1. Maastricht University Medical Centre; Department of Neurosurgery; P. Debeyelaan 25 Maastricht Netherlands 6229 HX
2. Maastricht University Medical Centre; Department of Orthopaedics; PO Box 5800 Maastricht Netherlands 6202 AZ
3. Maastricht University; Department of Epidemiology; PO Box 616 Maastricht Netherlands 200 MD
4. Cleveland Clinic Foundation; Department of Neurosurgery; S-80, 9500 Euclid Avenue Cleveland OH USA 44195