Posterior Cord of the Brachial Plexus

fasciculus posterior plexus brachialis

The posterior cord is formed by all three posterior divisions (C5-T1) and lies behind the second part of the axillary artery. It gives off the upper subscapular, thoracodorsal and lower subscapular nerves, and ends as the axillary and radial nerves. Between them, these supply almost every extensor muscle of the upper limb.

Anatomical Data

Course & Distribution

Origin	Posterior divisions of all three trunks (C5-T1)
Course	Lies behind the second part of the axillary artery deep to pectoralis minor and divides at its lower border into the axillary and radial nerves
Motor	Subscapularis and teres major (upper and lower subscapular nerves) Latissimus dorsi (thoracodorsal nerve) Deltoid and teres minor (axillary nerve) Triceps, brachioradialis and the extensors of the wrist and fingers (radial nerve)
Sensory	Skin over the lower deltoid (superior lateral cutaneous nerve of the arm) Back of the arm and forearm and the lateral dorsum of the hand (radial nerve branches)

Clinical Relevance

Clinical Notes

A posterior cord lesion causes weakness of shoulder abduction (deltoid), shoulder adduction and internal rotation (latissimus dorsi, teres major, subscapularis), and elbow, wrist and finger extension, with numbness over the outer shoulder and the back of the arm and forearm. Pressure in the axilla from badly fitted crutches can compress the posterior cord or the proximal radial nerve. The cord is also at risk in anterior shoulder dislocation and in proximal humerus fractures.

Pathology

Common Injuries & Conditions

Crutch Palsy

Leaning on axillary crutches compresses the posterior cord and radial nerve in the axilla, producing wrist drop and often triceps weakness. Correct crutch height and bearing weight through the hands usually prevents it, and recovery is common once pressure stops.

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