[1] Univ Oslo, Fac Med, Oslo, Norway
[2] Oslo Univ Hosp, Dept Neurosurg, Oslo, Norway
[3] Oslo Univ Hosp, Dept Res & Innovat, Div Clin Neurosci, POB 4956, N-0424 Oslo, Nydalen, Norway
[4] Arctic Univ Norway, Inst Clin Med, Tromso, Norway
[5] Univ Hosp North Norway, Dept Neurosurg, Tromso, Norway
[6] Univ Hosp North Norway, Norwegian Registry Spine Surg NORspine, Tromso, Norway
[7] Oslo Metropolitan Univ, Fac Hlth Sci, Dept Rehabil & Technol, POB 4,St Olavs Plass, N-0130 Oslo, Norway