1. The Hospital for Sick Children, Great Ormond Street, London WC1N 3JH, UK.
2. Epidemiology and Public Health, Institute of Child Health, 30 Guilford Street, London WC1H 1EH, UK.
3. Department of Orthopaedics, Malmö University Hospital, S-205 02 Malmö, Sweden.
4. Blackburn Royal Infirmary, Bolton Road, Blackburn BB2 3LR, UK.
5. Coventry and Warwickshire Hospital, Stoney Stanton Road, Coventry CV1 4FH, UK.