The serial radiographs of 64 children with unilateral developmental dislocation of the hip (DDH) were reviewed in order to follow the natural hip development with age after a concentric reduction.

All patients were treated with a closed (22 patients) or open reduction (42 patients) and were followed up to the age of skeletal maturity. The radiographic measurements including the acetabular angle (AA), Sharp angle (SA), center-edge angle (CEA), acetabulum-head index (AHI) and neck-shaft angle (NSA) were obtained in 6 age groups; 0-2 years, 2-4, 4-8, 8-12, 12-16, 16 and over.

The rate of acetabular development evaluated by both the AA and SA, was greatest before 4 years of age and continued to a lesser extent until skeletal maturity. The neck-shaft angle also continuously and gradually improved after the concentric reduction until skeletal maturity. Acetabular coverage of the femoral head, which was evaluated by the CEA and AHI, reached the normal level in the 8-12 year age group and was maintained a normal level through to skeletal maturity.

The hip dysplasia continued to improve until skeletal maturity. Therefore, it is recommended that secondary surgery should not be performed if the X-ray shows continuous improvement and a concentric reduction is maintained.