1. Department of Diagnostic and Biological Sciences, School of Dentistry, University of Minnesota, 515 Delaware Street SE, Minneapolis, MN 55455, USA;
2. Institute of German Dentists, Universitätsstr. 73, 50931 Köln, Germany; and
3. Department of Restorative Dentistry, School of Dental Sciences, Framlington Place, University of Newcastle upon Tyne, Newcastle upon Tyne NE2 4BW, UK