Simulation of acetabular coverage of femoral head using anteroposterior pelvic radiographs.

We developed a computer program to perform three-dimensional analysis of the acetabular coverage of the femoral head using two anteroposterior (supine and erect) radiographs of the hip. The center of radiation was marked, and an additional vertical line was placed on radiographs in the neutral standing position, permitting estimation of change in pelvic tilt and its effect on acetabular coverage. We studied 64 normal hip joints, 82 acetabular dysplastic hip joints, and 15 hip joints that had undergone rotational acetabular osteotomy (RAO). In normal hips, the pelvis tilted posteriorly on standing, and the anterior acetabular coverage decreased, but the extent of reduction was not significant. However, in dysplastic hip joints, the pelvic tilt changed from the posterior to anterior direction and from the painful side to the nonpainful side with progression of osteoarthritis. While RAO provided sufficient correction of anterior coverage for acetabular dysplasia, lack of posterior coverage was sometimes observed.