Risk Factors and Cup Protrusion Thresholds for Symptomatic Iliopsoas Impingement After Total Hip Arthroplasty: A Retrospective Case-Control Study

Background Groin pain and functional limitations may occur after total hip arthroplasty (THA) due to iliopsoas impingement (IPI). We aimed to determine the risk factors and thresholds of cup protrusion for symptomatic IPI. Methods This retrospective case-control study evaluated 569 primary THAs performed by a single surgeon in a single center. In all THAs, the cup was inserted with the assistance of the computed tomography–based navigation system. Twenty-two THAs (3.9%) were identified to have symptoms of IPI and included in the IPI group. Control group 1 comprised 547 THAs. Control group 2 (66 THAs) was formed by recruiting 3 data-matched controls per patient in control group 1. Risk factors for IPI were assessed in the IPI group and control group 1. Further analysis was performed in the IPI group and control group 2 to evaluate cup protrusion parameters measured on computed tomography images and radiographs. Results Multivariate logistic regression analysis confirmed that the anterolateral approach was associated with a higher risk of IPI (odds ratio [OR], 4.20); however, its goodness of fit was low. Axial protrusion length ≥12 mm (OR, 23.29; sensitivity = 72.7; specificity = 91.0) and sagittal protrusion length ≥4 mm (OR, 61.60; sensitivity = 86.4; specificity = 78.8) were determined as independent predictors of symptomatic IPI. In the linear regression analyses, higher native acetabular version and lower cup anteversion and inclination were related to cup protrusion. Conclusions This study determined the thresholds of cup protrusion length for IPI and identified associated factors increasing cup protrusion, which are useful for surgeons in determining cup position and alignment.