[1] Great Ormond St Hosp Sick Children, Dept Orthopaed Surg, Great Ormond St, London WC1N 3JH, England
[2] Univ Oxford, Nuffield Dept Orthopaed Rheumatol & Musculoskelet, Oxford OX3 7LD, England
[3] North Middlesex Univ Hosp, Dept Acute Internal Med, London N18 1QX, England
[4] Stoke Mandeville Hosp, Dept Plast & Reconstruct Surg, Aylesbury HP21 8AL, Bucks, England
[5] UCL, London WC1E 6BT, England