The first knee replacement was performed in 1968. As with hip replacements, the earliest attempts to rebuild the knee were crude and unsuccessful. At one point, artificial knees were just hinges, but the work of John Charnley and others has led to more elegant replacements. Today, recipients of knee replacements can choose a model that is suited for their gender, age, weight and activity level.
In the future, though, joints like the hip and the knee may be able to rebuild themselves. In July 2010, researchers announced they'd rebuilt a rabbit's joint with stem cells. These researchers inserted scaffolding into the rabbit's thigh, and material on the scaffolding stimulated cell growth. The rabbit regained the ability to move and bear weight on the joint. This finding could have an immense impact on the aging human population, as joint replacement surgery becomes increasingly likely the older a person gets. This won't be the last time stem cells come up in this list -- joint regeneration is but one way that these cells may be able to rebuild our bodies in the future.