Author Contributions: Drs Back and Curtis and Ms McCown had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis. Study concept and design: Back, Engelberg, Reinke, Wenrich, McGrath, and Curtis. Acquisition of data: Back, Young, McCown, Engelberg, McGrath, and Curtis. Analysis and interpretation of data: Back, Young, McCown, Engelberg, Vig, Reinke, Wenrich, McGrath, and Curtis. Drafting of the manuscript: Back, Young, and Reinke. Critical revision of the manuscript for important intellectual content: Back, Young, McCown, Engelberg, Vig, Reinke, Wenrich, McGrath, and Curtis. Statistical analysis: Engelberg and Reinke. Obtained funding: Engelberg, Wenrich, and Curtis. Administrative, technical, and material support: Young and McCown. Study supervision: Back, Engelberg, McGrath, and Curtis.