The effect of age on inflammatory responses and nerve root injuries after lumbar disc herniation - An experimental study in a canine model

Study Design. An experimental investigation on the effect of age on patholo gic events surrounding the herniated disc and at the adjacent nerve root. Objectives. To investigate the role of age on the inflammatory responses an d nerve root damage surrounding a sequestered lumbar disc fragment using a dog model, Summary of Background Data. Lumbar disc herniation is manifested in patient s by variable clinical findings, natural history, and resorption phenomena in which the variability is particularly noted among patients with differen t ages. There are no previous reports on the effect of age on pathologic ev ents induced by the herniated disc. Methods. Six beagle dogs, including two animals of each age group of 6, 12, and 24 months (human equivalent ages of 10, 15, and 24 years), were used i n this study. The dogs underwent L4-L5, L5-L6, and L6-L7 laminotomy and dis cectomy under general anesthesia. An autologous intervertebral disc from th e tail was divided into anulus fibrosus and nucleus pulposus fragments. The anulus fibrosus and nucleus pulposus fragments were placed in the anterola teral epidural space of LS-LS and L6-L7, respectively. The L4-L5 discectomy site served as a control. Dogs were killed at 12 weeks after surgery. The lumbar spine was removed an bloc, and histologic sections were prepared con secutively and examined. Results. In the nucleus pulposus group at L6-L7, neovascularity, and intens ive infiltration of lymphocytes, macrophages, and fibroblasts were observed surrounding the nucleus pulposus fragment in the 24-month-old group only. Degenerative changes of the nerve root fibers were observed in the 24-month -old group only. In the control and anulus fibrosus groups at L4-L5 and L5- L6, there were no marked inflammatory reactions in all age groups, The nerv e root fibers around the anulus fibrosus were normal in all age groups. Conclusions. There is an effect of age on the inflammatory response and ner ve root injury caused by the herniated disc. The apparent neuroprotective m echanism in the young animal, and the apparent inflammatory and resorption changes of the nucleus pulposus fragment in the older animal are quite intr iguing.