ELECTROMYOGRAPHIC RESPONSE OF THE PORCINE MULTIFIDUS MUSCULATURE AFTER NERVE-STIMULATION

Study Design. In this study, a porcine model was used to study whether a nerve reaction in the anulus fibrosus of a lumbar disc or in a face t joint capsule could cause a muscular response in the multifidus musc ulature. Objectives. To determine if there is an interrelationship bet ween the intervertebral disc and facet joint innervation and the multi fidus musculature as a possible pain mechanism. Summary of Background Data. The innervation of the anulus fibrosus of the intervertebral dis c and the capsule of the facet joint is well described in the literatu re, although the functions of these nerves are poorly understood. An i nterrelationship between this innervation and the paraspinal musculatu re has not been previously described. Methods. Fifteen adult pigs were used to measure the electromyographic response in the multifidus musc ulature to electrical stimulation of the lateral region of the disc an ulus and the facet joint capsule in the L1-L2 motion segment. Motor un it action potentials were recorded using three sets of bipolar needle electrodes placed into the deepest fascicles of the multifidus, bilate ral to the L2, L3, and L4 spinous processes. The effect of lidocaine i njection into the facet joint and subperiosteal muscle detachment on t he electromyographic response were studied. Results. Stimulation of th e disc anulus fibrosus induced reactions in the multifidus on multiple levels and on the contralateral side, whereas stimulation of the face t joint capsule induced reactions predominantly on the same side and s egmental level as the stimulation. Introduction of lidocaine into the facet joint resulted in a significantly reduced electromyographic resp onse to either stimulation, with the most drastic reduction seen when stimulating the facet joint capsule. Subperiosteal detachment of the p araspinal muscles prevented any muscular response. Conclusions. Stimul ation of the disc and the facet joint capsule produced contractions in multifidus fascicles. The clinical implications are that there may be interactive responses between injured or diseased structures, i.e., d isc or facet joints and the paraspinal musculature. Activation of the multifidus muscles may have a stabilizing effect, constraining the mot ion of the lumbar spine. Longstanding muscular contraction may produce ischemic conditions and may be a potential source of pain.