[1] Thomas Jefferson Univ, Dept Orthopaed Surg, Philadelphia, PA 19107 USA
[2] Thomas Jefferson Univ, Dept Neurol Surg, Philadelphia, PA 19107 USA
[3] Rothman Inst, 925 Chestnut St,5th Floor, Philadelphia, PA 19107 USA
[4] Univ British Columbia, Dept Orthopaed, Div Spine, Vancouver, BC, Canada