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 facet joint capsule could cause a muscular response in the multifidus musculature. Objectives. To determine if there is an interrelationship between the intervertebral disc and facet joint innervation and the multifidus musculature as a possible pain mechanism. Summary of Background Data. The innervation of the anulus fibrosus of the intervertebral disc and the capsule of the facet joint is well described in the literature, although the functions of these nerves are poorly understood. An interrelationship between this innervation and the paraspinal musculature has not been previously described. Methods. Fifteen adult pigs were used to measure the electromyographic response in the multifidus musculature to electrical stimulation of the lateral region of the disc anulus and the facet joint capsule in the L1-L2 motion segment. Motor unit action potentials were recorded using three sets of bipolar needle electrodes placed into the deepest fascicles of the multifidus, bilateral to the L2, L3, and L4 spinous processes. The effect of lidocaine injection into the facet joint and subperiosteal muscle detachment on the electromyographic response were studied. Results. Stimulation of the disc anulus fibrosus induced reactions in the multifidus on multiple levels and on the contralateral side, whereas stimulation of the facet joint capsule induced reactions predominantly on the same side and segmental level as the stimulation. Introduction of lidocaine into the facet joint resulted in a significantly reduced electromyographic response to either stimulation, with the most drastic reduction seen when stimulating the facet joint capsule. Subperiosteal detachment of the paraspinal muscles prevented any muscular response. Conclusions. Stimulation 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., disc or facet joints and the paraspinal musculature. Activation of the multifidus muscles may have a stabilizing effect, constraining the motion of the lumbar spine. Longstanding muscular contraction may produce ischemic conditions and may be a potential source of pain.