Study design: A porcine model was used to study whether muscular activation in the paraspinal muscles caused by nerve stimulation in the anulus fibrosus of a lumbar intervertebral disc could be altered by saline injection into the zygapophysial (facet) joint.
Objectives: To elucidate possible mechanisms regarding the nerve pathways and interactions between the intervertebral disc, zygapophysial joints, and the paraspinal musculature.
Summary of background data: The physiologic basis for chronic low back pain, including muscular spasm, is uncertain. Although extensive research involving the lumbar motion segments and the surrounding tissues has been performed, the neuromuscular connection has not been sufficiently investigated.
Materials and methods: Twenty-three adolescent pigs were used to measure the electromyographic response in the paraspinal musculature to electrical stimulation of the posterolateral L3-L4 anulus fibrosus, before and after introduction of physiologic saline into the zygapophysial joint. Motor unit action potentials were recorded using three sets of needle electrodes placed into the deepest fascicles of the multifidus, bilateral to the L4 and L5 spinous processes, and into the central longissimus musculature, bilateral to the L4 spinous process.
Results: Stimulation of the nerves within the posterolateral anulus of the disc elicited reactions in the paraspinal muscles, namely the lumbar multifidus and longissimus. Introduction of physiologic saline into the zygapophysial joint resulted in a reduction in the motor unit action potential amplitude. This reduction was manifested as an immediate and constant reduction, a graded reduction, or a delayed reaction, during which the reduction occurred an average of 5 minutes after the saline injection.
Conclusions: Introduction of physiologic saline into the zygapophysial joint reduced the stimulation pathway from the intervertebral disc to the paraspinal musculature. The zygapophysial joints may therefore have a regulating function, controlling the intricate neuromuscular balance in the lumbar motion segment.