Is the pelvis stable during supine total hip arthroplasty?

Intra- operative changes in pelvic position during total hip arthroplasty (THA) can affect acetabular orientation. We evaluated these changes during supine THA using a proprietary mobile application called PelvicTracker. Twenty- two patients undergoing THA using direct anterior approach were included in the study. In the sagittal plane, the pelvis was extended (anterior tilt) as compared to the start of surgery in 19/ 22 hips at the time of cup implantation (mean extension: 3.1°; range: 1°-6°). In the transverse plane, the pelvis was rolled to the opposite side of surgery in 12 hips (mean roll: 2.8°; range: 1°-5°), to the same side in 8 hips (mean roll: 3.9°, range: 1°-9°) and unchanged in 2 hips at the time of cup implantation. Predicted change in cup version of ≥5° due to changes in pelvic position was seen in 7/22 (32%) patients. Although minor, changes in pelvic position do occur during supine THA which may affect acetabular orientation.