Errors in measurement of acetabular index

Errors in determining the acetabular index can be induced either by incorrectly positioning the child for radiographs or by inter- or intraobserver errors. From postmortem radiographic studies, we have determined the magnitude of these errors. The error caused by pelvic rotation is ±3° if the obturator foramina ratio is kept within 0.5 to 2. If pelvic flexion/extension is confined to ±10°, the error induced by flexion/extension is ±3°. The intraobserver error was ±2°, and the interobserver error was ±3°. Under these circumstances, the total error is ±5°. We have not been able to find a satisfactory way of limiting the flexion/extension to ±10°. In some circumstances, particularly if a child is distressed, the flexion/ extension may be 20° from neutral; under these circumstances, errors as large as 10° can occur. Surgeons should be aware that very large errors can occur when the acetabular index is measured.