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<embedvideo service="youtube" Alignment="right">https://youtu.be/HPhiH_Jct0g</embedvideo>[[File:Fig 1 OK sign AIN.jpg|thumb|Figure 1]][[File:Fig 2 OK sign high median neuropathy.jpg|thumb|Figure 2]]The OK (pinch, straight thumb) sign refers to an inability to form the “OK” sign. Patients with weakness of the flexor pollicis longus and the flexor digitorum profundus to the index finger are unable to oppose the tips of the thumb and index finger to form a proper circle, but make a triangle instead, touching the finger pads.(1,2) These muscles are innervated by the anterior interosseous nerve, and the OK sign may be seen in either anterior interosseous or high median neuropathy.
The median nerve may rarely be entrapped proximally where it passes between the two heads of the pronator teres or by the ligament of Struthers, an anomalous fibrous band running from a distal humeral supracondylar spur to the medial epicondyle. Anterior interosseous neuropathy palsy often occurs as an isolated manifestation of neuralgic amyotrophy.(3) Recent MRI evidence indicates the process in many cases of AIN palsy predominantly involves AIN fascicles in the main trunk of the median nerve in the upper arm.(4)