Preliminary thoughts, when I knew less about fallen London, were that they’d been something of a casualty during the Starved War and had a resulting Shaping done to try to mitigate the damage. The hip and the thigh were still flesh, but the femur joint itself was bone porcelain. There would have been some kind of Correspondence etched into the places where the ball made contact with the body to keep it stable and functional.

In a BJD, the elastic is threaded through the body and the ball into the leg hollow. What would be threaded through the ball here would be the Nerves of the leg. This was prone to suck and be painful as you can imagine. The leg could be stiff and the character would limp from time to time as well as use a cane with regularity.