1. Department of Neurosurgery, National Hospital Organization Beppu Medical Center, Beppu, Oita, Japan
2. Department of Neurosurgery, Oita Prefectural Hospital, Oita, Japan
3. Department of Neurosurgery, Nakatsu Neurosurgical Hospital, Nakatsu, Oita, Japan
4. Department of Neurosurgery, Oita University Faculty of Medicine, Hasama, Oita, Japan.