1. Research Unit of Population Health, Faculty of Medicine, University of Oulu , PO Box 5000, FIN-90014 Oulu , Finland
2. Medical Research Center, Oulu University Hospital, University of Oulu , PO Box 8000, FI-90014 Oulu , Finland
3. Faculty of Medicine, University of Oulu , PO Box 8000, FI-90014 Oulu , Finland
4. Oral and Maxillofacial Department, Oulu University Hospital , PO Box 10, FI-90029 OYS, Oulu , Finland