1. 1 Department of Prosthodontics, Peking University School and Hospital of Stomatology, Beijing, PR China.
2. 2 Center of Digital Dentistry, Peking University School and Hospital of Stomatology, Beijing, PR China.
3. 3 Clinical Sciences Department, College of Dentistry, Ajman University, Dubai, UAE.
4. 4 Department of Reconstructive Dentistry and Gerodontology, School of Dental Medicine, University of Bern, Berne, Switzerland.