1. Department of Prosthodontics, The Affiliated Hospital of Qingdao University, Qingdao 266000, P. R. China
2. School of Stomatology, Qingdao University, Qingdao 266003, P. R. China
3. State Key Laboratory of Oral Diseases, National Clinical Research Center for Oral Diseases, Department of Prosthodontics, West China Hospital of Stomatology, Sichuan University, Chengdu 610041, P. R. China
4. Applied Oral Sciences and Community Dental Care, Faculty of Dentistry, The University of Hong Kong, Hong Kong SAR 999077, P. R. China
5. Division of Paediatric Dentistry and Orthodontics, Faculty of Dentistry, The University of Hong Kong, Hong Kong SAR 999077, P. R. China
6. Department of Dental Materials and Prosthodontics, School of Dentistry of Ribeirao Preto, University of Sao Paulo, Ribeirao Preto 14040904, SP, Brazil