1. Faculty of Odontology; University of Iceland; Reykjavik; Iceland
2. Department of Fixed and Removable Prosthodontics and Dental Material Science; University of Zürich; Zürich; Switzerland
3. Department of Social and Preventive Medicine; University of Bern; Bern; Switzerland
4. Department of Oral Implantology and Prosthetic Dentistry; Academic Center for Dentistry Amsterdam; Amsterdam; Netherlands