1. Department of Orthopaedic Surgery, Sapporo Medical University, School of Medicine, S-1, W-16 Chuo-ku, Sapporo 060-8543, Hokkaido, Japan.
2. Division of Orthopaedic Surgery, Sapporo Minami National Hospital, Shirakawa 1814, Minami-ku, Sapporo 961-2276, Hokkaido, Japan.