Accepted for Publication: December 9, 2010.
Author Contributions:Study concept and design: Yang. Acquisition of data: Guo and Yang. Analysis and interpretation of data: Guo, Zhang, Zhou, Lan, Wu, and Yang. Drafting of the manuscript: Guo and Yang. Critical revision of the manuscript for important intellectual content: Guo, Zhang, Zhou, Lan, Wu, and Yang. Administrative, technical, and material support: Guo, Zhang, Zhou, Lan, Wu, and Yang. Study supervision: Zhang and Yang.
Financial Disclosure: None reported.