Fair warning: IANAL

http://openprosthetics.wikispot.org/Federal_Regulation_of_Pr...

"...regulated as Class I (exempt) medical devices."

As long as the new device, "...is intended for the same use and the same user type as existing products, and the device operates on the same fundamental scientific technology."

This means that most prosthetic devices are not explicitly regulated and are free from regulatory overhead in nearly all cases, especially the 3d-printed ones. While one can speculate on why they chose not to regulate these I think we can assume it's because ostensibly there's no danger to the end-user.