Rotations of the pelvis and acetabular component orientation: a mathematical model

Background Since optimal positioning of total hip components has not been accurately established yet and several studies show that dislocation may occur within properly implanted total hip prostheses, we thought it would be interesting to evaluate the influence of spatial positioning of the pelvis on relative variations in orientation of the acetabular component. Methods For the purpose of this study, we developed a mathematical model of the pelvis to measure the influence of the pelvic movements on anteversion and inclination of an acetabular cup arbitrarily implanted with 10° of anteversion and 45° of inclination. We particularly evaluated the influence of −15 to 15° pelvic rotation around the three spatial axes. Results When considering the anteroposterior axis, the ranges of variation were 30° and 6° for inclination and anteversion, respectively. When considering vertical and mediolateral axes, the magnitude of variation was 30° for anteversion and 2° for inclination. Conclusions The present study demonstrated the close relationship between acetabular cup anteversion and pelvic rotations in all planes. In contrast, measurement of acetabular cup inclination was mainly related to rotation around the anteroposterior axis. The influence of pelvic position on acetabular cup alignment requires very precise CT measurement protocols for accuracy and reproducibility.