The impact of functional sagittal pelvic tilt on CT-based hip dysplasia diagnostics in adults: a cross-sectional 3D simulation study

BACKGROUND AND PURPOSE: Accurate acetabular coverage assessment is essential for diagnosing adult hip dysplasia. Computed tomography (CT) best evaluates 3-dimensional acetabular morphology but is typically acquired supine, while symptoms arise during weightbearing. This study evaluates how sagittal pelvic tilt affects CT-based acetabular parameters in adults with hip dysplasia and whether pelvic tilt causes clinically relevant measurement changes. METHODS: 3D-models from supine CT of 29 adults with hip dysplasia (lateral center-edge angle (LCEA) ≤ 25°) yielded 39 affected hips. LCEA, inclination (AI), anterior/posterior/horizontal acetabular sector angles (AASA, PASA, HASA), acetabular version (AV), extrusion index (EI), and total and loadbearing coverage were measured. Pelvic tilt was simulated in 5° increments from 30° anterior to 30° posterior around the hip centers and parameters were remeasured in each position. Differences < 2.5° were considered clinically irrelevant. RESULTS: Median supine LCEA was 21°, AI 50°, AASA 43°. Across -30° to +30° tilt, AASA/loadbearing coverage decreased, PASA/AV increased, LCEA/EI showed parabolic patterns, and HASA/total coverage remained stable. From supine to simulated upright, paired mean differences were: LCEA -1° (95% confidence interval [CI] -2 to 0), AASA -4° (CI -5 to -3), PASA +4° (CI 4 to 5), HASA 0° (CI -1 to 1), AV +4° (CI 3 to 4), AI +1° (CI -1 to 3), EI +2% (CI 0 to 4), total coverage 0% (CI 0 to 0), and loadbearing coverage -2% (CI -2 to -2). CONCLUSION: Functional sagittal pelvic tilt resulted in generally small changes in CT-based acetabular measurements in dysplastic hips, which may be clinically relevant in borderline cases, particularly for AASA, PASA, acetabular version, and EI, whereas LCEA and AI were not significantly affected.