1. Department of Orthopaedic Surgery, Faculty of Medicine and Graduate School of Medicine, Hokkaido University, Sapporo;
2. Spine Center, Hakodate Central General Hospital, Hakodate;
3. Hokkaido Spine Study Group, Sapporo, Hokkaido, Japan
4. Department of Orthopaedic Surgery, Hokkaido Orthopaedic Memorial Hospital, Sapporo;
5. Department of Orthopaedic Surgery, Hokkaido Spinal Cord Injury Center, Bibai;
6. Department of Orthopaedic Surgery, Eniwa Hospital, Eniwa;
7. Department of Orthopaedic Surgery, Sapporo City General Hospital, Sapporo;
8. Department of Orthopaedic Surgery, Kushiro Rosai Hospital, Kushiro; and