1. Department of Orthopaedic Surgery, Slagelse Hospital, Naestved, Slagelse and Ringsted Hospitals, Slagelse, Denmark;
2. Department of Orthopaedic Surgery, Copenhagen University Hospital Hvidovre, Copenhagen, Denmark;
3. The Harris Orthopaedics Laboratory, Department of Orthopaedic Surgery, Massachusetts General Hospital, Boston, Massachusetts, USA;
4. Department of Clinical Epidemiology, Aarhus University Hospital, Aarhus, Denmark