Quote Originally Posted by GSDOG
Almost sounds like a torn cruciate (ACL), which would possible require surgery or at the very least total confinement until it healed. but like catlady711 said you cant rule out arthritis. but the non-weight bearing makes me think cruciate. being 13 years old surgery could be tricky. I am interested to hear what your vet says.
I thought about ACL, however we mostly see that in dogs, I don't know how common it is in cats, but if this were a dog it would be one of my guesses. I'd like to find out what the vet says too.