This invention relates to a a locking system (402) for retaining a prosthetic implant to a vertebrae. The system comprises a first fastener (406, 410) for securing said prosthetic implant to said vertebrae and a second fastener (404, 408) for securing said prosthetic implant to said vertebrae; said second fastener (406, 410) being arranged so as to engage said first fastener (406, 410) to retain said first and second fasteners in a tightened position.