what is UCL caused by
Pay Now. When an acute sprain or tear on the ulnar collateral ligament (UCL) on the ulnar side of the meta-carpal-phalangeal (MCP) joint of the thumb occurs, it is called a UCL injury.CL injuries are commonly caused by injury or trauma in which the thumb is bent away from the hand at the MCP joint. This can happen in sports like skiing or hockey, or in any situation in which a fall is landed on an outstretched hand.