Chronic inflammation and compression of the dorsal root contribute to sciatica induced by the intervertebral disc herniation in rats

The pathophysiological mechanisms underlying sciatica and back pain are not well understood. In the present study, a sciatica model was developed to investigate the contributions of inflammation and compression of the dorsal root (DR). The procedure used autologous disc to apply direct pressure to the L5 DR (disc compression, DC group). For control, five additional groups were included: (1) mechanical compression of L5 DR without disc (compression, CP group); (2) epidurally placed disc without mechanical compression (disc group); (3) epidurally placed nucleus pulposus (NP) without mechanical compression (NP group); (4) epidurally placed annulus fibrosus (AF) without mechanical compression (AF group) and (5) sham group. The paw withdrawal latency to heat stimulation, paw withdrawal threshold to mechanical stimulation, body weight, and motor function were determined pre- and post-surgery. It was observed that all experimental groups with the exception of the sham group showed a progressive and prolonged mechanical hyperalgesia with the DC group having the strongest effect. Furthermore, the disc group showed a greater mechanical hyperalgesia with earlier onset in comparison with the CP group and disc, AF, and NP groups developed thermal hyperalgesia in addition to mechanical hyperalgesia following surgery. Finally, rats in all groups showed normal motor function and body weight increase. These data suggest that this model is suitable to investigate the mechanisms of sciatica and inflammation as well as mechanical compression is involved in the pathogenesis of this condition. Moreover, AF and NP may contribute similarly to the development of sciatica and back pain. (C) 2003 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.