1. Carolina Neurosurgery and Spine Associates, Charlotte, North Carolina;
2. Atrium Musculoskeletal Institute, Charlotte, North Carolina;
3. Center for Disc Replacement at Texas Back Institute, Plano, Texas;
4. Cedars-Sinai, Los Angeles, California;
5. Spine Institute of Louisiana, Shreveport, Louisiana;
6. Orthopaedic Institute of Western Kentucky, Paducah, Kentucky;
7. Steadman-Hawkins Clinic, Vail, Colorado; and
8. Texas Back Institute Research Foundation, Plano, Texas