Hi, * As per my clinical experience, the ankle spasms with heaviness legs and difficulty in maintaining balance sound in concern with neurologic issue. * The knee prosthesis does not sound to be of concern in this case. * There can be problem at lower spinal or nerve plexus bundle track from the coccygeal region. * However, a detailed evaluation with - peripheral nerve sensations and motor examination - electromyography and nerve conduction study - MRI lower lumbar spine will help to reveal best of the reasons in the balance issue. * If any of the reports are handy, kindly share with me during follow up discussion so that better guidance can be figured out ahead. Hope I have answered your query. Let me know if I can assist you further. Take care Regards, Dr. Bhagyesh V. Patel