A motor branch of the posterior interosseous nerve supplying the extensor carpi ulnaris, arising as one of the first branches after the PIN exits the arcade of Frohse in the radial tunnel. Its preservation or loss in PIN palsy determines ulnar wrist extension function.
The nerve to ECU is one of the proximal PIN motor branches, arising early after the nerve exits the supinator. In PIN palsy, if the ECU branch is involved, the wrist deviates radially on attempted extension (ECRL without ECU). If ECU is preserved, wrist extension is more balanced. EMG of ECU precisely maps the PIN injury level relative to the individual motor branch takeoffs.
ECU motor branch loss in posterior interosseous nerve palsy producing radial wrist deviation on extension from unopposed ECRL action, localising the PIN injury to at or proximal to the ECU branch takeoff within the radial tunnel.
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