Anterior Acetabular Coverage of Borderline Hips Is Overestimated on Standing Antero-posterior Pelvic Radiographs.

Abstract

We sought to: (1) develop a new method in measuring spino-pelvic tilt (SPT) from antero-posterior pelvic (AP pelvis) radiographs and computed tomography scans, and examine its reliability and validity; (2) use Z-scores to evaluate patients' SPT based on an established asymptomatic norm; and (3) examine whether a compensatory mechanism of anterior pelvic tilt exists for borderline hips, denoted as differences in anterior wall indices both between standing and supine positions, and between borderline hip dysplasia (BHD) patients and asymptomatic norms. After the inclusion and exclusion criteria, 24 patients were finally included. Patients who had unilateral or bilateral borderline hips were included. A supine computed tomography scan, standing AP pelvis, and standing full spine radiograph were obtained. Digitally reconstructed radiographs with various SPTs were generated. Regression models were fitted to predict SPT for standing AP pelvis, and subsequently to calculate Z-scores based on an established asymptomatic norm. Wilcoxon signed rank tests were applied to test the deviation of SPT from the asymptomatic norm. Generalized estimating equations were used to model Z-scores for SPT and between-position/between-population anterior acetabular coverage. The SPT prediction had excellent reliability with intraclass correlation coefficient > 0.99 (0.99 to 1.00) and interobserver reliability with intraclass correlation coefficient > 0.99 (0.99 to 1.00). The mean Z-score was -1.09 (0.94), and its difference from zero was statistically significant (P < 0.001). The generalized estimating equation modeling reveals a negative correlation between SPT and anterior acetabular coverage compensation, denoted as differences in anterior wall indices, both between standing and supine in BHD patients and between standing BHD patients and the standing asymptomatic norm. Patients who have BHD have greater anterior pelvic tilt compared with the asymptomatic norm. Anterior acetabular coverage may be overestimated on the standing AP pelvis for BHD patients.