1. Medical Faculty; Department of Prosthodontics and Biomaterials; RWTH Aachen University; Aachen Germany
2. Medical Faculty; Department of Medical Statistics; RWTH Aachen University; Aachen Germany
3. Private Office; Düren Germany
4. Department of Prosthodontics, Propaedeutics and Dental Materials; Christian-Albrechts University at Kiel; Kiel Germany