These data suggest measurement of the AAA on a plain CT scan used in current practice is biased. In patients with recurrent posterior dislocation from a sitting position, accounting for the functional variations in measurement of the position of the acetabular cup provides more relevant information regarding component positioning.

The acetabular anteversion angle varies according to the position of the pelvis. The objective goal of our study was to investigate changes in pelvic orientation after total hip replacement for primary osteoarthritis. We studied 89 patients who underwent total hip replacement for primary unilateral osteoarthritis. Lateral pelvic X-rays that included the hips were performed pre-operatively and one year post-operatively. Reference values were calculated by carrying out the same analysis in 100 asymptomatic healthy volunteers. Pelvic orientation was analyzed using the sacral slope. Patients having surgery for osteoarthritis had a decreased pelvic range of motion pre-operatively and post-operatively when compared to healthy volunteers. Post-operatively, this range of motion increased by 3 degrees but remained lower than the norm. Compared to asymptomatic healthy volunteers, patients affected by osteoarthritis had a posterior pelvic extension that decreased post-operatively but did not return to norm. This post-operative pelvic inclination generates a significant decrease in the final cup anteversion and thus may predispose to posterior dislocation. As this post-operative alteration to pelvic orientation cannot be anticipated, computer-aided surgery for cup positioning may not improve the accuracy of the acetabular anteversion in some patients.

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