Ultrasonography may be misleading in the diagnosis of ruptured and dislocated ulnar collateral ligaments of the thumb.

To find out if a non-invasive technique (ultrasonography) was able to identify a dislocated ligament of a metacarpophalangeal joint of the thumb with instability of the ulnar collateral ligament, 14 consecutive patients with clinical rupture of the ulnar collateral ligament were examined with ultrasonography before the ligament was explored. At exploration all ligaments were ruptured, and only five out of the 14 were dislocated. Ultrasonography recognised only two of the dislocated ligaments, and in half of the 14 patients the ultrasound scan gave incorrect information about the position of the ligament. We conclude that ultrasonography is not adequate for identifying dislocated ulnar collateral ligaments of the metacarpophalangeal joint of the thumb, and exploration is indicated in those with clinical instability in which palpation either suggests a dislocated ligament or is inconclusive.