Radial Nerve: Anatomy, Course, Branches, Function, Imaging

Topic

Radial nerve

The radial nerve is the largest branch of the posterior cord of the brachial plexus (C5–T1). It is a mixed motor and sensory nerve that supplies the posterior compartments of the arm and forearm, responsible for extension of the elbow, wrist, and fingers, and sensation to the posterior limb and dorsum of the hand.

It is the continuation of the posterior cord, coursing posterior to the humerus in the radial (spiral) groove, before passing anteriorly at the lateral elbow. The radial nerve then divides into superficial (sensory) and deep (motor/posterior interosseous) branches near the lateral epicondyle.

Due to its long, exposed course around the humerus, the radial nerve is vulnerable to compression and trauma, particularly in humeral shaft fractures and entrapment syndromes.

Synonyms

Musculo-spiral nerve (historic term)
Posterior cord branch of the brachial plexus

Origin and Course

Origin: Posterior cord of the brachial plexus (roots C5–T1) in the axilla, posterior to the axillary artery.
Course:
- Axilla: Lies posterior to the axillary artery, accompanied by the posterior cutaneous nerve of the arm.
- Arm: Enters the posterior compartment through the triangular interval (between long and lateral heads of triceps), then descends obliquely along the radial groove of the humerus with the deep brachial nerve.
- Distal arm: Pierces the lateral intermuscular septum to enter the anterior compartment, lying between brachialis and brachioradialis.
- At elbow: Lies anterior to the lateral epicondyle, where it divides into:
  - Superficial branch: Sensory, runs under brachioradialis to dorsum of hand.
  - Deep branch (posterior interosseous nerve): Motor, passes through the supinator muscle to posterior forearm.

Branches

In the axilla:
- Posterior cutaneous nerve of arm
- Branch to long head of triceps
In the arm:
- Inferior lateral cutaneous nerve of arm
- Posterior cutaneous nerve of forearm
- Muscular branches to triceps and anconeus
At the elbow:
- Brachioradialis and extensor carpi radialis longus branches
- Terminal branches: superficial radial nerve (sensory) and posterior interosseous nerve (motor)

Relations

In axilla: Posterior to axillary artery; medial to long head of triceps
In radial groove: Lies between the long and lateral heads of triceps
In distal arm: Between brachialis (medial) and brachioradialis (lateral)
At lateral epicondyle: Anterior to the capsule of the elbow joint
In forearm:
- Superficial branch: Deep to brachioradialis, lateral to radial artery
- Deep branch: Pierces supinator to enter posterior compartment

Nerve Supply and Distribution

Motor:
- Arm: Triceps brachii, anconeus
- Forearm: Brachioradialis, extensor carpi radialis longus
- Deep branch (posterior interosseous nerve): All extensor muscles of wrist and digits
Sensory:
- Posterior arm and forearm
- Dorsum of the hand (lateral side)
- Dorsal aspect of lateral 3½ digits (proximal phalanges only)

Function

Motor function:
- Extension of elbow, wrist, and fingers
- Supination of forearm (via supinator and biceps)
Sensory function:
- Cutaneous innervation of posterior arm, forearm, and dorsum of the hand
Reflex contribution:
- Afferent and efferent limb of triceps jerk reflex

Clinical Significance

Radial nerve palsy:
- Caused by compression (“Saturday night palsy”) or humeral shaft fracture
- Presents with wrist drop, loss of finger extension, and sensory loss on dorsum of hand
Posterior interosseous nerve syndrome:
- Pure motor weakness of extensors without sensory deficit
Superficial radial neuropathy (Wartenberg syndrome):
- Sensory loss or paresthesia over dorsal thumb web space due to compression near wrist
Iatrogenic injury:
- Occurs in humeral fracture fixation, intramuscular injections, or surgical dissection
Imaging relevance:
- MRI and CT neurography essential for localizing nerve entrapment, injury, or tumor infiltration

MRI Appearance

T1-weighted images:
- Normal nerve: Intermediate signal intensity (similar to muscle but slightly brighter)
- Surrounding fat: Bright, sharply delineating nerve contour
- Pathology: Loss of normal fascicular pattern, focal thickening, or hypointense discontinuity in laceration
- Fat planes: Help differentiate nerve from adjacent muscle and vessels
T2-weighted images:
- Normal nerve: Intermediate-to-low signal, darker than fat but slightly brighter than muscle
- Pathology: Bright hyperintense signal in neuritis, edema, or compressive neuropathy
- Useful for detecting signal change in entrapment zones (spiral groove, supinator canal)
STIR:
- Normal nerve: Intermediate-to-dark signal
- Abnormal nerve: Bright hyperintense signal in inflammation, trauma, or edema
- Surrounding muscles: May show edema or denervation signal changes in acute palsy
Proton Density Fat-Saturated (PD FS):
- Normal: Intermediate-to-dark signal with clear fascicular texture
- Pathology: Bright hyperintensity or focal thickening in compression or injury
- Excellent for assessing subtle neuritis, traction injury, or postoperative scarring
T1 Fat-Sat Post-Contrast:
- Normal nerve: Minimal or no enhancement
- Inflamed or neoplastic nerve: Patchy or nodular enhancement
- Post-traumatic neuroma: Focal enhancing mass at injury site
- Entrapment neuropathy: Enhancement at compression points (e.g., supinator tunnel)

CT Appearance

Non-Contrast CT:

Nerve appears as a soft-tissue density structure adjacent to humerus or radial artery
Indirect signs: bony callus, fracture fragments, or soft-tissue swelling causing compression
Chronic injury may show atrophy of innervated muscles or fatty infiltration

Post-Contrast CT (standard):

Nerve itself shows minimal enhancement
Enhancement of surrounding soft tissue may suggest neuritis, entrapment, or fibrosis
Excellent for correlating osseous and hardware-related causes of radial neuropathy

MRI images

MRI image

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