1. Institute of Dentistry, University of Helsinki, and Department of Oral and Maxillofacial Diseases, Helsinki University Central Hospital, PB 41, FIN-00014 Helsinki, Finland;
2. Faculty of Odontology, Complutense University, Ciudad Universitaria, E-28040 Madrid, Spain; and
3. Boston University Goldman School of Dental Medicine and
4. Harvard School of Public Health, Boston, MA 02215-1204, USA;