PKR 4 years pain

I am a 57 year old male I had a PKR, 4 years ago and have been in pain ever since then They did the op - about 3 years too late - in the meantime, my tendons and ligaments stretched too far to be recoverable - so now, I have a fixed knee but it hyperextends and causes me pain I was wondering whether anyone had any fixes for this ? The knee has affected my gait over the last 10 years which has taken out my back/spine - this is painful when trying to walk and means wheelchair !