The Innervation of the Lumbar Zygapophysial Joints With Special Reference to Low Back Pain

There has been considerable interest recently in the role of the lumbar zygapophysial joints and the structures innervated by the posterior primary rami in the pathogenesis of low back pain, and in the effects of facet joint injections. Accordingly, the anatomy of the zygapophysial joints of the lumbar region and the posterior primary rami which supply them was studied by microdissection in 28 dissecting room cadavers and a short series of operative specimens. In a further two intact cadavers, injections simulating clinical facet joint injections were carried out. Our study provides new data on the branching pattern, distribution and course of the posterior primary rami. The posterior primary rami of L1-4 form medial, intermediate and lateral branches. Each medial branch supplies two zygapophysial joints, the adjacent joint and the joint one level below and ramifies in the multifidus. The posterior primary ramus of L5 divides into two branches, medial and intermediate. In one case there was objective evidence of entrapment of the medial branch of the posterior primary ramus of L2 by the mamillo-accessory ligament. Controversy also exists regarding the innervation of the synovial folds. These intra-articular synovial folds were studied using both histology and immunohistochemistry, to identify PGP 9.5 and Substance P. The zygapophysial joint capsule has not only been found to be well innervated, but substance P which is associated with nociception, has also been localized in the joint capsule and its synovial folds. The experiments with facet joint injections in the cadaver suggest that injected material spreads from the joint into the epidural space and the paravertebral muscles. These results put the diagnosis and treatment of pain arising from the zygapophysial joint on a firmer structural basis