Stability of Acetabular Axis after Total Hip Arthroplasty, Repeatability Using CT and A Semiautomated Program for Volume Fusion

Purpose: To validate a CT method for detecting changes in acetabular cup orientation after THA. Material and Methods: 26 CT examinations were obtained from a pelvic model with an uncemented acetabular cup. The model position was altered between acquisitions, but the cup axis angle vis‐à‐vis the pelvis was maintained. Data sets were combined into 37 pairs, each containing a unique positioning error. The pelvi in different examinations were fused, creating transformed volumes. Landmarks corresponding to the cup before and after fusion were placed interactively by two independent examiners. The orientation of the acetabular axis was calculated for each volume and compared across volumes. Results: Before fusion the mean angle error between the acetabular axes was 4.17° (SD ± 1.95°). After fusion the mean angle error was 0.36° (SD ± 0.17). The 95% repeatability limits were below 0.7°. There was no significant interobserver difference. Analysis of the cup landmarking pattern by condition numbers and individual landmark errors showed stability. Conclusion: Non‐invasive fusion of CT volumes and a stable landmarking pattern for the acetabular cup outperforms routine plain radiography in detecting changes in the orientation of the acetabular axis over time. The method delivers both visual and numerical output and could be used in clinical practice.