Poor katze. Was it hip dysplasia that prompted the surgery? Get well soon, kitty.
It was severe arthritis, which they could see in xays -- she was limping badly from the pain. After she recovers, heals, she may still limp a little, but not from pain.
Basically, joint pain is gone because there is no joint anymore -- the femur is joined to the pelvis by soft tissue/ligaments only.