Reconsidering flexion-extension imaging: the emerging role of supine MRI and upright radiographs in isthmic lumbar spondylolisthesis

Segmental instability guides surgical decision-making in isthmic lumbar spondylolisthesis (ILS). Despite concerns regarding radiation exposure, reproducibility and applicability in patients with pain or limited mobility, flexion–extension (FE) radiographs remain the standard for instability assessment. Combining upright radiographs with supine magnetic resonance imaging (MRI), both routinely obtained in clinical practice, may offer a comparable, lower-radiation alternative. This study aimed to compare segmental motion at L5/S1 between FE and upright–supine (US) imaging and to evaluate their ability to detect radiographic instability. In this retrospective cross-sectional study, consecutive patients surgically treated for isolated L5/S1 ILS were screened for eligibility. Segmental motion was assessed using FE radiographs and US imaging, including measurements of dynamic sagittal translation and the proportion of patients identified with radiographic signs of instability. Preoperative pain intensity was assessed using the Numeric Rating Scale (NRS). Of 126 screened patients, 78 met inclusion criteria (59% female; median age 48.5 years; median BMI 26.4 kg/m²). Dynamic sagittal translation was significantly greater on US than on FE (p < 0.001). US imaging identified more cases of radiographic instability (p < 0.001). Only US-measured translation was associated with preoperative low back pain intensity (p = 0.025) and was inversely associated with intervertebral disc degeneration (p = 0.014). US imaging detected a higher proportion of radiographically unstable ILS cases than FE radiographs. Furthermore, only US-derived translation was associated with pain intensity, suggesting greater clinical relevance. Combining upright radiographs with supine MRI may offer a practical, low-radiation alternative for preoperative instability assessment.