How To Prevent Polio Stricken Legs From Further Extending Backward?

I am 54 years old. I contacted polio on both legs when I was 3 years old. I was able walk and climb stair without the help of bracelet and walking stick until In 2007, my right knee felt painful and extended backward. Now, I have to wear the leg bracelet to prevent the right knee from over-extend and walking stick to support me when I am walking.
I have consulted doctor and specialist and this was due to muscle weakness that cause the pain and over extend in the knee. The x-ray shown no sign of arthristic on the knee. I have to live with it and nothing the doctor and specialist can do about it. I felt very discourage and demoralize. I told myself never to give up. I am searching the web-site to find way to strengthen my right leg muscle and may possible to stand and walk with ease and without the walking stick. Thanks! Vincent Tan