Diagnostic value of upright-supine imaging and its association with postoperative outcomes in degenerative lumbar spondylolisthesis-a 2-year patient-reported outcome measure-based comparison with flexion-extension radiographs.

BACKGROUND CONTEXT: The optimal surgical treatment of degenerative lumbar spondylolisthesis (DLS) is a subject of ongoing debate, particularly regarding the choice between decompression-only and fusion surgery. Flexion-extension radiographs (FER) are commonly used to assess segmental instability and guide this decision, but in symptomatic patients, their reliance on active patient motion can lead to underestimation of instability that may be associated with postoperative outcomes. Upright-supine imaging (USI), which compares upright radiographs with supine magnetic resonance imaging, assesses translational slip dynamics under physiological loading conditions and may offer a more sensitive alternative. However, the clinical and outcome relevance of USI-based instability assessment across different surgical strategies is unclear.

PURPOSE: To compare the association of USI- and FER-derived instability measures with postoperative improvement in disability and low back pain (LBP) and to evaluate whether the clinical impact of instability differs between decompression-only and fusion surgery in patients with DLS.

DESIGN

Retrospective cohort study.

PATIENT SAMPLE: A total of 254 patients with DLS treated by decompression-only surgery or fusion surgery.

OUTCOME MEASURES: Functional disability and LBP were assessed using the Oswestry Disability Index (ODI) and numeric rating scale preoperatively and at 2-year follow-up.

METHODS

Slip dynamics were quantified on FER and USI as the between-position difference in relative translation (ΔRT). Associations between ΔRT on FER (ΔRT_FER) and USI (ΔRT_USI) and postoperative outcomes were evaluated using multivariable regression models. Interaction analyses were performed to assess procedure-dependent effects. Receiver operating characteristic analyses explored procedure-specific ΔRT_USI thresholds for clinically meaningful improvement.

RESULTS

USI identified a significantly higher proportion of unstable segments than FER in both decompression-only and fusion cohorts (both p<.001). ΔRT_USI was significantly associated with both absolute and relative postoperative improvement in ODI and LBP (both p<.001), whereas ΔRT_FER showed no significant association with outcomes in this cohort (all p>.2). The effect of USI-derived instability demonstrated inverse associations by surgical strategy, with greater ΔRT_USI associated with less improvement after decompression-only surgery and greater improvement after fusion (p<0.001 for both ODI and LBP). Receiver operating characteristic analyses identified exploratory procedure-specific ΔRT_USI thresholds, with a cutoff of 7.9% for decompression-only surgery and cutoffs of 5.7% (ODI) and 5.3% (LBP) for fusion.

CONCLUSIONS

In this cohort, USI detected segmental instability that was not captured by conventional FER and was associated with postoperative outcomes. The impact of instability on clinical improvement differed according to surgical strategy, suggesting that USI-derived slip dynamics may help risk-stratify patients in preoperative assessment. Procedure-specific ΔRT_USI thresholds identified in this study may support a more stratified approach to surgical decision-making; however, these findings should be considered hypothesis-generating and warrant prospective validation in independent cohorts.