Does Acetabular Coverage Vary Between the Supine and Standing Positions in Patients with Hip Dysplasia?

BACKGROUND Although variation in physiologic pelvic tilt may affect acetabular version and coverage, postural change in pelvic tilt in patients with hip dysplasia who are candidates for hip preservation surgery has not been well characterized, and its clinical importance is unknown. QUESTIONS/PURPOSES The aim of this study was to determine (1) postural changes in sagittal pelvic tilt between the supine and standing positions; (2) postural changes in the acetabular orientation and coverage of the femoral head between the supine and standing positions; and (3) patient demographic and morphologic factors associated with sagittal pelvic tilt. METHODS Between 2009 and 2016, 102 patients underwent pelvic osteotomy to treat hip dysplasia. All patients had supine and standing AP pelvic radiographs and pelvic CT images taken during their preoperative examination. Ninety-five patients with hip dysplasia (lateral center-edge angle < 20°) younger than 60 years old were included. Patients with advanced osteoarthritis, other hip disease, prior hip or spine surgery, femoral head deformity, or inadequate imaging were excluded. Sixty-five patients (64%) were eligible for participation in this retrospective study. Two board-certified orthopaedic surgeons (TT and MF) investigated sagittal pelvic tilt, spinopelvic parameters, and acetabular version and coverage using pelvic radiographs and CT images. Intra- and interobserver reliabilities, evaluated using the intraclass correlation coefficient (0.90 to 0.98, 0.93 to 0.99, and 0.87 to 0.96, respectively), were excellent. Demographic data (age, gender, and BMI) were collected by medical record review. Sagittal pelvic tilt was quantified as the angle formed by the anterior pelvic plane and a z-axis (anterior pelvic plane angle). Using a 2D-3D matching technique, we measured the change in sagittal pelvic tilt, acetabular version, and three-dimensional coverage between the supine and standing positions. We correlated sagittal pelvic tilt with demographic and CT measurement parameters using Pearson's or Spearman's correlation coefficients. RESULTS Although functional pelvic tilt varied widely among individuals, the pelvis of patients with hip dysplasia tilted posteriorly from the supine to the standing position (mean APP angle 8° ± 6° versus 2° ± 7°; mean difference -6°; 95% CI, -7° to -5°; range -17° to 4.1°; p < 0.001; paired t-test).The pelvis tilted more than 5° posteriorly from the supine to the standing position in 39 patients (60%), and the change was greater than 10° in 12 (18%). In the latter subgroup of patients, the mean acetabular anteversion angle increased (22° ± 5° versus 27° ±5°; mean difference 5°; 95% CI, 4°-6°; p < 0.001) and the mean anterosuperior acetabular sector angle notably deceased from the supine to the standing position (91° ± 11° versus 77° ± 14°; mean difference -14°; 95% CI, -17° to -11°; p < 0.001; paired t-test). Postural change in pelvic tilt was not associated with any of the studied demographic or morphologic parameters, including patient age, gender, BMI, and acetabular version and coverage. CONCLUSIONS On average, the pelvis tilted posteriorly from the supine to the standing position in patients with hip dysplasia, resulting in increased acetabular version and decreased anterosuperior acetabular coverage in the standing position. Thus, assessment with a supine AP pelvic radiograph may overlook changes in acetabular version and coverage in weightbearing positions. We recommend assessing postural change in sagittal pelvic tilt when diagnosing hip dysplasia and planning hip preservation surgery. Further studies are needed to determine how postural changes in sagittal pelvic tilt affect the biomechanical environment of the hip and the clinical results of acetabular reorientation osteotomy. LEVEL OF EVIDENCE Level IV, diagnostic study.