Iliopsoas impingement on the acetabular component:: Radiologic and computed tomography findings of a rare prosthesis complication in eight cases

Objective: To describe the radiographic and computed tomography (CT) scan features of iliopsoas impingement on the acetabular component (IPI) in total hip arthroplasty. Method: Eight cases of iliopsoas impingement were diagnosed and confirmed by surgical revision. The plain film radiograph findings and CT scan findings were compared with those of two control populations (8 patients with painless total hip prosthesis and 16 patients with other complications confirmed by surgical revision). Results: An oversized cup was found in four hips with IPI (50%) and in one hip with another complication (6%). Coronal inclination of the acetabular cup was normal in the IPI group and in the two control groups, with a similar mean cup size. All patients with IPI had an acetabular cup overhang of more than 12 mm. Conversely, in the two control groups, the overhang, present in three and two cases, respectively, was always less than 8 mm. Iliopsoas bursal effusion was present in four hips with IPI and three hips with other complications and was never present in normal prostheses. Conclusion: More than a 12-mm overhang of the acetabular cup is a sensitive and specific parameter for diagnosing IPI.