Natural history of hip instability in infants (without subluxation or dislocation): a three year follow-up

BACKGROUND The natural history of hip instability (without subluxation or dislocation) and treatment in infants remain controversial. We performed a retrospective cohort case-only study with blinded, prospectively collected data to assess normalization of the acetabular index in consecutive untreated infant hips with sonography instability. METHODS Consecutive hips meeting inclusion criteria were followed by sonography/radiography and data analyzed using tabular and regression models. RESULTS In 48 hips, acetabular index measured by radiography normalized within 3 years of age without treatment. Normalization by age occurred: 7 months in 35%, 12 months in 67%, 18 months in 75%, 24 months in 81%, and 36 months in 100%. Two patterns of normalization of the acetabular index were observed: group I showed ossification in a physiological range of normal by 7 months of age, and group II had delayed ossification with later normalization of the acetabular index measurement. Breech presentation (p =0.013) and cesarean delivery (p =0.004) statistically directly correlated with a later normalization. CONCLUSIONS The natural history of infant hip instability (without subluxation or dislocation), which is reduced at rest and unstable with stress as diagnosed by the Harcke method of sonography, has spontaneous normalization of the acetabular index within 3 years of age. We suggest three patterns of acetabular ossification in unstable infants' hips: (I) normal ossification, (II) delayed ossification with normalization of the acetabular index by age 3 years, and (III) defective secondary centers of ossification with an upward tilt of the lateral acetabular rim in adolescence.