Spine
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A prospective study. ⋯ 3.
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Retrospective study. ⋯ 3.
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Retrospective study of data collected prospectively by the American College of Surgeons National Surgical Quality Improvement Program. ⋯ 3.
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Retrospective cohort study. ⋯ 3.
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A retrospective study. ⋯ Level 3.