The supinator wraps around the proximal radius and the posterior interosseous nerve pierces through it under the Arcade of Frohse (the fibrous proximal edge of the supinator). This is the most common site of PIN entrapment.
| Origin | Lateral epicondyle, radial collateral ligament, annular ligament, and supinator crest of the ulna |
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| Insertion | Lateral, posterior, and anterior surfaces of the proximal radius — below the radial tuberosity |
| Nerve Supply | Posterior interosseous nerve (C7, C8) — the nerve pierces through the supinator (Arcade of Frohse) |
| Blood Supply | Radial recurrent and posterior interosseous arteries |
| Actions | Supinates the forearm — the only forearm supinator active in all elbow positions; At slow speeds, the primary supinator; at high speeds, biceps supplements it |
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PIN syndrome (posterior interosseous nerve entrapment at the Arcade of Frohse) produces finger extension weakness without wrist extension loss (the ECRL is innervated proximal to the arcade). The Arcade of Frohse is the fibrous leading edge of the superficial head of the supinator. Surgical decompression releases this arcade.
Not easily palpated separately from brachioradialis. Tested by resisted supination with elbow extended.
Posterior interosseous nerve compression under the supinator Arcade of Frohse producing finger drop without wrist drop, managed by surgical release of the fibrous arch.
