The 2008 Frank Stinchfield award: variation in postoperative pelvic tilt may confound the accuracy of hip navigation systems.

Most computer navigation systems used in total hip arthroplasty integrate preoperative pelvic tilt to calculate the anterior pelvic plane assuming tilt is constant; however, the consistency of pelvic tilt after THA has never been proven. Therefore, using a modern comprehensive gait analysis before and after arthroplasty we sought to compare (1) dynamic pelvic tilt changes and (2) pelvic flexion/extension range-of-motion changes. Twenty-one patients who underwent unilateral THA were prospectively studied. Quantitative pelvic tilt changes (in the sagittal plane) and pelvic range of flexion/extension motion relative to a laboratory coordinate system were compared using a computerized video motion system. Mean gait pelvic tilt was 13.9 masculine +/- 4.8 masculine (range, 1.73 masculine-23.1 masculine) preoperatively, 12.5 masculine +/- 4.5 masculine (range, 1.4 masculine-18.7 masculine) 2 months postoperatively, and 10.5 degrees +/- 5.5 masculine (range, -2.36 masculine-19.2 masculine) 12 months postoperatively. A significant proportion (31%) of patients had more than a 5 degrees difference between preoperative and 12-month postoperative measurements and the variability was spread over 20 degrees . Significant dynamic changes in pelvic tilt occurred after THA. While navigation clearly improves the anatomical position of the component during THA, the functional position of the component will not always be improved because of the significant change between preoperative and postoperative pelvic tilt.