FP1.6 How much do functional and anatomical acetabular coverage indices differ when standing in adults with hip pain?

Abstract

Abstract ICLB01 - The Instability Masterclass: Correcting Bone, Biology, and Bad Outcomes, Session Room 6 (BMCC Level 3), October 8, 2026, 10:30 - 12:30 Background Acetabular coverage is traditionally assessed using the lateral center-edge angle (LCEA) and acetabular index (AI). However, functional coverage evaluation with standing radiographs is influenced by coronal plane pelvic obliquity (PO), which may alter acetabular coverage measurements. Questions/Purposes This study therefore addressed the following questions: (1) What is the magnitude and direction of PO in a unilaterally symptomatic young-adult hip population? (2) How do functional (horizontal-referenced) and anatomical (inter-teardrop-referenced) LCEA and AI compare in symptomatic hips? (3) What is the effect of PO magnitude and direction on differences between functional and anatomical acetabular coverage indices in the symptomatic hip? Patients and Methods In this retrospective, consecutive, cohort study, 493 patients with unilateral hip pathology who underwent standing anteroposterior pelvic radiography were included. In each hip, LCEA and AI were measured using functional (horizontal-referenced; fLCEA, fAI) and anatomical (inter-teardrop-referenced; aLCEA, aAI) measurement indices. PO was defined as the angle between the inter-teardrop line and the horizontal reference line. Differences between constructs were quantified as ΔLCEA and ΔAI (anatomical minus functional). Coverage reclassification represented discordance between indices across LCEA (<20°, 20°-25°, 25°-40°, >40°) and AI (<0°, 0°-10°, >10°) categories. Results Mean PO magnitude was 1.54° ± 1.21° (0°–6°); 197/493 (40.0%) hips had PO 2° to <5°, and 14/493 (2.8%) had PO ≥5°. Mean ΔLCEA and ΔAI magnitude were 1.62° ± 1.27° and 1.63° ± 1.27°. Each 1° increase in PO magnitude increased ΔLCEA magnitude by 0.89° (95% CI, 0.86–0.93; p < 0.001; R2 = 0.86) and ΔAI magnitude by 0.90° (95% CI, 0.87–0.93; p < 0.001; R2 = 0.88). PO magnitude and direction altered acetabular coverage relative to aLCEA and aAI, leading to acetabular coverage reclassification in 69/493 (14.0%) symptomatic hips for LCEA when assessed functionally. Conclusions Functional and anatomical indices therefore yield discordant coverage estimates and have the potential to reclassify acetabular coverage. As standing radiographs are increasingly used to depict coverage under weight-bearing conditions, functional indices may be particularly informative with important implications in patients with mild deformity.