Visual intraoperative estimation of cup and stem position is not reliable in minimally invasive hip arthroplasty

Background and purpose - In hip arthroplasty, acetabular inclination and anteversion-and also femoral stem torsion-are generally assessed by eye intraoperatively. We assessed whether visual estimation of cup and stem position is reliable. Patients and methods - In the course of a subgroup analysis of a prospective clinical trial, 65 patients underwent cementless hip arthroplasty using a minimally invasive anterolateral approach in lateral decubitus position. Altogether, 4 experienced surgeons assessed cup position intraoperatively according to the operative definition by Murray in the anterior pelvic plane and stem torsion in relation to the femoral condylar plane. Inclination, anteversion, and stem torsion were measured blind postoperatively on 3D-CT and compared to intraoperative results. Results - The mean difference between the 3D-CT results and intraoperative estimations by eye was -4.9 degrees (-18 to 8.7) for inclination, 9.7 degrees (-16 to 41) for anteversion, and -7.3 degrees (-34 to 15) for stem torsion. We found an overestimation of>5 degrees for cup inclination in 32 hips, an overestimation of >5 degrees for stem torsion in 40 hips, and an underestimation