1. Department of Periodontics; Peking University School and Hospital of Stomatology, National Engineering Laboratory for Digital and Material Technology of Stomatology, Beijing key Laboratory of Digital Stomatology, 22 Zhongguancun Avenue South; Haidian District Beijing 100081 PR China
2. Department of Restoration Dentistry, School of Dentistry; University of Washington, 1959 NE Pacific Street, HSC-D770; Seattle Washington 98195-7456 USA