Implant Orientation Measurement After THA Using the EOS X-Ray Image Acquisition System.

Abstract

We investigated the accuracy of measuring implant orientation after THA in standing position using EOS system (EOS Imaging Inc., Paris, France). Ninety patients who underwent THA were subjected to this study by comparing angles measured by EOS system and those measured from CT scans using 3D image analyzing software, ZedHip (LEXI, Tokyo, Japan). The radiographic cup inclination and anatomical cup anteversion were measured with respect to the anterior pelvic plane (APP) coordinate. The femoral stem antetorsion was analyzed by measuring the angles between the stem neck axis and the post-condylar axis in the femoral functional axis coordinate.The differences (mean ± SD) (range of 95%CI) between angles measured by EOS system and those from CT scans in the cup inclination, cup anteversion, and stem antetorsion were - 2.3° ± 2.7° (-2.8°∼ - 1.7°), -0.1° ± 5.0° (-1.2°∼0.9°), and - 1.3° ± 6.5° (-2.7°∼0.1°), respectively. Cup inclination measured on 14 hips, cup anteversion measured on 28 hips, and stem antetorsion measured on 27 hips were classified as outliers whose differences were over 5°. Difficulties in defining the reference points for APP correlated with the incidences of the outliers in cup orientation measurements.We could not set new reference points on the 3D bone surface models reconstructed by EOS system, so we have to use reference points defined on 2D images. In addition, the APP coordinate in EOS system was not the same as the standard definition. EOS system may not be used to measure the implant positions after THA until these problems will be improved.