Estimating within-patient sagittal pelvic tilt change from routine anteroposterior hip radiographs in patients undergoing total hip arthroplasty

Aims Changes in sagittal pelvic tilt influence functional cup orientation in total hip arthroplasty (THA). Although lateral spinopelvic radiographs are the reference standard, they are not routinely obtained, whereas anteroposterior (AP) hip radiographs are widely used. This study evaluated whether within-patient pelvic tilt changes can be accurately estimated using AP radiographs. Methods This retrospective observational study included 116 patients who underwent primary THA (derivation cohort). Supine and standing AP radiographs and CT data were acquired preoperatively. CT-derived synthetic radiographs were generated and adjusted to match the pelvic foramen ratio (PFR) measured on AP radiographs, yielding adjusted synthetic (AS) radiographs. For the supine-to-standing transition, the change in anterior pelvic plane tilt (ΔAPPt) measured on AS radiographs was used as the reference outcome, and the change in pubic symphysis-to-transischial line distance (ΔPSTI, mm) on AP radiographs was used as the predictor. A linear regression model was developed to estimate ΔAPPt from ΔPSTI. Internal validation was performed using fivefold cross-validation. Independent temporal validation was performed in a separate cohort (n = 80). Results The derived equation was (ΔAPPt = −0.74 × ΔPSTI + 1.2). Fivefold cross-validation showed a mean error of 0.01° (root mean square error (RMSE) 1.4°, mean absolute error (MAE) 1.0°; 95% limits of agreement (LoA) −2.6° to 2.7°). Absolute error was ≤ 3° in 112 out of 116 (96.6%) cases and ≤ 5° in 116 out of 116 (100%). Independent temporal validation showed a mean error of 0.2° (RMSE 1.3°, MAE 0.9°; 95% LoA −2.4° to 2.8°). Absolute error was ≤ 3° in 75 out of 80 cases (93.8%) and ≤ 5° in 80 out of 80 (100%), within an observed ΔAPPt range of −15.3° to 8.0°. Conclusion A regression equation based on ΔPSTI estimated supine-to-standing within-patient ΔAPPt from routine AP hip radiographs with a clinically acceptable agreement (95% LoA approximately ± 3°). Cite this article: Bone Jt Open 2026;7(10):1280–1287.