
Well, outside of the low-hanging arrow to the knee joke, I've noticed that a lot of people don't treat healing as a magical cure-all for all your problems. There's a bit of leeway to say that your character was injured so grievously that even after getting healed, their leg still doesn't work quite right, or they just didn't have access to a healer at the time and it had to heal naturally. If it's your backstory, you've got plenty of wiggle room. As for the injury itself, I'm not a huge expert on niggling injuries, but maybe focus on nerve damage? Certainly, I think a bad burn could accomplish that.