1. Department of Operative Dentistry, Preventive Dentistry and Endodontics, The Affiliated Stomatology Hospital of Kunming Medical University, Kunming, Yunnan 650500, P.R. China
2. Department of Operative Dentistry, Preventive Dentistry and Endodontics, Affiliated Stomatology Hospital of Sun Yat-sen University, Guangzhou, Guandong 510055, P.R. China
3. Guanghua School of Stomatology, Sun Yat-sen University, Guangzhou, Guandong 510055, P.R. China