1. Department of Clinical Epidemiology, Aarhus University Hospital, Olof Palmes Alle 43-45, 8200 Aarhus N, Denmark.
2. Departments of Medicine and Community and Family Medicine, Section of Biostatistics and Epidemiology Dartmouth Medical School, 1 Medical Centre Drive, 8th Floor Rubin Lebanon, New Hampshire 03755, USA.
3. Department of Orthopaedic Surgery, Odense University Hospital, Sdr. Boulevard 29, 5000 Odense, Denmark.