Inclination of the acetabular cup in erect posture radiographs

Inclination of the acetabular component was measured in a series of 100 patients who underwent total hip replacement. The radiographs were taken with the patients in erect posture. Two different reference lines were used in the measurements: the ischial reference line, and the true horizontal line perpendicular to a plumb-line used as a vertical reference line. The values measured using these two reference lines correlated well with each other, though there were important differences caused by pelvic tilt. Often an inclination that was within the normal range when measured, using the ischial tuberosity line as a reference, shifted excessively to horizontal or vertical when control measurements were performed using the true horizontal line as the reference. We conclude that inclination of the acetabular cup should be measured in erect posture radiographs, because pelvic tilt increases the degree of inclination on the side of pelvic adduction.