Abstract
BACKGROUND: It is unclear how radiographic measurements of cup position are sensitive to deviations from a perfect AP pelvis image. PURPOSE: To quantify changes in radiographic measurements of cup abduction angle due to pelvic tilt or obliquity. METHODS: Part A, a retrospective comparison of radiographic cup abduction angle measurements from intraoperative and post-operative radiographs of 23 patients undergoing THA. Part B, a pelvic sawbones model was used to quantify changes in radiographic measurement of cup abduction angle due to known changes in pelvic tilt or obliquity. RESULTS: Part A, a perfect AP pelvis was obtained in just 30% of intraoperative radiographs. The mean intraoperative cup angle measurement was underestimated by 3.4° compared to post-operative standard radiographs. In Part B, pelvic tilt caused cup abduction angle measurement to decrease on inlet view and increase on outlet view. Pelvic obliquity caused cup abduction angle measurement to decrease on obturator oblique view and increase on iliac oblique view. CONCLUSIONS: A trend exists toward slight underestimation of cup abduction angle measurement using intraoperative radiographs. Pelvic tilt or obliquity alters the measured cup abduction angle in known directions.