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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° (‐20.5° to 24.5°), ‐1.3° (‐30.2° to 27.9°) and 0.6° (‐42.0° to 41.3°) in the three positions, respectively. The mean sagittal pelvic rotation from supine to standing was ‐5.5° (‐21.8° to 8.4°), from supine to flexed seated was ‐3.7° (‐48.3° to 38.6°) and from standing to flexed seated was 1.8° (‐51.8° to 39.5°). 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° 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.