Correspondence to: Dr. PENG Jiang, Institute of Orthopedics, Chinese People's Liberation Army General Hospital, Beijing 100853, China (Fax: 86-10-66939205. Email: shibilu301@gmail.com); Dr. MA Xin-long, Department of Spine Surgery, Tianjin Hospital, Tianjin 300211, China (Email: maxinlong8686@sina.com)