1 Department of Geriatric Dentistry, School and Hospital of Stomatology, Peking University, Beijing 100081, People's Republic of China.
2 Department of Prosthodontics, School and Hospital of Stomatology, Peking University, Beijing 100081, People's Republic of China.
3 State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People's Republic of China.
4 Department of Orthodontics, School and Hospital of Stomatology, Peking University, Beijing 100081, People's Republic of China.
5 Author to whom any correspondence should be addressed.