1. Department of Orthopaedics, Schoen Klinik, Duesseldorf 40549, Germany
2. Department of Orthopaedics and Trauma Surgery, University Hospital of Cologne, Cologne 50931, Germany
3. Department of Orthopaedic Surgery, Hannover Medical School, Hannover 30625, Germany
4. Institute of Medical Statistics, Informatics and Epidemiology, University Hospital of Cologne, Cologne 50937, Germany