Validation of a new 2.5D radiographic index evaluating acetabular coverage using ACX software

Background: Several radiographic signs have been described to assess acetabular coverage. However, plain radiographs only have 2 dimensions (2D) and cannot accurately show acetabular coverage. Questions/purposes: We developed the ACX Dynamics software to calculate the radial centre-edge angle (RCEA), which represents the acetabular coverage of the femoral head at each acetabular edge point on the radial plane. This study validated the accuracy of the RCEA, as calculated by ACX Dynamics, as a quantitative parameter for acetabular coverage. Patients and methods: We reviewed the anteroposterior (AP) pelvic radiographs and computed tomography (CT) of 650 hips from 325 patients who presented with chief complaint of symptoms at the hip joint. Of 109 hip radiographs (68 patients) that satisfied the criteria, 50 randomised, blinded AP pelvic radiographs were chosen. We determined the absolute RCEA difference (°) [= RCEA ACX (°) – RCEA CT (°)], determined the correlation between RCEA ACX (°) and RCEA CT (°), and examined the RCEA’s intra-observer and inter-observer reliability in 50 hips. Results: The absolute RCEA difference from A45° to P75° was 1.9–3.1°. The correlation between the RCEA ACX (°) and RCEA CT (°) was > 0.7 in all lesions (p < 0.001). Using the intraclass correlation coefficient, the intra-observer reliability of the RCEA was 0.83–0.97 in the whole range, which is a highly reproducible and reasonable parameter, and the inter-observer reliability was > 0.80 in A45°–P0°. Conclusions: The RCEA ACX (°) can be used as a simple quantitative parameter for assessing acetabular coverage using AP pelvic radiograph.