The estimated yearly prevalence of low back pain (LBP) has been reported to be 15-20% in the United States with an annual cost of $20-50 billion. Chemical irritation, compression, or tension on the spinal nerve roots are the major sources of LBP. The aims of the current study were to determine the mechanical properties of spinal nerve roots in vitro at various strain rates and also to establish a relationship in vivo between strain and strain rate and resulting functional and morphological injuries.

In the first part of the study, biomechanical tests were performed on 96 fresh nerve roots, from 12 adult male Sprague Dawley rats. The average failure load of 5.7±2.7 g and 13.9±7.5 g were observed for the 0.01 mm/sec (n=31) and 15 mm/sec (n=37) strain rates, respectively. Failure stress and E values were 257.9±111.3 KPa and 1.3±0.8 MPa for the 0.01 mm/sec rate and 624.9±306.8 KPa and 2.9±1.5 MPa for 15 mm/sec rate, respectively. These values were significantly higher (p = 0.000) at 15 mm/sec rate than at 0.01 mm/sec rate. Also, the root failure force was directly proportional to its cross-sectional area and proportional limit force was the maximum force sustained by the roots. Overall, the mechanical properties exhibited by the nerve roots were much weaker than those reported for peripheral nerves.

In the second part, a total of 72 L5 dorsal roots from 36 adult male Sprague Dawley rats were used. Each root was subjected to a predetermined strain (<10%, 10-20% and >20%) and stretched at a specific displacement rate (0.01 mm/sec, 1 mm/sec and 15 mm/sec). Results showed that the morphological and functional nerve root injuries were strain and rate dependent. The threshold strains were 16%, 10% and 9% at 0.01 mm/sec, 1 mm/sec and 15 mm/sec rates, respectively, for 50% occurrence of complete functional impairment of the nerve roots. Thus, high strains at low rates caused complete conduction block in the roots, while similar block was observed at lower strains at higher rates. The extent of morphological injuries including the impaired axoplasmic transport (IAT), axotomy and hemorrhage were observed concomitant with functional injury and were also strain and rate dependent. Overall, results from this study provide further insight into the mechanical, functional and structural changes in nerve roots. Also, this model serves as a good model for traumatic axonal injury. Thus, the reported data would help to better understand axonal injury mechanisms and tolerance.