1 *Division of Spine, Department of Orthopaedics, University of British Columbia, Vancouver, British Columbia, Canada †Departments of Orthopaedic Surgery and Neurological Surgery, Thomas Jefferson University and The Rothman Institute, Philadelphia, PA ‡Department of Neurological Surgery, Thomas Jefferson University, Philadelphia, PA §Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, New Haven, CT ¶British Columbia Children's Hospital, Vancouver, British Columbia, Canada ‖Departments of Surgery and Clinical Neurosciences, University of Calgary, Calgary, Alberta, Canada **Department of Neurosurgery, Brigham & Women's Hospital, Harvard Medical School, Boston, MA; and ††Department of Orthopaedic Surgery, Feinberg School of Medicine, Northwestern University, Chicago, IL.