Background It is difficult to assess the orientation of the acetabular component on routine radiographs. We present a method for determining the spatial orientation of the acetabular component after total hip arthroplasty (THA) using computed tomography.Patients and methods Two CT-scans, 10 min apart, were obtained from each of 10 patients after THA. Using locally developed software, two independent examiners measured the orientation of the acetabular component in relation to the pelvis. The measurements were repeated after one week. To be independent of the patient position during scanning, the method involved two steps. Firstly, a 3D volumetric image of the pelvis was brought into a standard pelvic orientation, then the orientation of the acetabular component was measured. The orientation of the acetabular component was expressed as operative anteversion and inclination relative to an internal pelvic reference coordinate system. To evaluate precision, we compared measurements across pairs of CT volumes between observers and trials.Results Mean absolute interobserver angle error was 2.3° for anteversion (range 0-6.6°), and 1.1° for inclination (range 0-4.6°). For interobserver measurements, the precision, defined as one standard deviation, was 2.9° for anteversion, and 1.5° for inclination. A Studentʼs t-test showed that the overall differences between the examiners, trials, and cases were not sig-

This method has a significantly higher accuracy than routine plain radiography in detecting acetabular cup migration and could be used in clinical practice. It gives both a visual and a numerical correlate to migration.

The accuracy of this non-invasive method out-performs routine plain radiography. The method gives both visual and numerical correlates to migration and can be used in clinical practice.

Movement of loose acetabular cups during torsion loading could be detected using CT volume registration. It was sensitive to cup movement in 3 out of 4 cases of loose cups. The method was specific and yielded no false positive results.

The true 3D position of the centers of cup and prosthetic head can be detected using CT. Spatial relationship between the components can be analyzed visually and numerically.