1. Department of Neurological Surgery, Washington University School of Medicine, St. Louis, MO 63110, USA
2. University of Pittsburgh School of Medicine, Pittsburgh, PA 15213, USA
3. Department of Orthopedic Surgery, University of Maryland Medical System, Baltimore, MD 21201, USA
4. Department of Orthopedic Surgery, Washington University School of Medicine, St. Louis, MO 63110, USA