Posterior interosseous nerve !

The posterior interosseous nerve is the deep branch of the radial nerve in the forearm; it moves the muscles that straighten the fingers and thumb.

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What it is

The deep branch of the radial nerve that passes through the supinator; it can be squeezed there (radial tunnel) and must be protected in distal biceps and radial head surgery.

What it does

Powers the finger and thumb extensors.

Key facts

Roots: C7–C8, deep branch of the radial nerve
Moves: Supinator, ECRB, extensor digitorum, EDM, ECU, APL, EPB, EPL, extensor indicis
Feeling from: No skin (wrist joint only)

Relations

Course: Arises from the radial nerve in front of the lateral epicondyle, passes under the edge of ECRB and the arcade of Frohse into supinator, winds round the lateral side of the radial neck between its layers, emerges in the back of the forearm, runs on the interosseous membrane and ends in the floor of the fourth extensor compartment
In front: At the radial tunnel, the radial recurrent vessels (leash of Henry) and the edge of extensor carpi radialis brevis
Deep: The neck of the radius, with only the deep layer of supinator between them
Superficial: The superficial extensors (extensor digitorum, extensor digiti minimi, ECU) once it leaves supinator

How to find it

Not palpable. In a posterior (Thompson) approach it is found as it leaves the distal edge of supinator between ECRB and extensor digitorum; at the wrist its terminal branch lies on the floor of the fourth extensor compartment, under the extensor digitorum tendons.

Examination

Resisted finger extension at the MCP joints with the wrist supported: weakness with intact sensation and wrist extension points to the PIN
Resisted middle finger extension: pain over the radial tunnel, distal to the lateral epicondyle, suggests radial tunnel syndrome rather than lateral epicondylitis
Tenderness over the supinator, a few finger-breadths distal to the lateral epicondyle, rather than at the epicondyle itself

On imaging

Ultrasound follows it from the radial nerve split into the supinator under the arcade of Frohse: look for swelling just before the arcade or a compressing mass (ganglion, lipoma, elbow synovitis). MRI shows denervation oedema, later fatty atrophy, of the extensors. After a Monteggia injury or radial head dislocation, radiographs show the injury that can stretch it.

Common problems

Injured with fractures of the humerus shaft, where it wraps around the bone: wrist drop.
'Saturday night palsy' from pressure on the arm during deep sleep.
Radial tunnel syndrome: aching in the forearm from compression near the supinator.

How it is diagnosed

Exam of wrist and finger lifting strength and of feeling on the back of the hand. EMG and nerve conduction studies.

Treatment

A wrist splint and watching for recovery; most pressure and fracture-related injuries recover.
Nerve exploration, repair or tendon transfers when it does not recover.

Clinical notes

Same tunnel, two pictures: radial tunnel syndrome is pain without weakness; PIN syndrome is weakness with little pain
At risk with radial head and neck fractures, two-incision distal biceps repair, plating of the proximal radius and posterior forearm approaches
Masses at the elbow (lipoma, ganglion, rheumatoid synovitis) can compress it and present as painless finger drop
Its terminal branch in the fourth compartment is divided in wrist denervation and can serve as a graft for digital nerves

Anatomical variants

The arcade of Frohse, the upper edge of supinator, is often tendinous in adults, and the nerve may pass between or through the two heads of supinator

Memory aid

Radial tunnel compression sites, 'FREAS': Fibrous bands, Recurrent radial vessels (leash of Henry), ECRB edge, Arcade of Frohse, Supinator distal edge.

Lateral epicondylitis (tennis elbow)

Test yourself

Think of the answer, then tap the question to see it.

How does posterior interosseous nerve palsy look?

Fingers and thumb drop at the MCP joints, but the wrist still extends in radial deviation (ECRL spared, ECU lost) and sensation is normal.

Which fracture-dislocation classically injures the posterior interosseous nerve?

The Monteggia fracture-dislocation: the dislocated radial head stretches the nerve.

Under which structure does the posterior interosseous nerve enter supinator?

The arcade of Frohse, the upper edge of supinator, often tendinous in adults.

How does radial tunnel syndrome differ from posterior interosseous nerve syndrome?

Same tunnel, two pictures: radial tunnel syndrome is pain without weakness; PIN syndrome is weakness with little pain.

Why is the forearm pronated in lateral approaches to the radial head and neck?

Pronation moves the posterior interosseous nerve away from the surgical field.

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2): ramus profundus nervi radialis TA2 6436
Official Spanish (DeCS): Ramo profundo del nervio radial; nervio interóseo posterior del antebrazo
English (TA2): Posterior interosseous nerve

Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Radial Nerve; Anatomy, Shoulder and Upper Limb, Forearm Nerves.

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Representative model, for educational purposes only.

Posterior interosseous nerve highlighted on the Durgical 3D model