Peter Tang, MD, MPH The ulnar and radial collateral ligaments are primary stabilizers of the thumb metacarpophalangeal (MP) joint. Injury to these ligaments can lead to instability and disability. Stress testing is essential to establish the diagnosis. Complete tear is diagnosed on physical examination when the proximal phalanx of the thumb can be angulated ulnarly or radially on the metacarpal head by 30° to 35° with the MP joint in either zero degrees of extension or 30° of flexion. Lack of a firm end point or angulation measuring >15° on stress testing compared with the contralateral thumb MP joint are also indicative of complete tear. Partial ligament injuries may be managed nonsurgically, but complete tears are usually managed surgically. Various techniques are used to reattach the ligament to bone, including suture anchors and, less commonly, repair of midsubstance tears. Options for managing chronic injuries include ligament repair, ligament reconstruction with a free tendon graft, and arthrodesis of the MP joint.