1. Department of Orthopaedics, University Medical Center, P O Box 85500, 3508 GA, Utrecht, The Netherlands.
2. Julius Centre for Health Sciences and Primary Care, University Medical Center, PO Box 85500, 3508 GA, Utrecht, The Netherlands.
3. Department of Orthopaedics, Sint Antonius Hospital, PO Box 2500, 3430 Nieuwegein, The Netherlands.