Acetabular cup anteversion measurement in total hip arthroplasty: reliability of metal artifact reduction MRI compared with CT.

Objective: The acetabular cup version in patients with total hip arthroplasty (THA) is a key parameter influencing hip stability and functional outcomes. Although CT remains the reference standard for assessing cup orientation, MRI with metal artifact reduction techniques is increasingly used for evaluating postoperative pain. This study compared MRI- and CT-based measurements of acetabular cup version regarding agreement, reproducibility, and reliability.

Material and methods: Patients who underwent THA between 2015 and 2025 with postoperative CT and MRI were retrospectively analyzed. MRI was performed using optimized metal artifact reduction sequences. Acetabular cup version was measured on axial high-bandwidth T1-weighted turbo spin-echo sequences and on corresponding CT scans by two musculoskeletal radiologists. Inter- and intra-reader, as well as inter-modality agreement, were assessed using intraclass correlation coefficients (ICC). Bland-Altman plots evaluated systematic bias.

Results: Thirty patients were included (mean age, 64.1 years; 14 women). Mean acetabular version was 31.2° (standard deviation (SD), 9.4-10.3) for CT and 30.1-30.2° (SD, 8.9-9.3) for MRI for both readers. Inter-reader agreement was almost perfect for CT (ICC, 0.96) and substantial for MRI (ICC, 0.76). Intra-reader agreement was almost perfect for both modalities (ICC, CT, 0.99; MRI, 0.94). Bland-Altman analysis showed no statistically significant differences between CT and MRI measurements with a slight positive bias for MRI (mean pooled difference, 1.1°; p = 0.058; limits of agreement, -4.7 to 6.8°) and almost perfect agreement for both readers (ICC, 0.86-0.90).

Conclusion: MRI measures acetabular cup version with excellent reliability and close agreement with CT, reinforcing MRI's role in postoperative THA evaluation.