[1] Hokkaido Univ, Fac Med, Dept Orthopaed Surg, Sapporo, Hokkaido, Japan
[2] Hokkaido Univ, Grad Sch Med, Sapporo, Hokkaido, Japan
[3] Hakodate Cent Gen Hosp, Spine Ctr, Hakodate, Hokkaido, Japan
[4] Hokkaido Orthopaed Mem Hosp, Dept Orthopaed Surg, Sapporo, Hokkaido, Japan
[5] Hokkaido Spinal Cord Injury Ctr, Dept Orthopaed Surg, Bibai, Hokkaido, Japan
[6] Eniwa Hosp, Dept Orthopaed Surg, Eniwa, Japan
[7] Sapporo City Gen Hosp, Dept Orthopaed Surg, Sapporo, Hokkaido, Japan
[8] Kushiro Rosai Hosp, Dept Orthopaed Surg, Kushiro, Japan
[9] Hokkaido Spine Study Grp, Sapporo, Hokkaido, Japan