[Diagnosis of rupture of the ulnar collateral ligament of the metacarpophalangeal joint of the thumb. Presentation of a simple clinical and radiological test].

When a normal thumb is attached to a rubber band and pulled in a radial direction, the MP joint adopts an attitude of abduction, internal rotation and, most often, slight flexion. Under the same conditions, rupture of the ulnar collateral ligament (UCL) expresses itself by an increase of abduction of at least 10 degrees, by an increase of flexion of at lest 10 degrees and, frequently more evidence, is provided by a decrease of rotation of at least 20 degrees. These clinical observations were also found on unembalmed corpses. They allow one to detect easily and without local anesthesia the recent and old ruptures of the UCL.