The largest brachial plexus branch (C5–T1), winding the humeral spiral groove before dividing into superficial sensory and deep posterior interosseous branches at the elbow, supplying all upper limb extensors.
Humeral fracture or spiral groove compression causes wrist drop. Posterior interosseous nerve entrapment at the arcade of Frohse causes lateral forearm pain without wrist drop.
Wrist drop, finger extension weakness, and lateral forearm paraesthesia from spiral groove compression — usually recovers spontaneously over weeks to months.
Lateral forearm aching with finger extension weakness but preserved wrist extension from entrapment beneath the arcade of Frohse.
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