1. Department of Biomedical Engineering; Duke University; Durham North Carolina
2. Department of Orthopaedic Surgery; University of California; Box 0514, 513 Parnassus Ave., S1164 San Francisco California 94143
3. Department of Orthopedic Surgery; Duke University Medical Center; Box 3093, 375 Medical Science Research Building Durham North Carolina 27710
4. Department of Neurosurgery; Duke University Medical Center; Durham North Carolina