A "Sprain" of the Ulnar Collateral Ligament of the thumb as an acute or subacute injury (current), as opposed to a chronic condition, often goes by the name of Game Keeper's Thumb &/or Skier's Thumb. As a current injury, this would be S63.64 _ _ (Sprain of the Metacarpophalangeal Joint of the thumb, but without specifying radial or ulnar collateral ligaments). However, if this is an old sprain/ligamentous injury, i.e. a chronic condition, then it would come under M24.2: Disorder of the Ligaments (Chronic Instability secondary to old ligamentous injury). The 5th Character would be 4 for hand, and the 6th Character would be for laterality.
Hope this helps.
Respectfully submitted, Alan Pechacek, M.D.
icd10orthocoder.com