Our barn cat has a bum leg also. Can’t find a thing wrong with it. Does not even act like it hurt when I examine it. Weird. He won’t use it. He is in crate for a few days.He has some weakness in a leg. I thought he had a bumble foot or a thorn in it but there is nothing that we can find physically wrong with it. He's been off of it and that has to help. He was free ranging 24/7.