Posterior interosseous nerve

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Posterior Interosseous Nerve (PIN)

PIN anatomy showing radial nerve bifurcation, arcade of Frohse, and course through supinator

Surgical anatomy of the PIN: the radial nerve divides into the superficial radial sensory nerve and the deep motor branch (PIN), which enters the supinator through the arcade of Frohse. ECU = extensor carpi ulnaris, EDC = extensor digitorum communis, ECRB/ECRL = extensor carpi radialis brevis/longus.

Origin and Course

The PIN is the deep motor branch of the radial nerve (C5-C8, T1), which arises from the posterior cord of the brachial plexus. The radial nerve divides just proximal to the lateral epicondyle into:

A superficial sensory branch (continues down the radial side of the forearm)
A deep motor branch - this becomes the PIN

Before becoming the PIN, the deep branch innervates the extensor carpi radialis brevis and gives branches to the supinator. It then pierces the supinator muscle through the arcade of Frohse - a fibrous tendinous arch at the proximal edge of the superficial layer of supinator. After emerging from between the two layers of the supinator muscle, it is formally called the posterior interosseous nerve and runs along the dorsal surface of the interosseous membrane.

The PIN terminates distally as articular branches that pass deep to the extensor pollicis longus to reach the wrist joint.

Gray's Anatomy for Students, p. 2722-2728
Imaging Anatomy Vol. 3, p. 3161

Muscles Innervated

The PIN supplies all extensor muscles of the forearm except those already innervated by the radial nerve proper proximal to the bifurcation:

Muscle	Action
Supinator	Supinates forearm
Extensor digitorum communis (EDC)	Extends MCP joints of fingers 2-5
Extensor digiti minimi	Extends little finger MCP
Extensor carpi ulnaris (ECU)	Extends and adducts wrist
Abductor pollicis longus (APL)	Abducts thumb
Extensor pollicis brevis (EPB)	Extends thumb MCP
Extensor pollicis longus (EPL)	Extends thumb IP joint
Extensor indicis proprius	Extends index finger independently

Spared (innervated proximal to PIN origin): brachioradialis, extensor carpi radialis longus (ECRL), and frequently extensor carpi radialis brevis (ECRB).

Key Anatomical Landmark: Arcade of Frohse

The arcade of Frohse is a fibrous arch formed by the proximal edge of the superficial layer of the supinator muscle. It is the most common site of PIN compression. The nerve is vulnerable here, especially with repetitive pronation/supination. This fibrous arch is present in ~30% of people as a thick tendinous band (vs. a fleshy edge), making compression more likely in those individuals.

Imaging Anatomy Vol. 3, p. 3161

PIN Syndrome (Posterior Interosseous Nerve Entrapment)

Sites of Compression (4 classic locations in the radial tunnel)

Fibrous edge of the extensor carpi radialis brevis origin
Adhesions around the radial head / radiocapitellar joint
Radial recurrent arterial fan (leash of Henry)
Arcade of Frohse (most common) - as the PIN enters the supinator; occasionally at the distal border as the nerve exits

Causes

Trauma (fracture-dislocations of the elbow, radial head fractures)
Inflammatory - especially rheumatoid arthritis (synovitis at the radiocapitellar joint)
Space-occupying lesions (lipoma, fibroma, ganglia, hemangioma of supinator, aneurysm of the posterior interosseous artery, schwannoma)
Repetitive occupational or athletic stress (violinists, orchestra conductors, swimmers, frisbee players, tennis players)
Volkmann ischemic contracture
Iatrogenic (radial head resection)
Campbell's Operative Orthopaedics 15th Ed, p. 8896-8920
Localization in Clinical Neurology 8e, p. 1695-1699

Spinner's Classification

Two types of PIN entrapment:

Complete - all PIN-supplied muscles paralyzed (EDC, EIP, EDM, ECU, APL, EPB)
Partial - only one or a few muscles are paralyzed

Clinical Features

Feature	Detail
Pain	Forearm pain, often over the radial tunnel (4-5 cm distal to lateral epicondyle)
Wrist extension	Preserved but deviates radially (ECU weak, ECRL/ECRB intact)
Finger extension	Loss of MCP extension ("drop-finger" deformity) - IP extension preserved via intrinsics
Thumb	Difficulty extending at IP joint, abducting thumb
Sensory loss	None - PIN is purely motor
Reflexes	Normal

Key distinguishing point: because ECRL is spared (branches off before PIN) but ECU is paralyzed, the wrist deviates radially on attempted extension. This differs from a high radial nerve palsy (wrist drop) where the wrist cannot extend at all.

An isolated "drop-thumb" deformity can occur if only the descending distal branch is compressed within the supinator (affecting EPL, EPB, APL, and sometimes EIP).

Rheumatology 2-Volume Set 2022, p. 2893-2904
Localization in Clinical Neurology 8e, p. 1703-1707

Diagnosis

Clinical: Radial tunnel tenderness (distal to lateral epicondyle), pain on resisted supination, pain on resisted extension of the long finger (middle finger test)
EMG/NCS: Confirms denervation in PIN-supplied muscles; rarely diagnostic for radial tunnel syndrome (pain only), but useful to exclude radiculopathy
MRI: Identifies space-occupying lesions; can show muscle denervation changes
Differential: Lateral epicondylitis (tennis elbow), posterior cord lesion, C7 radiculopathy

Note: A 2011 study found that 84% of PIN syndromes actually had proximal radial nerve lesions at the upper arm level (partial fascicular lesions) rather than focal compression at the arcade of Frohse - making neuroimaging an important complementary tool. (Localization in Clinical Neurology 8e, p. 1701)

Treatment

Conservative (first-line)

Rest, NSAIDs, activity modification
Splinting in wrist/finger extension
Corticosteroid injection into the radial tunnel
Physical therapy

Surgical

Indicated when conservative management fails (typically after 3-6 months), or when a space-occupying lesion is identified
Surgical decompression of the PIN at the radial tunnel - releasing all four potential compression sites (Campbell's 15th Ed, p. 8950-8956)
For complete PIN palsy with no recovery after 3-6 months, nerve exploration and repair/grafting may be needed
Tendon transfers can restore function if nerve recovery is not expected

Summary: PIN vs. Radial Nerve Palsy

	High Radial Nerve Palsy	PIN Syndrome
Wrist extension	Lost (wrist drop)	Radially deviated (preserved but weak)
Finger MCP extension	Lost	Lost
Brachioradialis	Weak	Normal
Sensory loss	Dorsal hand/thumb	None
Reflexes	Brachioradialis reduced	Normal

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