The best position for after a surgical repair of this kind is an abduction. For the uninformed, this is when a limb is kept away from the midline of the body. This will help keep the prosthetics from slipping out of place by keeping a little pressure on them. When hips and knees are replaced, they are often replaced by metal pieces, which can come loose after surgery if the patient isn’t resting properly.
Though it doesn’t happen often nowadays, these positions help prevent what could happen if the patient were not to be careful. Or, at least, mitigate the results.
This term in this situation does not apply to the situation when someone is taken away by force, usually from parents or friends by a stranger or the government. (Medical abduction is a term in that sense, but not for this question.)