Three-dimensional in vivo difference between native acetabular version and acetabular component version influences iliopsoas impingement after total hip arthroplasty

The potential influence of acetabular component orientation on iliopsoas impingement in total hip arthroplasty (THA) has not been previously quantified. The aim of the present study was to utilize pre- and post-operative CT-based 3D models to quantify iliopsoas impingement on acetabular components, and to identify any potential factors associated with iliopsoas impingement. Iliopsoas muscle was modelled from pre-operative CT scans and transferred to the post-operative 3D models in 19 THAs. The volume and the area of the overlap between iliopsoas muscle and acetabular cup (iliopsoas volume & area) was measured on axial and sagittal images. Most protruded lengths of cup uncovered by acetabular bone were measured on axial sagittal scan of CT scans. Version of acetabulum, acetabular cup, and the difference between the two (version difference) were also measured with cup inclination and size. Linear regression analysis was performed to identify any factor influencing iliopsoas impingement. Iliopsoas impingement volume and area were 100.6 +/- 226.1 (range, 0.0-663.9) mmA(3) and 52.6 +/- 102.0 (0.0-342.3) mmA(3), respectively. The protruded lengths on axial and sagittal view were 6.9 +/- 5.3 (0.0-16.0) and 2.1 +/- 2.7 (0.0-8.0). Linear regression model showed that version difference was significantly related to the iliopsoas impingement volume and area (beta = -0.709, p = 0.041 for volume, and beta = -0.684, p = 0.047 for area). The results of this study demonstrate that iliopsoas impingement on acetabular components was influenced by the version difference between pre-operative acetabular bone and acetabular component rather than the magnitude of post-operative cup version alone.