Assessment of the axial rotation of the pelvis with the EOS® imaging system: intra- and inter-observer reproducibility and accuracy study

PurposeA source of radiological bias occurs when the axial rotation of the pelvis is disregarded in hip and spine biomechanics analyses. The EOS imaging system (EOS Imaging, France) offers the possibility of detecting and measuring the axial rotation of bones. Reproducibility and accuracy have not been documented in the case of the pelvis.MethodsA dry pelvis has been X-rayed with the EOS system every 5° from 30° left to 30° right according to a laser line reference goniometer. Three observers have measured the rotation. One observer did it 3 times. The intra- and inter-observer reproducibility and the accuracy have been calculated using the root mean square standard deviation calculation. The relationship between the axial rotation and the offset between the left and right acetabulae on the lateral view was investigated.ResultsThe 95 % CI was ±0.23° for the intra-observer and ±0.33° for the inter-observer reliability. The mean of signed differences between the software calculation and the actual axial rotation of the pelvis was −0.39° (SD 0.77°). The lateral acetabular offset was proportional to the sin of the rotation. Approximately, 30 mm offset corresponded to about 10° rotation.ConclusionsThe 3D slot scanning imaging system demonstrated significant reproducibility and accuracy for the assessment of the axial rotation of the pelvis.