Upright supine imaging can detect clinically relevant occult instability in degenerative lumbar spondylolisthesis - A two-year PROM based comparison with flexion extension radiographs.

INTRODUCTION

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.

RESEARCH QUESTION: 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.

MATERIAL AND

METHODS

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 ΔRT_USI cutoffs.

RESULTS

ΔRT_USI independently predicted postoperative improvement in ODI (p < 0.001) and LBP (p < 0.001), whereas ΔRT_FER was not associated with outcomes. ROC analysis identified a ΔRT_USI 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).

DISCUSSION AND

CONCLUSION

USI detects clinically relevant occult instability missed by FER and independently predicts outcomes after decompression-only surgery. A ΔRT_USI threshold of approximately 8% serves as a quantitative marker supporting USI for preoperative assessment in DLS.