One of the more
evocative images medical science has produced. It becomes increasingly disturbing the more I think about it.
In fact it's not quite so interesting:
It's an
orthopaedic term referring to an artificial replacement (
prosthesis) for the head of a bone, e.g. the
femur. The head (I understand) is the
ball in a '
ball and socket ' joint such as the
hip or
shoulder. This is the kind of thing you're likely to land up with if you need
hip replacement.
Also apparently the title of a song by
Green Day.