How about the finger he broke in detox - could have healed crooked. Or the self-inflicted cuts on his arm in the Tritter arc, they could have left scars. Would there be any sign of his head injury/DBS from the bus? or a scar from the motorcyle crash in Locked In. Do burns leave scars? His hand was badly burnt in 97 seconds (though miraculously healed by the next episode!) I've seen heaps of people put callouses on his palm from the cane use, also guitar players usually have callouses on their finger tips I believe.