Lateral approach to the sciatic nerve block in the popliteal fossa: correlation between evoked motor response and sensory block.

BACKGROUND AND OBJECTIVES: The purpose of this study was to identify which of two motor responses of the foot (plantar flexion versus dorsiflexion) best predicts complete sensory blockade of the sciatic nerve when is used for lateral popliteal sciatic nerve block.

METHODS: Thirty American Society of Anesthesiologist physical status I or II patients scheduled for foot and ankle surgery under lateral popliteal sciatic nerve block were enrolled in the study. During each block, the needle was placed to evoke one of the following motor responses of the foot: plantar flexion or dorsiflexion. Thirty milliliters of 0.75% ropivacaine was injected after the motor response was elicited at <0.5 mA. The sequence of elicited motor response was randomized. Sensory blockade of the areas of the foot innervated by the deep peroneal, superficial peroneal, posterior tibial, and sural nerves was checked in a blinded manner. Time required for onset of sensory and motor block of the foot was recorded.

RESULTS: The 2 groups were similar with regard to demographic variables and type of surgery. The total of nerves blocked (deep and superficial peroneal, posterior tibial, and sural nerves) after elicited plantar flexion was greater (complete sensory block in 58 of 60 nerve distributions) than after elicited dorsiflexion (34 of 60 nerve distributions) (P <.05). Onset of complete sensory and motor blockade of the foot was faster after elicited plantar flexion (16.6 +/- 5.1 minutes, 20.1 +/- 5.1 minutes, respectively) than after elicited dorsiflexion (24.3 +/- 5.1 minutes, 28.1 +/- 5.0 min, P <.05).

CONCLUSIONS: After stimulation of the sciatic nerve, plantar flexion better predicts complete sensory blockade of the foot than dorsiflexion when using the lateral approach to the popliteal fossa. The findings of the present study apply to a single injection of 30 mL of ropivacaine 0.75%.