Pelvic tilt measurement before and after total hip arthroplasty.

INTRODUCTION Most computer-assisted navigation systems used in total hip arthroplasty (THA) reference the anterior pelvic plane, which connects the anterior superior iliac spines and the pubic symphysis. The pelvic tilt is defined as the angle between this anterior pelvic plane (APP) and a vertical line in the standing position. Important interindividual variations of this angle have been reported and may affect final functional anteversion of the acetabular cup. The preoperative value of the pelvic tilt has been included in computer-assisted navigation systems to improve acetabular cup positioning. However, there is no data available which strongly confirms the consistency of this angle for each individual after hip prosthesis implantation. HYPOTHESIS The orientation of the APP in the standing position is not significantly modified after THA. OBJECTIVES To evaluate in a prospective manner, the reproducibility of pelvic tilt measurement and its variability between THA preoperative and 3-year postoperative measurements. MATERIALS AND METHODS A lateral teleradiograph of the pelvis and dorsolumbar spine was obtained in the standing position preoperatively and 3 years after THA. Fifty patients undergoing THA performed by a single operator via an anterolateral approach (26 males and 24 females) were included prospectively. The pelvic tilt was measured on radiographs by two independent observers. The angle was defined as positive in case of pelvis retroversion relative to the vertical plane and negative in case of anteversion. Bland-Altman analysis was used to assess levels of agreement between both operator measurements while preoperative and last follow-up measurements were compared using the Student t-test for unpaired samples. RESULTS The level of agreement between measurements of both operators was satisfactory. Mean preoperative pelvic tilt was 4.68 degrees +/-0.68 S.D. (-6 degrees to 14 degrees), and 4.78 degrees +/-0.64 S.D. (-5 degrees to 14 degrees) at last follow-up. The mean difference between preoperative and last follow-up measurements was 3 degrees +/-0.3 S.D. There was no statistically significant variation between preoperative and 3-year follow-up values (p>0.05). Ninety-five percent of the patients had less than a 5 degrees difference between both measurements while 5% had a difference ranging from 5 degrees to 10 degrees ; none of the patients reported a variation greater than 10 degrees . DISCUSSION Our findings show no significant variation in pelvic tilt between preoperative and 3-year follow-up values after THA. Therefore, the individual preoperative value of this angle should be integrated to achieve proper acetabular cup placement during THA especially when using computed assisted navigation based on the APP.