1. Department of Orthopaedic Surgery Rush, University Medical Centre, 1725 W. Harrison Parkway, Chicago, Illinois 60612, USA.
2. University of Oxford, Kellogg College, Division of Health Sciences, Wellington Square, Oxford OX1 2JA, UK.
3. Department of Orthopaedic Surgery, Emory University, 59 Executive Park South, Atlanta, Georgia 30329, USA.
4. Department of Orthopaedic Surgery, Thomas Jefferson University and the Rothman Institute, 925 Chestnut Street, Philadelphia, Pennsylvania 19107, USA.