1. Department of Neurosurgery, University of Kentucky, Lexington, Kentucky, USA;
2. Creighton University School of Medicine, Phoenix, Arizona, USA;
3. Department of Neurosurgery, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, Arizona, USA;
4. Department of Biostatistics, University of Kentucky, Lexington, Kentucky, USA;
5. Center for Research and Training in Neurosurgery, Department of Neurosurgery, Hospital Universitario de la Samaritana, Bogota, Colombia