Comparative Analysis of Relaxed- and Flexed-Seated Radiographs for Assessing Spino-Pelvic Mobility in Total Hip Arthroplasty

BACKGROUND: This study evaluated the comparative value of relaxed- and flexed-seated lateral radiographs in assessing spino-pelvic mobility in patients undergoing total hip arthroplasty (THA). METHODS: A prospective cohort of 200 consecutive patients undergoing primary THA for osteoarthritis underwent preoperative standing, relaxed-seated, and flexed-seated lateral radiographs using a standardized protocol. There were 10 patients who were excluded for incomplete imaging, leaving 190 for analysis. The mean age was 66 years (range, 35 to 90), and the mean Body Mass Index (BMI) was 28.0 (range, 20.1 to 41.0). Measurements included sacral slope (SS), lumbar lordosis (LL), pelvic incidence (PI), and pelvic tilt (PT). Patients were also classified according to the hip-spine-classification. RESULTS: The mean PI was 52.7° ± 12°. The mean standing SS was 36.4°, decreasing to 15.1° in the relaxed-seated and increasing to 29.1° in the flexed-seated position. Based on standing-to-relaxed-seated change, 14.2% were classified as stiff (types 1B/2B). Using standing-to-flexed-seated views, stiffness increased to 52.6%. Reclassification between seated postures occurred in 39.5% of patients, predominantly from mobile (1A/2A) to stiff (1B/2B) phenotypes. Flatback deformity (PI-LL greater than 10°) was present in 28.4% and associated with a higher stiffness rate on flexed-seated imaging (63 versus 27.8%). The sacral slope correlation between relaxed and flexed postures was moderate (r = 0.48). CONCLUSION: Flexed-seated radiographs classify a greater proportion of patients as having reduced spino-pelvic mobility compared with relaxed-seated imaging, with reclassification observed in a substantial proportion of cases. The two postures show only moderate correlation, indicating that relaxed-seated imaging does not reliably predict flexed-seated behavior. These findings highlight that spino-pelvic assessment is posture-dependent and suggest a role for flexed-seated imaging in selected patients, although the impact on component positioning and clinical outcomes requires further study.