EP1.39 Comparison of methods for assessment of the sagittal orientation of the acetabulum

Abstract Introduction The position of the acetabulum in the sagittal plane has been ignored in the field of hip imaging and preservation surgery. The purpose of the present study was to compare reliability of two methods to assess the sagittal orientation of the acetabulum (SOA) in a mixed population of patients with hip dysplasia, normal and excessive acetabular coverage. Methods The study group was composed of 63 patients/hips, 21 with acetabular dysplasia (lateral center edge angle ≤18°), 21 with acetabular overcoverage (LCEA ≥39°), and 21 with normal acetabular coverage (LCEA > 25 and < 39 degrees). Three independent readers measured the SOA using 3D CT using two techniques: (1) the horn method, which calculates the angle between a horizontal line and a line connecting the distal-most points of the articular acetabular horns, and (2) the acetabular center method, which measures the angle between a vertical line and a line through the center of an ellipse fitted to the acetabular rim and the midpoint of the acetabular notch. Both measurements were standardized using the anterior pelvic plane (APP) as a reference for pelvic tilt. The ICCs were calculated using a two-way random effects model with average measures (ICC(2,k)), where each subject's ratings from all three raters were modeled as a random sample. Intra-rater reliability was also calculated comparing measurements of the same reader performed more than 2 weeks apart. Results Inter-rater reliability was higher for the horn method in the entire sample (0.88) compared to the center method (0.83). In subgroup analyses, the horn method demonstrated superior consistency in hips with normal coverage (0.79 vs. 0.67) and overcoverage (0.88 vs. 0.71). In contrast, the acetabular center method showed slightly higher reliability in patients with developmental dysplasia of the hip (DDH) (0.89 vs. 0.86). Intra-rater reliability for the entire sample was 0.91 for the horn method and 0.88 for the center of acetabulum method. Conclusion The horn method demonstrates superior and more consistent inter-rater reliability across all hip morphologies compared to the acetabular center method. While both techniques are reliable for DDH patients, the horn method is the more robust approach for standardizing SOA measurements in normal and overcovered hips.