Can EOS and lateral pelvic radiographs reliably identify patient with reduced pelvic roll back at risk for dislocation ?

Hip instability and dislocation post-THA is a prevalent complication leading to revision surgeries. Patients are considered “high-risk” if the sit-to-stand sacral slope change is less than 10 degrees. The current study analyses if lateral radiographs and EOS imaging result in similar measurements and identify the same at-risk patients. In a retrospective study consisting of 59 hips (58 patients), spinopelvic measurements were obtained from lateral sitting and standing pelvic radiographs and EOS sitting and standing (whole body) films taken during the same preoperative assessment. Measurements on each images set were performed to assess variation in measurements between the two modalities. The average measured sacral slope difference between the two imaging methods was 4.5° (standing) and 6.6° (sitting), with a sit-to-stand difference of 7.2°. Lateral radiographs identified 16 patients at high-risk for dislocation and EOS identified 15 high risk patients; however, only 9 patients overlapped between the two modalities. There was a significant postural difference between the two modalities: average spinal angle difference was 10.4°(p= 0.0026).and femoral angle off the horizonal was 7.4°(p= 0.16). There are discrepancies between EOS and conventional lateral radiographs in measuring sacral slope and change in sacral slope from sitting to standing and both imaging modalities might identify different high-risk patients for dislocation. Efforts need to be made to standardize the patients’ position at the time of imaging.