Correspondence to: Dr. HE Bao-rong, Department of Spine Surgery, Hong Hui Hospital, Xi'an Jiaotong University College of Medicine, Shaanxi 710054, China (Tel: 86-29-87800640. Fax: 86-29-87800002. Email: hebr888@163.com) Dr. HAO Ding-jun, Department of Orthopaedics, Hong Hui Hospital, Xi'an Jiaotong University College of Medicine, Xi'an, Shaanxi 710054, China (Tel: 86-29-87894724. Fax: 86-29-87800002. Email: haodinjun@126.com