Standing-to-horizontal imaging reveals greater translation than flexion-extension radiographs for assessing lumbar instability in lumbar degenerative spondylolisthesis.

Background: Flexion-extension radiographs are commonly used to assess lumbar instability in degenerative spondylolisthesis, but may underestimate mobility due to limited motion and muscle guarding. This study compared position-dependent translation and instability classification on (1) standing flexion-extension radiographs and (2) standing neutral-to-horizontal computed tomography (CT)/magnetic resonance imaging (MRI), and evaluated associations with postoperative patient-reported outcome measures (PROMS) change. Methods: In this retrospective single-center cohort study, surgically treated lumbar spinal stenosis patients with degenerative spondylolisthesis were assessed with standing neutral, standing flexion and extension radiographs, and horizontal CT and/or MRI within 6 months preoperatively (n = 227). Outcome measures were translation (millimeters [mm]), instability (Δ translation ≥4 mm), numeric rating scale (NRS) back/leg pain, and Oswestry Disability Index. Translation was measured on each modality. We compared mean translation change for flexion-extension and for standing neutral-to-horizontal CT/MRI. Correlation and agreement (overall agreement; Cohen's κ) were calculated. PROMS change was analyzed with regression adjusting for surgical technique. Results: Mean translation change was smaller on flexion-extension radiographs (1.4 ± 2.1 mm) than on standing neutral-to-horizontal comparisons (2.7 ± 2.6 mm for CT and 2.7 ± 2.3 mm for MRI; p < .001). Instability prevalence was 12.8% by flexion-extension and 26.6-30.4% by standing-horizontal comparisons, with limited agreement (overall agreement 62.6%-68.2%, κ ≈ 0). Standing extension translation correlated most strongly with horizontal CT/MRI (r = 0.78 and r = 0.80). PROMs improved at a mean follow-up of 16.1 ± 13.8 months; instability was not independently associated with PROMS change. Conclusions: Standing neutral-to-horizontal comparisons show larger position-dependent translation and often reclassify instability compared with flexion-extension radiographs. When horizontal CT/MRI is available, routine flexion-extension radiographs may add limited incremental information; standing extension radiographs may be a practical alternative when advanced imaging is unavailable. Imaging-defined instability was not consistently associated with midterm PROMS improvement.