Mike Shedlock has a piece describing some amateur-build prostheses, constructed using the 3D printer. He points out that the amateur-build devices are out-performing the commercial ones and are priced at pennies on the dollar.
The difference is that the 3D printed devices and the commercial ones is that the former were designed and built by amateurs and the commercial ones are approved by the FDA, whose charter is to insure that NOTHING ever gets to market. Hobbyists are permitted to tinker pretty much to their hearts content as long as they don't try to actually sell their product.
Give a little girl a prosthetic arm today and tomorrow she becomes Doc Octopus out hunting down Spiderman.