Characterising acetabular component orientation with pelvic motion during total hip arthroplasty

Introduction: Suboptimal acetabular component position can result in impingement, dislocation, and accelerated wear. Intraoperative pelvic motion has led to surgeon error and acetabular cup malposition. This study characterises the relationship between pelvic rotation and postoperative acetabular cup orientation. Methods: A device was constructed to allow cadaveric pelvis rotation along three axes about an acetabular cup in fixed orientation. The acetabular cup was fixed in space at 40 degrees of radiographic inclination and 15 degrees of anteversion relative to the anterior pelvic plane to represent consistent surgeon intraoperative placement. Active marker clusters were fixed to surgical equipment while the cadaveric pelvis was cemented with passive reflective markers, both identified with the Optotrak Certus motion capture system. The reamed cadaveric pelvis was rotated along three axes from -45 degrees to 45 degrees of roll, -30 degrees to 30 degrees of tilt, and -35 degrees to 35 degrees of pitch. The change in component inclination and anteversion was recorded at each 5 degrees interval. Using computed tomography 3D reconstruction, the experimental setup was duplicated computationally to assess against a greater range of pelvis and implant sizes. Results: Radiographic anteversion and inclination showed a non-linear relationship dependent on pelvic roll, tilt, and pitch. Radiographic anteversion changed -0.59 degrees, 0.76 degrees and 0.01 degrees while radiographic inclination changed 0.23 degrees, 0.18 degrees and 1.00 degrees for every 1 degrees of pelvic roll, tilt and pitch, respectively. Computationally, anteversion changed -0.61 degrees, 0.75 degrees and 0.00 degrees while inclination changed 0.22 degrees, 0.19 degrees and 1.00 degrees for every 1 degrees of pelvic roll, tilt and pitch, respectively. These results were independent of cup and pelvis size. Conclusions: Intraoperative pelvic motion can significantly affect final cup position, and this should be accounted for when placing acetabular components during total hip arthroplasty. Based on this study, intraoperative adjustment of the acetabular component position based on pelvis motion may be implemented to improve postoperative component position.