The purpose of this study was to evaluate functional ultrasound as a tool for detecting an ulnar collateral ligament injury of the thumb. The feasability of using ultrasound for imaging the thumb joint space was tested in a pilot study, using ultrasound and dissection in 14 cadaveric hand specimens. To test this method clinically, both metacarpophalangeal joints of the thumb in 461 healthy volunteers were examined using ultrasound (11 MHz) under radial stress. The distance between the innominate tubercle of the first metacarpal head to the proximal phalanx was measured. The mean distance between the first metacarpal head and the proximal phalanx (n = 461) was 4.5 mm (standard deviation, 0.65 mm) on the right side and 4.6 mm (standard deviation, 0.61 mm) on the left side. These data were compared with data of 25 patients with an operative diagnosis of rupture of the ulnar collateral ligament of the thumb. The difference in joint space between the injured and uninjured sides was 2.25 mm (standard deviation, 0.46 mm). The metacarpophalangeal joint space can be reproducibly detected on high-frequency ultrasound. An increased gap seen on ultrasound is indicative of a rupture of the ulnar collateral ligament of the thumb.
