The Acetabular Index and the Physeal Angle of the proximal femur are a radiographic assessment of the morphology of the acetabulum and the proximal physis, respectively. Their values to decrease with age and it remains unknown whether any correlation exists between them or if weightbearing has any influence. X-rays belonging to 30 infants (60 hips), 4 boys and 26 girls, were studied between 2003 and 2006, measuring the Acetabular Index (AI) and the Femoral Proximal Physeal Angle (PPA). Measurements were taken using a goniometer (error ± 1°). All the cases had ultrasound scans at 4 months of age, with alpha angles smaller than 50° (Graf type IIa) and cephalic coverage between 33% and 50%. Anteroposterior hip X-rays were taken at 3 months (pre-weightbearing) and 4–10 months (post-weightbearing). Statistics: t-Test and correlation.

The AI was 21.5° (19.5° boys, 21.8° girls) pre-weightbearing and 20.9° (20.8° boys, 21° girls) post- weightbearing. The PPA was 76.5° (75.9° boys, 76.6° girls) pre-weightbearing and 74.9° (75.5° boys, 74.8° girls) post-weightbearing. AI and PPA decreased pre- and post- weightbearing, 2′8% and 2′1% respectively. The decrease was considered significant in the PPA (p = 0.02), especially in girls (p = 0.009), and not significant in the IA. Differences were found between sexes: the AI increased in boys (+6.3%) and decreased in girls (−8.3%), and the PPA decreased in both boys (−0.5%) and girls (−2.3%). The side had no influence. No relevant correlation was found between AI and PPA, both pre- (r = − 0.15, p = 0.27) and post- weightbearing (r = − 0.24, p = 0.07).

We did not find any relevant correlation between IA and PPA values, neither previous to weightbearing, nor in the months after weightbearing occurs. The measured angles suffered a decrease after weightbearing but the only significant decrease was in the PPA.