1. Department of Periodontology, General Hospital of Ningxia Medical University, Yinchuan, Ningxia 750004, P.R. China
2. Department of Oral and Maxillofacial Surgery, General Hospital of Ningxia Medical University, Yinchuan, Ningxia 750004, P.R. China
3. Department of Prosthodontics and Oral Implantology, General Hospital of Ningxia Medical University, Yinchuan, Ningxia 750004, P.R. China