Being forced into work made me lose my leg

Hello I was called in to an assessment back in 2016 whilst waiting for a simple operation and they judged I was fit for work , on their decision I was sent to all types of courses to find employment at the same time my doctor was telling me to stay off my leg. When the operation came a month later the simple tear I had in my knee had cysts around it due to overuse and the operation went terribly wrong causing infection hence losing the use of my left leg. It doesnt end their I  now just received the result from my latest assessment saying nil points again and ime heartbroken at this I feel they are responsible for the loss of my leg now their judging me fit for work again how can they make such disgraceful decisions.