Tilt and rotation correction of acetabular version on pelvic radiographs.

Anteroposterior pelvic radiographs are the gold standard of imaging for mechanical hip problems. However, correct interpretation is difficult because the projected morphologic features of the acetabulum and nearly all routinely used hip parameters depend on individual pelvic position, which can vary considerably during acquisition. We developed software that recreates the projected acetabular rim and the measured hip parameters as if obtained in a standardized orientation. The vertical and horizontal distances between two easy identifiable points were used as indicators of tilt and rotation. These points were the middle of the sacrococcygeal joint and the middle of the upper border of the symphyseal gap. Calibration of the indicators was achieved by means of serial pelvic radiographs of 20 cadaver pelves. Validation of tilt indicator in 100 patients and a theoretical error analysis revealed that for accurate tilt prediction an additional one-time lateral radiograph of the pelvis is mandatory. The computer-assisted method allows standardized evaluation of anatomic morphologic differences of femoral coverage (dysplasia, retroversion), making their clinical relevance for development of early osteoarthritis more valuable.