Risk factors for iliopsoas impingement after total hip arthroplasty using a collared femoral prosthesis

Background The relationship between collar design of a femoral component and iliopsoas impingement (IPI) after total hip arthroplasty (THA) is still underrecognized. The purpose of our study was to determine the possible risk factors for IPI related to the femoral component, when using a collared femoral prosthesis. Methods A total of 196 consecutive THA patients (206 hips) using a collared femoral prosthesis were reviewed retrospectively after exclusion of the factors related to acetabular component and femoral head. The patients were divided into +IPI and -IPI group according to the presence of IPI. Radiological evaluations were performed including femoral morphology, stem positioning, and collar protrusion length (CPL). Multivariate regression analysis was performed to assess the risk factors for IPI. Results At a minimum follow-up of 1 year, IPI was observed in 15 hips (7.3%). Dorr type C proximal femur was found in nine hips (60%) in the +IPI group and in 28 hips in the -IPI group (14.7%,p< 0.001). The mean stem anteversion in the +IPI group was significantly greater than that in the -IPI group (19.1 degrees vs. 15.2 degrees,p< 0.001), as well as the mean CPL (2.6 mm vs. - 0.5 mm,p< 0.001). The increased stem anteversion (OR= 1.745,p= 0.001) and CPL (OR= 13.889,p= 0.001) were potential risk factors for IPI. Conclusions The incidence of IPI after THA is higher than expected when using a collared femoral prosthesis. Among the factors related to collared femoral prosthesis, excessively increased stem anteversion and prominent collar protrusion are independent predictors for IPI. In addition, high risk of IPI should be carefully considered in Dorr type C bone, despite that femoral morphology is not a predictive factor.