1. Department of Orthopedics, University Hospital Duesseldorf, University of Duesseldorf, Duesseldorf, Germany.
2. Department of Neurosurgery and Neuroradiology, University Hospital Zurich, Zurich, Switzerland.
3. Department of Orthopedic Surgery and Traumatology, Hospital Kemperhof-Koblenz, Koblenz, Germany.