Henrik Malchau1,2, Martyn L Porter3. 1. Harvard Medical School, Boston, MA, USA. 2. Harris Orthopaedic Laboratory, Massachusetts General Hospital, Boston, MA, USA. 3. The Centre for Hip Surgery, Wrightington Hospital, Hall Ln, Appley Bridge, Wigan, Lancashire, WN6 9EP, UK. martynporter@hipkneeclinic.com.