1. Department of Neurosurgery, Tomakomai City Hospital, No. 5-20, 1-chome, Shimizutyou, Tomakomai, Hokkaido 053-0034, Japan
2. Department of Neurosurgery, Kashiwaba Neurosurgical Hospital, No. 7-20, 15-chome, 1-jyou, Tsukisamuhigashi, Toyohira-ku, Sapporo, Hokkaido 062-5813, Japan