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.