1. Department of Orthopaedic Surgery, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts 02114, USA.
2. Deparment of Orthopaedics, Rheumatology and Musculoskeletal Sciences Nuffield Orthopaedic Centre, Headington, Oxford OX3 7LD, UK.
3. Smith & Nephew Orthopaedics Ltd., Implant Development Centre, Warwick CV34 6WG, UK.