Natural history of hips with borderline acetabular index and acetabular dysplasia in infants.

Management of clinically stable hips with an increased age-related acetabular index (AI) remains controversial. The authors' purpose was to document the natural history of such hips. Sixty-eight clinically stable hips with an increased age-related AI were followed up for a mean of 9.5 years. Four patterns of evolution were observed: type 1 (25 hips) had rapid progression to normal range in the first 2 years; type 2 (19 hips) had slow improvement, with AI values reaching the normal range between 4 and 11 years; type 3 (20 hips) had improved hip morphology with persistence of minor deviations from the normal values; and type 4 (4 hips) showed no improvement. Initially, none of the hips with type 1, 2, or 3 pattern showed radiologic signs of displacement; in contrast, all our type 4 hips were radiologically eccentered. Clinically stable and radiologically well-centered hips with an increased age-related AI improved spontaneously without treatment. For these cases, clinical and radiologic follow-up is all that is needed.