1. Nuffield Orthopaedic Centre, Headington, Oxford OX3 7LD, UK.
2. Nuffield Department of Anaesthetics, John Radcliffe Hospital, Oxford OX3 3DU, UK.
3. University of Warwick, Warwick Medical School, Clinical Sciences Building, Clifford Bridge Road, Coventry CV4 7AL, UK.
4. Pain Relief Clinic, Churchill Hospital, Headington, Oxford OX3 7LJ, UK.