1. Department of Neurosurgery, University of Pennsylvania, Philadelphia, Pennsylvania;
2. Cumberland Spine and Neurosurgery, Cookeville, Tennessee;
3. Maine Medical Partners Neurosurgery and Spine, Scarborough, Maine;
4. NeuroSpine Center of Wisconsin, Appleton, Wisconsin; and
5. Division of Neuroradiology, Department of Radiology, University of Pennsylvania, Philadelphia, Pennsylvania