Since you have already had one knee replaced this might not apply but...
Before they put anything into your body find out what they are going to use and be damned sure you are not allergic to it. I was the one guy in 10,000 that is deathly allergic to Cobalt and, you guessed it, they put in a Cobalt Chromium knee. My body was rejecting that knee before I was off of the operating table.
Six miserable, hellish months of failed PT later I fired the surgeon and got a second opinion. He quickly diagnosed the problem and they had to cut my leg off a second time and replace with a ceramic knee. Spent 31 days in the hospital, 17 the first time and 14 the second. I don't blame the VA, just my bad luck but beware.
Stuff happens.
:S