1. Department of Orthopaedics Uppsala University Hospital, SE-751 85 Uppsala, Sweden.
2. University Hospital for Orthopaedic Surgery Friedrichsheim, Johann WolfgangGoethe-University, D-60528, Frankfurt am Main, Germany.
3. Department of Biomathematics Hospital of the Johann WolfgangGoethe-University, D-60590, Frankfurt am Main, Germany.