1. Department of Orthopedics, Yonsei University College of Medicine, Seoul 03722, Republic of Korea
2. Department of Spine Surgery, Cheil Orthopedic Hospital, Seoul 06075, Republic of Korea
3. Department of Orthopedics, Anam Hospital, Korea University College of Medicine, Seoul 02841, Republic of Korea
4. Department of Biostatistics, Korea University College of Medicine, Seoul 02841, Republic of Korea