1. Department of Neurological Surgery, Carolina Neurosurgery and Spine Associates and Neurological Institute, Carolinas Healthcare System, Charlotte, North Carolina;
2. Orthopaedics of Steamboat Springs, Steamboat Springs, Colorado;
3. Department of Neurologic Surgery, Mayo Clinic, Rochester, Minnesota; and
4. Department of Biostatistics, Vanderbilt University School of Medicine, and Departments of
5. Neurological Surgery,
6. Orthopedic Surgery, and
7. Physical Medicine and Rehabilitation, Vanderbilt University Medical Center, Nashville, Tennessee