Influence of the sagittal balance of the spine on the anterior pelvic plane and on the acetabular orientation.

The Anterior Pelvic Plane (APP), defined by the anterior superior iliac spines and the pubic tubercle, was commonly used as reference for positioning and postoperative evaluation of the orientation of the acetabular cup in total hip arthroplasty. APP was assumed to be vertical, but was not observed always so, mostly because of associated spinal diseases inducing perturbations in the harmony of the sagittal balance of the pelvi-spinal unit. Consequently a sagittal rotation of the pelvis occurs, and so a tilt of the APP which alters directly the orientation of the cup in upright position. An analysis of the APP tilt related to the sagittal balance of the spine was provided and its implication on the cup orientation. It appeared essential for an individual adjustment of the cup positioning to avoid a functional mal-position which can lead to an increased risk of dislocation and impingement.