57 Aziliz Guezou-Philippe Figure 1. (top) Definition of, from left to right, the implants, the cup orientation and the pelvic tilts. (middle) Computation of the functional safe zone: for each position, the cup orientations safe from impingement are computed and form a “static” safe zone; the functional safe zone is then defined as the intersection of the sitting, standing and supine safe zones. (bottom) Effect of some parameters on the functional safe zone. Figure 2. Determination of the patient reference plane: landmarks of one side of the pelvis are acquired using a localized ultrasound probe; then a statistical shape model is registered to the acquired landmarks; finally, the anterior pelvic plane orientation is determined on the fitted model. The acetabular cup can then be positioned and implanted regarding this reference plane.