Full-thickness defect involving the distal end of the ulnar collateral ligament at the thumb metacarpophalangeal joint. 1st cine-loop shows the defect as an echogenic line due to non-bony avulsion of the ligament. The torn ligament is located deep to the adductor aponeurosis. 2nd cine-loop shows normal gliding of the aponeurosis over the torn ligament during interphalangeal joint flexion-extension. 3rd cine-loop during controlled stress shows joint space widening.