Spine
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Case control comparative series. ⋯ 3.
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This is a retrospective cohort study. ⋯ 4.
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Retrospective study. ⋯ 3.
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A retrospective review of a prospectively collected database, the Nationwide Inpatient Sample (NIS), years 2003 to 2012. ⋯ 3.
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A retrospective study. ⋯ 4.