1. University of Groningen, Department of Oral Health Care and Clinical Epidemiology, Centre for Dentistry and Oral Hygiene University Medical Centre Groningen, Groningen, The Netherlands
2. University of Groningen, Department of Oral and Maxillofacial Surgery, BB70, University Medical Centre Groningen, PO Box 30001, NL 9700 RB, Groningen, The Netherlands