1. Neurosurgical Department, Otaru General Hospital, Otaru, Hokkaido, Japan
2. Spine Center, Department of Neurosurgery, Sapporo Teishinkai Hospital, Sapporo, Hokkaido, Japan
3. Hokkaido Neurosurgical Memorial Hospital, Sapporo, Hokkaido, Japan
4. Department of Neurosurgery, Faculty of Medicine, Hokkaido University, Sapporo, Hokkaido, Japan