Upright Supine Imaging Can Detect Clinically Relevant Occult Instability in Degenerative Lumbar Spondylolisthesis - A Two-Year PROM Based Comparison with Flexion Extension Radiographs

Flexion-extension radiographs (FER) are widely used to assess segmental instability in degenerative lumbar spondylolisthesis (DLS) and guide surgical decision-making, as dynamic instability may be associated with suboptimal outcomes after decompression only. However, FER rely on patient effort, and pain or limited mobility can reduce reproducibility and underestimate instability. Upright-supine imaging (USI), comparing upright radiographs with supine MRI, may better capture slip dynamics, but its clinical value remains undefined. To evaluate whether USI-derived segmental instability provides additional prognostic information beyond FER for predicting postoperative improvement in disability and low back pain (LBP) following decompression-only surgery for DLS. This retrospective analysis of prospectively collected data included 92 DLS patients undergoing decompression-only surgery. Slip dynamics were quantified as between-position difference in relative translation (ΔRT) on FER and USI. Multivariable regression assessed associations between ΔRT and two-year outcomes (Oswestry Disability Index ODI, LBP numeric rating scale). ROC analyses determined optimal ΔRTUSI cutoffs. ΔRTUSI independently predicted postoperative improvement in ODI (p<0. 001) and LBP (p<0. 001), whereas ΔRTFER was not associated with outcomes. ROC analysis identified a ΔRTUSI threshold of 7. 9% as optimal (ODI: AUC=0. 87, sensitivity=0. 86, specificity=0. 81; LBP: AUC=0. 62, sensitivity=0. 9, specificity=0. 48). USI identified instability in 22. 8% of patients, uncovering occult instability that FER missed. USI-unstable patients showed significantly less postoperative improvement (p<0. 001). USI detects clinically relevant occult instability missed by FER and independently predicts outcomes after decompression-only surgery. A ΔRTUSI threshold of approximately 8% serves as a quantitative marker supporting USI for preoperative assessment in DLS. • ΔRTUSI predicted 2-year ODI and LBP improvement after decompression-only. • ΔRTFER was not associated with postoperative outcomes. • USI revealed instability missed by FER in 22. 8% of patients. • A 8% ΔRTUSI cutoff identified patients at risk for inferior improvement.