Course
The axillary nerve takes a short course from the back of the axilla, under the shoulder joint and round the humerus, to end in deltoid.
In the axilla
It arises from the posterior cord behind the axillary artery, lateral to the radial nerve, and runs down on the front of subscapularis. At the lower border of that muscle it turns backwards, passing just below the inferior part of the shoulder joint capsule.
Through the quadrangular space
The nerve leaves the axilla through the quadrangular space with the posterior circumflex humeral artery and vein. Seen from behind, the space is bounded by teres minor above (subscapularis and the joint capsule from the front), teres major below, the long head of triceps medially and the surgical neck of the humerus laterally. The nerve gives an articular branch to the shoulder joint here.
Round the humerus
Behind the humerus the nerve divides. The anterior branch winds round the surgical neck with the posterior circumflex humeral vessels, deep to deltoid, and runs forward to its anterior border. The posterior branch supplies teres minor and the posterior part of deltoid, then curves round the posterior border of deltoid as the superior lateral cutaneous nerve of the arm.
Branches and supply
The axillary nerve supplies two muscles, one joint and one patch of skin.
| Branch | Supplies |
|---|---|
| Articular branch | Inferior part of the shoulder joint capsule |
| Anterior branch | Anterior and middle parts of deltoid; small cutaneous twigs over the front of the muscle |
| Posterior branch | Teres minor and the posterior part of deltoid |
| Superior lateral cutaneous nerve of arm | Skin over the lower half of deltoid, the regimental badge area |
Deltoid is the prime abductor of the arm from about 15 degrees onwards, after supraspinatus has started the movement, and it also flexes and extends the shoulder through its anterior and posterior fibres. Teres minor externally rotates the arm and helps hold the humeral head in the glenoid as part of the rotator cuff.
Relations
The axillary nerve lies against bone and capsule for most of its course, which is why fractures and dislocations injure it.
- Axilla: behind the third part of the axillary artery, lateral to the radial nerve, on subscapularis.
- Shoulder joint: directly below the inferior capsule as it turns backwards, where a dislocating humeral head stretches it.
- Quadrangular space: above the posterior circumflex humeral artery, which accompanies it throughout.
- Humerus: on the back and lateral surface of the surgical neck, deep to deltoid.
The triangular space, medial to the long head of triceps, transmits the circumflex scapular artery, not a nerve. The triangular interval, below teres major, transmits the radial nerve and profunda brachii artery.
Clinical relevance
The axillary nerve is the nerve most often injured in shoulder trauma, and the injury shows as a flat shoulder and weak abduction.
Causes
- Anterior dislocation of the shoulder: the humeral head drops below the glenoid and stretches the nerve where it lies against the inferior capsule.
- Fracture of the surgical neck of the humerus: the nerve and posterior circumflex humeral artery lie on the bone.
- Quadrangular space syndrome: compression by fibrous bands or hypertrophied muscle, seen in overhead athletes.
- Surgery: deltoid-splitting approaches, arthroscopic portals and inferior capsular procedures. A deltoid split carried too far down from the acromion divides the anterior branch and denervates the muscle in front of it.
- Intramuscular injection: an injection placed low in the deltoid risks the nerve.
Findings
Deltoid wastes and the rounded contour of the shoulder flattens, making the acromion prominent. Abduction is weak beyond the first few degrees, and external rotation is slightly weak, since infraspinatus still works. Sensation is reduced over the regimental badge area. Sensory loss is an unreliable guide to motor recovery, so deltoid contraction should be felt directly, and axillary nerve function is recorded before and after any reduction of a dislocated shoulder.