Departments of Preventive Dentistry, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, 2-5-1 Shikata-cho, Kita-ku, Okayama, 700-8558, Japan. dekuni7@md.okayama-u.ac.jp. Advanced Research Center for Oral and Craniofacial Sciences, Okayama University Dental School, Okayama, Japan. dekuni7@md.okayama-u.ac.jp. Section of Preventive and Public Health Dentistry, Division of Oral Health, Growth and Development, Faculty of Dental Science, Kyushu University, Fukuoka, Japan. Mannoucho Kokuho Soda Dental Clinic, Nakatado, Japan. Departments of Preventive Dentistry, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, 2-5-1 Shikata-cho, Kita-ku, Okayama, 700-8558, Japan.