This patient’s weakness in her wrist extensor muscles with intact cutaneous sensation suggests a pure motor neuropathy that is sometimes associated with dislocation of the radial head.
The posterior interosseous nerve is a motor nerve that branches off from the radial nerve in close proximity to the radial head, which makes it susceptible to injury due to radial head dislocation or subluxation. Occasionally, posterior interosseous neuropathy can develop years after a fracture of the forearm or elbow. Compression of this nerve frequently impairs extension of the fingers (more than the wrist) because function of the extensor carpi radialis longus, which is innervated directly by the radial nerve, is preserved. The unopposed action of this muscle further explains the radial deviation seen on examination. Finally, this patient’s normal sensory examination is one of the hallmarks of posterior interosseous neuropathy because this nerve does not provide cutaneous innervation.