1. The Swedish Hip Arthroplasty Register;
2. Department of Orthopaedics, Institute of Surgical Sciences, Sahlgrenska University Hospital, Göteborg, Sweden;
3. Norwegian Arthroplasty Register, Department of Orthopaedic Surgery, Haukeland University Hospital, Bergen;
4. Department of Clinical Medicine, University of Bergen, Bergen, Norway;
5. Department of Clinical Epidemiology, Aarhus University Hospital, Aarhus;
6. Danish Hip Arthroplasty Register, Department of Clinical Epidemiology, Aarhus University Hospital, Aarhus, Denmark;
7. Department of Orthopaedic Surgery, Traumatology and Clinical Institute, Odense University Hospital, and University of Southern Denmark, Odense;
8. Department of Orthopaedics and Traumatology, Turku University Hospital, Turku;
9. The Finnish Arthroplasty Register, Finland.