Radial Nerve

Root Origin

• Spinal nerve roots: C5, C6, C7, C8, T1
• Plexus: Brachial plexus
• Cord: Posterior cord (the radial nerve is the larger terminal branch; the smaller is the axillary nerve)
• Type: Mixed (motor and sensory)

Course

1. Posterior cord. The radial nerve is the direct continuation of the posterior cord. It begins in the axilla, posterior to the axillary artery and anterior to the subscapularis, latissimus dorsi, and teres major.

1. Axilla. The nerve passes posterior to the brachial artery and exits the axilla between the long head of the triceps (medially) and the humerus (laterally). It gives off the posterior cutaneous nerve of the arm before entering the radial groove.

1. Radial groove (spiral groove). The nerve enters the radial groove on the posterior surface of the humeral shaft and spirals from medial to lateral, traveling between the medial and lateral heads of the triceps. It is in direct contact with the periosteum of the humerus here — this is why mid-shaft humeral fractures so commonly damage the nerve. Along this course it supplies all three heads of the triceps.

1. Lateral arm. The nerve pierces the lateral intermuscular septum approximately 10 cm above the lateral epicondyle, entering the anterior compartment of the arm. It travels between the brachialis (medially) and brachioradialis (laterally), giving off branches to brachioradialis and ECRL before reaching the elbow.

1. Anterior elbow. At the lateral elbow, anterior to the lateral epicondyle, the nerve divides into its two terminal branches: the posterior interosseous nerve (deep branch — motor) and the superficial branch (sensory).

1. Posterior interosseous nerve (PIN). The motor branch enters the forearm by passing between the two heads of the supinator muscle, diving through the arcade of Frohse — a fibrous arch at the proximal edge of the supinator's superficial head. This is the primary entrapment site for conditions/radial-tunnel-syndrome. After exiting the supinator, the PIN descends along the posterior interosseous membrane, supplying every extensor in the forearm.

1. Superficial branch. The sensory branch descends the forearm beneath the brachioradialis, then emerges superficially by wrapping around the radius approximately 8 cm above the wrist. It crosses the anatomical snuffbox tendons and supplies the dorsal hand and proximal phalanges of the thumb, index, middle, and radial half of the ring finger.

Motor Distribution

Arm — Main Trunk

Forearm — Main Trunk (Before Division)

Forearm — Posterior Interosseous Nerve (Deep Motor Branch)

Memory aid: The radial nerve supplies every muscle on the back of the arm and forearm — if it extends something, the radial nerve innervates it. The only exception: lumbricals and interossei extend the IP joints through the extensor expansion, and they are median/ulnar.

Sensory Distribution

• Posterior cutaneous nerve of the arm. Branches in the axilla. Supplies the skin over the posterior arm (triceps region).
• Lower lateral cutaneous nerve of the arm. Branches near the radial groove. Supplies the lateral arm below the deltoid.
• Posterior cutaneous nerve of the forearm. Branches in the radial groove. Supplies a narrow strip of skin down the posterior forearm from elbow to wrist.
• Superficial branch of the radial nerve. The terminal sensory branch supplies the dorsal hand — specifically the dorsum of the thumb, index finger, middle finger, and radial half of the ring finger proximal to the fingernails. The dorsal fingertips distal to the DIP joint are supplied by the median and ulnar nerves, not the radial nerve.
• Clinical significance: Radial sensory territory covers the dorsal hand but stops short of the fingertips. Numbness on the dorsal hand with wrist drop confirms a radial nerve lesion above the elbow. Numbness on the dorsal hand without wrist drop (motor sparing) suggests compression of the superficial branch alone — Wartenberg syndrome.

Entrapment Sites

1. Radial Groove (Spiral Groove Compression)

• Location: Mid-shaft of the humerus, where the nerve lies directly against the bone in the radial groove
• Structure: The nerve is pressed against the humeral periosteum. External compression (arm draped over a chair, crutch pressure) or humeral shaft fractures damage the nerve at this level
• Condition: Saturday night palsy (compression), Holstein-Lewis fracture syndrome (fracture-related)
• Presentation: Wrist drop (cannot extend the wrist or fingers) with preserved triceps function — because the branches to triceps leave the main trunk before the radial groove. Numbness over the posterior forearm and dorsal hand. The classic "Saturday night palsy" occurs when a person falls asleep with the arm draped over a hard surface, compressing the nerve against the humerus.
• MT relevance: Saturday night palsy is usually a neurapraxia (bruising without nerve disruption) and recovers fully in weeks to months. Recognize the pattern: wrist drop with intact triceps and sensory loss over the dorsal hand. Fracture-related radial nerve palsy requires medical referral.

