1. Department of Orthopaedics, Sorlandet Hospital Kristiansand, Norway;
2. University of Oslo, Oslo, Norway
3. Martina Hansens Hospital, Gjettum, Norway;
4. Department of Neurosciences, Rheumatology Division, INM, Norwegian University of Science and Technology, Trondheim, Norway;
5. Department of Orthopaedics, Sorlandet Hospital Arendal, Norway;
6. Oslo Centre of Biostatistics and Epidemiology;
7. Department of Orthopaedics, Oslo University Hospital, Ullevål;