The needle is initially advanced to place the tip between the axillary artery and the posterior cord of the brachial plexus, dorsal to the axillary artery (dashed arrow). Spread of local anesthetic should be visualized during injection, aiming for U-shaped spread lateral, posterior, and medial to the artery. If spread is inadequate on the medial side, withdraw the needle and redirect to deposit local anesthetic there (arrow).