When the ulnar collateral ligament in the thumb is injured, the middle thumb joint becomes painful and swollen, and the thumb feels weak when you pinch or grasp. You may see bruise-like discolorations on the skin around the joint. The loose end of the torn ligament may form a bump that can be felt along the edge of the thumb near the palm of the hand. A torn ligament makes it difficult to hold or squeeze things between your thumb and index finger.