1. Department of Orthopaedic Surgery, KKR Sapporo Medical Center, 6-3-40, Hiragishi 1-jo, Toyohira-ku, Sapporo, Hokkaido 062-0931, Japan
2. Department of Orthopaedic Surgery, Faculty of Medicine and Graduate School of Medicine, Hokkaido University, Nishi 7, Kita 15, Kita-ku, Sapporo, Hokkaido 060-8638, Japan