Flexion-extension standing radiographs underestimate instability in patients with single-level lumbar spondylolisthesis: comparing flexion-supine imaging may be more appropriate.

Background: Generally, most spine surgeons agree that increased segmental motion viewed on flexion-extension radiographs is a reliable predictor of instability; however, these views can be limited in several ways and may underestimate the instability at a given lumbar segment.

Methods: Consecutively collected adult (≥18 years old) patients with symptomatic single-level lumbar spondylolisthesis were reviewed from a two-surgeon database from 2015 to 2019. Routine standing lumbar X-rays (neutral, flexion, extension) and supine lumbar MRI (sagittal T2-weighted imaging sequence) were performed. Patients were excluded if they had prior lumbar surgery, missing radiographic data, or if the time between X-rays and MRI was >6 months.

Results: All 39 patients with symptomatic, single-level lumbar spondylolisthesis were identified. The mean age was 57.3±16.7 years and 66% were female. There was good intra- and inter-rater reliability agreement between measured values on the presence of instability. The slip percentage (SP) difference was significantly highest in the flexion-supine (FS) (5.7 mm, 12.3%) and neutral standing-supine (NS) (4.3 mm, 8.7%) groups, both of which were significantly higher compared with the flexion-extension (FE) group (1.8 mm, 4.5%, P<0.001). Ventral instability based on SP >8% was observed more frequently in FS (79.5%) and NS (52.6%) groups compared with FE group (16.7%, P<0.001). No statistically significant correlation was found between SP and disc angle for all radiographic views.

Conclusions: Comparing standing lateral and flexion X-rays with supine MRIs provides higher sensitivity to assess instability than standard flexion-extension radiographs. The FS and NS comparisons also show greater slip percentage differences at higher slip grades, but not at different lumbar levels. These changes are not dependent on age or gender.