If you retired at 55 or younger and had no more than 2 million bucks (in & outside of 401k) to start (and no pension), please tell me 1) how you are investing the amount and how much you withdraw annually, 2) how many years you've been in retirement, 3) any regrets you've had with regards to...
Before you spend alot of $$ or surgey, read this - http://www.medscape.com/viewarticle/478840
It is mostly about the back but the knee is also referenced. Dr John Sarno's view into why we get pain is worth learing about. Get his book. I treated my back pain using his methods.