Ultrasound-Guided Anterior Approach to a Sciatic Nerve Block: Influence of Lower Limb Positioning on the Visibility and Depth of the Sciatic Nerve

Abstract

Objectives: We aimed to identify the optimal lower limb position for an ultrasound (US)-guided anterior approach to a sciatic nerve block.

Methods: We included 45 patients who met the following criteria: American Society of Anesthesiologists physical status of 1 to 3, age between 18 and 80 years, and scheduled to undergo knee surgery that required a sciatic nerve block. The lower limbs of each patient were placed in the following 4 positions: N, neutral; ER, external rotation of the hip (angle, 45°); ER/F15, ER (angle, 45°) and flexion (angle, 15°) of the hip; and ER/F45, ER (angle, 45°) and F (angle, 45°) of the hip. An investigator acquired US scans of the sciatic nerve in each position, and the visibility score and depth of the sciatic nerve from the skin were analyzed.

Results: The visibility scores were significantly higher in positions ER/F15 and ER/F45 than in positions ER and N (P < .0001). However, there was no difference between the visibility scores in positions ER/F15 and ER/F45 (P = .0959). The depth of the sciatic nerve from the skin decreased with ER and an increase in the F angle of the hip (overall P < .0001).

Conclusions: Based on the visibility score and depth from the skin, ER of the hip to 45° with a greater F angle (45° versus 15°) of the hip appears to be the optimal position for an US-guided anterior approach to a sciatic nerve block.

Keywords: anterior approach; sciatic nerve block; supine position; ultrasound.