1. Department of Orthopedic Surgery, University of California San Francisco, San Francisco, CA, USA
2. Department of Neurological Surgery, University of California San Francisco, San Francisco, CA, USA
3. Department of Orthopedic Surgery, Boston Medical Center, Boston, MA, USA
4. Stritch School of Medicine, Loyola University, Maywood, IL, USA
5. Department of Neurological Surgery, University of California San Diego, La Jolla, CA, USA
6. Department of Orthopedic Surgery, University of Utah, Salt Lake City, UT, USA