1. Department of Oral and Maxillofacial Surgery, University Medical Centre Groningen, University of Groningen, P.O. Box 30.001, 9700 RB Groningen, The Netherlands
2. Department of Oral and Maxillofacial Surgery, Medical Centre Leeuwarden, P.O. Box 888, 8901 BR Leeuwarden, The Netherlands
3. Department of Oral and Maxillofacial Surgery, Rijnstate Hospital Arnhem, 6800 TA Arnhem, P.O. Box 9555, The Netherlands
4. Department of Oral and Maxillofacial Surgery, Amphia Hospital Breda, Postbus 90.158, 4800 RK Breda, The Netherlands
5. UMCG Centre for Dentistry and Oral Hygiene, Department of Oral Health Care & Clinical Epidemiology, University Medical Centre Groningen, University of Groningen, P.O. Box 30.001, 9700 RB Groningen, The Netherlands