Long-term evaluation of motor function following intraneural injection of ropivacaine using walking track analysis in rats

Background. There is a paucity of data regarding neurologicfunction following nerve injury. Our objective was the long-termevaluation of motor function following intraneural injectionof ropivacaine in rats using the sciatic function index (SFI),derived from walking track analysis. Methods. Rats were randomly assigned to one of four groups of13 animals each. A needle was inserted under magnification intothe left sciatic nerve and 0.2 ml of normal saline, formalin15%, ropivacaine 0.2 or 0.75% were injected intraneurally. Theright side was sham operated. Walking track analysis was performedthe day before and on days 1, 4, 7, 11, 15, 18, 21, and 67 followingintraneural injection. At the end of the experiment (day 67)a semi-quantitative evaluation of neuropathologic changes wasperformed by three independent observers. Results. Animals treated with saline and ropivacaine (0.2 and0.75%) had no detectable impairment of motor function at anytime point. In contrast, rats treated with formalin had a completeloss of motor function immediately after the intraneural injection,which persisted until day 21 and returned to normal by day 67.Important histopathologic changes (score=2) with excellent inter-observeragreement were seen only in the group treated with formalin.This applied to both axonal degeneration and Schwann cell densityevaluations. Conclusions. These findings suggest that intraneural injectionsof ropivacaine at concentrations routinely used in clinicalpractice appear to have no deleterious effect on sciatic nervemotor function in this experimental rat model.