Ultrasonic measurement of acetabular alpha angle to evaluate developmental dysplasia of the hip: differences between AIUM guideline and European consensus.

Purpose: To clarify the difference between the alpha angles measured by European method and by AIUM method.

Materials and methods: In this prospective study, infants less than 6-month-old with clinical suspicion of DDH were enrolled from the pediatric outpatient service in a large provincial teaching hospital from October 2022 to October 2023. The hips were examined according to the protocols described by AIUM guideline and by European consensus. The acetabular alpha angle was measured on the standard sections. Statistically significant difference between groups was determined by using a paired-sample t test.

Results: A total of 225 infants (median age, 4.0 months [interquartile range, 3.3-5.2 months]; 104 boys) were enrolled into the study. The mean value of alpha angle measured by AIUM method (62.1°; 95% CI 61.7, 62.4) was smaller than that by European method (63.4°; 95% CI 63.1, 63.7) (P < 0.0001). The 95% limits of agreement between alpha angles measured by the two methods were - 6.6 to 9.2 degree. Alpha angle measured by AIUM method could diagnose DDH with low sensitivity (56% [9 of 16 hips; 95% CI 31, 79]) and specificity (78% [300 of 384 hips; 95% CI 74, 82]) with Graf's technique as a reference standard. The position of transducer arranged on the skin of infant by AIUM method was 1.6 ± 0.3 cm back to the position by European method.

Conclusion: Alpha angle measured by AIUM method is smaller than that by European method and should not be applied to Graf typing of DDH.