula predominates, from a distant positive dipole at the same depth compared with a net positive potential for penetration depths exceeding 4 mm. The clinical portion of this study verifies the bipolar nature of the concentric needle electrode in detecting motor unit action potentials (MUAPs) with primarily an initial positive onset irrespective of recording depth. Refinements to the conceptualization of the nature and detection of MUAPs are discussed which are consistent with all the findings of the clinical and model study.