1. Department of Surgery, Division of Neurosurgery and Spinal Program, University of Toronto, Toronto Western Hospital, Toronto, Ontario, Canada;
2. Division of Biostatistics, University of Texas School of Public Health at Houston;
3. Department of Neurosurgery, The Methodist Hospital, Houston, Texas;
4. Department of Neurosurgery, University of Maryland, Baltimore, Maryland;
5. Department of Neurosurgery and Orthopedic Surgery, Division of Spinal Disorders, Thomas Jefferson University, Philadelphia, Pennsylvania;
6. Departments of Neurological Surgery and Orthopedic Surgery, University of Virginia Health System, Charlottesville, Virginia;
7. Department of Neurosurgery, University of Kentucky, Louisville, Kentucky;
8. Department of Neurosurgery and Miami Project to Cure Paralysis, University of Miami, Florida; and
9. Department of Neurosurgery, University of Texas Health Sciences Center, Houston, Texas