Ralph Mobbs, a neurosurgeon at the Prince of Wales Hospital in Sydney, made medical history in late 2015 when he successfully replaced two vertebrae with custom made prosthesis. The patient, in his 60s, suffered from Chordoma, a particularly nasty form of cancer that had formed on his top two vertebrae and threatened to cinch off his spinal cord as it grew. That would have left him a quadriplegic. Complicating matters, those top two vertebrae are what allow you to turn and tilt your head, so it's not like doctors can easily fashion a replacement out of bone grafted from another part of the patient's body. They have to fit perfectly and that's where the 3D printers come in.