Ultrasound (US) of the infant hip is used to diagnose developmental dysplasia of the hip (DDH). We present a new sonographic sign that describes the periphery of the femoral head and the acetabulum as two concentric circles.During 2008−2019, 3650 infants were referred for diagnosis of DDH. All underwent a clinical and US examination. We recorded the femoral head as the inner circle, within a fixed external circle, which was identified as the acetabulum. We analysed the clinical signs and risk factors. The US sign of two concentric circles was normal in 3522 infants and was classified as normal hip development. The alpha angle was >60° in 3449 (95%) infants. For the remaining 73 (5%) infants, the alpha angle was 50−60° and underwent further follow-up examination until the alpha angle was normalised. In 128 babies (3.5%), we detected the disruption of the concentric circle sign; the femoral head was found outside the acetabulum, which appeared with an upward sloping roof and the alpha angle was <50°. These infants had DDH and received appropriate treatment. Infants with a concentric circle sign and normal alpha angle are normal, whereas those with a disrupted sign are considered as having DDH.
Figure 1 Representative image of a concentric double circle. (a): The inner circle is the femoral head and the outer circle is the acetabulum. (b): The arrows point to the boundaries of the head and the acetabulum.
Figure 2 Normal concentric circles with the spherical femoral head lying inside the spherical subchondral acetabulum. (a) left and (b) right hip.
Figure 3 Bilateral normal ultrasound with the presence of ossific nuclei of the femoral head, showing the normal concentric circles sign. (a) left and (b) right hip.
Figure 4 Bilateral DDH with disruption of the double concentric sign on the left and right side. (a) The acetabulum is elliptic and the head has a spherical shape (b) The arrows point to the femoral head and the sloping acetabulum. There is increased distance between the head and the depth of the acetabulum.
