Correction of Pelvic Tilt and Pelvic Rotation in Cup Measurement after THA - An Experimental Study

Purpose Accurate assessment of cup orientation on postoperative pelvic radiographs is essential for evaluating outcome after THA. Here, we present a novel method for correcting measurement inaccuracies due to pelvic tilt and rotation. Method In an experimental setting, a cup was implanted into a dummy pelvis, and its final position was verified via CT. To show the effect of pelvic tilt and rotation on cup position, the dummy was fixed to a rack to achieve a tilt between +15 degrees anterior and -15 degrees posterior and 0 degrees to 20 degrees rotation to the contralateral side. According to Murray's definitions of anteversion and inclination, we created a novel corrective procedure to measure cup position in the pelvic reference frame (anterior pelvic plane) to compensate measurement errors due to pelvic tilt and rotation. Results The cup anteversion measured on CT was 23.3 degrees; on AP pelvic radiographs, however, variations in pelvic tilt (+/- 15 degrees) resulted in anteversion angles between 11.0 degrees and 36.2 degrees (mean error 8.3 degrees +/- 3.9 degrees). The cup inclination was 34.1 degrees on CT and ranged between 31.0 degrees and 38.7 degrees (m.e. 2.3 degrees +/- 1.5 degrees) on radiographs. Pelvic rotation between 0 degrees and 20 degrees showed high variation in radiographic anteversion (21.2 degrees-31.2 degrees, m.e. 6.0 degrees +/- 3.1 degrees) and inclination (34.1 degrees-27.2 degrees, m.e. 3.4 degrees +/- 2.5 degrees). Our novel correction algorithm for pelvic tilt reduced the mean error in anteversion measurements to 0.6 degrees +/- 0.2 degrees and in inclination measurements to 0.7 degrees (SD +/- 0.2). Similarly, the mean error due to pelvic rotation was reduced to 0.4 degrees +/- 0.4 degrees for anteversion and to 1.3 degrees +/- 0.8 for inclination. Conclusion Pelvic tilt and pelvic rotation may lead to misinterpretation of cup position on anteroposterior pelvic radiographs. Mathematical correction concepts have the potential to significantly reduce these errors, and could be implemented in future radiological software tools.