US examination of the hip for ddh is done to assess anatomy/morphology of the acetabulum and the stability of the joint during dynamic/stress imaging. Anatomy of the joint is assessed with static coronal and axial images that assess the depth of the acetabular cup and its relation to the labrum and location of the femoral head (whether or not it is dislocated out of the cup). There are four types of hip maturity according to the Graf classsification (type one is a normal hip, two can vary from physiologic immaturity to mild dysplasia, three is a dislocation, and four is a high dislocation). Joint stability is assessed by imaging the femoral head and acetabulum during a Barlow maneuver (laxity in the joint can be assessed visually and felt clinically by attempting to dislocate the hip). (1)
   On the US films, the femoral head and acetabular articulation is assessed with the alpha angle. Lines are often drawn directly on the film to delineate the articulation. The P line (Perkins line) is drawn vertically in a frontal plain through the iliac crest, tangential to the outer acetabular bony margin, and pointing down the shaft of the femur. The H line (Hilgenreiner line) is drawn perpendicular to the P line, in a horizontal orientation, through the triradiate cartilages (on diagram, where the ilium, ischium, and pubic bones intersect at the acetabulum, in a frontal plane). The A line is tangential to the bony acetabular roof, and forms the acetabular angle at its intersection with the H line. The alpha angle is the intersection between the A and P line. In a shallow acetabular joint, the alpha angle is decreased. Greater than or equal to 60 is a mature hip, 50-59 is physically immature or delayed ossification, 43-49 is subluxable, and less than 43 degrees is dislocated (with or without structural alteration) or severely dislocated. (2)