Investigating the correlation between facet joint fluid and instability in degenerative lumbar disease on flexion and extension radiographs: a retrospective study

Background: Degenerative lumbar disease is a leading cause of chronic low back pain and disability. While flexion-extension radiographs are traditionally used to assess spinal instability, they can underestimate subtle instabilities. Facet joint effusion detected on magnetic resonance imaging (MRI) has been proposed as a dynamic indicator of biomechanical instability, but its diagnostic utility and practical thresholds require further validation. Objectives were to investigate the correlation between the presence of facet joint fluid and spinal instability in patients with degenerative lumbar disease using dynamic flexion and extension radiographs. Methods: A retrospective study was conducted involving 132 patients diagnosed with degenerative lumbar disease. Imaging evaluations included supine MRI to assess facet joint fluid and upright flexion-extension radiographs to measure dynamic motion. Radiographic instability was defined as a sagittal translation greater than 3 mm or a segmental angulation greater than 20°. Clinical symptoms, such as pain location and duration, were also evaluated. Results: Of the 132 patients, 66 (50.0%) exhibited bilateral facet fluid, 32 (24.2%) had unilateral fluid, and 34 (25.8%) showed no facet fluid. Patients with bilateral facet fluid demonstrated the highest facet fluid index (0.58±0.04) and dynamic motion index (5.3±1.2), along with a greater slip percentage in flexion (8.4±1.1%). A striking association was observed between facet fluid presence and radiographic instability. Among patients with bilateral fluid, 93.9% had a sagittal translation >3 mm and 89.4% had an angular motion >20°. Furthermore, facet fluid presence significantly influenced clinical presentation, with 83.3% of patients with bilateral fluid reporting bilateral pain. Conclusions: The accumulation of facet joint fluid, particularly bilateral effusion, is strongly correlated with measurable radiographic spinal instability and specific clinical symptoms in degenerative lumbar disease. These findings suggest that MRI-detected facet fluid may serve as a useful adjunctive imaging marker of dynamic instability, providing valuable insights for early detection and surgical planning.