Deep Branch of the Radial Nerve: Anatomy, Course, Function, Imaging

Topic

Radial nerve (deep branch)

The deep branch of the radial nerve is the motor continuation of the radial nerve in the forearm. It originates near the lateral epicondyle of the humerus, passes through the supinator muscle, and supplies the extensor muscles of the forearm. After emerging from the supinator, it continues as the posterior interosseous nerve (PIN).

It provides motor innervation to most of the posterior compartment muscles of the forearm and has an important clinical role in radial tunnel syndrome and posterior interosseous nerve palsy, which cause weakness of wrist and finger extension.

Synonyms

Motor branch of the radial nerve
Radial nerve (deep branch)
Proximal segment of the posterior interosseous nerve

Origin, Course, and Termination

Origin: Arises from the radial nerve in the cubital fossa, anterior to the lateral epicondyle of the humerus.
Course:
- Passes between the two heads of the supinator muscle (forming the Arcade of Frohse) — a common site of compression.
- Winds laterally around the radius, within the supinator canal, to reach the posterior compartment of the forearm.
- Descends on the posterior surface of the interosseous membrane between the superficial and deep extensor muscles.
Termination: Continues distally as the posterior interosseous nerve, which ends in the dorsal wrist capsule and interosseous membrane.

Relations

Anteriorly: Brachialis, supinator, and radial recurrent vessels
Posteriorly: Supinator muscle and proximal radius
Laterally: Brachioradialis and extensor carpi radialis longus
Medially: Biceps tendon and brachialis near its origin
Distally: Lies between extensor digitorum and extensor carpi ulnaris muscles

Branches

Muscular branches:
- To the supinator (as it passes through it)
- To the extensor carpi radialis brevis
- To the extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, abductor pollicis longus, extensor pollicis longus and brevis, and extensor indicis (via the posterior interosseous nerve continuation)
Articular branches: To the elbow and wrist joint capsules

Function

Motor:
- Supplies the supinator and all extensors in the posterior forearm (except brachioradialis and extensor carpi radialis longus, which are supplied by the superficial branch).
- Enables wrist extension, finger extension, and thumb abduction/extension.
Sensory:
- The deep branch itself carries no cutaneous fibers, but contributes proprioceptive fibers to the wrist and elbow joint capsules.
Functional role:
- Critical for coordinated hand and wrist extension and stabilization during gripping and lifting.

Clinical Significance

Radial tunnel syndrome: Compression of the deep branch within the supinator (Arcade of Frohse) leads to forearm pain and weakness without sensory loss.
Posterior interosseous nerve palsy: More distal compression causing finger and thumb drop with preserved wrist extension.
Traumatic injury: May result from fractures of the proximal radius, surgical exposure, or iatrogenic causes.
Surgical relevance: Must be identified during approaches to the radial head, lateral epicondyle, or proximal radius.
Imaging importance: MRI and CT neurography are valuable for evaluating nerve entrapment, trauma, and postoperative scarring.

MRI Appearance

T1-weighted images:
- Normal nerve: Intermediate signal intensity, appearing as a slender tubular structure with smooth margins.
- Surrounding fat: Bright, creating contrast for clear nerve visualization.
- Entrapment or edema: Nerve enlargement with blurred margins and slightly increased signal.
- Fibrosis or chronic injury: Low signal intensity within nerve due to fibrotic change.
T2-weighted images:
- Normal nerve: Intermediate signal.
- Pathology: Bright hyperintense signal within the nerve and surrounding fascial planes in neuritis or compression.
- Muscle denervation: Early stages show mild hyperintensity; chronic stages show fatty infiltration (bright on T1).
STIR:
- Normal: Intermediate-to-dark nerve signal.
- Pathology: Marked hyperintensity in nerve or adjacent tissues, indicating edema or acute neuritis.
- Excellent for detecting radial tunnel syndrome and perineural inflammation.
Proton Density Fat-Saturated (PD FS):
- Normal nerve: Homogeneous intermediate-to-dark signal.
- Entrapped or injured nerve: Bright hyperintensity, fusiform enlargement, and loss of fascicular pattern.
- Distal denervation changes in target muscles appear as focal hyperintensity on PD FS.
T1 Fat-Sat Post-Contrast:
- Normal nerve: Minimal or no enhancement.
- Inflamed or compressed nerve: Focal or linear enhancement along its course through the supinator canal.
- Post-surgical or chronic scarring: Irregular enhancement in surrounding fibrotic tissue.

CT Appearance

Non-Contrast CT:

Nerve itself poorly visualized; may appear as a small soft-tissue density between supinator fibers.
Indirect signs: Atrophy or fatty infiltration in the supinator or extensor muscles indicating chronic denervation.
Bone changes: Evaluate for radial head or proximal radius fractures causing nerve entrapment.

Post-Contrast CT (standard):

Enhancing perineural tissues may indicate inflammation or fibrosis.
Entrapment neuropathy: Subtle soft-tissue thickening or narrowing of the supinator canal.
Useful for preoperative planning and assessing postoperative scarring or bone deformities compressing the nerve.

MRI image

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