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, and 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.

Children who present late with hip dislocation may require femoral osteotomy after reduction, to correct valgus and anteversion deformity of the femoral neck. After these procedures proximal femoral growth is unpredictable. We have studied proximal femoral growth in 40 children who had been treated by femoral osteotomy.

Preoperatively, the mean femoral neck-shaft angle was 5° greater on the affected side than on the contralateral side. Postoperatively, it was 28° less. There was progressive recorrection; after five years the angle was not significantly different from that on the contralateral side. In our series 70% of the capital epiphyses became abnormally shaped, taking the appearance of a ‘jockey’s cap’. All the growth plates became angulated but this corrected with time.

Correction of the neck-shaft angle probably results from the more normal mechanical environment provided by reduction. The abnormal radiographic appearance of the epiphysis and growth plate is probably due to the rotation produced by the osteotomy.