1. Spine Center and Department of Orthopedic Surgery, Seoul National University College of Medicine and Seoul National University Bundang Hospital, 166 Gumiro, Bundang-gu, Sungnam 463-707, Republic of Korea
2. Department of Orthopedic Surgery, Chung-Ang University Hospital, College of Medicine, Chung-Ang University, 102, Heukseok-ro, Seoul 06973, Republic of Korea