Plain radiographs are of limited value for diagnosis in the newborn child because the femoral head and acetabulum are largely cartilaginous. Ultrasound scanning is the investigation of choice to evaluate DDH in infants younger than six months of age and is useful to diagnose more subtle forms of the disorder when clinical exam is equivocal. It is also the only imaging modality that enables a three-dimensional real-time image of a neonate’s hip. This investigation is highly observer-dependent and may over diagnose dysplasia, especially in the first six weeks of life. Infants may be subjected to unnecessary treatment, but in many countries of the Western world, routine ultrasound screening is recommended. It is difficult to perform routine screening in developing countries due to limited resources and expertise. However, it is generally agreed that infants belonging to the high-risk group (as mentioned above) need to undergo a screening ultrasound to diagnose DDH.
As the child reaches three to six months of age, the dislocation will be evident on X-rays but the examiner must be familiar with landmarks of the immature pelvis in order to identify the abnormality. In infants, the upper femur is not ossified and most of the acetabulum is also still cartilaginous. There are several classic lines that are helpful in evaluating the immature hip.
Hilgenreiner’s line is a line through the tri-radiate cartilages. Perkin’s line, drawn at the lateral margin of the acetabulum, is perpendicular to Hilgenreiner’s line. Shenton’s line is a curved line that begins at the lesser trochanter, goes up the femoral neck, and connects to a line along the inner margin of the pubis. In a normally located hip, the medial beak of the femoral metaphysis lies in the lower, inner quadrant produced by the intersection of Perkin’s and Hilgenreiner’s lines. Shenton’s line is smooth in the normally located hip with no step off. In the dislocated hip, Shenton’s line has a step off because the femoral neck lies cephalic to the line from the pubis