Background: Surgical anesthesia for reconstructive ankle surgery requires sensory and motor block of all the terminal nerve distributions of the sciatic nerve. In this prospective observational study, we investigated the value of sensory and motor testing of the foot, after local anesthetic injection, for predicting complete sciatic nerve blockade and the duration of testing required for identifying incomplete anesthesia.
Methods: Sciatic nerve blocks (n = 180) using the infragluteal-parabiceps approach were performed in patients undergoing reconstructive ankle surgery. Levobupivacaine 0.625% with epinephrine 1:300,000 (0.4 mL/kg) was injected after obtaining an elicited motor response at <0.4 mA of plantar flexion or inversion. Pinprick sensory assessments were performed at intervals by an observer unaware of the elicited motor response in the distal cutaneous distributions of the superficial peroneal nerve, deep peroneal nerve, posterior tibial nerve, and sural nerve. Motor block was assessed using foot (plantar flexion and dorsiflexion) movement and toe movement. A complete block was defined as sensory and motor loss in all distributions of the sciatic nerve within 25 minutes of local anesthetic injection. The optimal sensitivity and specificity of various cutoff times of sensory and motor testing were determined by receiver operating characteristic analysis. The area under the curves was compared for equivalence using nonparametric methods. The cutoff times were determined as the point of intersection of the lines of sensitivity and specificity.
Results: The elicited evoked motor response before sciatic nerve block was plantar flexion in 87 patients and inversion in 93. Eighty-eight of 93 patients (94.6%) who had an elicited motor response of inversion and 49 of 87 (55.7%) who had an elicited motor response of plantar flexion achieved complete sciatic nerve block at 25 minutes. Area under the curves were not different among testing paradigms. Receiver operating characteristic analysis identified optimal testing times of 4 minutes for the sural and 6 minutes for the posterior tibial nerve with an elicited motor response of inversion and 6 minutes with an elicited motor response of plantar flexion. No subject with an incomplete block achieved sural anesthesia by 10 minutes.
Conclusion: Sural anesthesia assessed at the lateral heel and the lateral aspect of the foot and the fifth toe identified within 4 to 6 minutes demonstrated a similar posttest predictive value as anesthesia in the distributions of the posterior tibial and peroneal nerves or motor movement of the foot at later intervals. In addition, failure to achieve sural anesthesia within 10 minutes was predictive of block failure.