1. Health Promotion Sciences Unit, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK.
2. Nuffield Department of Orthopaedic Surgery, Nuffield Orthopaedic Centre, Windmill Road, Headington, Oxford OX3 7LD, UK.
3. Queen Alexandra Hospital, Cosham, Portsmouth PO6 3LY, UK.