Variations of pelvic anteversion in the lying and standing positions: analysis of 24 control subjects and implications for CT measurement of positionof a prosthetic cup

The position of the acetabular implant plays a dominant role in the displac ement of a total hip prosthesis. CT allows precise measurement of the posit ion of the cup, but the influence of pelvic rotation on this measurement is unknown. The aim of this study was to determine, in a group of healthy sub jects, whether a pelvic equilibrium exists specific to each individual, and whether this is constant over time on the one hand and between the standin g and lying positions on the other. The study concerned 15 men and 9 women with a mean age of 31 years. Each subject had strictly lateral radiographs of the pelvis, lying and standing, repeated at two different times. Pelvic version was measured in these radiographs. Each individual had a pelvic pos ition constant over time, both in the lying and standing positions, However . there were important variations of the position of the pelvis during pass age From the lying to the standing position: 22 patients had retroversion o f the pelvis by a mean of 7 degrees (2-18 degrees) and 2 others had an ante version of 3 degrees. These major variations of the pelvic position between the standing and lying positions explain why CT studies made in the lying position do not allow for the anteversion of the cup in the standing positi on, which is close to the dynamic situation during which displacement may o ccur. Thus, an excessive anteversion of the cup may be masked when the scan is made in the lying position, since in this position the anteversion of t he pelvis leads to retroversion of the cup. The error may reach 20 degrees, so that we recommend that CT measurements made without allowing for the po sition of the pelvis should be interpreted with caution.