Midfemoral nerve block for foot surgery: Is there an anatomical-clinical correlation between motor response and latency?

BACKGROUND AND OBJECTIVE: The latency times of midfemoral sciatic nerve blocks vary greatly. This study investigated the correlation between the type of motor response to nerve stimulation on the one hand and latency and block efficacy on the other. PATIENTS AND METHODS: We enrolled 215 consecutive patients (184 women) undergoing orthopedic foot surgery. A tourniquet was applied above the malleolus. The puncture location was found by palpating to locate the groove between the vastus lateralis and biceps femoris muscles, at the mid-point of the line between the posterior edge of the greater trochanter muscle and the insertion of the biceps femoris muscle in the popliteal fossa. A solution of equal proportions (1:1) of 1.5% mepivacaine (with bicarbonate 1:10) and 0.75% levobupivacaine was injected at a dose of 0.45 mL.kg(-1) (maximum 40 mL) using a 10-cm needle. Nerve stimulation was applied at 100-300 ms, 0.2-0.4 mA, and 2 Hz. Latency was classified as response in less than 15 minutes, in 15 to 30 minutes, or later than 30 minutes. RESULTS: The evoked motor response was inversion in 30 patients, flexion or extension in 38, plantar flexion in 101, dorsiflexion in 37, and eversion in 9. Shorter latencies (15 minutes) were observed in all patients with inversion or flexion/extension and in 84 (83%) of the 101 patients with plantar flexion. Mid-range latencies were observed in 13% of those with a plantar flexion response and in 29.7% of those with dorsiflexion. All 9 patients with eversion and 17 (45.9%) of the 37 patients with dorsiflexion had the longest latencies. The surgical block was complete for all patients. CONCLUSIONS: This approach provides an effective block with minimum latency in patients who have a flexion or extension motor response in the foot and/or fingers, inversion, or plantar flexion, which assumes that the injection has reached the common trunk of the sciatic or tibial nerve. However, a longer latency is associated with a peroneal motor response, particularly eversion.