1. Department of Neurosurgery, Spine Center, Seoul National University Bundang Hospital, Seoul National University College of Medicine, Seongnam;
2. Department of Neurosurgery, Konyang University Hospital, Konyang University College of Medicine, Daejeon;
3. Medical Research Collaborating Center, Seoul National University Bundang Hospital, Seongnam, Republic of Korea; and
4. Department of Orthopaedic Surgery, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania