1. Department of Preventive and Community Dentistry, The Nippon Dental University, School of Life Dentistry at Niigata, Niigata City 15103/951-8151, Japan
2. Department of Pediatric Dentistry, The Nippon Dental University, School of Life Dentistry at Niigata, Niigata City 15103/951-8151, Japan
3. Oral Environment and Community Dental Health, The Nippon Dental University Graduate School of Life Dentistry at Niigata, Niigata City 15103/951-8151, Japan