Strain and excursion in the rat sciatic nerve during a modified straight leg raise are altered after traumatic nerve injury

Purpose: This study investigated the biomechanics of the sciatic nerve with hind limb positioning in live and euthanized Sprague-Dawley rats after traumatic nerve injury. Methods: With radiographic analysis, sciatic nerve excursion and strain were measured in situ during a modified straight leg raise, which included sequential hip flexion and ankle dorsiflexion. Comparisons were made between nerves in uninjured, sham-injured and mild crush-injured rats at the 7-day and 21-day recovery times. Results: Significant strain and proximal excursion of the sciatic nerve were observed in all groups during hip flexion, and additional increased strain was noted during dorsiflexion. Seven days after nerve injury, strain increased significantly during hip flexion (17.64 ± 14.12%; p = 0.0091) and dorsiflexion (22.56 ± 15.47%; p = 0.0082) compared to the sham-injured controls. At 21 days after injury, the strains were similar between the injured and sham-injured groups. Conclusions: Nerve bed elongation during straight leg raise causes sciatic nerve strain and excursion towards the moving joint with the greatest movement nearest the moving joint. In the first week after injury, the maximal strain exceeded the level previously shown to impair nerve conduction and circulation.