Predictability of Acetabular Component Angular Change with Postural Shift from Standing to Sitting Position

Background: The angles of the acetabular component of a total hip replacement change with body postural changes, and this change can affect stability and wear. We sought to correlate the intraoperative angles of inclination and anteversion of the cup with the changes in these angles when patients moved from standing to sitting and determine if these changes were predictable. Methods: Eighty-five patients (eighty-five hips) had sagittal (lateral) spinopelvic radiographs made while they were standing and while they were sitting before and after undergoing total hip replacement. The spinosacral tilt and the pelvic tilt were measured on these radiographs. The angles of acetabular inclination and anteversion achieved at surgery changed during sitting. Each patient was classified according to the stiffness of the spine/pelvis as measured by the change in posterior sacral or pelvic tilt between the standing and sitting positions. The magnitude of change of the sagittal cup position (termed ante-inclination) was correlated to the stiffness classification of the pelvis. An experimental phantom model reproduced possible combinations of intraoperative inclination and anteversion and correlated them to sagittal ante-inclination according to pelvic tilt. Results: The pelves with normal stiffness tilted posteriorly 20 degrees to 35 degrees with the postural change from standing to sitting. Ante-inclination of the acetabular cup averaged 29.6 degrees +/- 8.4 degrees (95% confidence interval [CI] = 13.1 degrees to 46 degrees) with standing and 54.6 degrees+/- 10.2 degrees(95% CI = 44.4 degrees to 64.8 degrees) with sitting. The stiff pelves had a mean of 4 degrees less tilt than those with normal stiffness and 13 degrees less than the hypermobile pelves with the postoperative sitting position. The phantom model showed ante-inclination could be predicted by measuring the preoperative degrees of change in sacral/pelvic tilt from standing to sitting. Conclusions: Ante-inclination during sitting results in a more vertical acetabular cup, which can result in hip instability, especially drop-out dislocation, and edge-loading wear. Patients with supine coronal cup inclination of >= 50 degrees and anteversion of >= 25 degrees and those with a hypermobile pelvis are at risk. It is the pelvic spatial position during postural change that creates the postoperative consequences of the surgical cup placement.