Does lumbar facet fluid detected on magnetic resonance imaging correlate with radiographic instability in patients with degenerative lumbar disease?

Study Design. Retrospective radiographic/imaging study. Objective. The purpose of this study was to analyze the association between lumbar facet fluid on MRI and sagittal instability on flexion lumbar radiographs in patients with degenerative disease at L4-L5. We hypothesized that the amount of facet fluid on MRI correlates with instability on the flexion radiograph. Summary of Background Data. Although never formally studied, it has been suggested that lumbar facet fluid detected on MRI is indicative of spinal segment instability. Methods. Patients who underwent laminectomy or laminectomy and fusion for the treatment of degenerative lumbar disease at L4-L5 at our institution between 2002 and 2004 and who had preoperative lumbar MRI and anteroposterior and flexion-extension radiographs available for review were study eligible. Axial T2 MRI images through the L4-L5 facets were analyzed for facet fluid. The facet fluid index was calculated, i.e., the ratio of the sum of the width of fluid in each facet ( bilateral) to the sum of the width of each facet (bilateral). Instability on the flexion radiograph was measured as percent anterior slip at L4-L5. Results. Fifty-one patients were included in the study, 28 (55%) of whom had facet fluid noted on MRI. Of those patients who did have facet fluid on the MRI, 23 of 28 had instability on the flexion lumbar radiograph and 5 of 28 had no instability. The mean facet fluid index and percent anterior slip for the 28 patients with MRI facet fluid was 0.12% and 11.1%, respectively. There was a positive linear association between these values (Pearson correlation coefficient of 0.90, P < 0.001). The positive predictive value of L4-L5 facet fluid on MRI as an indicator of radiographic instability was 82%. Conclusions. There is a close linear association between the facet fluid index and the amount of radiographic instability at L4-L5. Facet fluid on MRI should raise high suspicion of lumbar instability.