1. Department of Prosthetic Dental Medicine, Faculty of Dental Medicine, Medical University-Sofia, 1, Georgi Sofiiski Boulevard, 1431 Sofia, Bulgaria
2. Department of Oral Function and Prosthetic Dentistry, College of Dental Science, Radboud University Nijmegen Medical Centre, 25 Philips van Leydenlaan, 6525 EX Nijmegen, The Netherlands
3. Department of Preventive and Restorative Dentistry, College of Dental Science, Radboud University Nijmegen Medical Centre, 25 Philips van Leydenlaan, 6525 EX Nijmegen, The Netherlands