*2.5.2.2 Axillary nerve compression neuropathy (Quadrilateral space syndrome-QSS)*

**Anatomy:** The quadrilateral space (QS) (**Figure 5**) is a space in the posterior aspect of the shoulder and bordered medially by the the long head of the triceps, laterally by the medial edge of the surgical neck of the humerus and inferiorly by the teres major and latissimus dorsi muscles and superiorly by the the teres minor muscle or the glenohumeral capsule. The QS contains the posterior circumflex humeral artery and the axillary nerve. Axillary nerve originates from the posterior

#### **Figure 5.**

*The schematic drawing of entrapment site (quadrilateral space) in axillary nerve compression neuropathy AN: Axillary nerve, Tm: Teres minor muscle, Tr: Long head of the triceps, TM: Teres major muscle, H: Humerus, D: Deltoid muscle).*

cord of the brachial plexus (C5-C6). It then runs along the inferolateral edge of the subscapular muscle and curves downward from the glenohumeral joint capsule to reach the OS. It divides into anterior and posterior branches in the space. The anterior branch curves together with the posterior circumflex artery around the humeral neck and reaches the deltoid muscle. At the acromion level, the nerve gives branches to supply the anterior deltoid and cutaneous branches that spread over the skin covering the deltoid muscle. The posterior branch innervates the teres minor and the posterior deltoid muscle and gives off branches to the skin over the distal part of the deltoid and the upper part of the long head of triceps [8, 16, 26, 29, 30].

**Description:** Quadrilateral space syndrome or axillar nerve compression neuropathy is a condition characterized by compression of the posterior humeral circumflex artery and axillary nerve in the quadrilateral space while the shoulder is in abduction and external rotation [29, 30].

**Causes:** Fractures of the upper limb, improper use of crutches, casts, fibrous bands, or inferior (from 9 to 7 o'clock positions) paraglenoid cysts may cause stretching injuries or stenosis of the quadrilateral space and OS contents may be compressed in QS. As a result, axillary neuropathy develops due to compression. Fibrous bands are the most common cause of compression in the QS. Also, space-occupying lesions in the QS (paralabral cyts, bony fracture fragments, being tumors), venous dilation and muscle hypertrophy have been implicated cause of cases of QSS [8, 26, 29, 30].
