**3.5 US-guided approach to the AN and ICBN in the axillary fossa**

The AN provides motor innervation to subscapular, teres major and minor, and deltoids muscles. The nerve branches before entering the quadrangular space. The anterior division of the AN originates the first articular branch, which ends in the anteroinferior capsule; blocking the nerve by the posterior approach can provide incomplete analgesia.

The sensitive skin supply of the medial aspect of the arm is provided by MBCN, ICBN, and variable branches of the intercostal nerves [39].

The AN run into the inferolateral margin of the subscapular muscle and enters the quadrangular space (QS) (limits: upper, teres minor muscle; inferior, teres major muscle; medial, long head of the triceps muscle; lateral, surgical neck of the humerus; anterior, insertion of the subscapular muscle on the minor tuberosity). The subscapular muscle, the upper edge of the teres major muscle, and the humerus are the sonographic marks that lead to the identification of the AN. The ICBN originates mainly from the second intercostal nerve, with variable contributions from intercostal nerves T1, T3, and T4. It is identified in the axillary subfascial space, along.

with fat, lymph nodes, and other cutaneous branches of the upper intercostal nerves. After crossing the axillary subfascial space, it courses on the surface of the latissimus dorse muscle, covered by the superficial axillary fascia [40].

With the arm abducted 90o , the BP is identified in the armpit (anterior to the teres major and the tendon of the latissimus dorse muscles, seen in short axis) (Position 1, **Figure 11**). The probe moves slightly in a proximal direction (position 2, **Figure 11**) towards the QS, which is identified as soon as the upper edge of the teres major muscle deepens. At this point, the AN appears as an oval honeycomb structure, accompanied by the posterior circumflex artery of the humerus (although it has an inconsistent course and presence). The elevation of the arm from 90 <sup>o</sup> to 180 <sup>o</sup> brings the nerve closest to the skin by closing the quadrangular space.
