EXPERIMENTAL LUMBAR RADICULOPATHY - BEHAVIORAL AND HISTOLOGIC-CHANGESIN A MODEL OF RADICULAR PAIN AFTER SPINAL NERVE ROOT IRRITATION WITH CHROMIC GUT LIGATURES IN THE RAT

Objective. The recently proposed animal model of lumbar radiculopathy was used to investigate behavioral consequences and histologic changes in spinal nerve roots, dorsal root ganglia, and spinal nerves after t he L4, L5, and L6 nerve roots were loosely ligated with either silk or chromic gut sutures in an attempt to better understand the pathophysi ologic mechanisms that give rise to pain associated with lumbar radicu lopathy. Summary of Background Data. Little is known about the pathoph ysiologic mechanisms that give rise to pain associated with lumbar rad iculopathy. The recently proposed animal model of unilateral lumbar ra diculopathy, which demonstrated an association with motor paresis and thermal hyperalgesia of the affected hind limb and showed evidence of spontaneous pain has been demonstrated, may serve as a vehicle to allo w direct investigation of the nature of the. pathophysiological mechan isms associated with lumbar radiculopathy. Methods. Three distinct tre atments of the nerve roots were initially investigated: 1) a sham inte rvention, where the surgery simply exposed the nerve roots and dorsal root ganglion followed by standard closing procedures; 2) 4-0 silk lig ature, where two loose ligatures of 4-0 silk were placed around the ne rve roots; and 3) 4-0 chromic gut 2, where four 0.3 cm pieces of 4-6 c hromic gut were laid adjacent to the nerve roots and secured by two lo ose ligatures of 4-0 chromic gut. Study Design. ANOVA techniques were used to test for differential effects across time for the three treatm ent groups in terms of animal function. A qualitative analysis of the histology of the ipsilateral and contralateral nerve roots, dorsal roo t ganglia, and spinal nerves was done to correlate histologic changes with behavioral changes. Results. Behavioral results were consistent w ith the previous study. Rats treated with chromic gut, but not silk, r eliably demonstrated a prolonged thermal hyperalgesia that was maximal 2 weeks after surgery and lasted for up to 12 weeks. These behavioral changes, however, were not correlated with histologic changes in myel inated fiber content in the L4, L5, and L6 nerve roots, dorsal root ga nglia and spinal nerves, the ipsilateral spinal nerved, dorsal root ga nglia, and nerve roots of rates ligated with silk or chromic gut showe d similar, significant, decreased in the number of large diameter myel inated fibers. Conclusions. These results suggest that mechanical cons triction of the L4, L5, and L6 spinal nerve roots, as evidenced by a l oss of myelinated fibers, is not sufficient to produce the behavioral effects associated with this model of lumbar radiculopathy. It is hypo thesized that chemical factors from the chromic gut play a role in the pathophysiology and development of the behavioral, but not histologic al, changes in this model of lumbar radiculopathy.