Femoral Stem Collar Overhang Is Associated with Iliopsoas Impingement After Direct Anterior Total Hip Arthroplasty

INTRODUCTION: Iliopsoas impingement (IPI) is an underrecognized complication following total hip arthroplasty (THA), typically attributed to anterior acetabular component overhang. As modern implants have shifted toward collared stem designs, an overhanging femoral collar may also contribute to IPI. We aimed to determine whether the degree of femoral stem collar overhang is associated with IPI in direct anterior approach THA. METHODS: We retrospectively reviewed 985 primary direct anterior THAs performed with triple-tapered collared femoral stems at a single academic center. All IPI cases were identified using diagnosis codes and surrogate markers (iliopsoas injections, metal artifact reduction sequence magnetic resonance imaging, or arthroscopic tenotomy),and then confirmed through chart review. Radiographic measurements included femoral collar overhang, neck height, and anterior cup overhang at six weeks, with collar overhang reassessed at one year. Collar overhang was analyzed continuously and dichotomously using a three-millimeter (mm) cutoff determined by receiver operating curve (ROC) analysis. Multivariable logistic regressions assessed the association of collar overhang with IPI, adjusting for age, sex, and body mass index (BMI). RESULTS: Within one year of surgery, 4.8% of hips (47 of 985) met criteria for IPI. At one-year follow-up, mean collar overhang was significantly greater in the impingement group (1.90 versus 1.42 mm, P = 0.036), and a larger proportion had overhang ≥ three mm (29.8 versus 13.7%, P = 0.003). In multivariable analysis, collar overhang ≥ three mm was associated with more than double the odds of IPI (Odds Ratio [OR] 2.2, P = 0.024). Cup overhang was minimal and not significantly different in either group. CONCLUSION: In direct anterior THA with collared femoral stems, increased collar overhang was associated with IPI. A threshold of greater than three mm may identify hips at elevated risk. Further research is needed to reduce IPI risk.