Axillary Nerve - Anatomy

Innervation
Motor deltoid anterior and posterior branches of axillary nerve teres minor posterior branch of axillary nerve Sensory superior lateral brachial cutaneous nerve lateral shoulder sensation
Origin
Brachial plexus middle trunk - posterior divison - posterior cord C5 and C6 fibers
Course
Arises from the posterior cord of the brachial plexus anterior to the subscapularis muscle and posterior to the axillary artery courses along anterior subscapularis to inferior edge passes beneath glenohumeral joint capsule gives off articular branches that supply the capsule Travels through the quadrangular space location lateral to triangular space and superomedial to triangular interval boundaries superior - teres minor inferior - teres major medial - long head of triceps brachii lateral - surgical neck of the humerus contents axillary nerve posterior circumflex humeral artery divides into anterior and posterior branches within the quadrangular space Terminal branches anterior branch wraps around the surgical neck of the humerus, running in the deep deltoid fascia with the posterior circumflex humeral artery supplies the anterior deltoid muscle traditional "safe zone" from lateral acromion is 5 cm anterior branch has been shown to run 5-7 cm distal to the lateral edge of the acromion arm abduction to 90° decreases the distance between acromion and nerve by 30% damage to nerve with a muscle split will denervate the anterior deltoid terminates in small cutaneous branches supplying the anterior and anterolateral shoulder posterior branch supplies the teres minor and posterior deltoid muscles branch to the teres minor is closest to glenoid labrum and most susceptible to injury during arthroscopy pierces the deep fascia terminates as the superior lateral brachial cutaneous nerve supplying the lateral shoulder
Clinical Conditions
Quadrangular space syndrome compression of the axillary nerve within the quadrangular space overhead athletes muscle hypertrophy or paralabral cysts present with vague posterior or posterolateral shoulder pain weakness +/- atrophy of teres minor and deltoid muscles most treated successfully without surgery but can undergo nerve decompression in recalcitrant cases Iatrogenic injury arthroscopic surgery the posterior branch has an intimate association with the inferior glenoid and capsule can be damaged during arthroscopic HAGL repair or inferior capsule release Hornblower's test detects teres minor weakness place shoulder in 90° abduction and 90° external rotation positive if patient unable to maintain external rotation, arm falls into internal rotation