Largest branch of brachial plexus. Main nerve of posterior compartments of arm and forearm.

Origin and Root Value

Course

Axilla

• Posterior to 3rd part of axillary artery
• Leaves via lower triangular space (boundaries: teres major above, long head triceps medially, shaft of humerus laterally) with profunda brachii artery

Radial Groove

• Between lateral + medial heads of triceps; directly on posterior humerus
• Accompanied by profunda brachii artery → injury in mid-shaft humeral fracture

Lateral Side of Arm

• Pierces lateral intermuscular septum ~5 cm above lateral epicondyle
• Descends between brachialis (medial) and brachioradialis + ECRL (lateral)

Cubital Fossa — Medial to Lateral Order

Median nerve → Brachial artery → Biceps tendon → Radial nerve (MBBR)

Divides just below lateral epicondyle into:

1. Deep branch → posterior interosseous nerve (motor)
2. Superficial branch (sensory)

Branches and Motor Supply

Axilla

• Posterior cutaneous nerve of arm; nerve to long + medial head of triceps

Radial Groove

• Nerve to lateral + medial head of triceps; nerve to anconeus; lower lateral cutaneous nerve of arm; posterior cutaneous nerve of forearm

Below Groove (Before Division)

• Brachioradialis; ECRL; small branch to lateral brachialis; articular branches to elbow

Deep Branch / Posterior Interosseous Nerve

• ECRB; Supinator; Extensor digitorum; EDM; ECU; APL; EPB; EPL; Extensor indicis
• No cutaneous distribution; articular branches to wrist + carpal joints

Superficial Branch

• Lateral 2/3 dorsum of hand; dorsal surface of thumb; proximal dorsal surfaces of index, middle, lateral ½ ring finger
• Autonomous area: 1st dorsal web space

Applied Anatomy

Axillary injury (Crutch Palsy)

• Triceps paralysed + wrist drop + finger/thumb extension lost
• Sensory: posterior arm + forearm + lateral dorsum of hand

Radial Groove Injury (Saturday Night Palsy)

• Cause: mid-shaft humeral fracture; compression during sleep/intoxication
• Wrist drop; triceps largely spared (branches to long + medial heads arise proximal to groove)
• Sensory: 1st dorsal web space most reliable

Posterior Interosseous Nerve (PIN) Palsy

• Cause: radial tunnel entrapment, arcade of Frohse, neck of radius fracture-dislocation
• Finger/thumb extension weakness; wrist extension preserved but radially deviated (ECRL intact, ECU paralysed)
• No cutaneous sensory loss

Wartenberg’s Syndrome

• Superficial radial nerve compressed beneath brachioradialis near wrist
• Pain + tingling over dorsoradial hand; no motor weakness; mimics de Quervain’s but purely sensory

Lesion Level Summary

The radial nerve is the largest branch of the brachial plexus and the main nerve of the posterior compartments of the arm and forearm. Its spiral course around the humerus makes it clinically important in mid-shaft humeral fractures and compression palsies.

Core exam idea:

Posterior cord → axilla → radial groove → lateral arm → cubital fossa → deep motor branch + superficial sensory branch

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Origin and Root Value

The dominant functional roots are C7 and C8, but the nerve carries fibres from all five roots of the brachial plexus.

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Relations in the Axilla

The radial nerve arises in the axilla posterior to the third part of the axillary artery.

It leaves the axilla through the lower triangular space with the profunda brachii artery.

Lower triangular space boundaries:

Do not confuse this with the quadrangular space, which transmits the axillary nerve and posterior circumflex humeral vessels.

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Course of the Radial Nerve

1. Axilla

In the axilla, the radial nerve lies posterior to the third part of the axillary artery and anterior to the posterior wall of the axilla.

Branches in the axilla:

• Posterior cutaneous nerve of arm
• Nerve to long head of triceps
• Nerve to medial head of triceps

2. Upper Arm

The radial nerve enters the posterior compartment of the arm through the lower triangular space with the profunda brachii artery.

It passes between the long and medial heads of triceps and reaches the radial groove.

3. Radial Groove

In the radial groove, the nerve:

• Lies directly on the posterior surface of the humerus
• Passes downwards and laterally
• Runs between the lateral and medial heads of triceps
• Is accompanied by the profunda brachii artery

This close bony relation explains injury in mid-shaft fracture of the humerus.

Branches in the radial groove:

• Nerve to lateral head of triceps
• Additional branch to medial head of triceps
• Nerve to anconeus
• Lower lateral cutaneous nerve of arm
• Posterior cutaneous nerve of forearm

4. Lateral Side of Arm

About 5 cm above the lateral epicondyle, the radial nerve pierces the lateral intermuscular septum and enters the anterior compartment of the arm.

It descends between:

• Brachialis medially
• Brachioradialis and extensor carpi radialis longus laterally

Branches here:

• Brachioradialis
• Extensor carpi radialis longus
• Small branch to lateral part of brachialis, mainly proprioceptive/minor motor
• Articular branches to elbow joint

5. Cubital Fossa and Termination

In the cubital fossa, the radial nerve lies deep in the lateral part of the fossa, lateral to the biceps tendon.

