1. Department of Life, Health and Environmental Sciences University of L'Aquila L'Aquila Italy
2. Clinic for Preventive Dentistry, Periodontology and Cariology University of Zurich Zurich Switzerland
3. Department of Oral Surgery, Center of Dental Traumatology University Center for Dental Medicine Basel UZB Basel Switzerland
4. Department of Operative Dentistry and Periodontology Center for Dental Medicine Faculty of Medicine University of Freiburg Freiburg Germany
5. Institute for Medical Biometry and Statistics University of Freiburg Freiburg Germany