1. Division of Spine, Department of Neurosurgery, Duke University, Durham, NC 27710, USA
2. Department of Neurological Surgery, Washington University, St Louis, MO 63110, USA
3. Division of Spine, Department of Orthopedic Surgery, Duke University Medical Center, Durham, NC 27710, USA
4. Department of Neurosurgery, Brigham and Women’s Hospital, Boston, MA 02115, USA