Accuracy of measuring acetabular cup position after total hip arthroplasty: comparison between a radiographic planning software and three-dimensional computed tomography

Purpose Various methods are available for measuring acetabular cup position after total hip arthroplasty (THA) on standard anterior-posterior (AP) radiographs. We compared the accuracy of a commercial radiographic planning software program with that of three-dimensional computed tomography (3D-CT) scans. Methods We obtained plain AP radiographs and 3D-CTs from 65 patients after THA. In addition to calculating cup anteversion and inclination with 3D-CT, we determined the cup position using the radiographic planning software program mediCAD (R) 2.5 (Hectec, Niederviehbach, Germany). Furthermore, we compared the measurements using the inter-teardrop and bi-ischial lines as pelvic landmarks. Results The mean difference in anteversion between 3D-CT and mediCAD (R) software was 0.1 degrees using the inter-teardrop line (standard deviation [SD], 8.8 degrees; range, -21 degrees to 23 degrees; p = 0.97) and 0.4 degrees using the bi-ischial line (SD, 8.8 degrees; range, -23 degrees to 21 degrees; p = 0.72). Inclination showed a mean difference of 0.6 degrees using the inter-teardrop line (SD, 4.4 degrees; range, -9 degrees to 21 degrees; p = 0.24) and 0.5 degrees using bi-ischial line (SD, 4.6 degrees; range, -9 degrees to 22 degrees; p = 0.35). The means for absolute differences were 7.2 degrees for anteversion and 3.1 degrees for inclination. With regard to using the bi-ischial or inter-teardrop line, no significant difference was found between the two pelvic landmarks. The intra-class correlation coefficient (ICC) was analysed for anteversion and inclination using either the inter-teardrop line or the bi-ischial line as radiographic baseline. Conclusions A radiographic planning software program (mediCAD (R)) is a helpful tool for measuring cup inclination on AP radiographs. With respect to anteversion, measurements are rather susceptible to mistakes with mean inaccuracies of over 7 degrees. Thus, 3D-CT remains the "gold standard" if a lower tolerance limit (+/- 3 degrees) is required for more complex biomechanical evaluations. As a pelvic landmark, the interteardrop line is preferential to the bi-ischial line because of its lower impact on the position of the pelvis