The purpose of this study was to characterize somatosensory units of the lumbar facet joint, which may play a central role in idiopathic low-back pain. A laminectomy was performed on the lumbar spine of adult male New Zealand White rabbits. Receptive fields of mechanosensitive afferent units were investigated in the lumbar facet joint and adjacent surrounding tissues, and electrophysiological recordings were obtained from filaments of the dorsal root.

Twenty-four units were identified in the region of the facet joint:​ ten, in the capsule of the joint;​ twelve, in the border regions between capsule and muscle or tendon;​ and two, in the ligamentum flavum. Of these units, two had a conduction velocity that was slower than 2.5 meters per second (group IV), fifteen had a velocity ranging from 2.5 to twenty meters per second (group III), and seven had a velocity faster than twenty meters per second. Seven units had a von Frey threshold of more than 6.0 grams, thirteen had a threshold of less than 6.0 grams, and four were not examined. Seven units in the facet joint responded to movement of the joint. Fourteen other mechanosensitive units were found in the muscle, tendon, and interspinous ligament;​ seven had a conduction velocity of 2.5 to twenty meters per second, and seven had a velocity that was faster than twenty meters per second.

CLINICAL RELEVANCE:​ The present study showed that the facet joint contains group-III and group-IV mechanosensitive units with low to high thresholds and that several units responded to movement of the joint. Units that have a high threshold and a slow conduction velocity may serve as nociceptors and units that have a lower threshold, as proprioceptors. On the basis of the neurophysiological characteristics of high-threshold group-Ill units, the results suggest that the facet joint is probably a source of low-back pain.