1. Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109;
2. Department of Biomedical Engineering, University of Michigan, Ann Arbor, MI 48109
3. Department of Biomedical Engineering, University of Michigan, Ann Arbor, MI 48109; Department of Surgery, University of Michigan, Ann Arbor, MI 48109
4. Spine Research Laboratory, Department of Neurosurgery, University of Michigan, Ann Arbor, MI 48109;
5. Spine Research Laboratory, Department of Neurosurgery, University of Michigan, Ann Arbor, MI 48109
6. Department of Orthopaedic Surgery, University of Michigan, Ann Arbor, MI 48109; Department of Biomedical Engineering, University of Michigan, Ann Arbor, MI 48109 e-mail: