Millsaps Knee problems - question

Just watched the interview with Millsaps on Transworld, and he talks about his serious knee issues and upcoming surgery. Sounds like a logical choice to get it repaired ASAP. The audio wasn't too good, but he did discuss the possibility of a knee replacement in the future. I'm assuming that he means a possible replacement AFTER his career is over. My question is, "Is it possible to race at a high level with a knee replacement?" My understanding is that an artificial knee...and surrounding bone and tissue....would not hold up to the physical demands of motocross.