Study design: Retrospective radiographic review.
Objective: To determine the incidence of exaggerated fluid signal in lumbar facet joints on magnetic resonance imaging (MRI) and evaluate the correlation of this finding with radiographic evidence of instability.
Summary of background data: Lumbar spine MRI is routinely used to evaluate low back pain. The presence of exaggerated fluid in the facets joints on MRI has been correlated with dynamic instability. To our knowledge, this correlation has never been evaluated in an objective fashion.
Methods: One hundred thirty-four consecutive lumbar MRIs obtained by a single surgeon were selected for review. Studies were evaluated for exaggerated fluid in the facets on axial views. Standing plain films of all patients were then evaluated to determine the incidence of spondylolisthesis for patients with and without exaggerated fluid in the facets on MRI.
Results: Of 134 consecutive MRIs, 118 were available for review. Sixteen had exaggerated fluid in the facets on axial images (13.6%). Only 2 of these 16 (12.5%) had spondylolisthesis appreciable on the MRI at that level. In contrast, 8 of the 16 (50.0%) had spondylolisthesis at the level of exaggerated fluid when the corresponding radiographs were reviewed. Thus, spondylolisthesis was suggested in 6 of 14 cases (42.9%) when the exaggerated fluid sign was present but spondylolisthesis was not evident on the supine MRI. In comparison, in the population without exaggerated fluid, only 1 in 102 (0.9%) showed a slip on plain film that was not observed on MRI. This difference was statistically significant (P<0.001).
Conclusions: Exaggerated fluid in the facets seen on axial MRI is significantly suggestive of spondylolisthesis on standing films, even if this is not appreciated on the supine sagittal MRI sequences. This is the first time that the relative incidence of subtle instability with and without this finding has been evaluated to our knowledge.