1. Leiden University Medical Center; Department of Neurosurgery; PO Box 9600 Leiden Netherlands 2300 RC
2. Maastricht University Medical Centre; Department of Orthopaedics; PO Box 5800 Maastricht Netherlands 6202 AZ
3. Sint Maartenskliniek; Department of Research, Development and Education; PO Box 9011 Nijmegen Netherlands 6500 GM
4. Radboud University Nijmegen Medical Center; Department of Neurosurgery; PO Box 9101 Nijmegen Netherlands 6500 HB
5. Sint Maartenskliniek; Department of Orthopedics; PO Box 9011 Nijmegen Netherlands 6500 GM
6. University Medical Center Utrecht; Department of Orthopedics; Utrecht Netherlands