Posterior Cord of the Brachial Plexus A Radial Plus Palsy

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The posterior cord OF THE BRACHIAL PLEXUS comprises the posterior division of all three trunks: the upper, middle, and lower trunk. It usually contains fibers from C5 to C8, and the presence of T1 fibers is variable.

Palsy of the posterior cord is called Radial Plus Palsy.

What is Radial Plus Palsy?
Radial Plus Palsy involves radial nerve palsy plus axillary nerve palsy, because these are the two terminal branches of the posterior cord.

Symptoms include:
Weakness of the triceps, wrist, and finger extension, indicating that the injury to the radial nerve is located above the innervation of the triceps.
Radial nerve injury occurs in the axilla or higher, leading to wrist and finger extension weakness.
Cutaneous sensory deficits where the superficial branch of the radial nerve supplies sensation.
Axillary nerve injury causes:
Weakness of shoulder abduction due to deltoid muscle paralysis
Loss of sensation around the lateral shoulder
Teres minor weakness (important for external rotation of the shoulder)
Since the radial and axillary nerves are the two terminal branches of the posterior cord, their dysfunction indicates a posterior cord injury.

Other Branches of the Posterior Cord
Upper subscapular nerve → Innervates subscapularis muscle
Thoracodorsal nerve → Innervates latissimus dorsi, a muscle used in flap surgeries for lower extremity fractures (e.g., distal tibia) and shoulder tendon transfers for young patients with irreparable rotator cuff tears.
Lower subscapular nerve → Innervates subscapularis and teres major
Some people use the mnemonic "STARS" to remember the branches of the posterior cord:

S → Subscapular (Upper) Nerve
T → Thoracodorsal Nerve
A → Axillary Nerve
R → Radial Nerve
S → Subscapular (Lower) Nerve
Possible Exam Question:
A patient has a bullet injury to the axilla or a brachial plexus injury from a clavicle fracture. They present with weakness in the deltoid, triceps, and wrist extension. Which part of the brachial plexus is likely injured?
✅ Answer: The posterior cord
QUIZZES
1. What forms the posterior cord of the brachial plexus?
A. Posterior division of the upper trunk only
B. Posterior division of the lower trunk only
C. Posterior divisions of the upper, middle, and lower trunks ✅
D. Posterior division of the lateral cord
Explanation: The posterior cord is formed by the posterior divisions of all three trunks (upper, middle, and lower).

2. What are the two terminal branches of the posterior cord?
A. Median and musculocutaneous nerves
B. Ulnar and radial nerves
C. Axillary and radial nerves ✅
D. Ulnar and axillary nerves
Explanation: The posterior cord gives rise to the axillary nerve and radial nerve as its two terminal branches.

3. What muscle is primarily responsible for shoulder abduction and is affected in axillary nerve injury?
A. Supraspinatus
B. Deltoid ✅
C. Teres major
D. Latissimus dorsi
Explanation: The axillary nerve innervates the deltoid, which is responsible for shoulder abduction.

4. Which nerve from the posterior cord is used for muscle flap surgeries in limb reconstruction?
A. Axillary nerve
B. Radial nerve
C. Thoracodorsal nerve ✅
D. Lower subscapular nerve
Explanation: The thoracodorsal nerve innervates latissimus dorsi, a muscle commonly used in reconstructive flap surgeries.

5. Which muscle is weak if the axillary nerve is injured?
A. Teres major
B. Deltoid ✅
C. Trapezius
D. Pectoralis major
Explanation: The axillary nerve innervates the deltoid muscle, which is responsible for shoulder abduction.

6. Where would sensory loss occur in a patient with axillary nerve damage?
A. Medial forearm
B. Lateral shoulder ✅
C. Palmar thumb
D. Dorsum of the hand
Explanation: The axillary nerve supplies sensation to the lateral shoulder region.

7. Which nerve innervates the triceps and is responsible for elbow extension?
A. Median nerve
B. Ulnar nerve
C. Radial nerve ✅
D. Musculocutaneous nerve
Explanation: The radial nerve innervates the triceps, allowing for elbow extension.

8. Which muscle is NOT affected in a posterior cord injury?
A. Deltoid
B. Triceps
C. Biceps brachii ✅
D. Latissimus dorsi
Explanation: The biceps brachii is innervated by the musculocutaneous nerve, which comes from the lateral cord, not the posterior cord.

9. What is the primary function of the upper subscapular nerve?
A. Innervates subscapularis ✅
B. Innervates teres major
C. Innervates infraspinatus
D. Innervates supraspinatus
Explanation: The upper subscapular nerve innervates the subscapularis muscle, which contributes to internal rotation of the shoulder.

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Posterior cord brachial plexus, Radial nerve palsy, Axillary nerve injury, Brachial plexus injury, Wrist drop causes, Shoulder abduction weakness, Thoracodorsal nerve function, STARS mnemonic brachial plexus, Triceps weakness causes, Deltoid paralysis