Predictors of Iliopsoas Impingement after Total Hip Arthroplasty with Overhanging Acetabular Components

BACKGROUND: Cup protrusion has been recognized as the most sensitive and specific radiological parameter to predict iliopsoas (IP) impingement after total hip arthroplasty (THA): it is a necessary but not sufficient factor. We aimed to investigate the following: (1) whether radiological differences exist between protruding cups associated with symptomatic IP impingement and asymptomatic ones; (2) which positional parameters correlate with symptomatic IP impingement in THA with cup protrusion. METHODS: This retrospective case-control study included 35 noncemented THAs with CT-confirmed IP impingement (clinical, radiographic, and arthroscopic diagnosis) and 17 asymptomatic noncemented THAs with comparable protrusion and ≥5 years of follow-up. Postoperative CT scans were evaluated for cup protrusion in all planes, inclination, anteversion, cup tilt, femoral and acetabular offsets, eccentric reaming, leg length discrepancy (LLD), and stem antetorsion. Groups were comparable for age, sex, side, diagnosis, and protrusion magnitude. RESULTS: Symptomatic hips showed markedly greater acetabular diameter mismatch (1.7 mm vs-1.8 mm; Cohen's d = 1.16), higher cup anteversion (19.8° vs 11.5°; d = 0.88), greater sagittal tilt (19.9° vs 11.2°; d = 0.81), and smaller LLD (1.4 vs 6.8 mm; d = 0.73). In the exploratory Firth penalized logistic regression model, acetabular diameter mismatch was the only variable associated with symptomatic IP impingement (OR 1.33, 95% CI, 1.09 to 2.26), whereas anteversion, tilt, and LLD did not retain independent associations. CONCLUSION: Among THAs with comparable cup protrusion, cup oversizing showed the strongest and most consistent association with symptomatic IP impingement, whereas increased anteversion and sagittal tilt appeared to modulate the mechanical consequences of protrusion. LEVEL OF EVIDENCE: Level of Evidence: III.