High variability in sagittal pelvic rotation in functional positions - can we still simply follow Lewinneck safe zone for acetabular cup positioning?

Abstract

Introduction: The sagittal rotation of pelvis changes during daily activities, which potentially influences the functional anteversion and inclination of the acetabular cup in patients having total hip arthroplasty(THA). The aim of this study was to measure changes in pelvic tilt between different functional positions. Methodology: Lateral images of consecutive patients, who were planned to have THA in study institution, were investigated with EOS imaging system in standard low-dose protocol. Pelvic tilts, defined as the angle between coronal plane and anterior-pelvic plane, were measured using Centricity system at three functional positions - standing, seating and flexed seating by two independent observers. Anterior pelvic tilt was assigned a positive value. Results and Analysis: Pelvic tilts of 52 patients were measured (mean age: 60.7 ±14.3; sex: M: F = 24:28). Mean pelvic tilts were 14.4° (-21.5° to 46.4°), -0.85° (-31.6° to 38.0°) and 10.5° (-20.5° to 47.2°) at the positions of standing, seating and flexed-seating respectively. The mean rotation changes from seating to flexed-seating was 11.4° (-54.9° to 50.7°), from flexed-seating to standing was 3.9° (-41.4° to 40.3°) and from standing to flexed-seating was -3.87° (-44.3° to 41.4°), and from flexed-seating to sitting -11.4° (-50.7° to 54.9°). In 20 patients (38%), the extent of sagittal pelvic rotation could lead to functional malorientation of cup when position changing from flexed-seating to standing. Discussion and Conclusion: High variability of the sagittal pelvic rotation in functional positions among Chinese was observed. Cup positioning based on Lewinneck safe zone alone in supine position may fail to predict clinically significant changes in sagittal pelvic rotation when patients resuming functional activities.