Well, lose an arm does not necessarily mean that if fell off. It just means that his arm is unusable. Maybe his arm broke and can't heal (as nerfix said) but I bet its more common that there was some infection or problems with bloodstream in that arm that made it 'wither off' and unusable (look at me, I'm sounding like bad version of dr. House
). He may still have it 'attached' to his torso you know.
