Long-term effect of sciatic nerve block with slow-release lidocaine in a rat model of postoperative pain.

BACKGROUND Postoperative pain management is important for preventing perioperative complications. The authors examined the effectiveness of controlled-release lidocaine for sciatic nerve block in a rat model of postoperative pain. METHODS The authors created a novel slow-release lidocaine sheet (SRLS) with polylactic-coglycolic acid. In male Sprague-Dawley rats (postoperative pain model), the authors applied the SRLS, lidocaine alone, or polylactic-coglycolic acid (control) near the ipsilateral sciatic nerve just before making the paw incision. Mechanical hypersensitivity was assessed using von Frey filaments, and c-fos expression was examined in the spinal cord dorsal horn at segments L4-L5. Neurotoxicity and muscle toxicity were also evaluated via histopathology. RESULTS The SRLS (30%, w/w) continuously released lidocaine for 1 week in vitro. The withdrawal threshold in the SRLS-treated group was higher than that in the control group at all time points measured (2 h to 7 days). The withdrawal threshold in the lidocaine-treated group was higher than that in the control group only at 2 h after paw incision. The mean number of c-fos immunoreactive neurons in the SRLS-treated group was lower than in the control group at 2, 5, and 48 h after paw incision and lower than in the lidocaine-treated group at 5 and 48 h after paw incision. On histopathology, signs of inflammation were only slightly present in the muscle and nerve tissues of the SRLS-treated group. CONCLUSIONS Single treatment with the SRLS inhibited hyperalgesia and c-fos expression in the spinal cord dorsal horn for 1 week. Slow-release local anesthetics are promising for the management of postoperative pain.