2. Lateral Intermuscular Septum

• Location: Where the nerve pierces the lateral intermuscular septum, approximately 10 cm above the lateral epicondyle
• Structure: The septum opening can compress the nerve, particularly in the setting of fractures with callus formation or after surgical plating of the humerus
• Presentation: Similar to radial groove compression — wrist drop with triceps sparing — because the compression occurs below the triceps branches
• MT relevance: Rare in isolation. Most relevant post-surgically. If a patient has radial nerve symptoms after humeral fracture fixation, the hardware or callus at the septum may be the culprit.

3. Arcade of Frohse (Radial Tunnel)

• Location: Proximal forearm, where the posterior interosseous nerve passes through the arcade of Frohse — the fibrous proximal edge of the supinator's superficial head
• Structure: The arcade is a fibrous arch (present in about 30% of people; muscular in the rest). It compresses the purely motor PIN, not the sensory branch.
• Condition: conditions/radial-tunnel-syndrome / posterior interosseous nerve syndrome
• Presentation: Two presentations depending on severity. Radial tunnel syndrome: lateral forearm pain without motor loss — essentially lateral elbow pain that does not respond to lateral epicondylalgia treatment. The pain is located approximately 3-4 cm distal to the lateral epicondyle, over the radial tunnel, rather than directly at the epicondyle. Posterior interosseous nerve syndrome (more severe): finger drop and weak wrist extension with radial deviation (ECRL is spared because it is innervated above the tunnel, so the wrist deviates radially when attempting extension). No numbness — the PIN is purely motor.
• Key differentiator from lateral epicondylalgia: Tenderness is maximal over the supinator, not the lateral epicondyle. Resisted middle finger extension (which tenses ECRB through the radial tunnel) reproduces the pain. Standard lateral epicondylalgia treatment (eccentric loading, grip work) does not resolve symptoms.
• MT relevance: Supinator release and forearm extensor work are first-line manual approaches. Use caution with deep work directly over the radial tunnel — the nerve is superficial to the supinator and easily irritated.

4. Wartenberg Point (Superficial Branch)

• Location: Distal forearm, where the superficial branch of the radial nerve emerges between the brachioradialis and ECRL tendons approximately 8 cm above the wrist
• Structure: The nerve becomes superficial by wrapping around the radial side of the forearm. It can be compressed by tight watchbands, handcuffs, or pronation with wrist flexion (which stretches the nerve against the radius)
• Condition: Wartenberg syndrome (cheiralgia paresthetica)
• Presentation: Burning pain or numbness over the dorsal thumb and first web space. No motor loss (superficial branch is purely sensory). Often confused with de Quervain tenosynovitis because both produce radial wrist pain.
• MT relevance: Remove external compressors (watchband, bracelet). Nerve gliding exercises and soft tissue release of the brachioradialis-ECRL interval can reduce symptoms. Differentiate from de Quervain's: Finkelstein test is negative in Wartenberg syndrome.

Clinical Tests

Related Nerves

• anatomy/nerves/axillary-nerve — The radial nerve's smaller sibling from the posterior cord. A posterior cord lesion affects both — producing deltoid weakness (axillary) plus wrist drop (radial). If radial nerve deficits are accompanied by deltoid weakness and regimental badge numbness, the lesion is at the posterior cord, not the radial groove.
• anatomy/nerves/median-nerve — Lateral and medial cords. The median nerve runs alongside the radial nerve in the arm but supplies the opposite compartment (flexors vs. extensors). Their sensory territories meet at the dorsal fingertips — the radial nerve supplies the dorsal hand proximal to the nails, while median digital branches supply the fingertips.
• anatomy/nerves/ulnar-nerve — Medial cord. Shares the dorsal hand sensory territory with the radial nerve (ulnar side vs. radial side). Also provides the interossei, which extend IP joints via the extensor expansion — a function that persists even when the radial nerve is damaged.