1. Department of Orthopaedic Surgery, Skaraborgs Sjukhus Kärnsjukhuset, 451 85 Skövde, Sweden.
2. Oxford Orthopaedic Engineering Centre, Nuffield Department of Orthopaedic Surgery, Oxford Orthopaedic Engineering Centre, Nuffield Orthopaedic Centre, Windmill Road, Headington, Oxford OX3 7LD, UK.