Originally Posted By: tanstaafl.
Yes, I know -- they don't know for sure how long these replacements will last, but consensus seems to be at least 20 years. Since technical/medical progress over time has historically been asymptotic by nature, I am quite convinced that 20 years from now the problem of replacing worn-out prosthetic knees will have been solved quite handily, probably through stem-cell regeneration techniques.
Before I graduated, I toyed with the idea of making the jump into Biomedical Engineering for a graduate degree, specifically using advanced aerospace materials for joint replacements. My guess is the only limitations are financial.

The Doc's concern is that you can only replace the knees twice, then there isn't enough bone left and they have to fuse the leg together. That would suck.