1. Stomatology Medical Center of Zhongshan People's Hospital, Zhongshan, Guangdong 528403, P.R. China
2. Department of Respiratory Disease, Zhongshan People's Hospital, Zhongshan, Guangdong 528403, P.R. China
3. School/Hospital of Stomatology, Guizhou Medical University, Guiyang, Guizhou 550004, P.R. China