Evaluation of Femoro-Epiphyseal Acetabular Roof Index for Assessing Hip Instability in Hip Disorders

Introduction The Femoro-Epiphyseal Acetabular Roof (FEAR) index is a proposed radiographic marker of hip instability, particularly in borderline developmental dysplasia of the hip (BDDH). Its diagnostic value in other hip morphologies and its association with patient-reported outcomes (PROMs) remain unclear. This study evaluated the FEAR index for assessing hip instability across different acetabular morphologies by analyzing its associations with radiographic measures, dynamic ultrasonography, and PROMs. Materials and Methods We retrospectively studied 171 female patients (200 hips) who underwent radiographs and ultrasonography from April 2020 to March 2024. Hips were classified by lateral center-edge angle (LCEA) as normal (≥ 25°), BDDH (20°–25°), or developmental dysplasia of the hip (DDH; <20°). Radiographic measures included the FEAR index, LCEA, vertical-center anterior angle (VCA), and head lateralization index (HLI). Dynamic instability was assessed by femoral head translation distance (FTD) on ultrasonography. Correlation between FEAR and other parameters were tested with Spearman’s rank. Receiver operating characteristic (ROC) analysis identified the FEAR cut-off for instability (FTD ≥ 2 mm). PROMs were compared between groups using the Japanese Orthopaedic Association Hip Disease Evaluation Questionnaire (JHEQ) and Numerical Rating Scale (NRS). Results The FEAR index correlated significantly with LCEA, VCA, HLI, and FTD across all morphological groups. ROC analysis identified 5.2° as the optimal cut-off (AUC, 0.787). Patients with FEAR ≥ 5.2° had lower JHEQ scores and higher NRS scores than those with FEAR index < 5.2° ( p  < 0.05). Conclusions The FEAR index is a simple, clinically useful radiographic marker of hip instability across BDDH, DDH, and normal hips. Its strong associations with dynamic ultrasonography and PROMs support its value as a practical tool for comprehensive hip instability assessment.