Variation in functional pelvic tilt in patients undergoing total hip arthroplasty

Aims The pelvis rotates in the sagittal plane during daily activities. These rotations have a direct effect on the functional orientation of the acetabulum. The aim of this study was to quantify changes in pelvic tilt between different functional positions. Patients and Methods Pre-operatively, pelvic tilt was measured in 1517 patients undergoing total hip arthroplasty (THA) in three functional positions - supine, standing and flexed seated (the moment when patients initiate rising from a seated position). Supine pelvic tilt was measured from CT scans, standing and flexed seated pelvic tilts were measured from standardised lateral radiographs. Anterior pelvic tilt was assigned a positive value. Results The mean pelvic tilt was 4.2 degrees (-20.5 degrees to 24.5 degrees), -1.3 degrees (-30.2 degrees to 27.9 degrees) and 0.6 degrees (-42.0 degrees to 41.3 degrees) in the three positions, respectively. The mean sagittal pelvic rotation from supine to standing was -5.5 degrees (-21.8 degrees to 8.4 degrees), from supine to flexed seated was -3.7 degrees (-48.3 degrees to 38.6 degrees) and from standing to flexed seated was 1.8 degrees (-51.8 degrees to 39.5 degrees). In 259 patients (17%), the extent of sagittal pelvic rotation could lead to functional malorientation of the acetabular component. Factoring in an intra-operative delivery error of +/- 5 degrees extends this risk to 51% of patients. Conclusion Planning and measurement of the intended position of the acetabular component in the supine position may fail to predict clinically significant changes in its orientation during functional activities, as a consequence of individual pelvic kinematics. Optimal orientation is patient-specific and requires an evaluation of functional pelvic tilt pre-operatively.