The OK sign test is a clinical test to evaluate for potential injury of the anterior interosseous nerve.
Gross anatomy
The anterior interosseous nerve (roots C8-T1) is a branch off the median nerve (roots C6-T1), arising just after the median nerve passes through the two heads of the pronator teres in the proximal forearm. The anterior interosseous nerve provides motor innervation to 3 muscles 1:
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flexor digitorum profundus (lateral half)
the medial half is innervated by the ulnar nerve
Clinical presentation
The patient is typically asked to oppose their thumb with the second digit. With an anterior interosseous nerve injury, opposition of the thumb and second digit can still be possible. However, paresis of the flexor digitorum profundus (lateral part) and flexor pollicis longus muscles can lead to the absence of flexion of the second digit distal interphalangeal joint and at the thumb interphalangeal joint.
The result is more of a 'pinched' appearance of the thumb and second digit, resembling the closing of a pair of tongs. It can be a subtle finding that may warrant further investigation.
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