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Systematic review and meta-analysis.To assess how confidently low back pain (LBP) can be attributed to abnormalities on magnetic resonance imaging (MRI), and thereby explore the potential value of MRI abnormalities in refining case definition for mechanical LBP in epidemiological research.Most epidemiological studies of mechanical LBP have defined cases only by reported symptoms, but it is possible that the potency of causes differs depending on whether there is demonstrable underlying spinal pathology.We reviewed the published data on MRI abnormalities, looking for data on the repeatability of their assessment, their prevalence in people free from LBP, and their association with LBP. Where data were sufficient, we calculated a summary estimate of prevalence in people without LBP and a meta-estimate of the odds ratio for the association with LBP. A formula was then applied to estimate the corresponding prevalence rate ratio, assuming 3 possible prevalence rates for LBP in the general population.Data were most extensive for disc protrusion, nerve root displacement or compression, disc degeneration, and high intensity zone, all of which could be assessed repeatedly. All were associated with LBP, meta-estimates of odds ratios ranging from 2.3 (nerve root displacement or compression) to 3.6 (disc protrusion). However, even for disc protrusion, estimates of the corresponding prevalence rate ratios were mostly less than 2.MRI findings of disc protrusion, nerve root displacement or compression, disc degeneration, and high intensity zone are all associated with LBP, but individually, none of these abnormalities provides a strong indication that LBP is attributable to underlying pathology. This limits their value in refining epidemiological case definitions for LBP.