I just read a very good book about caring for the knees, which, of course doesn't make me an expert. But one interesting point it made about knee replacements is that the artificial knee, based on current technology, lasts around 20 years, and once THAT goes, it is much more difficult for surgeons to replace it than it was to "install" the first artificial knee. Therefore, doctors often try to discourage younger patients from having the procedure if it can be avoided.