Is flexion-extension imaging necessary for surgical decision-making in degenerative lumbar spondylolisthesis? Supine MRI and upright radiographs offer superior diagnostic value

Degenerative lumbar spondylolisthesis (DLS) is a major indication for lumbar surgery. Surgical decision-making, particularly regarding the need for fusion, often depends on radiographic detection of segmental instability. Flexion–extension (FE) radiographs remain the reference standard but are limited by radiation exposure, motion-related discomfort, and poor reproducibility, whereas upright–supine (US) imaging - combining upright lateral radiographs with supine MRI - may offer a comparable, low-radiation alternative using routinely acquired images. To compare segmental motion at L4/5 between FE and US imaging and evaluate their respective abilities to identify patients with radiographic signs of instability. In this retrospective cross-sectional study, 128 patients surgically treated for isolated L4/5 DLS were included. Segmental motion was analyzed using FE and US imaging by determining relative slippage, dynamic slip angle, and radiographic instability, defined as sagittal translation ≥8% and/or a dynamic slip angle ≥10°. Relative slippage was comparable between US and FE imaging (US: 7.43 ± 5.10%; FE: 7.29 ± 4.57%; p = 0.994). The dynamic slip angle was significantly higher in US imaging compared to FE imaging (US: 6.42 ± 4.20°; FE: 4.03 ± 2.98°; p < 0.001). US imaging identified more patients with radiographic signs of instability compared to FE imaging (US: n = 68, 53.1%; FE: n = 42, 32.8%; p < 0.001). US imaging identified a higher proportion of patients meeting established thresholds for radiographic instability compared to FE radiographs. Although FE imaging remains the diagnostic standard, US imaging may offer a practical, low-radiation alternative for preoperative assessment. Prospective investigations are warranted to confirm its clinical utility. • US imaging detected more patients with radiographic instability than FE radiographs. • Dynamic slip angle was significantly higher in US imaging than in FE imaging. • Relative slippage was comparable between US and FE imaging. • US imaging may provide a low-radiation approach for instability evaluation.