Ulnar collateral ligament (UCL) of the first metacarpophalangeal (MCP) joint is torn distally.
Simple UCL tear needs to be differentiated from Stenner lesion. Stenner lesion – aponeurosis of abductor pollicis longus gets trapped between torn UCL and metacarpal head. Original article by Dr. Stenner in 1962 has excellent drawings explaining this concept.
Stener B. DISPLACEMENT OF THE RUPTURED ULNAR COLLATERAL LIGAMENT OF THE METACARPO-PHALANGEAL JOINT OF THE THUMB. The Journal of Bone and Joint Surgery. British volume Vol. 44-B, No. 4. https://doi.org/10.1302/0301-620X.44B4.869