1. Institute of Dentistry, University of Oulu, PO Box 5281, 90014 Oulu, Finland
2. Oral and Maxillofacial Department, Department of Otorhinolaryngology, Keski-Pohjanmaa Central Hospital, Kokkola, Finland
3. National Institute for Health and Welfare (THL) of Finland, Helsinki
4. Department of Public Health Dentistry, University of Turku, Finland
5. Oral and Maxillofacial Department, Oulu University Hospital