Mechanical compression of the lumbar nerve root alters pain-related behaviors induced by the nucleus pulposus in the rat

The purpose of this study was to refine a method of nerve-root injury in th e rat to produce hyperalgesia, a pain-related behavior, and to determine if there were any relationships between the histological extent of nerve-root injury and the magnitude of hyperalgesia. Three methods were used to produ ce hyperalgesia: irritation of a nerve root by ectopic nucleus pulposus, si lk loop alone, or both silk loop and ectopic nucleus pulposus. Autologous n ucleus pulposus obtained from coccygeal intervertebral discs was relocated on the lumbar nerve roots after laminectomy. Two loops of 4-0 silk were pla ced around the exposed nerve roots. Hyperalgesia was measured preoperativel y and postoperatively. The distribution of myelinated axons in the dorsal n erve roots was evaluated histologically. Mechanical hyperalgesia was detect ed in rats in which autologous nucleus pulposus was applied to the nerve ro ot but not in those in which silk loops were used. Silk loops around the ne rve root resulted in thermal hyperalgesia only in rats in which autologous nucleus pulposus was applied to the nerve root. Fewer large myelinated fibe rs were seen in the rats in which silk loops were used. Although a silk loo p around the nerve root was not sufficient to produce hyperalgesia, supplem ental application of autologous nucleus pulposus to the nerve root produced thermal hyperalgesia. It is possible that mechanical constriction of the n erve root alters the pain-related behavior elicited by chemical factors fro m the nucleus pulposus.