1. Department of Neurosurgery, University Hospital Ulm, Albert-Einstein-Allee 23, 89081 Ulm, Germany
2. Department of Neurosurgery, Military Hospital Ulm, Oberer Eselsberg 40, 89081 Ulm, Germany
3. Department of Neurosurgery, Hospital Landshut, Robert-Koch-Strasse 1, 84034 Landshut, Germany
4. Institute for Epidemiology and Medical Biometry, University of Ulm, Schwabstrasse 13, 89075 Ulm, Germany