1. Department of Neurosurgery, University Hospital of Essen, D-45147 Essen, Germany
2. Department of Neurosurgery, Medical Center, University of Freiburg, D-79106 Freiburg, Germany
3. Institute of Medical Biometry and Statistics, Faculty of Medicine and Medical Center, University of Freiburg Institute for Medical Biometry and Medical Informatics, University Medical Center Freiburg, D-79106 Freiburg, Germany