1. Radboud University Nijmegen Medical Centre, Department of Neurosurgery, Nijmegen, The Netherlands
2. Radboud University Nijmegen Medical Centre, Department of Epidemiology and Biostatistics, Nijmegen, The Netherlands
3. Cleveland Clinic Foundation, Department of Neurosurgery, Cleveland, Ohio
4. Division of Neurosurgery and Spinal Program, University of Toronto and Krembil Neuroscience Center, Toronto Western Hospital, Toronto, Canada
5. South Denver Neurosurgery, Littleton, Colorado