Imaging characteristics of "dynamic" versus "static" spondylolisthesis: analysis using magnetic resonance imaging and flexion/extension films.

BACKGROUND CONTEXT: Traditionally, the "dynamic" and "static" types of spondylolisthesis have been lumped into a single group in the literature. The goal of this study was to define the radiographic characteristics of "dynamic" and "static" spondylolisthesis with the use of magnetic resonance imaging (MRI) and flexion/extension radiographs.

PURPOSE: Describe the characteristic findings present on MRI and flexion/extension radiographs that are associated with dynamic versus static spondylolisthesis.

STUDY

DESIGN

Retrospective radiographic/imaging study.

METHODS

From 2009 to 2011, patients who underwent elective primary posterior spinal fusion for the diagnosis of spondylolisthesis had their plain films assessed for the degree of spondylolisthesis and were designated "dynamic" or "static," as defined by historical measures. Axial and sagittal T2 MRIs were evaluated for associated facet fluid (FF), facet cysts, interspinous fluid (ISF), and facet hypertrophy. These finding were then statistically evaluated for associations between dynamic and static spondylolisthesis on flexion/extension radiographs and characteristic MRI findings.

RESULTS

Ninety patients were included in the study with 114 levels examined for spondylolisthesis. Patients with greater than 3 mm of instability on flexion/extension films were more likely to have FF (p=.018) and ISF (p<.001). Of the patients who had a greater than 3 mm of instability, 39.5% did not demonstrate spondylolisthesis on the sagittal MRI reconstruction. If ISF was present on MRI, there was a positive predictive value of 69.0% that there would be greater than 3 mm instability on flexion/extension films. Absence of FF on MRI had a positive predictive value of 75.6% for instability less than 3 mm on flexion/extension films. In the presence of ISF on MRI, the likelihood ratio of finding more than 3 mm of instability on flexion/extension films was 3.68. The presence of FF on MRI had a likelihood ratio of 1.43 for instability. A total of 36.8% of all spondylolisthesis reduced when supine on MRI.

CONCLUSIONS

The presence of FF and/or ISF is associated with instability greater than 3 mm in flexion/extension radiographs.