1. Department of Neurosurgery, Hokkaido University Graduate School of Medicine, Sapporo, Japan.
2. Department of Neurosurgery, Sapporo Azabu Neurosurgical Hospital, Sapporo, Japan.
3. Department of Orthopedic Surgery, Teine Keijinkai Hospital, Sapporo, Japan.
4. Department of Neurosurgery, Hokkaido Neurosurgical Memorial Hospital, Sapporo, Japan.