Localization of the ilioischial line on axial computed tomography images for preoperative planning of total hip arthroplasty

BACKGROUND: If the bony region indicating the ilioischial line is established on the preoperative axial computed tomography (CT) image, the distance between the simulated cup and the ilioischial line can be measured on this image so that the surgeon can use these data to define a more accurate preoperative two-dimensional (2D) template of total hip arthroplasty (THA). This study aimed to verify the hypothesis that on the CT axial image, the cortical bone area, indicated by the superimposition of a line (line α) with a perspective projection angle to the ilioischial line on radiography and tangent to the medial acetabular wall, is the cortical bone that represents the ilioischial line on radiography. METHODS: Study 1: If the two measured distances (distance A’ and distance B) are sufficiently equal, then the hypothesis can be supported. Distance A’ was calculated by multiplying the distance A, between the ilioischial line and the medial margin of the metal cup after THA measured at the level of the hip joint center on the pelvic radiograph, by 0.91 to correct for radiographic magnification. Distance B was defined as the distance between the medial margin of the metal cup and line α on the axial CT image at the level of the hip joint center. These two distances were measured for all 51 hip joints included in the study. Study 2: The difference between distance A and distance A(#) (distance A on the 2D template) was compared between the group containing 59 primary THAs in which distance B′ was measured (distance B in the simulation) and the control group containing 59 primary THAs. RESULTS: Study 1: The average distance for A’ was 4.5 ± 2 mm, and the average distance for B was 4.7 ± 2.1 mm. The difference between distances A and B was 0.2 ± 0.2 mm. Study 2: The mean difference between distance A and distance A(#) for the measurement and control groups was 1.8 ± 1.3 mm and 3.7 ± 2.4 mm, respectively (P < 0.001). CONCLUSIONS: The ilioischial line is located in the bony region where line α intersects the medial acetabular wall with a maximum overlap on axial CT images. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12891-022-06021-1.