The posterior interosseous nerve is the deep, predominantly motor branch of the radial nerve after it divides at the lateral elbow. It supplies all the forearm extensors except the extensor carpi radialis longus (which receives a direct radial nerve branch above the division). Its passage through the supinator muscle between the superficial and deep heads creates the radial tunnel where compression produces posterior interosseous nerve syndrome.
| Origin | Deep branch of the radial nerve, arising at the level of the lateral epicondyle |
|---|---|
| Course | Arises as the deep branch of the radial nerve in front of the lateral epicondyle between brachialis and brachioradialis, supplies extensor carpi radialis brevis and supinator, passes under the arcade of Frohse (the fibrous upper edge of supinator), winds around the lateral radial neck between the two layers of supinator, and runs on the posterior interosseous membrane to the wrist |
| Motor | |
| Sensory |
|
Posterior interosseous nerve syndrome produces weakness of finger and thumb extension without sensory loss, because the nerve has no cutaneous territory. The wrist can still be extended, but drifts toward the radial side, because extensor carpi radialis longus is supplied above the division and extensor carpi ulnaris is lost. The commonest compression site is the arcade of Frohse, the fibrous edge of supinator; other causes include lipomas and ganglia, radial head fracture or dislocation, and rheumatoid synovitis at the radial head. EMG shows denervation of the finger and thumb extensors with sparing of the radial wrist extensors. Radial tunnel syndrome is a related pain syndrome without weakness, with lateral forearm aching about 4 cm below the lateral epicondyle that worsens with resisted supination.
Compression, most often at the arcade of Frohse, causes painless weakness of finger and thumb extension without wrist drop or sensory loss; decompression is done if there is a structural cause or no recovery.
Lateral forearm aching over the supinator without measurable weakness, reproduced by resisted supination or middle finger extension, usually managed conservatively and occasionally by surgical release.
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