Spinopelvic mobility affects accuracy of acetabular anteversion measurements on cross-table lateral radiographs

Aims Cross-table lateral (CTL) radiographs are commonly used to measure acetabular component anteversion after total hip arthroplasty (THA). The CTL measurements may differ by > 10 degrees from CT scan measurements but the reasons for this discrepancy are poorly understood. Anteversion measurements from CTL radiographs and CT scans are compared to identify spinopelvic parameters predictive of inaccuracy. Methods THA patients (n = 47; 27 males, 20 females; mean age 62.9 years (SD 6.95)) with preoperative spinopelvic mobility, radiological analysis, and postoperative CT scans were retrospectively reviewed. Acetabular component anteversion was measured on postoperative CTL radiographs and CT scans using 3D reconstructions of the pelvis. Two cohorts were identified based on a CTL-CT error of > 10 degrees (n = 11) or < 10 degrees (n = 36). Spinopelvic mobility parameters were compared using independent-samples t-tests. Correlation between error and mobility parameters were assessed with Pearson's coefficient. Results Patients with CTL error > 10 degrees (10 degrees to 14 degrees) had stiffer lumbar spines with less mean lumbar flexion (38.9 degrees(SD 11.6 degrees) vs 47.4 degrees (SD 13.1 degrees); p = 0.030), different sagittal balance measured by pelvic incidence-lumbar lordosis mismatch (5.9 degrees (SD 18.8 degrees) vs -1.7 degrees (SD 9.8 degrees); p = 0.042), more pelvic extension when seated (pelvic tilt -9.7 degrees (SD 14.1 degrees) vs -2.2 degrees (SD 13.2 degrees); p = 0.050), and greater change in pelvic tilt between supine and seated positions (12.6 degrees (SD 12.1 degrees) vs 4.7 degrees (SD 12.5 degrees); p = 0.036). The CTL measurement error showed a positive correlation with increased CTL anteversion (r= 0.5; p = 0.001), standing lordosis (r= 0.23; p = 0.050), seated lordosis (r= 0.4; p = 0.009), and pelvic tilt change between supine and step-up positions (r= 0.34; p = 0.010). Conclusion Differences in spinopelvic mobility may explain the variability of acetabular anteversion measurements made on CTL radiographs. Patients with stiff spines and increased compensatory pelvic movement have less accurate measurements on CTL radiographs. Flexion of the contralateral hip is required to obtain clear CTL radiographs. In patients with lumbar stiffness, this movement may extend the pelvis and increase anteversion of the acetabulum on CTL views. Reliable analysis of acetabular component anteversion in this patient population may require advanced imaging with a CT scan.