Acetabular notch.

Ultrasound images of dysplastic and/or unstable hips often display an indentation or notch at the superolateral part of the acetabulum where the iliac wing joins the acetabular roof. We reviewed the ultrasound and subsequent radiographic examinations of 295 babies examined in our hip screening clinic. Of the hips with a notch demonstrable at the first ultrasound, 97% had a persistent notch at the second ultrasound and in 79% the notch was apparent on the 3-month radiograph. When the notch persists, we believe that it represents damage to the lateral acetabular ring epiphysis and delayed maturation of the lateral acetabulum

Microanatomy of the acetabular cavity and its relation to growth.

The anatomy and development of the growing acetabulum are not clearly understood. We dissected and studied histologically two acetabula from the pelvis of a three-month-old infant. Relative rates of growth at the different growth plates were assessed by comparing the height of the proliferative layer with that of the hypertrophic layer. The three bones which form the acetabulum are surrounded by growth plates on all sides except medially. These face towards the centre of the triradiate cartilage, the limbs of the triradiate cartilage and the articular surface and each may be divided into four distinct areas according to the orientation of its cell columns which reflect the direction of growth. Growth was particularly rapid at the ischial growth plates directed towards the centre and the articular cartilage, add on both sides of the anterior limb of the triradiate cartilage. These findings may explain the mechanism by which the acetabulum changes orientation and inclination with growth

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 degrees if the obturator foramina ratio is kept within 0.5 to 2. If pelvic flexion/extension is confined to +/- 10 degrees, the error induced by flexion/extension is +/- 3 degrees. The intraobserver error was +/- 2 degrees, and the interobserver error was +/- 3 degrees. Under these circumstances, the total error is +/- 5 degrees. We have not been able to find a satisfactory way of limiting the flexion/extension to +/- 10 degrees. In some circumstances, particularly if a child is distressed, the flexion/extension may be 20 degrees from neutral; under these circumstances, errors as large as 10 degrees can occur. Surgeons should be aware that very large errors can occur when the acetabular index is measured