1. Department of Orthopaedic Surgery, Hokkaido Spinal Cord Injury Center, 3-1 Higashi 4 Minami 1, Bibai 072-0015, Hokkaido, Japan
2. Department of Orthopaedic Surgery, Hokkaido University Graduate School of Medicine, Sapporo 060-0815, Hokkaido, Japan
3. Department of Orthopaedic Surgery, Faculty of Medicine, University of Tsukuba, Tsukuba 305-8575, Ibaraki, Japan