1. Leiden University Medical Center; Department of Orthopedics; Albinusdreef 2 Leiden Zuid Holland Netherlands 2333 ZA
2. University Medical Center Groningen; Department of Orthopedics; Hanzeplein 1 Groningen Groningen Netherlands 9713 GZ
3. Leiden University Medical Center; Department of Neurosurgery; PO Box 9600 Leiden Netherlands 2300 RC
4. School of Medicine, University Hospital of North Straffordshire; Medical Research Unit; Thornburrow Drive Hartshill Stoke on Trent UK
5. Sint Maartenskliniek; Department of Orthopedics; PO Box 9011 Nijmegen Netherlands 6500 GM