Medial-to-lateral order in the cubital fossa:

Median nerve → Brachial artery → Biceps tendon → Radial nerve

Memory: MBBR

The radial nerve divides just below the lateral epicondyle into:

1. Deep branch: motor branch; continues as posterior interosseous nerve
2. Superficial branch: sensory branch

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Branches and Distribution

Branches in the Axilla

Branches in the Radial Groove

Branches Below the Radial Groove

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Deep Branch and Posterior Interosseous Nerve

The deep branch is the motor terminal branch of the radial nerve.

Course:

1. Arises in the cubital fossa
2. Passes between the two planes of supinator
3. Winds around the lateral side of the neck of radius
4. Enters the posterior compartment of forearm
5. Continues as the posterior interosseous nerve

Important supplies:

• Extensor carpi radialis brevis
• Supinator
• Extensor digitorum
• Extensor digiti minimi
• Extensor carpi ulnaris
• Abductor pollicis longus
• Extensor pollicis brevis
• Extensor pollicis longus
• Extensor indicis

The posterior interosseous nerve is essentially motor but gives articular branches to wrist and carpal joints. It has no cutaneous distribution.

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Superficial Branch of the Radial Nerve

The superficial branch is the sensory terminal branch.

Course

Cutaneous Supply

It supplies:

• Lateral two-thirds of dorsum of hand
• Dorsal surface of thumb
• Proximal dorsal surfaces of index, middle, and lateral half of ring finger

The distal dorsal parts and nail beds of the lateral 3.5 digits are supplied mainly by the median nerve, not the radial nerve.

Autonomous sensory area: first dorsal web space.

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Muscles Supplied by the Radial Nerve

In the Arm

In the Forearm

Direct branches before division:

• Brachioradialis
• Extensor carpi radialis longus
• Small branch to brachialis

Deep branch / posterior interosseous nerve:

• Extensor carpi radialis brevis
• Supinator
• Extensor digitorum
• Extensor digiti minimi
• Extensor carpi ulnaris
• Abductor pollicis longus
• Extensor pollicis brevis
• Extensor pollicis longus
• Extensor indicis

Key exam distinction:

• Brachioradialis and ECRL are supplied by the radial nerve before terminal division.
• ECRB and supinator are supplied by the deep branch.
• Most remaining forearm extensors are supplied by the posterior interosseous nerve.

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Cutaneous Distribution

Because of overlap, the most reliable sensory loss in radial nerve injury is over the first dorsal web space.

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Applied Anatomy

Injury in the Axilla: Crutch Palsy

Cause:

• Pressure from an improperly used crutch
• Severe injury in axilla

Motor effects:

• Triceps paralysed → loss of elbow extension
• Wrist extensors paralysed → wrist drop
• Finger and thumb extensors paralysed
• Supination weak, especially with elbow extended

Sensory loss:

• Posterior arm
• Posterior forearm
• Lateral dorsum of hand
• First dorsal web space

This is the highest radial nerve lesion, so it produces the most extensive motor loss.

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Injury in the Radial Groove: Saturday Night Palsy

Causes:

• Mid-shaft fracture of humerus
• Compression of arm over the back of a chair during deep sleep or intoxication
• Prolonged external pressure on posterior arm

Motor effects:

• Wrist drop due to paralysis of wrist extensors
• Loss of finger and thumb extension
• Triceps usually spared or only weakened, because branches to long and medial heads often arise proximally

Sensory loss:

• Lower lateral arm and posterior forearm may be affected
• First dorsal web space sensory loss is the most reliable sign

Important exam phrase: Radial groove injury causes wrist drop with triceps largely spared.

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Injury Near the Elbow: Posterior Interosseous Nerve Palsy

Cause:

• Entrapment at radial tunnel
• Compression at Arcade of Frohse
• Fracture/dislocation around head or neck of radius

Features:

• Weak extension of fingers and thumb
• Weak extension at MCP joints
• Wrist extension may be preserved but deviates radially because ECRL is intact and ECU is paralysed
• No cutaneous sensory loss, because posterior interosseous nerve is motor
• Pain over proximal lateral forearm may occur in radial tunnel syndrome

This lesion does not produce a classic complete wrist drop if ECRL is intact.

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Superficial Radial Nerve Injury: Wartenberg’s Syndrome

The superficial radial nerve may be compressed as it passes beneath brachioradialis near the wrist.

Features:

• Pain, burning, tingling, or numbness over dorsoradial hand
• Sensory symptoms over thumb and first web space
• No motor weakness
• May mimic de Quervain’s tenosynovitis, but de Quervain’s causes tendon pain rather than pure sensory paraesthesia

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Wrist Drop

Wrist drop occurs because wrist extensors are paralysed while wrist flexors continue to act unopposed.

Consequences:

• Hand hangs flexed at the wrist
• Grip is weak because finger flexors work best when the wrist is extended
• MCP extension of fingers is lost
• Thumb extension and abduction are weak or lost

The level of the lesion determines whether triceps is involved:

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Rapid Revision Table

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