EP4.14 Introducing the Impingement Index: Patient and Three-dimensional CT Drivers of Hip Internal Rotations Validated Across a Prospective Multicenter Study

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ID: 331288
2026
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Abstract
Abstract Introduction Hip internal rotation at 90° flexion (IRF) is commonly restricted in femoroacetabular impingement syndrome (FAIS), but tools to comprehensively characterize patient-specific level of impingement using CT morphology remain limited. This study aimed to identify CT-based morphological factors associated with IRF in a multicenter prospective cohort and develop an Impingement Index incorporating head-neck morphology, acetabular morphology, and femoral version. Methods A prospective multicenter cohort of 527 hips with symptomatic FAIS treated with hip arthroscopy was analyzed. The primary marker of FAI was IRF. Patient and CT morphology measures - alpha angle, acetabular version (AV), radial acetabular coverage (RAC), and femoral version (FV), were evaluated at baseline for association with IRF using Spearman correlations. Univariate and hierarchical multivariate linear regression models were constructed to identify independent predictors and derive a composite Impingement Index. Model performance was assessed using adjusted R2, standard error of estimate (SEE), mean absolute error (MAE), and root mean squared error (RMSE). Results The cohort included 53.5% (282/527) females and had a mean age of 24.8±8.0 years. Mean IRF was 13.1°±10.1°, with mean flexion of 98.3°±10.1°. Within-domain clustering was observed for alpha angles from 1:30–3:00 (r=0.83–0.86; p<0.001) and RAC from 1:30–3:00 (r=0.75–0.90; p<0.001). Independent predictors of restricted IRF included male (b=−4.82; p<0.001), larger alpha angle at 3:00 (b=−0.21; p<0.001), and greater RAC at 2:00 (b=−0.31; p<0.001), whereas higher FV (b=0.39; p<0.001) and greater AV (b=0.12; p=0.042) predicted higher IRF. Femoral version was the strongest contributor, and its association with IRF was moderated by alpha angle at 3:00 (b=−0.012; p<0.001). The Impingement Index demonstrated significant predictive ability (r=0.661), with MAE 5.84° and RMSE 7.59° and allows for a predicted patient-specific IRF (with and without correction for age, sex, and Beighton score). Conclusions IRF is strongly correlated to underlying hip morphology including FV, alpha angle, AV, and RAC across a multicenter prospective cohort of FAI patients. The Impingement Index allows for a patient-specific assessment of predicted IRF, as well as contributions from different factors such as femoral version vs. alpha angle.
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Authors Jeffrey J. Nepple, Songyuan Gu, Zak Robben, Caroline E. Drain, Anchor Study Group, John C. F. Clohisy
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Journal Journal of hip preservation surgery
Year 2026
DOI
10.1093/jhps/hnag043.348
URL
https://doi.org/10.1093/jhps/hnag043.348
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