This is a great story.
As you can see from the picture above, Vincent the cat was severely injured as a kitten. Normally an animal this damaged might be euthanized or given a cart with wheels to support his back end and make him slightly mobile. While the "cart solution" might help a little bit, the animal is still dependent on human beings to attach the cart, etc.
However, the smart people at Iowa State University had a better idea.
In a procedure similar to dental implants. The vets surgically placed Titanium rods into both his femurs (thigh bones) which protruded outside the skin. the end outside the body was threaded and allowed for the "legs" you see in the video to be placed. As Vincent grows these "legs" can be unscrewed and replaced by longer ones.
Of course it isn't like the vets at Iowa State had a selection of "kitty femur implants" to choose from. So how did they do it? This is where it gets really cool. The implants were custom designed just for Vincent and then 3D printed by BioMedtrix.
I understand that orthopedic surgeons in Europe are now working with printed titanium for custom fitted hip replacements, so it seems like it is only a matter of time before things like this are commonplace.
At some point in the not too distant future, the idea of custom printed joints & prosthetics may be the standard of care.
For a detailed account of this story, check out TechCrunch.