1. State Key Laboratory of Military Stomatology & National Clinical Research Center for Oral Diseases & Shaanxi Key Laboratory of Oral Diseases; Department of Prosthodontics; School of Stomatology; The Fourth Military Medical University; Xi'an Shaanxi 710032 P. R. China
2. Guanghua School and Hospital of Stomatology; Sun Yat-sen University; Guangzhou Guangdong P. R. China
3. College of Stomatology; Xi'an Jiaotong University; Xi'an Shaanxi P. R. China
4. Guangzhou Institute of Oral Disease; Stomatology Hospital of Guangzhou Medical University; Guangzhou P. R. China
5. College of Graduate Studies; Augusta University; Augusta Georgia 30912 USA