Does facet joint inflammation induce radiculopathy? An investigation using a rat model of lumbar facet joint inflammation

Study Design. The association between lumbar facet joint inflammation and radiculopathy was investigated using behavioral, histologic, and immunohistochemical testing in rats. Objectives. To develop a rat model of lumbar facet joint inflammation and ascertain whether facet joint inflammation induces radiculopathy using this model. Summary of Background Data. Both mechanical and chemical factors have been identified as important for inducing radiculopathy. In lumbar spondylosis, facet joint osteophytes may contribute to nerve root compression, which may induce radiculopathy. Furthermore, inflammation may occur in the facet joint, as in other synovial joints. Inflamed synovium may thus release inflammatory cytokines and induce nerve root injury with subsequent radiculopathy. Methods. A piece of gelatin sponge containing complete adjuvant was inserted into the L5-L6 facet joint in rats ( arthritis group). Saline was used in the control group. Mechanical allodynia was determined using the von Frey test. Inflammatory cells infiltrating the epidural space were counted, and changes in cartilage were assessed histologically. Tumor necrosis factor (TNF)-alpha-immunoreactive cells in the L5 dorsal root ganglion were counted. Results. Mechanical allodynia was observed in the arthritis group from day 3, gradually recovering during the observation period. Significantly larger numbers of inflammatory cells had infiltrated the epidural space by days 3 and 7 in the arthritis group than in controls. Numbers of TNF-alpha-immunoreactive cells were significantly increased at days 1 and 3 in the arthritis group compared with controls. Predominantly small nociceptive neurons were stained. Conclusions. When inflammation was induced in a facet joint, inflammatory reactions spread to nerve roots, and leg symptoms were induced by chemical factors. These results support the possibility that facet joint inflammation induces radiculopathy.